What a difference a filter can make

Another and probably last attempt at M42 this year. This this time armed with a CLS filter and F6.3 focal reducer

M42. Stack of five 90 second frames using a Canon 450D and CLS filter at f6.3. Stacked in Deep sky stacker. CGE 11 a prime focus.

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Here is one of the individual frames.
The frames seem to have a "skyglow" around them (maybe due to haze). I'm not sure how to remove this.

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John - very nice images.

I suspect, based on the individual frame you provide, that vignetting may be an issue. What calibration frames do you use - flats, darks, flat darks, bias etc... Flats will help remove vignetting.

I would have thought "skyglow" would affect the entire frame rather than just the central portion. I susepct vignetting because your frame looks exactly like the frames I get out my C14. Flats alleviate the problem. If you don;t use them, let me know and I can give you the process for taking and using them.

Cheers
Dave

Nice shots John,

I agree with Dave about the vignetting.

Here is a link to a discussion you may find useful:

www.astrochat.co.uk/forum/viewtopic.php?t=4759

Best regards

steve

Hey John, the detail is good in that individual frame, and I do agree with both Dave and Steve, it's vignetting which is causing the 'problem'.

I'm interested in taking flats myself, so i'll have a look at that discussion too...

Keith..

I had a look at that (comprehensive) article. Very good.

It did omit two issues - one important, one less so.

The least important issue, and if you want to be a pedant, is to darks for your flats. Yes, it sounds mad but don't forget, that whenever you take images you should subtract darks - including for flat frames. Bear in mind the flat-darks should be taken at the same temp and exposure duration of the flats. In practise, I've found flat-darks are not that important. Unless you're a pedant

The more important issue is the duration of your flat exposures. This is tricky unless you know the full-well capacity (FWC) of your CCD chip. You want your flats to cover about 30 to 70% of the FWC. Why? If you saturate the chip, your flat will look like a completely white frame and be of little use. For the DSLR's, I've really no idea what the best exposure time is. Experiment. Start at say 1 second and increase it until you get something in between a completely black and completely white frame. If you flat image shows vignetting and lots of dust donuts without you having to stretch the levels, this is probably a good flat.

Don;t forget, as you are shooting t-shirt flats using the twilight sky, you'll need to adjust your exposure times to compensate for the sky getting darker (in the evening) or brighter (in the morning). Of course, if your're shooting a t-shirt flat using a light source, this doesn't matter. But light sources are typically non-uniform and will diminish the quality of your flats.

I like T-Shirt flats - easy and quick. Don't forget to shoot flats for each filter (if any) that you have in your imaging train.

One final point, reading the above may fill you full of dread - don't panic. It sound more complicated than it is. If you can shoot light frames, flats require only a teensy weeny bit more work. But you will be rewarded with much much better images. It really is well worth the effort.

Of course, if you're still stuck, let us know here and we'll try and help some more.

Here is another article that describes calibration frames in a little more detail: www.aavso.org/observing/programs/ccd/manual/3.shtml

Dave

Thanks everyone. I hadn't taken any flats which probably explains the result. The stacked image itself does seem to have quite a bit of good data and detail (reasonable tracking etc...). Is there anything I can do to save the image (make it better)?