A little bit longer now... M33 with the Astro-Physics refractor.

December 9-10. The forecast calls for three more nights before I have to tear things down for rain. With the newly organized cables, I don't think that's going to be the imposition it once was.

Tonight I'm again going to run down the battery using the ST2000XM, but for variety's sake I'm putting the CCD behind the 5-inch at F6. In that configuration, the camera can be mounted behind the Maxbright diagonal to take up backfocus, shorten the moment arm, and lower the cg. It's a sweet deal. This is about as compact an imaging setup as I own (or could want). Rather than remount the guidescope atop the 5-inch for the ultimate in small, I reached for the ultimate in convenience: I pulled the 200mm on its dovetail rail off the G11, clamped the rail-mounted A-P in its place, and rebalanced the DSBS plate. Hollywood does stuff right.

The tangent-arm micrometer works so well to focus the 200mm that I plan to adapt the idea (and the hardware) to the A-P. It might need just a little lathe work, and I might already have the metal stock required (nope, simpler than this). Tonight, however, finding focus with a 2-inch aftermarket focus knob should be manageable. Film at eleven.

After much fumbling, I managed to put M33 where I wanted it on the chip (next time, take the time to align the finder, guider, and main instrument -- do it with eyepieces if you don't want to use run-time; 762mm isn't "long" but it's longer than the 200mm). Thirty minutes of power lost. I focussed the CCD and began a series of 15, ten minute exposures. Decent focus at F6 is a cinch (perfect focus, which I did not get, is never a cinch). The first frame was a mess. I adjusted guiding parameters and changed to the Clear-IR filter so I could get faster results to try to fine tune the setup. Guiding errors are still high (absolute values of 2 - 4 on both axes) and most frames are useless. One hour lost.

I shifted the counterweights out slightly so that the drive has to lift some weight, changed the guiding speed, adjusted the aggressiveness settings, recalibrated, and tried again. Looked better, with guide errors usually under 1 pixel with excursions to 1.5 pixels. 90 minutes of power wasted. I restarted my 150 minute sequence, expecting the battery to allow me to get 10 frames at best.

Things did not improve. In fact, they went sour quickly. Corrections rose to +/-2.5 in each axis. Two more frames were useful, barely. I decided the battery was getting weak and switched to household current at 2h30m. That wasn't it; I still got mediocre results.

The best I could do with the sum of the night's best frames is at right. Significantly, there are no larger versions offered. You're not missing much. Those "mottled" arms should be "starry," dark lanes should spin almost all the way into the core, and the foreground stars should be sharp and small rather than flat-topped and sharp-edged.

I looked at the Crab briefly on the screen. Could not get reasonable tracking. I adjusted guiding parms; I rotated the guide camera; I tried different parts of the sky; I rebalanced the mount. I discovered I was again off the pole; how? Nevermind, just fix it, and bolt things together better when you set it up again. I realigned (which made a substantial difference, of course). I tried different guiding rates, calibration times, aggressiveness settings. In the end, the best results came after moonrise, after midnight, with these settings:

G11: rate 0.3x, east heavy.
Maxim: guide calibration 6 seconds in X & Y; minimum move = .01s, maximum move = .50s; settling time = 2s; X and Y aggressiveness = 5.

I aimed into an anonymous, moonlit field in Gemini and watched half an hour's worth of guiding corrections: a few tenths of a pixel from start to finish. So what's different? Most importantly: Nail the polar alignment and be sure it doesn't move. By now you know what the guider calibration looks like when polar alignment is off, so when you see that pattern, just stop fiddling and re-do it. Then start with the settings above and hope for the best.

December 10: If something works once, try it again. The micrometer tangent trick turns out to be easy to install on the refractor. Two hose clamps, and we're almost there. Photos to come after the JB Weld cures on the latest bracketry. It's a two-stage "welding job" so don't be holding your breath. The polar alignment problems appear to begin with an insufficiently clamped joint between the tripod and the G11. I cranked it down tight, but next time the rig is apart, add a shim.

Astro-Porn
Also a record of the setup for next time
Long side of ST2000XM frame is oriented N/S.

 

Messier 33.
5-inch F6 Astro-Physics refractor.
4x600s, Clear (IR-blocked) filter.
ST2000XM @ -35C.
Worked over in Maxim and Photoshop CS2.

 

 

 

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Text & Photos by David Cortner
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