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Old 06-03-2011, 12:07 AM
apaulo (Paul)
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Freaky Flats

Hi guys

I am simply stumped! I took some images tonight and some flat frames
Stacked them in deep sky stacker and the result was something I have never seen. Has anyone got an idea to what is going wrong? The iso is the same as well as the focus. DSS is a little unstable I reckon but someone hopefuly has come across this problem before.
Regards Paul.
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Old 06-03-2011, 08:46 AM
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rcheshire (Rowland)
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Probably stating the obvious, but the curved lines at the bottom look typically like uncropped post alignment overlap - except that you'd expect them to be straight. The curvature in the vicinity seems to have blurred the image. Is DSS attempting to align to something other than a reference image - if that's how it works?
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Old 06-03-2011, 10:32 AM
apaulo (Paul)
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alignment

Hi Rowland,

If you look closer you will notice around a star (where the problem occurs) there is an echo of red and green star, surrounding the star itself, so it maybe an alignment issue. The problem doesn't occur when the flat is NOT added to the stacking process!.

Regards Paul.
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Old 06-03-2011, 11:47 AM
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rogerg (Roger)
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I think I need more info ...
- how many flat frames?
- can you show us a sample flat? and the combined flat?
- using filter wheel or single shot colour?

The flats shouldn't be aligned, were they simply stacked as they were taken? and were they stacked at the same orientation (having auto rotation turned off if it's a DSLR, to be sure).
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Old 06-03-2011, 04:11 PM
apaulo (Paul)
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flats to view

HI Roger

Yes the rotation was the same, i took 10 flats but the method used via live view and the histrogram was adjusted to a third of the way from the left. Here are the flats you requested.
The actual size of the files are large so i have reduced the size so to easily upload to this forum.
Regards Paul.
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Old 06-03-2011, 04:48 PM
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rogerg (Roger)
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hmm... not sure how much help I'm going to be, apologies if you already know all this, but see if any of this is useful ...

The two flats look quite different. That's odd if they're taking one after the other. It could be just the conversion down to 8bit compressed jpg, but still, they look quite different. The first is generally more even, the second has the brighter area down te bottom corresponding more closely with your end resultimage. The second has big steps of gradient, not a smooth gradient (might be the JPG, but if it's not that'd be a problem). Overall there's a lot of reasons why they could be different but if I took a series of flats and two varied that much when looking at the RAW files I'd throw them away and question what's wrong. Perhaps compare your set and pick a sub-set of flats which look similar. I compare them by stretching the levels but not saving that.

With my ST7 I take flats which have about 25,000 ADU out of 65000, which fits reasonably with your aim of 1/3 down the histogram I think. With my DSLR I just put it on P. This results in a neutral brightness but a lot brighter than yours and probably a bit brighter than my ST7 flats. [edit: I can't remember now, I might under-expose by a stop or two]. They work very effectively. As long as your not clipping data I don't think it matters if they're brighter (just got to make sure they're not over exposed and not under exposed). I would try brighter ones in your situation as you're having troubles.

I don't think you've mentioned what camera/filterwheel you're using. Still would be helpful to know. I would think the reasons would be different if you're doing seperate sets of colour flats for filters.

I think it's clear the flats have been applied too aggresively. They're having too much effect. Not sure if there's a setting you can change for that in DSS or if you should adjust the images before use.

To get the offset of colours you have on stars I'd think you'd need the colours to be offset in the flats. hmm, or perhaps one of the actual light images was accidentily used as a flat and not aligned? that'd produce a bizzar result perhaps like this.

hmm, that's my almost-random thoughts for now...
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Old 06-03-2011, 06:51 PM
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tlgerdes (Trevor)
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I have seen this on occasion with DSS. I dont think it is your flats causing it, but more likely some sort of alignment error.

I had it in a widefield milky way shot, tried everything I could in re-stacking it but nothing helped, so I just cropped.
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Old 07-03-2011, 09:56 AM
apaulo (Paul)
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all cropped up and no were to go!

HI Trevor and Roger.

I knew there would be someone over time popping up saying they have suffered the same problem!. Trevor I have uninstalled DSS from the harddrive and the registery. I have reinstalled DSS and the problem has mostly gone. I can now add flats to any images apart from the ones where I have used different iso flats to different iso images.

I suggest you give that a go. I think DSS is a great program but there are plenty of bugs within it.

So thank you Roger and Trevor for your kind help regarding this matter.

Regards Paul.
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Old 07-03-2011, 03:31 PM
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These are not flats, they look like darks or biases.

Flats are taken at dusk or dawn with a white T shirt or material over the end of the scope or with a flat light box of some sort.

They look usually white and show perhaps some vignetting depending on your setup.

How did you take these? What exposure, was it taken during dusk or with a light box etc?

Here's a sample of one of my flats that worked well:

http://www.pbase.com/image/132995797

greg.
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Old 07-03-2011, 06:33 PM
TheDecepticon
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I have had this issue with DSS and trying to stack my gray scale frames for LRGB conversion, where it smears a certain section of each finished frame. I gave up looking for the problem in the end and started using MaxIm as I had that already and have never looked back, using all the same frames I did in DSS. You may want to look at CCD Stacker also, but there are others too.
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Old 08-03-2011, 06:56 AM
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gregbradley
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I read the rest of the thread and I see you are on top of the issue. Ignore my post, your flats must have just turned out dark in the posting process.

Greg.
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Old 08-03-2011, 10:44 AM
apaulo (Paul)
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what fflats?

Hi Greg,

No problems mate, yes the transfer wasn't the best. although I think you have a point, my flats need to be more exposed! This problem occurred two weeks ago and I thought my flats back then were over exposed, so I backed off a bit. I use the "P" option on the dslr and adjust the A/E until I get the histogram one third from the left using Live view. I will amp it up a bit and see the results.
As I said before, I had gathered DSS may of been the problem, by simply reinstalling this problem usually remedies the issue.
So a big thank you to everyone who contributed, I appreciated your help

Regards Paul.
By the way he is the link of the finished product of the orginal photo.

http://www.mpas.asn.au/_astro/galler...g2_itemId=3165

Paul.
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