Tuesday, July 3, 2012
Unknown Arthropod: Pycnogonida?
For something completely different.
One day in May last year, I was walking the dogs along a Bass Strait beach, just near Torquay (Victoria, Australia). There is a little fan shaped cove with rocky promontories at either end. The sun was warm, the sea lapping gently on the shore, at low tide. Perhaps it was the light, but as I traversed this section of the beach I noticed a small creature, perhaps 5 - 6mm (leg tips,) hurrying along the waterline, occasionally covered by a light wash. Once the water had receded, this leggy little organism, having resisted the water flow, continued on its way.
A wisp of life, it seemed to be feeding near the low tide water line over smooth packed sand. It hurried here and there, stopping suddenly, then moving on with equal energy. Taking a camera with me that day was fortuitous. I managed to squeeze off one shot in focus while chasing this little fellow around the beach, losing sight of him, standing up to find him again, a bright little speck on the sand.
I made several inquiries to various institutions without much joy. Without a ’sample’ there was little to identify. Eventually, however, and almost a year later, it was suggested that the arthropod is most likely a species of Sea Spider or Pycnogonida, which inhabit the oceans from the shore to the deep, worldwide.
I’m intrigued by what appears to be a proboscis attached by a socket to the front of a dorsal appendage sweeping backward over the body, like a trowel handle. The other legs appear to be arranged asymmetrically. The abdomen gives the appearance of a terrestrial Daddy Long Legs. The eyes, blue and arranged either side of the base of the dorsal appendage. Fascinating little guy!
Wednesday, May 16, 2012
The Bow Tie Focus Mask: an hybrid diffraction mask
The Bow Tie focus mask, described here, is derived from Carey and Lord focus masks, which are types of diffraction gratings, similar to the well known Bahtinov mask.
The bow tie mask was purpose designed to suit a small aperture, short focal length lens. The four obstructions are intended to produce splayed double spikes, similar to the Carey mask, while eliminating the grating typical of focus mask designs. The wide obstructions and absence of grating increases the brightness of the diffraction spikes - discernible with a small lens.
The junction of the obstructions also provides an area of certainty. A central spike perpendicular to the double splay is generated at focus. This spike is not present otherwise. Another phenomenon of this design is the presence of red and/or blue fill within the splay of each pair of spikes.
The bow tie mask is easy to make. A flat section of rigid plastic is easily cut to shape with a hobby knife and steel rule. The clear plastic can be coated with black indelible marker. Sharp straight edges are essential.
Using the bow tie mask is straightforward. Equal spacing of each pair of spikes and the presence of the perpendicular spike indicate focus.
The star used for test images is Alpha Centauri.
Sunday, December 11, 2011
Canon 1000D/XS/Kiss F DSLR cooling modification - images and overview
Designing and building a DSLR cooling system was a way to satisfy my technical curiosity. There is room for improvement in several areas. This should get you started.
Note: It’s advisable to check back here from time-to-time, because I am always thinking of something to add; such as
“don’t upload code to the Arduino while the cooling system is operating - testing for instance - it breaks the MOSFET”; and
Make sure to GND the camera chassis.
Warning: Electrical shock - this design utilizes high ampere electrical current to power the Thermoelectric module (8A 12v). Please do not attempt this project without appropriate knowledge and precautions.
The idea of cooling a DSLR was too interesting to pass up. Of the Canon DSLR’s, the 1000D/XS/KissF is one of the easiest to adapt and is relatively inexpensive. I found it to be a robust little camera, particularly in view of the punishment it endured through several modifications.
Note: Upgrade to the latest firmware
There is little point to cooling without replacing the factory fitted infrared blocking filter, with an astronomical filter, or removing it completely.
Astrodon Inside filter purchased from Hap Griffin. Gary Honis’ modification instructions
Note: the low-pass/antialiasing filter is sometimes touted as a substitute astronomical filter, but also softens focus. All the filters can be removed if using reflecting optics. Refractors, including cameras lenses, need a replacement for the infrared blocking filter, to achieve focus.
Consistent with the above, good focus and proper image calibration and dithering, which is not compulsory, improves signal to noise ratio significantly. The most favourable treatment of DSLR data is adequate dithering.
Very little processing should be required to produce a good looking image with good quality data - taking care of the detail before getting out at night.
These images were captured with this setup - Canon 200mm prime lens f5.6 and f6.3.
The cooling system
Please feel free to change any or all of this.
UPDATE: The circuit below is an improved version of the MOSFET current control. It incorporates two snubbers - voltage clamps - and one inductor. The 17.9R resistor represents the TEC (resistance). I will update the PCB, but for now;
1. Connect a 100uH inductor in series with the MOSFET drain to TEC negative lead.
3. Make two RCD (resistance capacitor diode) snubber networks and place one before the 100uH inductor on the MOSFET drain side to GND; and
4. Connect the second snubber to the 12v supply to positive TEC lead and (as shown) behind the first snubber diode; and
5. Replace the 750ohm pull down resistor with 100kohm 1/4 watt. Caution: depending on the MOSFET, the value of the pull down resistor may differ - in which case, experiment. The R value should permit the gate to go low/off, otherwise the MOSFET will remain on running at full pelt, irrespective of PWM cycles.
Please note: a second inductor 1nH may be placed between the 12v supply and snubber, if needed - EMI will decide this, depending on equipment etc (refer to sketch).
Low footprint arrangement - the smaller inductor may be omitted or use a low value if needed.
UPDATE: I have modified the cooling circuit, replacing a temperature sensor with a humidity/temperature sensor, while retaining the cold finger temperature sensor. The code has been modified also.
PLEASE NOTE: The code is set to default; that is, cold finger temperature will modulate automatically at dewpoint +2C. If you want to use push button control, go to the “void tempCont() {” section and comment out the line setPointT = Td +2; with // (that is, //setPointT = Td + 2;) - please read the code.
The main components of the cooling system are; the cold finger, sealant, thermoelectric cooling module (TEC), heat sink and cooling fan and electronics package and a power supply (PSU 12V 12.5A).
Note: The sensor electronic components, mounted on the PCB attached to the sensor assembly, are sealed with a layer of Blutak (green in this case) pressed on, excluding air from the region. This is overlaid and held in place by the metal cover. Note the top left and bottom right corners, the green, protruding from under the cover. Unfortunately, I didn’t take an image of this stage. It seems self evident from the image below.
Note: Switching power supplies are quite noisy and require a very large value capacitor and a low value capacitor across the supply leads to smooth the output.
Electronics
Please note: the component values listed in the BOM are in some cases Fritzing defaults. To avoid confusing readers, I have silk screened the values of each component where appropriate and these are included in the BOM under ‘Label’.
The main components of the electronics package are; temperature sensors, microprocessor board and software, logic level N channel MOSFET low rds(on), LED indicators and control buttons and an Arduino shield to accommodate components and wiring.
Note: the digital signals and analog signals have separate GND to avoid ground loops.
Temperature readings of the cold finger sensor, are erratic because of EMI. Capacitor values across the TMP36 Vs and Gnd pins is nominal 100nf -100uf.Experiment with values to smooth out wild fluctuations.
Arduino code - if using < Arduino 1.0 change extension to .pde and in the Bounce.cpp file, change Arduino.h to WProgram.h
Etching mirror pdf - Silk Screen (not essential)
NOTE: Solder 8 amp wires between J1 and J2 +ve terminals and Q1 Gate and GND - PSU TEC connections. Alternatively increase the width of the trace in these areas. Or move the terminal off the Arduino shield.
Cooling is controlled by pulse width modulation (PWM) supplied by the microprocessor to the gate of a logic level N channel MOSFET (Low Rds on). Since modifying the circuit, PWM is now scheduled as a function of dewpoint, +2C. The circuit still retains the ability to register cold finger temperature to ambient temperature and set temperature as desired. Energy extracted from the cold-finger and that produced by TEC operation is dissapated by the heat sink, assisted by a cooling fan.
Note: EMI shielding, installed as shown and earthed, shrouds the black and red TEC wires. The braid was harvested from a length of 5mm coaxial cable removing the insulated wire core and outer insulation for greater flexibility.
Set-point temperature in the range ambient to ambient -30C, may be set by two push buttons, or just let the dew point calculation modulate temperature. B1 increments set-point by -2C; B2, -1C. Pressing B1 and B2 simultaneously, registers set point to ambient temperature, resetting the system. Note: irrespective of button pushing, maximum differential never exceeds ambient -30C. LED’s indicate set-point temperature and maximum differential.
Notes
Temperature is maintained by dithering either side of set-point , +/- 0.33C, indicated by a flickering LED. At start-up there is a little hysteresis, indicated by temperature overshooting then undershooting set-point. However, the system is statically stable. If power is removed from the TEC and cooling fan, cold finger temperature response is very slow - the system does not respond rapidly to sensor fluctuations.
Maximum temperature differential is 34C. 30C is a practical working range. Cooling time increases exponentially with increasing differential. All the same, cooling is very rapid, a few minutes at most.
The electronics package may not be optimal, and while based on careful research, I’m not an EE. The same can be said of my heatsink calculations - over-engineered. EMI was a significant problem that was resolved expediently, by grounding the camera chassis, tying a capacitor from the MOSFET gate to GND and shielding the TEC wires.
The switchmode power supply and switching of the TEC, generating EMI through its wiring, were probably the major source of noise. Ground looping was resolved by separating the analog sensor GND from the digital/control channel.
At the heart of the system, an Arduino board provides all the computing power necessary. The analog and digital GND’s must converge at the board. There are 3 GND pins on the standard board.
Essential Reading
The most informative resource for TEC science is the TellurexFAQ. The Peltier (TEC) device is 12v 8a sealed with a QMax of 68.5C.ambient and cold finger temperature
Notes
Completing the imaging set up, remote control of the camera and control of the equatorial mount axii, RA and DEC, is scheduled by a separate microprocessor. Note: two Arduino’s provide the electronics control package separately, for cooling and imaging, which simplified writing the software. This is discussed in another post to come.
The shortcomings of the system are; the heat sink should be located in-line with the optical axis, to improve balance on the equatorial mount; there is no dew protection, except for the heatsink mounting plate, which collects dew and ice, preventing formation on the camera electronics (which are sealed where required). Dew forms on the face of the modified Ha filter without some form of prevention - electronic or dessicant.
There are other precautions that should be taken with this modification, such as; drying the modified sensor assembly in a sealed bag with dessicant, to prevent dew forming inside the assembly; following sealing with silicon; placing the MOSFET in the heatsink cooling fan airflow - a very/ultra-low rds(on) logic level MOSFET should not require a heatsink.
ambient and cold finger temperature
Other Projects
Stargazers Lounge. Without a TEC device.This one too.
Acknowledgements
Tuesday, November 29, 2011
Basic astrophotography image processing in GIMP - Part 2: increasing SNR (image alignment, integration and enhancement)
I thought this section deserved more attention. Leaving off in part 1, we discuss combining images - to use astrophotography jargon, stacking and aligning - more correctly, registration.
Please remember that these tutorials are intended for beginners, using very basic equipment and software. The methodology is the basics of image calibration and processing, but very much hands on, using what we have at our disposal.
Recapping, the purpose of combining images is to increase the signal to noise ratio (SNR); that is, less noise and more signal, improving the overall appearance of our combined final image - our integrated image (more jargon).
We are going to select the best light frames and combine them into a single image. But, noise reduction strategies start before uploading images to our computer. We employ a nifty method during image capture; that is, we make sure that our images are slightly offset one from the other during the imaging session (yet more jargon). The technical term for this is dithering, a science and a separate discussion altogether.
For our purposes however, we will take advantage of our fixed set up. We note that the stars move across the sky and change position from East to West at 15.0416 degrees/hour (the siderial rate), we let the stars drift across the camera sensor between exposures. Of course, after a while the object that we are imaging will drift out of view. For 6 or 10 images there should be no need to recenter our target.
In part 1 we exposed for 10 seconds. Adding a 3 second delay between exposures ensures that a few pixels separate the next image from the previous - in effect offsetting our images. Very crude dithering - effective all the same. And, furthermore, once complete, our total exposure time is 60 seconds vs 10 seconds. However, SNR increases by the square root of the number of combined images. 2 images increases SNR by 1.414 - approximating for our purposes.
So, starting where we left off in part 1, the image below shows the second and third images in our set of calibrated light images - we have already aligned the bottom and second image in the stack. In this case, the third image is selected with Mode set to Difference (and View 8:1, for clarity). This layer is transparent, showing the difference between the two images as they came out of the camera. We can use the drag tool to align the transparent (difference layer) with the image below.
And this is the result in Difference mode. The pixels have been aligned.
We then set Mode to Normal and select the image above, by selecting its ‘eye’ and highlighting the layer, setting its Mode to Difference. As before we drag the image into alignment with the image below, and so on up the stack.
Note: We loaded our images, File > Open as Layers, and need to deselect the ‘eyes’ of the images above the image that we are dragging so that it is visible.
The image below is the first of our image stack (the ‘eyes’ above it are deselected to make it visible). It’s noisy.
Lets see what happens when we average the images; that is, with Mode set to Normal for all images, (all ‘eyes’ selected), we set the Opacity slider of the bottom image to 100% - the default setting. Select the second image and set it to 50%, third to 25%, 4th to 12.5%, 5th to 6.3% and our 6th image to 3.1%.
As you proceed up the layers, note the change - dithering has been to good effect and pixels that were not removed during calibration are hidden behind good pixels. Additionally, because ambient noise is random the image is becoming less noisy. If we had 50 or 100 images, noise would be reduced even further. Still, for 6 images the result is impressive - as below - and much smoother.
Just to finish things off, Image > Flatten, to fuse all the layers together. Apply a sharpen algorithm to the luminous layer. This can be found at, FX-Foundry > Photo > Sharpen > Luminosity Sharpen. You can also use, Filters > Enhance > Sharpen (Smart Redux), or any of the available sharpen algorithms available for GIMP. Avoid the use of unsharp mask if you can. It too, tends to overdo the image (my personal view).
And here is our completed image.
For comparison, the image below is the final image from part 1, which is a single layer, as opposed to 6 layers in the image above.
Comparing the position of the constellation Orion on the frames shown, it should be evident that any one of our light images may be selected as the base or background image, framing the scene as preferred. Terrestrial objects do not align in any case, and we have to live with that.
The availability of free programs to perform calibration, registration and integration, and then using GIMP to finish off with brightness, contrast, colour and enhancement, makes the process much easier. (Keep in mind that images that contain terrestrial objects may interfere with alignment in some programs, essentially designed to align stars).
The next step perhaps, is to use RegiStax or Deep Sky Stacker (DSS) to do all the heavy lifting (calibration, registration and integration of our images) and follow up with GIMP. Now we are getting into serious amateur stuff. But, we can still use our fixed tripod/camera set up, to take beautiful shots of the Milky Way, well beyond the spectrum of the human eye.
Perhaps you need one of these.
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 camera platform used to track and capture images of celestial objects using long exposure times. The design is conventional and attempts to refine the tracking performance of the double arm drive. Hence, the Tangent Error Minimized “Preloaded” (TEM) Tracker - this is a prototype.
However, before proceeding this neat little design may be preferable for some readers. It is small and compact and can be driven using the Arduino electronics described later in this post, if desired. I really like what Gary Seronik has done with this design.
This how to is intended for a wide audience, consequently, there is lots of info and does not assume previous experience… Let’s get started.
Mathematics of the Double Arm Drives at Dave Trott’s site - for the mathematically literate.
A few useful notes?
The unit had to be easy to build and accurate. A steel rule, sharp pencil, basic tools and a small drill, should be all that’s needed. Having said that, built-in adjustments can be used to fine tune performance to overcome minor fabrication errors.
I’ve provided as much detail as possible, along with the Arduino code, PCB template and Eagle board, for those who would like to have a commercially made Arduino shield.
In hindsight, the conventional layout is best - the camera arm, as shown in the images of the prototype, is not too stable and needs restraining to prevent the camera and lens toppling.
Although stepper motors are reliable and accurate, vibration can be a problem. A solution is gearing, which also increases torque at the drive shaft. An ideal solution is the 5v 28BYJ-stepper (Note: although the 28BYJ is advertised as 64 steps, it is acutally 32 steps? Oh well! it’s very inexpensive and does a good job).
Besides the dimensions of the tracker, packing up the camera arm hinge with 4 thicknesses of 80GSM A4 paper (0.4mm), improves tracking overall.
Image Processing
The reader will want to process their images. A low cost solution is GIMP, however, Deep Sky Stacker and Star Tools is a more sophisticated image processing solution.
Basic image processing Part 1
Basic image processing Part 2
TEM Tracker
Front and rear views - the reduction drive is more effective. Increasing torque at the drive screw and minimizing stepper motor resonance.
The Lagoon Nebula M8 and M20 the Triffid Nebula. Composite of 9, 30 second frames.Tail of Scorpio toward the centre of the Galaxy - M7, M6, the Butterfly Cluster and Cats Paw Nebula - 21 30 second frames. Taken with a FujiFilm XPro1, 60mm, f/2.4, iso800 and preprocessing in Pixinsight (Deep Sky Stacker is free) and post processing in Star Tools. I went to the trouble of taking bias, dark and flat calibration frames.
The Equatorial Wedge (EW) provides adjustment of altitude (latitude), 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, as shown above.
Notes:
For simplicity of construction the Conventional Layout is recommended. Accurate dimensions and ensuring that the Tracker is flat when closed will ensure that it performs as expected. 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 the appendix;
Design and performance testing
The dimensions of the TEM Tracker provide for very accurate tracking in the first 15 to 20 minutes of operation and subsequent tracking error is minor to 60 minutes. A design goal was accurate tracking for up to 60 minutes. In practice, performance is very accurate up to 90 minutes.
Geometry
Some helpful 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 centers should line up when the Tracker is closed, except that the Camera-arm hinge is slightly elevated. The performance of the Tracker is predicated on this arrangement - its the zero datum.
and riser detail.
Straining at Gnats
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 tpm 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 and 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 contact points 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.
How did it shape up - 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 - true!
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
To show that the images are aligned and verify the ‘authenticity’ of tracking, in-camera software (CHDK) was used to combine/stack the Spica images. Compare the 5 sub exposures.
Spica 
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 ‘Arduino’ board, fitted with a motor shield provides very accurate and consistent motor speed. This arrangement was used to test the tracker. Alternatively, Google other types of conventional circuitry.
Polar Alignment
Planning is the key to acquiring quality exposures, which depends, in part, on proper polar alignment.
Device leveling, latitude setting and finding True North or True South (depending on hemisphere) is essential to accurate polar alignment - finding TN or TS can be the most difficult and frustrating nightly chore. Setting up references/datums during the day minimizes efforts in the dark when we should be imaging.
If you have access to Google Maps. TN/TS can be referenced to natural lines, buildings or fence lines, by measuring the angle between a reference line and TN/TS (which is, of course, vertically up and down the page (screen shot)).
Locate two legs of the tripod on the reference line and the third perpendicular to the reference line. Now point the axis of the drive arm hinge to TN/TS; that is, the angular difference between the reference line and TN/TS measured from Google map.
Next level the azimuth/base board of the tracker, set the latitude at your location by adjusting the altitude board up or down and check alignment with TN or TS for accuracy.
Having completed this task once, nightly set up at the same location and datums, perhaps marked on the ground, is a 3 minute job. If you have a GPS equipped phone/tablet, record the latitude and longitude of the location.
A polar alignment scope, if you have one, is the traditional polar alignment method - wide field imaging at short focal lengths is tolerant of small polar alignment error.
Shoot an image and check for drift - elongated stars. Make very small adjustments in azimuth (rotating the azimuth board) and latitude (adjusting the angle of the altitude board) to further improve polar alignment. That is, until stars are round for the chosen exposure time.
I haven’t tried this and you may prefer the curved rod tracker design.
Construction Notes
Providing the Critical Dimensions, Points of Rotation and other design conventions are observed, performance should be consistent in various configurations.
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, in particular the placement of hinges.
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.
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 - 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).
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.
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.
Note: Drive Shaft & Nut Assembly (replaced with a nylon sleeve and plastic tube insert tapped to 6mm)
Azimuth and Altitude
If intending to mount the drive on an adjustable (sturdy) tripod, the azimuth board may be omitted.
Camera Arm
Be careful of heavy telephoto lens that may topple the Camera Arm - restraint is necessary.
Electronics
EDIT: Update - for L293D read L293NE, which seems to run the stepper smoothly. Half stepping included in Arduino code - see acknowledgements for author credit.
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. 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 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.
Motors
A 5 volt 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.
Ebay has a plethora of unipolar 5v geared stepper motors for sale from Hong Kong (28BYJ-48 - advertised at 64 steps, it actually has 32 steps/rev and 1/64 gear ratio) - set the stepper speed and change the motorStep line of your Arduino script to suit your motor. Otherwise steppers come in various grades and steps - gearing of some type is highly recommended to reduce resonance.
Arduino motor shield
Arduino Resources
Direction and Kill Switch wiring
Arduino Code (incl. half stepping)
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 provides very accurate tracking up to 90 minutes, consistent with accurate polar alignment.
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. The spreadsheet enabled experimentation with various component dimensions.
Mike Mohaupt - whose Compact Imperial design prompted further research to optimise performance, which provided the data for 1/4” 20tpi dimensions.
Open source software (Linux) - Qcad.
Eagle PCB software and the Arduino Decimilia provided the tools to develop the electronics platform to drive the stepper motor.
Arduino half step library Note: The Stepper.cpp file above has been modified to suit Arduino 1.x (WProgram.h changed to Arduino.h)
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
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