Question about flatframes

[tuo]

Hello PixInsight Community,

I am making my first steps in CCD imaging since a couple of weeks, and also started using PixInsight at the beginning of this year (when I was still using a DSLR).

Currently, I am imaging with a x.6 reduced/corrected 9.25'' SCT and an ATIK 383L monochrome CCD with both LRGB and Narrowband filter sets.

The progress I am making is very satisfying for me, guiding works nearly perfect, focusing gets better and better etc.

But there seems to be one particual aspect where I am not on the right track, and that is flatframes.

Due to the optical train, basically looking like this:

SCT => Reducer => Focuser => Filterwheel => CCD

and sometimes

SCT => Focuser => Reducer => OAG => Filterwheel => CCD

of course I do get a fair amount of vignetting, in the later case paired with the shadow of the OAG mirror.
At first, I didn't care about the flats that much, until again and again I stumbled in PixInsight in the process that my "background" was too hard to extract (basically "real" background + vignetting + dust etc.).

So I started taking flatframes more seriously, taking them against sunsetting sky etc.

The results where better, but not to the point where I hoped they would be or where I saw data in the tutorials.

Thus, I invested in an illuminated flatfield-foil, and took the first flatfields with this the last two nights.

I tried to get my flatfields into the 25% to 50% of full well (that's what I red in a tutorial) of my CCD. Integrated them like the PixInsight-Tutorial tells, and calibrated my lights again.

But still, the flatframes seem to "not do enough". There is a difference between ticking the box for the master flat and selecting it and doing calibration without it, but the difference is not that great. I still struggle greatly with the extremely uneven background introduced by dust and vignetting.

What could be common errors I am doing?

best regards and thanks in advance to any and all help and time provided

ulrik

[sreilly]

You only use the master flat designation if you truly have made master flats. I use 30,000-34,000 ADU level for my flats using either my ST10XME of STL-11000. I use flat exposures that are 1-2 seconds each to avoid shutter artifacts. I use a rotator so I usually take 14 flats for each filter. 7 being with the East angle and then the West at +180 degrees. So essentially I have East and West side flats for the image sets. I also make the masters using this technique http://www.pixinsight.com/tutorials/master-frames/en.html I did find that I needed to add terms to the resulting Fits header to get them recognized using the BatchPreProcessing script. See http://pixinsight.com/forum/index.php?topic=5702.0 thread that covered this issue. Basically I added the filter keyword and value as well as Binning X and Binning Y.

HTH,

[sdh]

I have a few tips:

1) Try calibrating your master flat with itself. No bias, no dark, just flatten the flat. You should get a very even image. If you do not, then look carefully at your workflow.

2) If flattening your flat gives you an even image, try calibrating your images without any correction for bias or dark. Just flatten them. If that doesn't work, again, look carefully at your workflow. Try subtracting a stack of calibrated images from a stack of uncalibrated images, to see what the calibration is doing to your data. If just flattening does work, then try adding dark subtraction. Only change 1 variable at a time in your workflow.

3) My experience with moving mirror catadioptric scopes is that flats need to be shot each night, with all components in place, and essentially in the same focus as will be used for imaging.

Hope this helps.