Flats for solar imaging

[multiweb]

Recently I became aware of how important flat fielding dust bunnies is for solar imaging. What's the proper way to do it? Slightly defocus, or offset the sun or any kind of translucent shield in front? I'd be interested to hear what the common practice is out there? TIA.

[alocky]

I centre the scope on the brightest part of the solar disc, being careful to avoid any active regions and sunspots, then defocus enough for it to be featureless. So far this seems to eliminate dust spots, newtons rings and gradients.
Cheers,
Andrew.

[Merlin66]

There's a lot of info and discussion on taking and the application of solar flats over on the new Solar Chat forum.
Well worth checking out.
http://solarchat.solarastronomy.org/index.php/en/

[multiweb]

Thanks fo the links and tips guys. Here's a post that made sense to me and that I will try tomorrow. See below for the benefits of others.

1) after capturing a sequence or a series of sequences, without changing the camera orientation and, if it's the case, without changing ROI (frame size), move the scope to a featurless area and then defocus until you have a rather ... featurless picture on video. Then take a sequence of say 200-300 frames.
2) Take a new such sequence every time you change the camera orientation or frame size.
3) When you go about processing the sequences captured create a flat field frame using the software you normally use (I use avistack and that program has a feature that automatically creates flats from a given sequence).
4) in avistack in the processing parameters there's a "frame stacking" section. If you open that section by clicking on it, you'll find the flat field and dark frame area. double click on the flat field and there you'll find a menu that allows you to choose the appropriate flat for the sequence you are about to process. Select the flat field frame you just created or the appropriate one for that sequence or series of sequences and then let avistack do its thing.

[alocky]

I take a minute's worth of video at the end of every mosaic run, and put the lens cap on to do another minute of dark. Thanks to some of the guys here I've switched to Autostakkert from Avistack, which also lets you create a dark and a flat file, which you load into the code before processing the image runs and it manages it pretty much automatically.
I've got a couple of solar mosaics using this technique at
http://astrobin.com/users/alocky/
Including a venus transit one.
cheers,
Andrew

[multiweb]

Thanks Andrew. I downloaded Autostakkert. Pretty easy to use. It doesn't have the wavelet section that registar and avistack have though unless I'm missing something.

[jjjnettie]

You have to save your file and reopen it in Registax and then do your wavelets. :)

[multiweb]

This AVI sequence was taken with a DMK21AF04 at 30fps at 600mm FL 90mm refractor, white light and YUV codec, approx 1000 frame. Might have done something wrong there as it's a mono. :question: but I don't recall IC capture giving me any other option for the capture format.

The image size is 640x480px. I also wonder if the low resoltution contributed to the lines and blockiness when I ran registar wavelets on the stacked final picture (1st shot).

The second shot is the stack out of Avistack2.

The thrid shot is Autostakkert.

The 4th is the Autostakkert stack sharpened in PixInsight.

DO you know what caused the lines with Registax? Am I doing something wrong during data acquisition? Is it undersampled?

[multiweb]

Ha... I'm not sure how to do that. Open a single frame and apply wavelet only. :question:

[jjjnettie]

If you save your stacked image as a tif, just open it in registax and it will take you straight to the wavelets.

[multiweb]

Cool - will try that. :thumbsup:

[multiweb]

Found the wavelet for TIFFs in registax. Works quite well. :thumbsup:

I also checked the codec I used with IC capture and although I used Y800 for the acquisition I used another one for saving the AVI and somehow ended up making a color video (out of the mono cam) so it must have halved the res and stuck a bayer pattern in there which is what popped its ugly head when sharpening. I did another test this morning with the right codec and all is well. Much better. Pfeewh... for a sec I thought the DMK21 would be too low res to do decent solar imaging. :whistle: