Taking Flats - Will This Work?

[Knight of Clear Skies]

I took another Cygnus widefield using my 50mm lens this week which I'm quite pleased with, but I'd really like to apply some flats to it. I was wondering if this approach might work? The Moon should be visible this evening, so I could auto focus on it to find infinity. I could then switch to manual focus and take some flats in Av mode, using my computer screen as a light panel (one member mentioned they just open Notepad).

I can see a couple possible problems with this. Any dust bunnies may have shifted position, although as my camera doesn't have a sensor cleaner that might not be a big problem. However, I'm not sure if autofocus will take the focuser to the same position as I was using on Tuesday night, and I'm not sure how critical the focus is for flats. Has anyone tried anything like this please?

Ideally I'd just take some kind of light panel out to my dark site, but this would be a pain as I have to walk some distance and I'm already pretty loaded up. Alternatively, I could try marking out infinity focus on my lenses with a dab of paint, and try the daylight flats method.

[ollypenrice]

Exactly the same focus is ideal but you could give it a try, even just set to infinity or a tad short of it. I doubt your stellar focus is hugely far from the infinity mark? 

Large scale flattening of effects due to vignetting (though not dust bunnies so much) can be done successfully in Pixinsight using Automatic Background Extraction of Dynamic Background Extraction. You could always try the free trial download.

Olly

[Knight of Clear Skies]

Thanks, I'll give it a try and see what happens.

I doubt your stellar focus is hugely far from the infinity mark? 

My Canon lenses lack any markings on the focus ring. They are designed for auto-focus really, with manual an afterthought. As such they are quite sloppy.

[laser_jock99]

'Library Flats' are better than no flats at all. The main thing you're aiming to cure here is vignetting I guess ?

If you get the focus and F-stop the same then you might get usable flats.

[Leveye]

Just use a white T shirt and the sun the next day. The darks though should always be taken at the same temp and focal length preferable right after the lights. Just cap the scope yep.

[Mr Spock]

Camera lenses usually have fairly hefty vignetting wide open. If you don't want to get a light panel then the white t-shirt will do.

[Knight of Clear Skies]

Gave this a try by focussing on the Moon and then using my monitor as a light panel. Applying the resulting flats to my data didn't result in any noticeable improvement to the final image. Mind you, the flats I generated appeared uniformly grey to my eye, so perhaps my 50mm lens doesn't suffer much vignetting at f4?

I'll experiment some more, taking some more flats at different aperture settings with my collection of lenses to see how they perform. (If I can make a flat with bad vignetting I can then compare it with my f4 ones and have a better idea of what is going on). Perhaps I can open up the aperture a bit more to go deeper but then use flats to correct the image? I'm not sure where the sweet spot is for my kit.

Lots to learn but I'm enjoying myself.

The main thing you're aiming to cure here is vignetting I guess ?

I'm not quite sure, I thought it was part of the problem but perhaps not. My work in progress image is here, I've added a post to that thread about the issues I'm having.

[melsky]

If you have a fairly recent version of Photoshop and your using a DSLR with a camera lens you can use the RAW converter to automatically correct vignetting, CA and distortion (it reads the exif data for all the lens parameters) If your going to use DSS to stack your images don't use the distortion correction as this can produce a moire effect just save your files as 16bit tiffs, I find the PS converter also does a great job of taking out hot pixels as well.

Mel

[Knight of Clear Skies]

If you have a fairly recent version of Photoshop and your using a DSLR with a camera lens you can use the RAW converter to automatically correct vignetting, CA and distortion (it reads the exif data for all the lens parameters)

Now that is clever. Unfortunately I only have an old version, CS2.

[Knight of Clear Skies]

Looking at the flats I took more carefully I think they are no good. The master flat does show vignetting as I'd expect, but it is offset from centre. Possibly I wasn't holding the camera parallel to the screen or some background light interfered (I turned the lights off and pulled the curtains, but some light was still leaking into the room). I'll have to try this again more carefully.