Money for nothin'... that's the way you do it!   I want my IC 443.

December 11-12. I've been building the micrometer focusing attachment for the refractor, but it's not ready for tonight. Truth to tell, neither am I, but as the weather is about to close down on me for a few nights at least, I'd better image while I can. With the sequence command and the wifi connection, I can do a lot of work while watching data come off the chips.

I started late, about 9:00, and spent fifteen minutes on polar alignment, then half an hour lining up the finder, the guide scope, and the CCD. Another ten minutes focussing gave me a reasonably tight point spread function. I stopped fiddling with focus when I saw a PSF with a FWHM around 2.5 pixels (5 arc seconds; nothing to write home about). Nudging the knobs the slightest bit either way pushed FWHM values above 3, so I said good enough. (Was I looking at half flux diameters? I'll pay more attention when I can put these metrics to practical use.)

I'm imaging at F6 again tonight, which yields exactly 2 arcseconds per pixel. I returned to IC 443 and began shooting ten minute subframes about 10:00PM, having used up an hour of battery life on preparations. Tracking is much better tonight. I put this down to careful polar alignment and a decent starting point for guider calibration settings.

An hour into the run, I got a particularly poor frame so I recalibrated the guider. I increased the Y-axis calibration time to 15 seconds. I reset the delay after guider correction to 1 second. Guide exposures remain 2 seconds. Much better. It seems to help to let the tracking work through several cycles before opening the shutter; errors converge toward zero for ten or more guiding cycles (does Maxim refine calibration based on observed errors? and would I be better off running PEC along with the auto-guider?).

Ten minutes is not long enough to produce an optimim background level. Far from it; I'm only getting backgrounds of around 1100 ecu's. Something over 3000 is said to be best for the ST2000XM. So 30 minute subframes would be better all around. When the tracking is nailed, I'll be more willing to invest half an hour in individual subframes. Also, I'll likely lose the last frame to a depleted battery. Ten minutes is not too much to give up; half an hour's data would be tougher to sacrifice.

Tonight we begin to see what this outfit can do. Temperatures were 40F dropping to 35F during the session. The Peltier cooler kept the chip at minus 35C with 60-65% power. At 12:55AM, tracking errors went up; the guider reported "star faded;" the ST2000XM stopped reporting its chip temperature. Maxim seemed to think it was still making an exposure, but I doubted it. I turned off the guider to keep it from sending the odd command. It's 414s into a 600s frame. At the end of the exposure, Maxim reported "reading CCD camera" but never went on to "downloading image." 3 hrs 55 minutes run time tonight.

The last 9 frames turned out far better (sharper, deeper) than the first 6. A quick look image from the first two hours was promising, but the version shown here begins to suggest what's in the data. It combines all frames, those made with both good and mediocre tracking, each set combined and re-combined to take advantage of the strengths of the selected subframes.

Supernova Remnant IC 443.
2.5 hours, 15 x 600s. ST2000XM @ -35C.
All images aligned by auto star-matching and sigma-summed; best images aligned and summed; the two combinations then added in Maxim; the result stretched in Photoshop CS2.
5-inch F6 A-P refractor at F6.
10nm Schuler H-a filter.

 

Imaging station of the 21st century amateur astronomer.

 

This is more or less how it's supposed to work.
Remote PC screen display showing Maxim running on the notebook computer beside the telescope. Image is the most recent 10-minute download of IC 443. Graphs display tracking errors in X (Dec) and Y (RA). Scale is +/- 2 pixels in each axis. 1 pixel = about 4.5 arcseconds with the DSI behind a guide telescope with a 400mm FL.

 

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Text & Photos by David Cortner
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