I got a whole lot of holes . . .

Feb 19. Last night, I calculated from two defocussed star images that I needed to move the focal point out 2.7 inches to allow the Canon to come to focus while using the Feathertouch fine-focus adapter. So today I made four brackets out of oak and attached them to the back of the mirror support. The brackets feature a series of holes 1/2 inch apart. The idea was to step the mirror for and aft in the truss until near the ideal location. I did this by day while looking through the 20D's viewfinder at fine detail in high altitude clouds and in jet contrails. The first placement was wrong; then I adjusted in the wrong direction. On the third try, I got it just right. The camera focussed with the Feathertouch racked out just about a quarter inch. I should've believed my numbers. The change in spacing: -2.5 inches.

Next, make sure that 5 miles is as good as 50 million light years when it comes to finding focus. Then make sure that the ST2000XM also focusses. Adjust as needed. Then recut the mirror-side truss tubes and ditch that creaky bracket. Good news, as yet unverified, about the distance from the mounting flange to the sensor of different cameras:

Canon 20D: 44mm
ST2000XM / CFW-8a: 42mm
ST2000XM / CFW-8a / Mandel adapter: 46.5mm

If one focusses with even a little room to spare, so should the others.

Contrast by day is dreadfully low and will be until I get the tube partially enclosed or at the very least put a black background behind the secondary mirror. The situation should be much better in some parts of the night sky and I don't expect to have to enclose any part of the tube before trying it out for real.

I roughly collimated the primary and secondary. There's no sense being very careful with this since I am still mucking around with the mirror mount, so the laser, barlow, sight-tubes, et al stayed home. Things do adjust appropriately, and I am sure that proper alignment is somewhere within the available ranges of adjustment. Optical collimation is just one example of my most useful general engineering insight: If you can't make something to spec, make it adjustable.

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Text & Photos by David Cortner
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