The Starry Night, 34

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12/7/2010: Two new subjects and more light for IC 443. Tonight is clear and cold, with less wind and better seeing than last night. I took another cut at the Iris Nebula, did a short session with the California Nebula and set the telescope to work taking six more hours of the supernova remnant IC 443 while I got some sleep. The last four frames were increasingly damaged by dawn. It would be useful to know far below the horizon the Sun was when images began to deteriorate. Check the timestamp of frame IC443_ha_021 to find out. [The shutter opened @ 5:46, closed at 6:01; the Sun moved from -19 to -16 degrees of elevation during that interval. "Astronomical Twilight," when the Sun is within 18° of the horizon, really means something! That image could be pressed into service, but it is noisier than the one that preceded it. The following image (-16° to -13°) is much worse.] Shooting from twilight to twilight, I had the shutter open for over nine hours of useful exposures. Focusing and framing and guider-calibration took some of the remaining dark time, but all in all that seems to be a reasonably efficient use of a clear night.

First position. I centered the telescope on the Iris Nebula before twilight ended. I focused the CCD and calbrated the guider and started the first frame (G) of a 300s RGB triplet before the sky was completely dark. These I followed under a darker sky with three 900s color frames. After that, I collected luminance frames until the target dropped to an altitude of 35° in the northwest.


The Iris Nebula in Cepheus
8x900s L, 1x900s RGB
(2 hours of L, 15 minutes each of RGB)
Luminance and RGB data processed seperately,
combined in Photoshop.
It's worth reading up on what may be producing those
lovely colors. Aromatic carbon compounds, fullerenes, and diamonds.

Second position. I wanted to start gathering additional light to improve my IC 443 as early as possible, but the supernova remnant was still in the trees at the end of the Iris Nebula session. On a lark, I selected NGC 1499, one of E. E. Barnard's discoveries, the huge "California Nebula" north of the Pleiades. It was very close to the zenith. I took 4, 900s H-a frames for the nebula and finished with a set of 300s RGB images for the stars. The combination is not as good as it could be, but the monochrome image works well.

Ngc 1499 ha

NGC 1499, a nebula in Perseus
4x900s H-a (1 hour)


ngc 1499 color

4x900s H-a in red, with 1x300s RGB overlayed



Third position. I've added tonight's IC 443 H-a frames to yesterday's and cropped the result to show only the common areas.


ic 443

IC 443, the Jellyfish Nebula, a supernova remnant in Gemini
36x900s, 9 hours exposure


This morning, I covered the camera with a black shroud and capped the telescope before shooting fresh 300s and 900s darks. I barely made it: when the air temperature rose to 32°F, the cooler was working at 99% capacity to hold -40°. Last night, in 17°F air, I noted that the cooler was working at only 55% capacity to maintain the same set point. The overnight low was 12°F.


12/9/2010: Another unexpected clear evening. I'm going to try for more luminance exposure of the Iris Nebula to see if I can resolve more detail in the shell of dust surrounding the "iris" portion.

The mount dialed in Deneb in bright twilight. While the chip cooled to -40, I rough-focused on that star and refined the guide-scope's alignment. I shot 10 t-shirt flats (and I really did use a t-shirt this time: two layers over the objective, half-second exposures, averaged into a master flat for the L channel) while the sky was still much too light for imaging. Here's the flat, stretched without mercy to show the uneven illumination, dust motes, etc:


It's not as bad as all that -- I've scaled the image to show less than 1,000 levels out of 65,000 -- but still, the uneven illumination and other foibles are comparable in significance to the fainter outlying bits I'm trying to photograph in the field of NGC 7023. So if I stretch the image enough to show interstellar dust, I also put this crap on display. Out, out, damned spots! That's what a flatfield does.

I slewed to NGC 7023, calibrated the guider, then grew impatient to start something so I began the first 900s L frame at the beginning of nautical twilight which is probably much too soon. It will be an interesting data point, and it will keep me busy for 15 minutes, after which it will make more sense to take data. [Not bad, actually, a perfectly useful exposure. The second frame commenced wtih the sun 3.5 degrees further from the sky, and it's just fine.]

I got a few good frames, five or six in all, but clouds blew in and made a mess of others. The new flat frame and another hour of decent luminosity data combined to demonstrate that the nebulosity here is much more extensive than I knew. Here's a very small and very preliminary look made from 12 x 900s (3 hours) of luminance and 3, 15-minute color frames. When I show something this small, you can bet there are real problems with the image. Next clear and relatively moonfree night, make a point of getting better color data:




A good flatfield makes all the difference when pushing low-contrast subjects as hard as I am pushing this one. Color is very weak and is bogus in the outer nebulosity; I need more, a lot more of that. I think there is enough L here to work with. More is always better, but it's RGB exposures we need. (That green line in the upper right? Satellite.) A cloudy, moony spell is in the offing. Maybe some H-a imaging early next week of something else. The Iris Nebula will likely have to wait until after the Moon gets into the morning sky.

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                   © 2010, David Cortner