A real problem with flats : Need help

[Catanonia]

Guys,

I have been racking my head over this for months now and I just need some pointers.

I keep getting on my SuperNewt a strange dark arc and hotspot of over brightness on every single image, no matter what target, what moon condition, what light condition, it is always there on every single image in the exact same place.

I have attached 3 files.

1. Flat

2. Raw Image

3. Calibrated with the flat only

4. Number 2 stretched and DBE in PI to really show the problem

I have tried the following as it looks like a light leak, but I can't work it out.

1. Remove dew shield

2. Cover the rear mirror cell with a dark cloth to stop stray light getting in the back

3. Cover the whole CCD / OAG unit with black cloth in case it was getting in there.

4. Messed with all different methods of flats, from panels, sky flats, laptops.

5. Moved the TS OAG turrent incase that was causing relections being in wrong position in train.

None of these has worked, I always get the same pattern on every image so much so I have to push DBE on these images to tolerance of well over 5 from default of 0.5

My last resort will be to strip the scope and blacken the primary and secondary mirror edges.

Perhaps it is something to do with F2.8 and I am being to precise, but I hate having this arc across all of my calibrated images, I can process how I would like to.

It is a real pain, can anyone offer any suggestions please ?

[Astroscot2]

Very odd Steve, So sign of the Arc/spot on the flat or raw image ?Have you tried just the camera with no Filter wheel or OAG to eliminate these ? Does the Dark Arc or the bright spot move when the camera is rotated ? Other than that it may be a software issue, I really dont know.

[Mark72]

Your flat file looks a bit dark or may be my display is too dim.

Does it reach half of a histogram? How do you make flats?

Mark

[davew]

Apart from the word " Flat " is that a real flat ?

It's an RGB image and looks nothing like any flat I've seen before.

Could you re post a flat as it comes out of software ie. don't touch the mode and don't stretch it when you save to Jpeg.

I've a screen grab of your flat and one of mine. They don't look quite the same

Dave.

[Catanonia]

The images I have posted have been stretched and saved to JPG to be visible on the forum.

Flats are taken by a panel, laptop screen and even sky flats with a Teasshirt over the scope.

The problem is in the same place irrelevant to camera orientation.

[davew]

Is it the mono camera you are using ?

If so, does the histogram of the flats always have 5 distinct columns and show as an RGB image ?

That flat doesn't make sense to me unless you've altered it substantially when you wrote on it.

Dave.

[Catanonia]

Is it the mono camera you are using ?

If so, does the histogram of the flats always have 5 distinct columns and show as an RGB image ?

That flat doesn't make sense to me unless you've altered it substantially when you wrote on it.

Dave.

It is a mono CCD and perhaps the stretch and conversion to JPG has caused this.

I will get some more examples and post the originals on dropbox

[Catanonia]

Ok here are the original images

http://dl.dropbox.com/u/10801850/Astro%20Pictures/MASTER%20FLAT.tif flat. These were 31 sky flats combined together to make a master flat. Taken on same image train before session started

http://dl.dropbox.com/u/10801850/Astro%20Pictures/NGC7023-001L.fit raw image in luminance

http://dl.dropbox.com/u/10801850/Astro%20Pictures/NGC7023-001L_c.fit calibrated raw with the flat

Thanks for any help you can offer, should have posted these initially DOH

[davew]

I've downloaded the L fit. Software side looks ok now. I can't download the flat, I just get a 404 error.

The Lum channel doesn't show a distinct hot spot or dark arc.

I would like that Flat

Dave.

[Catanonia]

I've downloaded the L fit. Software side looks ok now. I can't download the flat, I just get a 404 error.

The Lum channel doesn't show a distinct hot spot or dark arc.

I would like that Flat

Dave.

uploading, will be done in about 5 mins

[davew]

Cheers.

I would like to get to the bottom of it ALMOST as much as you We image under the same skies.

Dave.

[Shibby]

Cat, have you tried the DBE on the flat (as with #4)? This would just help to prove that the anomaly isn't in the flats and therefore is nothing to do with how the flats are applied. In which case, I think you are right to chase a light leak... or some sort of amp glow?

[Dave Smith]

Dropbox no longer gives the 404 error but just hangs ? Have you re-uploaded it yet?

Dave

[davew]

I'm working on the downloads.

I've Pixel mathed the Lum and the flat. It appears there's no problem with the software. I get the same effect.

I'll carry on investigating shortly.

DBE is great but it would be nice to kill the problem at source. It would be a devil to sort with an image full of Nebulae.

Dave.

[Catanonia]

I'm working on the downloads.

I've Pixel mathed the Lum and the flat. It appears there's no problem with the software. I get the same effect.

I'll carry on investigating shortly.

DBE is great but it would be nice to kill the problem at source. It would be a devil to sort with an image full of Nebulae.

Dave.

Thanks mate, I am at a loss as to what it is. My guess would be the flats but I don't know.

[davew]

When I apply the flat ( divide ) I get the same results as you.

When I look at the difference in illumination between the light and flat I get this image below.

I can't find a darkening on your Lights.

It looks to me the flats and lights don't match. I don't know why that is. It may not be a software problem.

I even tried rotating the flat just in case. The dark arc isn't there but they still don't match well.

I hate to say it but Mark's ( Astroscot ) suggestion may be a good place to start. Eliminate things one by one before you pull the scope to bits.

Hopefully someone with more knowledge than me will be along soon !

Dave.

[ollypenrice]

Yves and I are struggling with flats on the ODK as well. It's the first time I've had any trouble with them and it does seem to be associated with reflectors, though that's an anecdotal observation. My random jottings are below.

The arc doesn't seem to be on the lights or the flats (though can you measure the relevant area of the flat to see if it really is there?) If it isn't in the lights or flats then surely it must be coming from the maths, in which case trying some assorted different stacking software would be worth a try.

If it really is there in the flats it will be a bright arc to look for and that could come from an internal reflection. Maybe dimming the panel a lot might help. I always do panel flats in the dark since stray light can mess with them.

Beware of the assumption that you have a well depth of 65000. Try some flats at a much lower value, say 8000.

Olly

[Mark72]

This is indeed a real problem with flat. You do not need even to apply it.

When you open master flat and apply STF you can see that illumination does go all the way to the top. It remains dark. If you check your light frame it is illuminated to the top.

So calibration will never work - the upper corner is always very light when divided by darker part of your flat. It looks as though when you make flats one part of your set up were not illuminated at all.

I'm not familiar with this camera. Does it have a shutter? How long do you expose your flat? It still does not reach the middle of the histogram. Try to expose it longer and check in PixiInsight with STF as to whether the illumination moves to the top.

Mark

[Mark72]

Sorry, When you open master flat and apply STF you can see that illumination does NOT go all the way to the to

[davew]

Good point re shutter Mark.

I'm also not familiar with the qhy9 so I've looked it up. It does have a shutter.

When I take flats with the 8300 chip I aim for around 8 / 9 seconds with an average ADU of 20000 or slightly less.

The lights here look ok.

We'll get there.

Dave.

I should also add that in my experience laptop screens don't work well with flats, especially narrowband. They seem to for some.

[Catanonia]

Thanks all, reading and still trying to digest.

The QHY9 Monon uses a mechanical shutter and perhaps the shapes are due to shutter interaction. I will try to dim even more and take flats. Problem is getting a dim enough light source (already 2 white teashirts over the scope) to give ADU readings around 23,000 and 3 - 4 seconds.

Will see if I can take much longer flats and see if that solves the problem.

In the meantime, still reading and working out what you guys have said, so many thanks.

Cat

[Mark72]

OK. It's clear now. I had the same issue with Atik. 3 seconds would not do. Nor would t-shirts. Take paper A4 or A3 format - a few lists will sort this out. Make sure exposure is 5-6 seconds. If you do this with artificial light one flat will do. If it's sky flats then take a break after each shot - 5 seconds at least.

Good luck. I hope you cam fix this soon.

Mark

[Catanonia]

OK. It's clear now. I had the same issue with Atik. 3 seconds would not do. Nor would t-shirts. Take paper A4 or A3 format - a few lists will sort this out. Make sure exposure is 5-6 seconds. If you do this with artificial light one flat will do. If it's sky flats then take a break after each shot - 5 seconds at least.

Good luck. I hope you cam fix this soon.

Mark

Thanxs Mark, will try this..

[iwatkins]

Wild card and probably nothing to do with this issue Cat, but years ago I had a very similar arc from a focus draw tube that had its Matt paint worn on one edge. Everything was fine while imaging, but the arc would appear in the flat due to the bright light source for the flats (which was a torch on a makeup mirror reflecting onto a white T shirt).

As I say, long shot.

Cheers

Ian

[ollypenrice]

Ian's above is the kind of thing I was thinking about as well regarding internal reflections.

I didn't realize you had a shutter so, yes, longer flats may help.

Karel Teuwen apparently told Yves that only sky flats work for his big telescopes.

Mark, could you explain why you need to take a break between sky flat exposures? I don't know about this. The trouble is that the sky brightness changes very fast at twilight.

Olly