Sunday, November 27, 2011
Tangent Error Minimized Tracker - A Double Arm Drive
Double Arm Drives have been used to photograph the night sky for over 20 years. Originally designed by Dave Trott, based on the Haig or Scotch mount (otherwise known as a Barn Door Tracker), the Double Arm Drive is a platform used to track and capture images of celestial objects, using slow shutter speeds necessary for this type of photography. The proposed design, while conventional, attempts to refine the tracking performance of the double arm design. Hence, the Tangent Error Minimized (TEM) Tracker - this is the prototype.
Basic image acquisition
Basic image processing
Notes:
Alternate methods of tangent error reduction in barn door drives include cams and varying motor speed. The point of the Tracker design, is to minimize tangent error while retaining simplicity of construction and operation.
The Tracker need only be driven at a steady rate of 1 rpm by hand or with a drive mechanism - in this case a programmable microprocessor (Arduino), precisely governing the speed of a stepper motor - Google
Vibration is inevitable when using stepper motors. A reduction system such as a gear box, gear train, or pulley system between the motor and drive shaft will reduce vibration significantly - Google.
TEM Tracker
View complete set of Tracker images
Front and rear views, with the original resonance dampers - the cork and spring arrangement is surprisingly effective. A reduction drive is more effective. The original build is shown here for illustration purposes.
The Equatorial Wedge (EW) provides active adjustment of altitude, particularly useful for refining tracking performance on the fly - limited to a range of latitudes in which the device is expected to be used. If attached to an adjustable tripod, directly to the Altitude board, the Azimuth board is not required, and may be omitted. Although, an EW is a more rigid design and easier to set up.
Notes:
For simplicity of construction the Conventional Layout is recommended - some cross referencing of the Tracker Plan is required. Observe the dimensions and ensure that the Points of Rotation are aligned when the Tracker is closed. All Tracker dimensions are metric (unless otherwise stated), including the Drive Shaft thread.
For the non-metric world, imperial measurements for use with the 1/4 inch 20 tpi drive screw can be found at the bottom of the page in Appendix;
The algorithm used to calculate the Tracker dimensions was kindly supplied by my brother. For any combination of Drive Shaft thread (metric or imperial) and rate of rotation, the algorithm calculates Critical Dimensions - that is, Drive Arm and Camera Arm length and corresponding hinge-to-hinge distance. Any mistakes are mine!
“Section 1” Development and Testing
Double arm design seems to be a set of compromises, trade offs, to minimize deviation about mean performance. An in-depth analysis can be found here.
The dimensions of the TEM Tracker are set at their current values because, among other things, they provide very accurate tracking in the first 15 to 20 minutes and subsequent tracking error is minor to 60 minutes. A design goal was accurate tracking for up to 60 minutes. In practice, accurate performance has been observed beyond 60 minutes.
A simple method to resolve tangent error during the final 40 minutes of tracking was not immediately evident. Varying motor speed or fitting a cam, while effective, is not preferred because of undesired complexity, whereas, constant motor speed can be replicated in a variety of ways.
An alternative method was found and, for want of better terminology, is simplicity itself. While experimenting with arms and arcs, using a 2D CAD program, the answer to the problem became evident. Raising the Camera Arm hinge tilts the arc followed by the Camera Arm backwards, reducing tangent error, retaining accuracy during the early stages of operation.
Geometry
Definitions
Siderial rate: The rate at which the Earth rotates on its axis - approximately 15.0416 degrees/hour.
Drive cycle: From boards closed to 60 minutes (zero to nominally, 15.0416 degrees).
Contact Point: The physical point at which the Drive Arm lifts the Camera Arm - 349.95mm (350mm).
Optimal Contact Point: The position at which the contact point ‘would’ intersect the Camera Arm, if it were to move (optimally) throughout the drive cycle. In practice, too complex.
Points of Rotation: Hinge and pinion centres should line up when the Tracker is closed, except the Camera-arm hinge which is slightly elevated. Why is it important? The performance of the Tracker is predicated on this arrangement - its the zero datum. What really matters is that the geometrical relationship between the components is retained.
Methodology
A spreadsheet was used to calculate Drive Arm and Camera Arm dimensions with tracking tolerances set to 4 decimal places of a degree, using the following fixed parameters;
motor speed, 1 rpm ; drive screw pitch, 1 thread/mm (6M (6mm) or 8mm fine - which has the same 1 tpmm pitch as 6M).
Camera Arm - Drive Arm Trend
Optimised angular displacement of the Camera Arm was calculated to 4 decimal places at 1 minute intervals for 60 minutes; i.e., 15.0416/60. Optimal contact points were determined to match the displacement of the Camera Arm at these intervals. The start and end points being 349.95 (350mm) and 347.11mm, respectively.
With the contact point fixed at 350mm (349.95mm) the Camera Arm is driven through 14.9517 degrees (in 60 minutes). If the contact point is fixed at 347.11 mm the Camera Arm is driven through 15.0416 degrees, which is optimal, but problematic because error is introduced during the early part of the drive cycle. The object is to drive the Camera Arm between these two points and take advantage of accurate performance at both ends of the drive cycle. This can be achieved by raising the Camera Arm hinge 0.4mm (4 thicknesses of 80gsm paper).
Calculating 30 contact points (two minute intervals) made it possible to verify the arc derived from the CAD program; angles subtended from the Camera Arm hinge to the Camera Arm arc correspond very closely to the optimal contact points.
Performance
A Canon G9, fitted with a 2x tele-converter lens with the camera lens set at 24x digital zoom, an approximate focal length of 1600mm, was used to take 10 x 64 second exposures (Spica, southern hemisphere) over 22 minutes, of which 5 were stacked, showing no apparent trailing. The others, subject to atmospheric distortion and vibration, due to construction faults, were discarded. Similarly, trailing was not apparent. Spica1 and Spica5 are the first and last in the series of 10 exposures.
Accurate tracking was observed > 30 minutes; that is, 15 minutes to resolve polar alignment using the drift method, 10 minutes to verify tracking and 22 minutes of photography, including a period of approximately 5 minutes where the setup was unattended after the shooting cycle was complete.
Spica - 5 x 64 second exposures over 22 minutes
Software control of motor speed is optimal because it eliminates a variable that tends to mask other errors, such as construction faults and/or poor polar alignment.
Programming an ATMEL ATMEGA168 microprocessor on an ‘Arduino’ board mated with a motor shield is an effective solution (if you know what this means, you may wish to skip this section). This arrangement was used to test the tracker.
Alternatively, inexpensive electronic control, using one of the many circuits to be found on-line may suffice. Other methods include, utilizing a 1rpm clock motor, or gearing down a DC or Servo motor. Otherwise, the device may be hand driven by reference to a time piece to produce acceptable wide field images.
Electronic control may not provide consistent performance, particularly where variations in ambient temperature affects the timing of the circuit (a function of resistance). There are ways to compensate for this, and it is recommended that if choosing this type of circuitry, adding temperature compensation is essential. However, using an oscillator/crystal type circuit is probably a better solution.
Polar Alignment
Accurate Polar alignment is essential! (Google)
Note: First, level the tracker, preferably with a circle type spirit level, or these days, a smart phone level. Align the axis of the camera arm roughly true South (SH), true North (NH). A compass with magnetic variation applied, Google map, survey maps, street directory, GPS may assist with the direction of the poles in relation to property boundaries.
I use the following procedure and find it more accurate than optical polar alignment for this type of mount.
1. Point the camera comfortably (for your neck and back) near the Celestial Equator, closer to Zenith than the horizon.
2. Expose for 10 seconds and check the image for star drift (trails instead of points of light) - adjust the tracker in Azimuth opposite to the star drift (horizontally) and Altitude (vertically). During the next exposure of 20 seconds the star trails will hopefully be shorter. If you adjust the wrong way the star trails will get longer, so adjust back the other way, plus a bit more.,
3. Repeat this process, increasing the length of exposure with each try. Eventually, tracking error will be minimized to allow for approximately 3 minutes. Don’t expect to achieve 10, 20 or 30 minutes, besides which there is no point. Testing was done with specially made vernier adjustment, specifically for verification purposes.
Start with 10 seconds (gross error check), increase to 20 seconds, then 30 seconds, achieving good tracking at each stage, and then increase to 60 seconds. If tracking is good at 60 seconds, there’s a very good probability that this is sustainable to 2 or 3 minutes.
4. Expose for 2 or 3 minutes, depending on whether you choose to expose for 2 or 3 minutes and check for drift. Resolving errors in Azimuth and Altitude.
Notes:
The reason this method is successful is that it accounts for refraction of the position of the Celestial Pole through the atmosphere.
If you’re in a suburban light polluted area, 3 minute exposures is as much as you will need (iso800). Any more and the sky glow will dominate the image, any less and the signal from the image you are taking will be drowned in noise.
If exposures are too long they will be over exposed. Lots of optimal exposures stacked together is the best method for a good image.
To verify the authenticity of tracking, in-camera software (CHDK) was used to combine/stack the Spica images (at that time they were not stacked using a computer program that automatically rotates and aligns images), further noise reduction and conversion to jpeg was done with GIMP. The more recent image was combined in GIMP. Very little adjustment was required to align the Spica images - a few pixels at most.
Spica
Note:
CHDK provides slow shutter speeds to 64 seconds (now much longer), besides other functions. Scripting allows automated multiple shots with a single press of the shutter. The process can be interrupted between exposures by pushing the shutter release button.
“Section 2” Design and Resources
Design criteria; construction possible using hand tools - a drill press is a handy addition;
simplicity retained throughout;
‘critical dimensions’ easily measured and reproducible;
tracking accurate up to 60 minutes - this seems adequate for most astrophotography;
final build tracking within 2 decimal places of a degree or better;
the device portable and easily deployed in the field;
lightweight and rigid.
Tracker Plan
Providing the Critical Dimensions, Points of Rotation and other design conventions are observed, performance should be consistent in various configurations.
Note:
The centre line through the Drive Arm hinge and the Drive Nut pinion, including the motor mount hinges (Points of Rotation), when the device is closed, represents the intersection of two planes. A third exists between the centre of the raised Camera Arm hinge and the Contact Point. These ‘virtual’ planes are the design datum; it is important that construction proceed with this concept in mind, otherwise design performance cannot be guaranteed.
Construction Tips
Not all pairs of boards are square, even if you had them cut at the timber yard. Choose the squarest end of each board and mark the edges; be careful to measure to these edges. The square ends should be at the drive end of the board the measurements referenced to a common datum. In other words, don’t introduce error during construction by assuming that the boards are square.
Preparing the drive end, before committing to other measurements, referenced to the centre of the Drive Shaft, is preferable, making sure that the 20mm (nominal) drive shaft holes in the top and bottom boards are aligned prior to marking the location of other components. That is, marking out the motor/drive shaft assembly end first, will minimize construction errors - readily corrected during this stage.
The boards pictured are 17mm ply coated with laminate - a cut-off picked up at a timber yard. This material is used for concrete form-work and is very stable - resists warping etc. The Camera Arm is cut out of the drive arm. 12mm waterproof ply is acceptable, but is getting a bit light.
Drive-end Construction
Notice that the motor is mounted on the top board and hinged. It may be mounted on the bottom board in a similar fashion - a matter of preference. Importantly, the centre of the drive shaft should be coincident with the centre of the motor mount hinge and the centre of the Drive Nut pinions. It may be necessary to ‘pack the motor up’ to provide clearance between the Drive Nut and the motor shaft.
An easy way to make Drive Shaft pinions, and have them match up with the Motor Mount hinges, is to cut the ends off the hinges to be used for the Motor Mount. The part with the pin is retained (see photo); additional holes are drilled to accept locking screws, once the Drive Nut pinions have been mounted and centered - use tape to hold things in place while drilling.
Another refinement is the use of springs on the pinions to minimize slack in the assembly. Alternatively, remove the pins and tap threads to fit grub screws for centering the Drive Nut (recommended). The most difficult task was drilling the holes in the sides of the Drive Nut and ensuring that they were concentric.
While it is important to ensure that everything is properly aligned during construction, it is recommended that the Tracker be started slightly open - say 10 - 15mm - to stabilize the drive shaft and pinion. With the Tracker closed the drive shaft tends to lean due to its proximity to the drive nut pinion assembly. A bit of triangularity stabilizes the drive.
Nylon nuts and bolts can be easily modified with side-cutters, and are useful replacements for hinge pins and pinions - they tend to reduce the transmission of motor resonance. Nylon threads are noticeably tighter.
Tip - place a small ball of Blutak on the end of the screw before pushing it into the hinge - this will further isolate hard surfaces without compromising rigidity.
Tracker - Drive End Assembly - Motor Mount - Contact Point
Drive Shaft & Nut Assembly (replaced with a nylon sleeve and plastic tube insert tapped to 6mm)
Azimuth and Altitude boards
If intending to mount the drive on an adjustable (sturdy) tripod, the azimuth board may be omitted. This part of the design is flexible.
Camera Mount
It is advisable to place a layer or two of Neoprene under the Camera Mount bracket to reduce transmission of resonance. Having said that, an improved Motor Mount is needed. Be careful of heavy telephoto lens that may topple the Camera Arm - restraint or a counter balance may be necessary.
Two hinges are better than one for the camera arm, tending to be more stable. A wider footprint for the camera arm and drive arm is recommended - use the alternate layout and build the unit a little wider for stability, but not so wide that it becomes cumbersome.
Electronics
The Printed Circuit Board (PCB) is designed as a motor shield and fits on top of the Arduino board. It utilises an L293D or SN754410NE H-Bridge bipolar stepper driver, and a ULN2003AN (or similar) to drive a unipolar stepper motor. Primarily, logic is used to control the motor function. A three position switch selects Forward, Stop and Reverse and a ‘Kill Switch’ stops the motor once the Drive Arm is back in the start position; the motor is held in position with its coils energised. Turn off supply power to rotate by hand, if necessary.
Limit (Kill) Switch
The L293D is probably a better choice because it has in-built protection to prevent damage to your Arduino from voltage spikes generated by the motor; the SN754410NE does not. However, the use of the Arduino pull-up resistors may well serve to provide additional protection; no problems have been experienced to date.
The L293D and SN754410NE use two separate power sources, one for the chip and one for the motor. As such, the motor shield is designed to provide several control configurations. For example, the SN754410NE may utilise a “power-off” kill switch, or the Arduino logic. Similarly, for the L293D, the board may also be configured to remove power from the logic and power supply. This is more derivative, through design evolution, than a deliberate feature.
The ULN2003AN Darlington Array, drives a 5 or 6 wire uni-polar motor. Changing the pin allocation in the ‘Global’ section of the ‘Wiring’ program is necessary with the current program.
Fitting a heat sink to the 780x (x = the motor supply voltage) IC and attaching a cooling fan will be necessary where more powerful stepper motors, consuming large amounts of current, are used. A 5 volt 4 wire 200 or 400 step bi-polar motor, or 5 or 6 wire unipolar is adequate for the job, unless you have other requirements. Besides, there are several motor shields available for the Arduino if you prefer an alternative, for some reason.
Motors
Stepper motors can be purchased at most electronic stores or on-line. Many sites speak of using motors from old dot matrix printers. A suitable motor with sufficient torque can be purchased for about $20. These motors usually have 200 steps/revolution. Less expensive hobby motors have as few as 48 steps (probably too coarse for digital photography at high magnification used without using a gear box). Don’t forget to change the motorStep line of your Arduino script to suit your motor. If using a gearbox of some kind, increase the speed of the motor for the gearbox reduction - for a ratio of 5:1: motorSpeed must be 5rpm to maintain 1rpm at the drive shaft.
Arduino motor shield
Arduino Resources
Direction and Kill Switch wiring
StepperDriver.brd (Eagle Board Milling)
StepperDriver.pdf (PCB Etching)
Notes:
Copy and Paste the Arduino code to your editor and upload to the board.
The PCB pdf file prints the actual size of the shield to fit the Arduino (Decimilia or similar) - it was printed directly from Eagle. Print to a transfer medium then iron onto a single sided board for etching. It may be wise to print to paper first, cut out, and check for fit with the Arduino board. A Laser printer is required, as well as a 1mm and 0.8mm drills, fine hacksaw and file to cut to shape.
Refer to the parts list and use the image of the Arduino Motor Shield for guidance (note the two jumpers - logic setup). The 100uf capacitor is nearest the diode and 4 pin connection header, the 1uf capacitor is at the back of the shield. The L293D (SN754410NE) is the IC to the front of the image/board. The ULN 2003AN is located at the back of the board.
The Direction Switch is an 8 pin 3 position sliding switch. Terminal layout as shown, is 3 + 1 and 1 + 3. The limit switch, when closed, sets Pin2 LOW. Note, that in the Stop and Reverse positions Pin 3 is always LOW. Forward, sets Pins 2 and 3 HIGH, overriding the limit switch.
If problems are experienced getting the stepper motor to rotate; i.e., it ticks one way then the other, the motor wiring will need rearranging in the socket. If the motor turns the wrong way, plug the socket in the opposite way.
If intending to have a board made commercially, use the “Eagle Board Milling” file.
The “PCB Etching” file has bigger pads to improve adhesion during image transfer (ironing) and provides more copper for better adhesion to the board.
Warning the program makes use of the pull up resistors on the Arduino board for voltage protection. No resistors have been used in this design. Use of the L293D is recommended because it has in-built protection.
“Section 3” Concluding
It has been 2 years since designing the Tracker, and it is safe to say that it will consistently provide accurate tracking, consistent with accurate polar alignment, for 3 - 4 minute exposures, which is adequate.
Demonstrated performance, more than 30 minutes, was achieved under controlled conditions, specifically to verify the design parameters - which it did quite well
Appendix
Large Imperial version:
Similar profile to the Metric version, for exposures up to and beyond 60 minutes - say 90 minutes.
Drive Arm hinge - Drive Nut pinion / Drive Shaft centre = 16 inches; Drive Arm hinge - Contact Point = 14 inches; Drive Arm and Camera Arm hinge = 4 inches. Pack up the Camera Arm hinge with 2 layers of 80 gsm paper, because the uncorrected error after 60 minutes is half that of the metric version.
Compact Imperial version (see Section 3 Acknowledgements):
Indicates superior tracking up to 40 - 45 mins with no camera arm correction (packing up, as in the tracker design) and may be ideal for hand driven exposures of shorter duration. A computerised motor driven version should demonstrate exceptional tracking to 42 minutes - more than enough.
DA hinge - DN pinion / DS centre = 14” ; CA hinge - CP = 12.92” ; DA hinge - CA hinge = 1.9”. No packing is required. Calipers may be useful for measuring down to 1/100”.
Acknowledgements
Dave Trott,, the original designer of the Double Arm Drive, proposed the concept in the Sky and Telescope magazine, 1988. Containing a wealth of information, his web-site is also beautifully designed.
My brother, the interested sceptic, and the brains behind the spreadsheet. Without whom this project may not have had the impetus to continue. The spreadsheet enabled experimentation with various component dimensions.
Mike Mohaupt - whose Compact Imperial design prompted further research to optimise the performance of his very accurate design which provided the data for 1/4” 20tpi dimensions.
Open source software (Linux) - Qcad, without which the idea would have escaped my attention.
Eagle PCB software and the Arduino Decimilia provided the tools to develop the electronics platform to drive the stepper motor.
Not forgetting Stellarium an excellent open source desktop planetarium.
GIMP the image manipulation program, another open source astronomical imaging tool.
The CHDK developers and many excellent sites devoted to digital astrophotography and Double Arm Drive design.
Licence
This work is licensed under a Creative Commons Attribution-Noncommercial 2.5 Australia License.
Disclaimer
The information on this site is provided in good faith. The author/owner of the material of this site accepts no responsibility for reader/user outcomes, of any nature, directly or indirectly associated with this and/or any other site associated with, or affiliated, by any means or interpretation. Please use the information freely, at your own risk.
Recent reading
Science
The Smithsonian Intimate Guide to the Cosmos - Visualising the Realities of Space (Dana Derry). Beautifully illustrated. A pictorial guide to the universe, with some informative and interesting comment.
The special theory of relativity (Albert Einstein - of course) - Does this really need an introduction? Time, it seems, shapes not only the universe constraining its limits, but shapes the lives of everyone.
General
The Sound of One Hand Clapping (Richard Flanagan) - Set in Tasmania. The style of writing is unusual but effective. A must read.
Galileo: A Dramatised Life (Gerald Smith) - An interesting approach that traces the life of Galileo in some detail.
Rogue Economics: Capitalisms new reality (Loretta Napoleoni) - If you want to know what drives world economics, Rogue Economics is a must.
Blind Man’s Bluff (Sherry Sontag and Christopher Drew, with Anette Lawrence Drew) - Blind Man’s Bluff traces the history of submarine espionage during the Cold War years. Fascinating reading.
The Brief Life of HMS Trooper (David Renwick Grant) - HMS Trooper served in the Mediterranean during World War II. A technical and personal look at the exploits of Trooper and its crew. If you have an interest in submarine life of that era, this book is a good resource while serving as a tribute to submariners, many of whom were lost as was the Trooper.
White Fang (Jack London) - A classic. More than a story of survival and the pressing need to live in a hostile environment. White Fang is a story about the nature of men, good and bad.
Philosophy and Theology
Bible A must read.
Word Studies in The Greek New Testament (Kenneth Wuest) Wuest has been the richest resource imaginable. Comprising 4 volumes, one of which is the Wuest version of the New Testament (available for multi-function devices, such as the Palm).
Wuest conveys the richness of the Greek language and in doing so elaborates the New Testament demonstrating its consistency.
The Republic (Plato)
Escape From Reason (Francis A. Schaeffer). Escape From Reason is an inspiring work. For those who recognise, or suspect, that discovering the true nature of one’s being is hampered by the conventions of modern thinking, Escape From Reason is a must read.
The Last Superstition (Edward Feser). A rebuttal of atheist writings such as The God Delusion (Richard Dawkins) and that of other prominent atheist authors. A demanding read, but well worth the effort for those wanting to acquire an alternative perspective. Feser exposes the flaws in naturalistic and modernist thinking.
Consciousness And the Existence of God (J.P. Moreland). Very much an academic work, arguing God’s existence in view of human consciousness. Moreland discusses the unlikely emergence of consciousness from physical processes, postulated by a naturalistic world view.
Kingdom Triangle (J.P. Moreland). A must read. Not so academic. Moreland addresses the crisis of this age from the perspective of competing philosophies, Scientific Naturalism and Post Modernism and elaborates the underlying issues that invade human thinking.
Moreland exposes the truths about modern thinking and its departure from God mindedness, demonstrating very clearly where humanity is at, in this day and age.
The Rough Guide to The Da Vinci Code (Michael Haag and Veronica Haag). More informative than the Da Vinci Code.
The God Delusion (Dawkins). One may ask, why list this here, among such worthy works of Christian insight. Richard is entitled to his views. No matter how much he may abhor theists.
Richard’s scornful, passionate objection and need to reconcile naturalism and theism, is an obvious and predictable imperative, with an equally obvious solution - reject theism.
The Scarlet Letter - Nathaniel Hawthorne. Included here, because it illustrates how man gets the Christian message wrong. Written in the 19th Century, Hawthorne illustrates the hypocritical, legalistic notions of 16th Century Puritans. Grace seldom abounds in a climate of self righteousness. The message forgotten so Readily amid the clamour of the fallen human condition.
Jane Eyre - Charlotte Bronte’ The Wikipedia entry describes Jane Eyre as early feminist writing and moralistic. Seen through secular eyes, that’s as close as one might get to the substance of Jane Eyre.
Given the propensity for secularism to misinterpret Christian concepts, indeed, Spirit led behaviour, in the midst of human weakness, it’s not surprising that Jane Eyre is interpreted as such - a poor substitute for the truths within.
Jane Eyre is however, an exposition of human frailty, beautifully characterized in all its light and shade. Above all a story of redemption and the enduring heart of the Christian soul, empowered by the Spirit of God, in the salvation of Christ.
As far as gender distinctions are concerned, Paul the apostle tells the Romans, that all are one in Christ, that distinctions are neither present nor made. Concluding, that whatever distinctions exist are those made by men, not the vestiges of Christ.
The Lost History of Christianity: The Thousand-Year Golden Age of the Church in the Middle East, Africa, and Asia–and How It Died (Philip Jenkins).
Tracing the decline of Christianity in the East.
The message of the gospel is simple - as Paul put it. “There is only one gospel… …Christ crucified…
Fast Infrared (FIR) how-to for applicable Toshiba laptops
I wrote this several years ago, once I figured out how to get IR working. It was primarily aimed at Slackware users, and now includes ubuntu 8.10, 9.04, 10.04 and 11.x - not tested. There seems no reason it shouldn’t work… However, it may be getting a little out of date and reference only.
IRDA (FIR mode) on Toshiba laptop with an smsc-ircc IrDA device and no BIOS setting.
Because the laptop came with IRDA, this was more of a challenge than anything else, and more difficult than first imagined. Most people get SIR working, I didn’t!
Acknowledgements to the various IR sites - GMane in particular.
——————————————————————–
If your laptop (Toshiba) is equipped with an “ISA bridge: Intel Corporation 82801DBM (ICH4-M) LPC Interface Bridge”, and a 24cc controller or similar, it will require the smsc-ircc2 kernel module driver. Patches are added from time to time and may be viewed on Gmane; http://blog.gmane.org/gmane.linux.irda.general
If you want to support a specific combination of bridge and controller Gmane may be a good place to start, to see if your combination is supported.
Distribution
Slackware 11.0, 12.0 and 12.1 running a recent 2.6.x kernel, and more recently ubuntu up to 11.x
Please read the documentation for your distribution.
NOTE: The smsc-ircc2 module is experimental and may break your system.
Software requirements
Latest irdautils, openobex and recent kernel (2.6.17.13 at the time of writing) (still working with 2.6.26.3 and ubuntu 2.6.27-9-generic - plus).
Kernel setup
Not an issue with ubuntu
Networking > IRDA (compiled as modules).
ISA and Serial support enabled (SIR capable).
The smsc-ircc2 module is experimental, therefore it is necessary to set > Code maturity level options > Prompt for development and/or incomplete code/drivers = y.
Please refer to the many howto’s on compiling and installing the linux kernel.
IrDA hardware
PCI by name (the relevant bits)
#lspci -v
00:1f.0 ISA bridge: Intel Corporation 82801DBM (ICH4-M) LPC Interface Bridge (rev 03)
Flags: bus master, medium devsel, latency 0
PCI by numbers
#lspci -v -n
00:1f.0 0601: 8086:24cc (rev 03)
Flags: bus master, medium devsel, latency 0
Install the software and then create the IrDA devices (linux irda howto - 2.6 kernel)
# mknod /dev/ircomm0 c 161 0
# mknod /dev/ircomm1 c 161 1
# mknod /dev/irlpt0 c 161 16
# mknod /dev/irlpt1 c 161 17
# mknod /dev/irnet c 10 187
# chmod 666 /dev/ir*
Set the aliases in /etc/modprobe.d - Kernel 2.6.x requires a separate entry eg. /etc/modprobe.d/smsc-ircc2 will do.
Regardless of options placed in modprobe.d, I chose to pass the required options during modprobe. It was impossible to load the module otherwise.
alias irda0 smsc-ircc2
alias tty-ldisc-11 irtty-sir
alias char-major-161 ircomm-tty
alias char-major-10-187 irnet
For information on your chip, run smcinit
NOTE: Other than for setup, never run smcinit to initialize the smsc-ircc IrDA device - it will prevent IR from working.
#smcinit -v
smcinit 0.5cvs
SIR ioport: 0×3f8
FIR ioport: 0×130
FIR interupt: 3
FIR DMA: 3
Detected IO hub vendor id: 0×8086
Detected IO hub device id: 0×24cc.
Detected Chip id: 0×7a
SIR ioport register write: 0xfe read: 0xfe
FIR interrupt register write: 0×3 read: 0×3
FIR ioport register write: 0×26 read: 0×26
FIR dma register write: 0×3 read: 0×3
Initialization of the SMC 47Nxxx succeeded
Windows device manager indicates the following values
I/O 02f8 - 02FF (SIR)
I/O 0130-0137 (FIR)
IRQ 07 (FIR)
DMA 01 (FIR)
Controller 24cc
There are differences in some values between Windows and smcinit. I used the smcinit values.
The SIR serial device in this case is /dev/ttyS0
but may vary depending on your hardware.
To confirm
#setserial /dev/ttyS0
/dev/ttyS0, UART: 16550A, Port: 0×03f8, IRQ: 4
From smcinit above, SIR ioport: 0×3f8 = /dev/ttyS0 Port: 0×03f8.
dmesg will provide the same information concerning your serial ports, however, it may be necessary to match the correct serial driver with the irda hardware, as they may vary from machine to machine.
To initialize FIR, first disable the serial device
#setserial /dev/ttyS0 uart none
or whatever your SIR port /dev/ttySX is.
Load the smsc-ircc2 module using the values provided by smcinit;
#modprobe smsc-ircc2 -v –ignore-install ircc_dma=3 ircc_irq=3 ircc_fir=0×130 ircc_sir=0×3f8
–ignore-install was not always necessary, but occasionally the module would not load without it? See “man modprobe” for details. ircc_dma=7 works also. Otherwise, the values are fixed as far as I can tell. -v for debugging.
dmesg
# dmesg grep | tail
Detected unconfigured Toshiba laptop with Intel 8281DBM LPC bridge SMSC IrDA chip, pre-configuring device.
Setting up Intel 82801 controller and SMSC device
Overriding FIR address 0×0130
Overriding SIR address 0×03f8
SMsC IrDA Controller found
IrCC version 2.0, firport 0×130, sirport 0×3f8 dma=3, irq=3
No transceiver found. Defaulting to Fast pin select
IrDA: Registered device irda0
Then
#irattach irda0 -s
If all has gone well you should see something similar to this in /var/log/messages
#tail /var/log/messages
Oct 20 16:32:38 localhost irattach: executing: ‘/sbin/modprobe irda0′
Oct 20 16:32:38 localhost irattach: executing: ‘echo xx > /proc/sys/net/irda/devname’
Oct 20 16:32:38 localhost irattach: executing: ‘echo 1 > /proc/sys/net/irda/discovery’
Oct 20 16:32:38 localhost irattach: Starting device irda0
“xx” the laptop - 1 device discovered.
Then run irdadump to verify the whole process. You should see your computer and any device that you used to test the link. In this case a Palm.
#irdadump
07:32:53.284907 xid:cmd 286e7df5 > ffffffff S=6 s=5 (14)
07:32:53.374893 xid:cmd 286e7df5 > ffffffff S=6 s=* xx hint=0400 [ Computer ] (18)
07:32:54.462330 xid:rsp 286e7df5 < 3ea004c9 S=6 s=5 zz hint=8220 [ PDA/Palmtop IrOBEX ] (20)
07:32:55.834526 xid:cmd 286e7df5 > ffffffff S=6 s=0 (14)
xx is the computer name, zz is the Palm username.
And, just to be sure, the following shows the Palm device.
# cat /proc/sys/net/irda/discovery
IrLMP: Discovery log:
nickname: zz, hint: 0×8220, saddr: 0×286e7df5, daddr: 0×3ea004c9
This start|stop|restart script is adapted from the slmodemd script. Added module loading and unloading, to ensure that all relevant modules are loaded before the smsc-ircc2 module, otherwise IR will not work, and to unload the modules when stopping IR, ready for the next start. Not really necessary, but it is cleaner and prevents problems.
NOTE: Don’t use this script in ubuntu
#!/bin/sh
# rc.irda
#
# Start irda
#
irda_start()
{
if [ -x /sbin/setserial ]; then
echo -n “Starting irda:”
/sbin/setserial /dev/ttyS0 uart none
/sbin/modprobe ircomm
/sbin/modprobe ircomm-tty
/sbin/modprobe smsc-ircc2 –ignore-install ircc_dma=3 ircc_irq=3 ircc_fir=0×130 ircc_sir=0×3f8
/usr/sbin/irattach irda0 -s
fi
}
irda_stop()
{
echo “Shutting down irda”
killall irattach
/sbin/rmmod smsc-ircc2
/sbin/rmmod ircomm-tty
/sbin/rmmod ircomm
/sbin/rmmod irda
}
irda_restart()
{
irda_stop
sleep 1
irda_start
}
case “$1” in
’start’)
irda_start
;;
’stop’)
irda_stop
;;
‘restart’)
irda_restart
;;
*)
echo “usage $0 start|stop|restart”
esac
Make it executable - as su or sudo
#chmod +x /etc/rc.d/rc.irda
Run as su or sudo
#/etc/rc.d/rc.irda
Irlan, irnet, rfcomm, phone and pda connections etc, are adequately explained in other tutorials.
I did manage to sync the Palm with my desktop. Set /dev/ircomm0 in Kpilot or Jpilot preferences. If you are using Gnome, it’s under Evolution, Edit>Synchronisation options… menu, or the Gnome Preferences menu.
ubuntu setup: up to and including 11.x should be OK.
1. Ubuntu kernel has smsc-ircc2 module configured.
2. In /etc/modprobe.d/irda-utils add line: alias irda0 smsc-ircc2
3. In /etc/default/irda-utils edit: DEVICE=”irda0” SETSERIAL=”/dev/ttyS0” SMCINIT=”no”
4. In /etc/init.d/irda-setup under FIR=”smsc-ircc2”; add line
OPTIONS=”–ignore-install ircc_dma=3 ircc_irq=3 ircc_fir=0×130 ircc_sir=0×3f8”
NOTE: Invoking smcinit or setting to “yes” in /etc/defaults/irda-utils prevents the operation of IR. Ensure other related modules are loaded before invoking /etc/init.d/irda-utils start.
Radio Astronomy - Square Kilometer Array (SKA)
Readers may be interested in the SKA (Square Kilometer Array) project under development in South Africa. A €150 billion project intended to be the most sensitive instrument available to radio astronomers to date, probing the depths and ‘extremities’ of the cosmos, costing ~€100 million annually to run.
It seems that humanity seeks to discover its origins, to confirm theories and ideas, explore new possibilities and learn more about its place in the universe. For some, confirmation of generally accepted theories such as evolution and the search for extraterrestrial life - for others verification of intelligence and purpose.
Richard Hawking’s once said that “understanding the intricate workings of the universe would be to know the mind of God - a supreme triumph of human reason”. The workings of a clock however, has little bearing on the ability of the user to tell the time - the intricacies are intended for that purpose - cosmologically, if only to sustain life and provide an avenue for consciousness…
Regardless of ones philosophical disposition, radio astronomy provides a window into the unknown regions of the cosmos. Should we inquire - is the money better spent? The argument that mankind has lived quite successfully without this knowledge is contextually correct, because the tools were not available, not because of a perception that knowledge of the cosmos has little relevance to everyday life - it has, because it’s the stuff that builds on the knowledge of who we are - and isn’t that what everyone wants to know?
Saturday, November 26, 2011
Basic astrophotography image processing in GIMP - Part 1: image calibration
It has been some time since I added anything useful to this blog, so here is a tutorial for beginners on astronomical image processing using GIMP. GIMP is open source and free. Photoshop is proprietary software and performs similar functions.
This tutorial is intended to provide a method for photographers of any skill level to try their hand at astrophotography, without the need for specialist equipment. Anyone with a digital camera and a tripod can easily take images of the night sky and produce satisfying results with basic, and in this case free software.
A very basic introduction
This tutorial uses jpeg images, because all digital cameras produce jpeg images. Serious astrophotographers, using DLSR cameras, shoot RAW. Forget about this for now. If you cant shoot RAW, jpeg is just fine.
There are several reasons for calibrating astronomical images. Primarily, to make our images look better, we want to reduce noise and retain detail in the final image, that is, we want to increase the Signal to Noise ratio (SNR). If we expose too long, finer detail is obliterated, too short and the image is dominated by noise. In any case, because we are taking images in low light (at night), noise is a problem. So what is the best exposure time to use?
If you have a tracking device that follows the stars (see the Tracker page, you may wish to build one) and the sky at your location is polluted by suburban lighting, and depending on the ISO setting, 1 - 5 minutes is usual. At a dark site (no suburban lights), exposures will be much longer - counter intuitive, perhaps, and a separate discussion altogether. Astrophotography can be complex. Here we wish to deal with the basics.
If you have a fixed tripod, you will most likely use a high ISO (1600 - 3200, perhaps higher), a short focal length lens, wide aperture and short exposures. At 24mm stars start to show trailing after about 10 seconds. This tutorial is based on the calibration of a single 10 second exposure - 24mm, f/4.0, iso1600. Don’t be put off by all of this. If you have any sort of camera with a manual setting, particularly the ability to take longer exposures, that will do for now - this tutorial is designed for you.
You will need to know where to find certain functions in GIMP. Referring to the File menu - I use the convention, File > Open as Layers, to indicate that you need to select File and Open as Layers. Another, Image > Flatten. And, Windows > Dockable Dialogs > Layers. Another, View > Zoom > 4:1 (400%). You will use all of these in this tutorial. Photoshop has something similar.
Why calibrate?
Digital images taken under low light conditions are noisy - produced by the electronic, thermal and optical properties of the camera, lens and ambient noise. Ambient noise changes from image to image (basically, a consequence of light conditions at the time). Because ambient noise is random, combining several images averages it out remarkably well, and we can pretty much forget about it for now.
Sources of noise…
Electronic (Bias) - when the camera takes a photo the sensor is activated electronically and this leaves a characteristic pattern (cross hatch - canon 1000D) of noise, easily seen in a low light image. We take a bias frame and subtract it from our image or images.
Thermal (Dark) - as the camera sensor acquires an image over a long duration it gets hot. Heat produces a characteristic thermal noise signature. We take a dark frame and subtract it from our image or images.
Optical (Flat) - the combined camera sensor and lens (optical train) has a characteristic appearance that shows up as dust spots, vignetting and variations in individual sensor pixels (not all pixels are equal in operation or light gathering capability). We take a flat frame and divide it into our image or images.
Ambient (Random) - reduced by combining images.
In-fact, it’s a little more complicated in that we take several bias, dark and flat frames and average each set to create a master bias, master dark and master flat, thereby obtaining a better average of each type of noise. And to complicate things further, bias, dark and flat noise is different for each exposure we take.
Let’s leave it at that. There is a good deal more to effective astro imaging. For now, we want to calibrate our newly acquired image and impress our friends and family - if indeed, they can be impressed.
The light image
We will use a single image in this example. The constellation Orion rising in the East. The Great Orion Nebula (M42) can be seen in the handle of Orions’ sword. A front yard perspective - tree, shrubs and power lines. The camera was mounted on a fixed tripod, lens focal length 24mm, iso1600, focal ratio, f/4.0, 10 seconds exposure time. Note the brown characteristic hue of light pollution. Because it’s a single exposure noise is quite evident.
The calibration frames
We begin by taking bias, dark and flat frames. Please note: the images shown have been processed to show the detail. Out of the camera, bias and dark frames appear as black frames. The flat will be a uniform light colour. I used Color > Auto > Equalize, to reveal the noise.
Bias - cover the lens (put the lens cap on, or cover with something that wont let light into the lens) and set the cameras’ fastest shutter speed - no need to change any other settings. Press the shutter release - 1 bias frame.
Dark - cover the lens as before and set the exposure time to that of the light frame. In this case 10 seconds. Press the shutter release - 1 dark frame.
Note the bias noise in the dark frame below. Because the dark is a 10 second exposure, there is not a lot of thermal noise and it looks very similar to the bias. Look closely and you will see the differences.
Flat - a bit more complicated, but well worth the effort, as you will see. This can be done in various ways, and each astrophotographer has their own true and tried method. For our purposes, we want to take an image of the camera sensor and lens, the inside the optics stuff. Focus must be the same as for the light frame. Here is an easy method, adequate for the purposes of this tutorial;
On your computer desktop open GIMP (Photoshop) and create a New frame. Maximize to fill the screen - the default is a white frame. If you wish, place a sheet of A4 paper over the computer display. Hold the camera lens (objective), so that it just touches the paper (as close as possible). Now, set the shutter speed to give an exposure (refering to the image preview histogram) of approximately 70 - 75% (make sure auto focus is off). Press the shutter - 1 flat frame.
Usually, and until familiar with your camera, taking flats may require some experimentation, till you get it right. This flat was taken at iso1600 and is quite noisy. But it’s OK to take flats at the lowest ISO provided by the camera. Note the vignetting at the corners (dark areas) and the dust spots on the face of the low pass filter, which is exposed to the air.
Calibrating the calibration frames
That’s right! First we must calibrate the calibration frames! You may have guessed it, but we need to subtract the bias from the dark, flat and light frames. This is easily done in GIMP (or Photoshop). With GIMP open on your desktop, File > Open as Layers, the dark and bias frames. You may need to go to the Windows > Dockable Dialogs > Layers, and open the Layers dialog, if it’s not already displayed.
It is inconvenient, but the first image opened as layers is automatically set as ‘Background’ - so we are left to deduce which is the bias and which is the other frame - there are only two, fortunately, but still use proper naming conventions; that is, bias, dark, light should be the names of our frames for this tutorial (it’s easy to mix them up otherwise). Set the dark frame as the background image (that is, at the bottom of the layer). The bias will be the image above. Highlight the bias and set Mode to Subtract. Now, flatten the image (Image > Flatten) and save as masterdark.jpg.
Do the same with the flat and light images, saving as masterflat.jpg and biassubtractedlight.jpg - we have our master dark and master flat frames and a bias subtracted light frame. The bias is of course the master bias and the only frame that does not require calibration in this instance.
Now, I have used different names for each image, because that’s what I was using when I created the screen shots for this tutorial. Still adhering to a naming convention that I understand, however, the names suggested above more accurately represent the state of calibration of the dark, flat and light frames.
For an imaging run of several hours, we might take 40 or 50 bias frames, 30 or 40 dark frames and 20 or 30 flat frames and combine the bias frames to create a master bias, and subtract it from the dark, flat and light frames, as we have done here. We may even shoot the flat frames at a lower ISO and require a separate set of bias frames with which to calibrate - naming conventions and ordering our folder structure is very important for smooth execution of the calibration and processing task.
Applying the calibration frames
Let’s keep in mind that the way in which we are approaching this task is slightly different to a sophisticated program such as Pixinsight. But not too different. The principles are essentially the same - we are working with what we have.
With GIMP on the desktop, File > Open as Layers’ the biassubtractedlight.jpg, the masterdark.jpg and the masterflat.jpeg. The biassubtracted light is set as the bottom image, next is the masterdark with Mode set to Subtract. The top image is the masterflat with Mode set to Divide. The result is shown below.
What have we done? Indeed, what have we done? Well, we have calibrated all the images by subtracting the master bias from each dark, flat and light frame and then subtracted the dark from the light frame, dividing the result by the flat frame.
Now! If we had a stack of light images, we would do the same for each and combine them to reduce the ambient noise. Effectively, we have increased the SNR. Take a look at the frame below. There are two very ugly blue (cold/dead pixels). My camera sensor has several of these, as well as bright red hot pixels (always on).
Voila! No blue pixel - it was definitely noise. After applying the master dark the image is looking better. The thermal, reddish appearance is diminished too.
One additional step will improve the result and get rid of that brown sky. If we normalize the masterflat we set the pixel values to upper and lower limits providing ourselves with some colour calibration (This can be done in situ by deselecting the ‘eye’ for the light and dark frames, leaving the flat as the active image. Then select Color > Auto > Normalize. The master flat can be normalized beforehand if desired. Don’t forget to re select the eyes for the light and dark frames.
The result
While originally a single frame, of only 10 seconds, the appearance is greatly improved. Contrast, brightness, smoothness and colour balance, particularly the sky, is a better representation of that captured by the camera sensor, which is much more sensitive than the human eye. Compare the 3 light images at each stage of processing and notice the steady improvement. Now it needs to be cropped… as you please.
A word on combining (stacking) images in GIMP
The composition of the example image does not lend itself to automatic alignment, even though there is a GIMP astrophtography package that has a stacking tool, among other things. Still, it is possible to stack images manually in GIMP. For calibration frames it’s very easy.
For example, if we did take several bias, dark and flat frames, File > Open as Layers, all the bias images and apply the Average script downloadable from the GIMP repository. Flatten and save as masterbias. Do the same with the darks and flats (now we’re getting into the jargon).
It’s a little different with light images because, the stars will have moved between exposures (this is actually a good thing for noise reduction - and similar to dithering). Part 2 covers stacking and alignment, noise reduction and basic image enhancement.