The Starry Night, 134

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01/09/2015. Comet Lovejoy has finally moved far enough north that I can see it above the pines from the backyard. Under variably clear skies, I snagged perhaps a hundred 30s subframes through clear and RGB filters. There are so many ways to combine them, but here's the best I've managed so far. This is the best run of 20 clear exposures with color from the best 3 consecutive RGB exposures, so it's only 10 minutes of luminance and 30 seconds of color in each channel.



Comet Lovejoy
AT10RC @ F5
20x30s C, 1x30s RGB


I don't understand the curved star trails. Why curved? Every other time I've stacked subframes of a comet, the star trails in the background have come out straight. It's something to ponder.

I refocused and recalibrated on Procyon then slewed to NGC 2238 --part of the Rosette Nebula in Monoceros-- where I left the telescope to its business until morning. This is the best image of the session:



NGC 2238 with problems
900s H-a


So continues tonight's theme of mysterious field rotation. When this mess came off the computer (after several minutes of multi-pixel guide corrections), I went back outside to check polar alignment. During my previous imaging session, I saw only sub-arcsecond corrections after 20-second guiding exposures. Had the cold weather heaved one of the mount's footers? Had a deer bumped it? Hard? But no. Slews and calibration runs suggested polar alignment was still good. The polar alignment scope showed a small (3-4 arc minutes) error, which I dialed out before resuming the run. In the morning, I went out to park the telescope and saw that the CCD was no longer rotated to 115 degrees.

The 2-inch snout had come undone. It was completely free and allowed the CCD to rotate slowly under the off-center weight of the camera's electronics and connections while constrained by the anti-gravity device (an elastic bungee cord) that assists the focusser in lifting the imaging train during "in" adjustments. The resulting trails are centered on the guide chip though the physical rotation is around the optical axis and the imaging chip. The geometry makes my head hurt. As for the curved trails in the comet photo, I need strong coffee.


01/10/2015. That's better. Among other efforts, I tried NGC 2238 in the Rosette Nebula again. The Moon is waning and not so much of a factor now, but the Hubble palette is still fun to work with.



NGC 2238
4x900s H-a, 3x900s O-III, 2x900s S-II
(total 2h45m)


A note on technique: I've been calibrating and combining the individual channels in Maxim DL then doing channel combination, deconvolution, noise suppression etc in PixInsight. The first step I'm doing in PI is stretching each channel to a similar histogram distribution, a peaked distribution centered around the 25% level. After that, whatever works. PixInsight permits me to edit the FITS header so I could (I suppose) save versions of the Clear-filter flat with spurious filter values; that would, I think, allow PixInsight to handle calibration, and with it, all other preprocessing steps.

Before I left the telescope to do its thing with the Rosette, I worked a bit on a mosaic of IC1805 and environs. Since the first time I shot this field, I've been entranced by the lacy dark clouds and UV-eroded forms near the bright cluster. Next page, please...


Except where noted, deep-sky photos are made with an SBIG ST2000XM CCD behind a 10-inch Astro-Tech Ritchey-Chretien carried on an Astro-Physics Mach1GTO. The CCD is equipped with Baader wide- and narrow-band filters. The internal guide chip of the CCD most often keeps the OTA pointed in the right direction (I'll let you know when an OAG or guidescope takes its place). Camera control and guiding are handled by Maxim DL 5.12. The stock focuser on the AT10RC has been augmented with Robofocus 3.0.9 using adapters turned on the lathe downstairs. A Canon 6D and a modded 50D find themselves mounted on an Orion 10" F4 Newtonian or carrying widefield glass on an iOptron Skytracker. Beginning in May 2013, PixInsight has taken over more and more of the heavy lifting -- alignment, stacking, gradient removal, noise-reduction, transfer function modification, color calibration, and deconvolution. Photoshop CS4 et seq and the Focus Magic plugin get their licks in, too.


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                   © 2015, David Cortner