Sea trials for the SN10.

Feb 27-28. There's a lot of Moon and a dull sky tonight. Not "hazy," just dull. A nice 3.5 magnitude sky, insofar as a 3.5 magnitude sky is ever nice.

The sky quality meter says 16.48 (mag per square arc second). The temperature is 50-ish, dewpoint at HKY 21F (RH=31%), but that big unprotected corrector collected dew just the same.

After the first hour, data quality began to decline, and by the end of the session, things were clearly not (clear). When I went out, the corrector looked white in the moonlight and I stopped the sequence three frames short; I'm surprised that the numbers describing the last few frames look as good as they do. I thought I would need to discard the last hour's take, but all frames are useable. The mrror and diagonal remained dry; it's just that corrector.

I was shooting in H-a because of the moonlight. I knew it would weaken the galaxy, but I thought M81 would produce a more vivid image than this. The H-II regions do sparkle nicely. Tracking worked reasonably well (after finally getting it established) and the Feather Touch did a fine job finding and holding focus with the ST2000XM hanging directly below. Some lessons from the session:

• Just because you can see stars in the guide exposures doesn't mean Maxim will guide on them. I wasted an hour arguing with it ("guidestar not found" "it's right there!" "guidestar not found") then offset to a bright star (24 UMa at M=4.7) on which I guided with 0.5s exposures which produced a flux in the 30,000 range. Could have gone shorter. I had been exposing 5-7s and getting fluxes in the low thousands. So I had probably been trying to guide on 10th magnitude stars. Pay attention to the numeric measure of signal strength; don't rely on (just) a clear image to find useful guidestars.
  
  
• The focusing mask I made for the 10-inch doesn't work very well: light passing the round aperture dominates images to such an extent that it's hard judge merger (fixed 2/28: I used the Dremel to cut approximately larger triangular apertures). I found optimum focus using FWHM stats and brightest pixel displays in Maxim; the numbers didn't look good compared to the A-P (FWHM values of 5-6 pixels at 1.3 arc second per pixel, yech!) but the results are reasonably sharp. We'll see if images improve with better collimation (2/28: I collimated using Tectron tools; things really weren't too far off; next I'll break out the laser). The 10th magnitude double tucked beside M81 was nicely resolved in all frames and combos (seperation 8 arc seconds). This is no challenge at all for masters of the craft, but it is reassuring to me.
  
  
• I'm having to extend the ST2000XM slightly in the focusser. Fix by machining a longer holder when you feel like it; it works fine as is.
  
  
• I tried using a barlow on the guide scope but had little success. I should have tried again after centering a bright guidestar.
  
  
• Pinpoint Astrometry says the SN10 has a FL of 1015mmm, exactly as advertised.
  
  
• A bigger finder scope would be welcome and there's a 10x60 straight-thru downstairs looking for work; also just count on replacing the cameras with eyepieces for finding targets and guidestars.
  
  
• Shoot some flats.

Clouds are in the forecast for the next few days. So I do not suggest breath-holding for the next attempts.

Messier 81
15x600s, SN10, ST2000XM, H-a, -35C, Sigma-Sum.

 

Sum of first 90 minutes' data.
(Trail of hot pixels just below spiral.)

 

Cropped, deconvolved, inverted.
90 minutes of data.