04/14/2013. Chasing threads with a pocket knife. This morning, I avoided webworking headaches by working on the telescope instead. I wanted to remove some metal from the camera-mounting hardware of the AT10RC (see previous page about inability to focus at minimum EFL in warm weather). I chucked the 2-inch adapter in the lathe and turned several hundredths off the shoulder that limits its in-travel. I said to myself, whatever you do, do not let the tool touch those threads. So of course while whittling aluminum off the adapter, I managed to shave the tops off the threads anyway.
Simplified schematic of 2-inch adapter at tailpiece of AT10RC
Red arrows indicate in-travel gained by turning down stock shoulder.
This led to my donning magnifying glasses, fixing a cup of coffee, and as patiently as possible stroking the valleys of the threads with a sharp pocket-knife, resculpting whatever burrs and egregious bends I could see or feel. And, voila! it threads on again and it goes in far enough that I am sure the F4.8 focus is back in play, whatever heat the summer brings. If, despite my confidence, that modification didn't buy enough additional in-travel, I can shave off that angled lip on the left side of the adapter and get several more hundredths. Q. D. Greene would not be pleased with my technique, and yet, "We got'er done."
That was this morning. Last night, things went much more smoothly. When I saw what a sparkler the sky was, I uncovered the telescope and was taking data twenty minutes later. Nevermind that I had to restart the guider after the second 900-second integration (I forgot to bin the guide chip 2x2; guidance was tight but "nervous" with the smaller pixels). The sky stayed clear and the optics undewed until the galaxy found the trees. The overnight returns were very good: 18 well-focused and well-guided 900s subs to add to my recent L- and RGB-captures. The result is sharp, detailed, and smooth:
27x900s L, 4x300s RGB
(total exposure: 7h45m @ F5.2)
There are gales of stars in those spiral arms! Right click and 'view image' for a 1024-pixel version. I am more or less satisfied with M81 (for now -- this subject rewards piling up tons of light). Next, I want to use the F4.8 EFL to complete the Trio in Leo which will be lost in spring twilight soon and lost in moonlight sooner.
04/16/2013: Yes the modified adapter does allow F4.8 imaging with plenty of room to spare for whatever heat the summer may bring. I tried it out under marginal skies (poor transparency) to see if I could capture anything that might help in completing the Trio in Leo montage. Tracking was fine, the results were not. I also set up a series of unguided LRGB images of Regulus (because it and little else was visible). Tonight's results were not so much "useful" as "instructive."
Proof of focus
20x20s L + 4x20s RGB, unguided