Wednesday, April 25, 2012

M100 - part 3

Green filter sub-exposure. Note the remaining dust rings on the background.

Ups. I discovered that my calibrated green filter subexposures suffer from remnant dust rings which are visible on the background at high contrast setting.

Suspecting MaxIm's auto-calibrate I tried to calibrate manually but it made no difference. Something must be wrong with my flats - what I do not know. I will ignore this for the time being and sort as usual. Will use 13 out of 32 subexposures. I rejected due to high background values (from passing haze) or due to fading of the M100 arm visibility.

Note: two different exposure times were used on the selected 13 sub-exposures: 1200sec and 900sec. That was not wise and will cost me when doing further processing. Still, despite these problems with my green data, I will press on.

Sunday, April 22, 2012

M100 - part 2

I have decided to try and blog each time I have worked on my M100 image processing project. Maybe someone will find it interesting?

First step was to collect all the files (lights, darks, flats, flatdarks) in one folder; this amounted to 1064 images! Next step is to use MaxIm's 'Set Calibration/Auto Generate' feature to automatically detect and group all calibration frames. I let MaxIm replace all the calibration frames with stacked master frames - the total number of images was now reduced to 204.

After calibrating the light frames and I saved them into another folder. I checked manually to see if MaxIm managed to do the flat field correction properly since I have seen problems with this before. Now, to my great relief, it worked!

Rejecting sub-exposures - on my setup this is typically due to haze and light pollution.
Next step is to load all images for a given filter into Mira and inspect them. Mira is not a well known program these days, but I like the 'image set' feature where you can load and animate a series of images in a single window and measure background levels in a user defined region. In my experience, a combination of background level measurements and visual inspect is the best method for deciding which images to reject and which ones to use. Drifting haze and clouds coupled with light pollution is my most common rejection reason. Today I managed to inspect the red series; out of 29 1200 second sub-exposures I chose to use 17 (see the image with this posting). I do not only reject on the basis of background level, but also if the target contrast is reduced. I rarely see bad images due to tracking issues or wind-buffeting on the AT8RC setup I'm using here. When my initial rejection has been done I'll measure the stellar FWHM in each image with CCDInspector.

Next time I will continue this task on the green, blue and luminance series - stay tuned!

New project - M100

Having finished work on narrowband imaging of the Bubble and Jellyfish nebulas I am now moving on to M100 with LRGB filters. The key difference here is more light and shorter exposures. I tried to get lots of data so that I could use only the best and get good SNR. Good SNR will hopefully enable deconvolution - my big quest these days is high resolution. Collecting the data yesterday I found that it amounted to eight nights and more than 1000 files incl. calibration frames!