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.

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!

-Mikael

Project for a cloudy day

Plot of the regions of sky where my imaging system cannot go without a pier flip.

I sometimes find that my scope can't point at the planned target due to a conflict between the camera and the tripod. The solution is to do a 'pier flip' - i.e. to rotate in RA by 180 degrees. However, after a some time tracking I'll need to flip back because now the camera-tripod conflict arises anew. I'd like to try and map out the regions of sky which are problematic so that I can take them into account when planning. Hence, during this cloudy day I set up in the living room and started mapping out what my limitations are. First, I set the scope on the west side of the mount when pointing at the meridian (I call this my 'normal' configuration), put the RA setting circle at 18hrs. Next, I put the dec. at 0,10,20,30..90 deg and for each value noted down at what (if any) RA value there was a camera-tripod conflict. I then did the same thing with the scope on the east side of the mount (I call this my 'pier flip' configuration). Using a planetarium program (time set to June 24 at midnight  for which RA=18hrs passes the meridian) I then translated my RA, dec. limitation values into az, alt. values. The result is shown in the figure where zenith is in the middle and the horizon runs around the circumference with azimuth angles indicated. For each configuration I have a region where I can't point, shown in red and blue. Now, if only I could figure out a way to use this information in an easy manner to predict if and when pier flips will be required for a given object on a given date. Does anyone know of a good way to do this?

Bubble in Hubble

Bubble nebula using the Hubble palette. Total exposure time is 33 hours.

Finally - I have managed to create an image which is somewhat of a finished product! Sometimes I nearly despair, feeling that I never reach this stage. Instead I fiddle around with calibration, stacking and analysis for weeks and weeks until I start diffusing into other projects, leaving much 'unfinished' work behind me. This morning I got the 'final' stacked, narrowband images of NGC7635 - the Bubble nebula - wrapped up and I vowed to attempt to make something pretty out of it by the end of the day. I chose to use the Hubble palette where SII, H-alpha, OIII are assigned to R,G,B respectively. I also have short exposure length RGB data, but have not yet processed this into something that could be used to create more natural looking stars.

The seeing in this image is ~3.2". It would have been nice to have 2" instead! I have not fallen for the temptation of sharpening and I don't think it would help much.

Earlier today I sent the narrowband images to my good friend, Henrik Bondo, who will attempt to process them into something that has more natural looking colors. So, for many reasons the image shown here is not the final one, after all. We can keep on working on our projects forever - and that's OK!

Full resolution JPG: click here
Some processing details: click here

High thin clouds on and off

Had the AT8RC setup out again last night. Prospects were not very good due to forecasted high, thin clouds and haze. In addition there was a 60% full moon in the sky. Before midnight I did IC417 in OIII light. Inspecting the calibrated images showed that the nebulosity was completely washed out. Sky flux this time was 60% higher than last time I shot IC417 in a moonless and less hazy sky.

After midnight the target was M100 in green light - haze seemed to be less severe then. Comparing to red light data from a few weeks ago with no moon and less haze suggested the images to be of reasonable quality, but not as good. The background ADU was around 2000, compared to ~1000 earlier. I'll do a critical comparison with future M100 green images to find out if last nights run is useful at all - I am not too optimistic!

Unresolved flatfield issue in MaxIm

Despite several days worth of testing, fiddling and communicating on Yahoo groups I have not managed to solve my problem with MaxIm. When combining individual flat field frames with sigma clipping or SD masking the resulting flat field calibration fails to remove dust doughnuts. Working with the same data in Mira I have no problems. I will now generate my dark corrected master flat field in Mira, then use it in MaxIm. In future sessions I will try to use Maxim again - if it continues to fail I'll have to contact Cyanogen directly. Irritating!!!