For scopes without a clock drive, or for Alt-Az mounted scopes, not to worry. Exposures are taken with the scope in a fixed position, and the object's image is allowed to drift past the CCD.
Construction is easier than you might think, and your results will be well worth the effort. Total cost for my ScanCam was about $210 including the scanner. With the cost of scanners falling, you should be able to build a monochrome camera for about $100, and a color one for about $150.
Any Twain compliant application such as Aldus PhotoStyler 2.0 SE, boundled with many scanners, can be used to acquire and process your images.
Field of view is about one inch at the focal plane. You can image the entire Moon if your focal length is less than 100 inches, or you can make multiple scans and stitch them together.
Sensitivity is about the same as many real-time CCD cameras. The first quarter Moon records well at f:31. Don't expect to record faint stars or nebulae. Exposure time for each line is quite short: around 5 to 10 milliseconds.
Image resolution is adjustable from 413 pixels at 100 dpi, to 3300 pixels at 800 dpi, in 8-bit greyscale, 24-bit color, and other modes.
I will be adding further images and information to this page as time permits. Hopefully I will be able to include all the information a person with modest electronic skills will need for successful construction and operation.
Anyone building a ScanCam is encouraged to submit your best images, and I'll include them here for the world to see. This information is freely shared in the sincere hope that it will encourage others to build their own ScanCam, and enjoy the benefits of wide-field high-resolution lunar and solar imaging.
If your browser doesn't support JPEG, load them to disk and use a JPEG viewer.
These images will probably look awful if your viewer or display adapter doesn't support 24-bit (16M) color.
1st quarter Moon - Greyscale (188K)Celestron-14 prime focus with 5" off-axis stop (~f:30)
This red-channel image shows more detail than the color
one below. Isolating one channel reduces the effects of chromatic abbaration
caused by atmospheric refraction.
1st quarter Moon - 24 bit color at half scale (60K)
Celestron-14 prime focus with 5" off-axis stop (f:31)
Although the Moon doesn't exhibit much color, recording
all three channels (RGB) allows subsequent separation into red, green,
and blue. Certain detail shows up better in some channels than others.
1st quarter Moon terminator - 24 bit color (130K)
Celestron-14 prime focus (f:11)
This image shows more shadow detail at the terminator,
while allowing brightly illuminated areas to saturate. Note the chromatic
aberration caused by atmospheric refraction. Some of the color in the highlight
areas is an artifact of the way some channels saturate before the others.
/Near Full Moon - 24-bit color at half scale (107K)
Celestron-14 prime focus with 4" off-axis stop (~f:39)
Stitch of two scans taken on May 31, 1996 in 24-bit color
mode at 200 dpi. Slightly 'stretched' to increase contrast.
When you receive your scanner, make sure that it works perfectly as a scanner. Scan a black-and-white photograph and verify that it records shadow detail. If a color model, use 16M color mode, and make sure that the result is shades of grey with very little color. This insures that the three color channels are tracking each other. Next, scan a white piece of paper at both the minimum and maximum 'lightness' settings. Verify that there are no bad pixels, which will appear as light or dark streaks in the direction of travel. At the minimum setting, the result should be a uniform shade of grey, and not some shade of puke. There should be no variation from side to side. Finally, scan something very black and verify that the result is very dark grey with a just a little noise present. A viewer with a histogram function should show most of the pixels at a low but non-zero value. A poorly adjusted scanner may not record shadow detail properly.
If any of these tests fail, return the scanner for a replacement. Once you disassemble it, you are on your own.
The three boards, not counting the power supply, can be removed from the scanner shell, leaving the ribbon cables intact. Protect the CCD from dirt and scratches by wrapping its board with lens tissue secured by a rubber band. Room light will not damage the CCD, but prolonged exposure to direct sunlight might damage the dyes used for the color filters.
Mounting the boards in an enclosure is an easy task unless you require the most compact design. I found that I could use a smaller box if I moved some of the electronic components to the other side of the boards. In particular there is a voltage regulator, which if remounted, allows the boards to be mounted in a small space with their controls accessible. Observe polarity when moving capacitors and wear a grounding strap to avoid static discharges.
All hand scanners have some means of sensing movement across the subject. The ENVColor scanner uses a roller which rotates a perforated wheel through an elaborate gear train. The rotating wheel interrupts light from a LED shining on a photodetector at a rate of 400 per inch of travel. Since the roller won't work on your telescope, a substitute must be found. The LED and detector (labeled PH1 on the board) should be removed, leaving its pads available for connection to the timing circuit which you must build. Only 3 connections between the scanner electronics and your timing circuit are needed. They are +12 volt power, Ground, and the timing output
For non-zero declinations, multiply the scan frequency by the cosine of the declination. Ignoring declination results in a worst case error of about 13 percent. Images will appear somewhat stretched in the RA direction. Use your image processing software to correct for this by scaling the appropriate dimension.
A nice feature of the scanner is that the scan frequency is independent of the dpi (dots per inch) setting, so you can change the resolution without having to also change the scan frequency.
There is, however, a maximum scan rate based on the dpi setting that you can use which limits the maximum useful focal length to about 300 inches at 800 dpi. At 100 dpi, you should be able to use up to 8 times this limit. The start scan button includes a LED which remains on for the duration of the scan. If you exceed the maximum scan rate, this LED will flash or go out entirely. This indicates the scanner's inability to operate at the selected rate.
Here is a table listing optimum scan frequency as a function of focal length. Select the next higher available frequencies with the rotary switches.
I chose not to tap the +5 volts on the scanner. Instead I included a separate +5 volt regulator (Q1) supplied by +12 volts. The controller supplies +12 volt power when you are ready to scan. I found that the start scan button could be permanently in the pressed state, allowing the exposure to be controlled entirely by the software. Pressing a button on the camera could cause vibrations which would be apparent in the image as serrated edges.
The scanner's original photodetector (PH1) operated as an emitter follower, ie. it sources current. Since TTL cannot source much current, a pull-up resistor (R1) is needed. R2 and LED1 are optional and merely provide a visual indication that the circuit is functioning. It flashes at the selected output frequency. Drivers for hand scanners require you to wait up to a minute for the lamp to 'warm up' before the scan begins. It appears that this wait is still useful, as it also allows the CCD to warm up. CCDs are very temperature sensative and without this waiting period, the effects are noticable as gradually increasing background noise.
A prototyping board (Radio Shack P/N 276-150) holds all of the components on one side and which are soldered on the other side. Three wires connect to the scanner bottom board and two others are from the rear panel mounted LED.
Here is the Schematic and Parts List and the layout of the timing circuit.
Information from Digi-Key's catalog for the Epson America SPG-8651 Crystal oscillator.
When you remove the optical encoder marked PH1, you will have four vacant pads. Connect the output from the timing circuit to the pad shown in the photos. Likewise when you remove the four pin header that used to supply +12 volts to the lamp power supply, you will be left with two +12V and two Ground pads. They are also shown in the photos.
The higher the resolution, the larger your image files will be. For example, at 100 dpi a 400 by 400 pixel image requires only 16K bytes for greyscale and 48K bytes for 24-bit color images. On the other hand at 800 dpi, corresponding to 3300 by 3300 pixels, greyscale images will require 10.9 megabytes, and 24-bit color images will need nearly 33 megabytes each.
The most sensitive test of focus is to maximize the brightness of stars. Scan a dense star field and focus not for minimum star size, but for maximum brightness. As you approach best focus, you will see more and more of the fainter stars appear.
If you need to reduce your aperture to control exposure, focus at full aperture. Any residual focusing errors are then minimized when you stop down.
Here is a look at my prototype camera Before GIF or 24-bit JPEG , During GIF or 24-bit JPEG , and After GIF or 24-bit JPEG modification.
The JPEG versions are better if your system supports 24-bit color.
my article in the Summer issue of CCD
Last Updated: June 16, 1996