CITSO Blog: Deeper Light at the Cabin in the Sky Observatory

CITSO Blog: "Deeper light" at the Observatory,
June 4-6 2010

Includes Pinwheel Galaxy -
SkyNews Photo of the Week #435!

We returned to our Cabin in the Sky on the weekend of June 4-6 2010, three weeks after we moved our vacation furniture in, and this was a chance for me to follow-up on the first light images from that trip with a first chance at "deeper light".

My goal was to spend both Friday and Saturday nights imaging a single object, which after much thought I decided would be M101, the Pinwheel Galaxy. With the rich knotted structures and brilliant bursts of burgundy lying in its magnificient blue spiral arms, it is one of the most beautiful galaxies in the sky. Despite its comparatively high (integrated) brightness relative to many deep-sky objects, it is a challenging target, given its large angular size (I knew that it would fill my CCD chip!), making for a relatively low surface brightness. Unfortunately, weather conditions turned out to be mixed, with clear skies at sunset on both Friday and Saturday raising my hopes, but with intermittent clouds that came in not long after it got dark both nights, necessitating a change in plans.

Nonetheless, I did manage a total integration of 105 minutes for M101 on Friday night (albeit with a good part of that shooting through thin cloud) for the following LRGB rendition, here showing only a part of the full frame. Click the image to pop-up the high resolution full-frame version. The individual frames were 5 minutes each in L (with 2x2 binning), and 3 1/3 minutes each in R, G, and B (with 4x4 binning). This image is definitely a first for me, both for attempting to capture the vivid colours and detail in M101, and for the total exposure time, the longest I've ever managed! Image capture and initial processing were done with MaxIm DL, with additional processing under Photoshop. Seeing conditions were a very modest 3" FWHM, and the image scale is 1"/pixel.

I had also planned to shoot M101 with the PlaneWave 0.66x focal reducer, so that I could fit the outer reaches of the spiral arms into the frame. Since my SBIG STL-4020M camera gives a very small 0.52"/pixel for the CDK17 without the reducer, there is no point operating at the full resolution most nights anyway, so I planned to attach the reducer on Friday night, and leave it on until a night of exceptional seeing comes along. However, I discovered that I was missing the proper screws, and had to wait for Saturday to attach the reducer. Heavy clouds ended up covering most of the Big Dipper once it got dark enough on Saturday, so I decided instead to go for M13, the Great Globular in Hercules, with that part of the sky having bigger and longer holes in the clouds. I could only manage 25 minutes of integration, but long enough to capture a fair bit of detail in this brilliant beast! In this case, only one frame in each of LRGB was useful, 10 minutes in L (1x1 binned), and 5 minutes each in R, G, and B (2x2 binned). This image is also a first on several levels, the first unbinned long exposure that I've shot with the camera, the longest guiding interval, and the sharpest globuar I've ever managed. Click the image to pop-up the high resolution full-frame version. Seeing conditions this night were slightly better, at about 2.5" FWHM, and the image scale is 0.75"/pixel (owing to the 0.66x reducer). I pushed deconvolution in MaxIm DL - perhaps a tad bit too hard ;).

I was also able to do some important additional calibration work this weekend. In particular, I finally was able to measure the colour response of the camera and filters, by imaging the double star 16 Cygni (apparently it's actually a triple star system, and the 2nd brightest star is reported to have an exo-planet). This star system is part of a recognized list of "white stars" suitable for colour calibration (MaxIm DL gives this table of recommended stars, based on a December 1998 Sky&Telescope article by Berry et al.). By the time I gave up on deep-sky imaging on Saturday, due to clouds, 16 Cygni was at about 80 degrees altitude, and well away from the heavy cloud, and so was ideal for this purpose (having two nearly identical white stars only about 30" apart is very handy as well, providing for a useful cross-check of the calibration). I found that only a modest re-tuning of colour is necessary, after applying auto-equalization of the image background under MaxIm DL. Nonetheless, the re-tuning has a very noticeable effect, and produced the colours in the M101 image above without further colour balance, with a result that agrees extremely well with other sources.

I also did another iteration of TPoint, this time with many more stars than on my first night (it's easy once Auto-Centre under MaxIm DL is calibrated), and the polar alignment at this point is off by only 2 arc-minutes. This encouraged me to auto-guide for 10-minutes for the 1x1 binned luminance for M13 (the issue being that auto-guiding eliminates tracking errors, but not field rotation). I plan to do another measurement of the polar alignment, before starting to compile a large and permanent TPoint map.