Concentric Rings in my Flats

[leon]

Hi Guys, I am a little puzzled, at the appearance of my final combined flat.

I take them as usual, about 15 minutes on the dark side of sun set, and convert to Tiff Format in Image Plus, and then combine them in Image Plus.

I have tried Medium Combine, and Average combine, and all in between, but the end result is the same.

The captured single images are all fine, a clear and a even flat field, they just look fine, but once combined the final image has concentric rings of a different shade starting from the center right out to the edges of the frame.

Has anyone else ever experienced this, or have any suggestions , it would be much appreciated

Leon

[[1ponders]]

Could you post a pic Leon?

[Bassnut]

when you say "even flat field" do you mean featureless?. If so, thats the problem, then the concentric circle will appear in your image, instead of the flat, where it should be, to cancel the one in your image out !.

[leon]

Ok Paul, have a look at this, it is a combine set of 19 flats taken tonight.

The flats were sky flats taken 15 minutes on the dark side of Sunset converted to tiffs and combined in Image Plus.

Leon :thumbsup:

[Bassnut]

Thats a very dim flat Leon, it looks so dim that the seperate circles are A/D conversion artifacts. Flats should be about a third way up the histogram (saturation).

[leon]

Fred, I agree that the combined flat is very dark and dim.

This happens when I convert them to 16 bit tiffs in image plus, unless i'm missing something, what is the answer. :shrug:

I'm sort of puzzled about this, more so, what are those rings of which i speak, i have never seen this before, could it be some reflection from inside the scope, do you think. :shrug:

Leon :thumbsup:

[Bassnut]

No, I dont think so. When you convert to 16 bit TIFF, are you presented with an option on wether they are scaled or not?. Unscaled, 12 bit images look much darker in 16 bit. Make sure you click the scaled option (or somesuch). And again, are the pics 1/3 to 1/2 way up the histogram to start with?.

[leon]

Fred, I will check out the scaled part of your answer, must say I have not checked that part of the process, however the histogram of the originals are usually no more that have way across the scale.

I see you said up the histogram, am I looking at this wrong, when the histogram appears after my shot it is half way across (horizontally) not (vertically)

Leon

[Bassnut]

half way horizontally. I dont want to lead you astray here, its been a while since I used IP. There is an automatic cal frame generation feature in IP, are you useing that? (V3.5)

[Ian Robinson]

Vignetting ???

Or

An electronic artifact due to how the chip behaves ???

Does it affect the final stacked image ?

[leon]

Ian, it dosen't seem to have any effect on the final image at all, it is only visible on the combined image, the original raws are fine, buggered if I know.

Fred I am using version 2.8, i have not updated to 3.5 as yet, didn't really see any need to.

leon

[[1ponders]]

I don't know Leon. I'll have to have a play with some of my flats to see if I can reproduce the same effect. It may simply be how your monitor is displaying a low contrast 16 bit image on an 8 bit monitor. :shrug: Though why it would only do it with your master and not the individual flats I'm not sure. Do you get the same effect if you use FITS instead of TIFS?

I don't know if this makes a difference but do you have adobe RGB checked in the System Options box or sRGB?

[leon]

Haven't tried Fits Paul, and the answer to your last question is yes, this is strange but i will get to the bottom of it some how.

It however has no effect on the final image, very strange, gee I seem to come up with some strange issues.

Leon.

[rogerg]

It looks to me 100% like an artifact brought in by conversion between bit depths. I'd expect this kind of "chunky gradient" if I did the following with a stacked image of a galaxy:
1. converted to 8bit from 16bit
2. adjusted levels & curves
instead of:
1. adjusted levels & curves
2. converted to 8 bit from 16bit

It's not clear to me what bit depth they're being captured at? I gather you're converting them to 16bit so I'm presuming they're not 16bit?

Can you work with them at their original bit depth? and file format?

I think it's a bit dark also, would increase the exposure time a bit. But very hard to accurately judge that without the originals, impossible to know what the conversion/processing has done.

Re noticing it - I wouldn't expect you to notice such fine gradient changes in the flat, in the end result image very easily. The change would be very subtle. Even with my flats which have quite a bit of contrast in some places I have to really push the image to see the effect that the flats are having.

Without the conversion problem it looks to me like your actual flat field would be quite smooth, with just vinyetting kind of affets in play, darkening the edges. Very nice compared to my flats which have dust etc :thumbsup:

[Bassnut]

Leon

Hang on, is the flat RAW or JPEG?.

[leon]

Roger and this will apply to Fred as well I presume.

I capture the original flats in raw format, and then convert them to 16 bit Tiff format, and then combine all the converted tiffs (Medium Combine) in Image Plus, to make my master flat, which I then gray scale, with luminence and gamma in Image Plus.

The flats are captured with the camera in AV mode, on the evening sky, the darker side of sunset.

Hope this helps.

Leon