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  #21  
Old 21-07-2015, 08:41 PM
Garbz (Chris)
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But can I do it again without the funny noise and vertical lines in the image this time. :-)
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  #22  
Old 30-07-2015, 08:14 PM
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I have had problems with flats in the past. So yes it can suddenly go haywire when some simple step has been omitted or a few dodgy individual flats got into the master flat. I was playing around with some flats just last night to do with another thread and simple minor changes can make it go weirdly wrong.

Best to list exactly what steps you are taking and with what camera and scope, software and how you take your flats - dusk with a white cloth over the end of the scope, illuminated panel etc.

Its impossible to comment without knowing what your current procedure is.

Here at least is my procedure:

1. I take 6 to 12 flats at dusk or sometimes during the day with a white cloth cover over the end of my scope. In my case my observatory which is painted black on the inside walls lets in a small amount of light to make taking a flat possible. These work well. Otherwise I take dusk flats. It can be a race to take them and you need a camera with fairly fast download speed as you can see the ADU falling between subs. I increase the exposure length to compensate between filters.

2. I take bias perhaps 12 to 16 at the same temp.
3. Darks are about 16 subs and at the same temp and binning. I use sigma reject combine for the master dark using CCDstack. I do the same for bias.
4. Flats on a camera with a physical leaf type shutter like Apogee, FLI need a minimum of about 4 seconds exposure otherwise you see the shutter in the flat.
5. When applying to the same dark to the lights I do not use scaling but I tend to leave scaling on in CCDstack and bias subtract on.
6. I do median combine for flats and when applying the master flat to the images I check the subtract bias from the master flat. You can subtract the bias from the subexposure flats when making a master flat. I don't know it makes too much difference but for some reason I got better results doing it as I mentioned.
7. I check to see my luminance is flat fielded well with a trial and go hunting for what is different if I don't get a good result. I often do this while the camera is imaging so I've got something to do whilst I am waiting.

With a Sony sensor then you really don't need to do any of the above if your sensor and filters are clean which I prefer (of course before you start imaging). A bias subtract may be all that is needed unless you have dust donuts. So the above is the procedure for a Kodak sensor which are noisier and need the above to clean up. Or for a setup that has significant vignetting or dust donuts.

Is this similar to what you are doing? Do you scale your darks? Are you flats at a consistent ADU level more or less? I go for around 20-30K ADU. These work well. I have had some flats that seem too bright and get reverse to vignetting. I have had bad calibration when the darks do not match the lights precisely on temp and duration on some touchy scopes. Most though are fairly tolerant. Are you using 1x1 binned flats on 2x2 subexposures? I believe you can do that with CCDstack. I am unsure how well it works but perhaps.

I have at times used one flat from red for all RGB if I didn't get good flats for the others and the other coloured filters are clean with no dust donuts. Not ideal as perhaps they have dust donuts but if everything is clean I've gotten away with it. I usually have a separate master flat for each filter imaged with in exactly the same orientation as the filter wheel (perhaps not so important to match the orientation of the scope just the filter wheel and of course nothing changed in the optical train as well as the scope is focused and not way out of focus which could shift things.

The first thing I would do is check the master flat and make sure it looks like a usual flat that has worked for you over the years.

Greg.
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  #23  
Old 30-07-2015, 09:04 PM
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I personally consider flats to be for a lot more than just removing dust donuts. Unlike most on here though, I am also uninterested in doing astrometry which requires a good flat field above all else.

For the "perfect" flat field it must be taken at the same focus as what you are imaging at because light fall off from the centre to the edges changes as the focus does. On a purely cosmetic level, the dust donuts change with focus as well.

As for configuring ADU levels, this is quite easily calculated on camera. Most cameras have a linear response for 75-90% of saturation, this is tested by gradually increasing exposure time (the same way you would normally do flat) and plotting exposure time against average ADU. Typically you want to do 50-75% of saturation for a flat field which equates to 24,000-40,000 but this depends on the camera. As Greg mentioned, have them last at least 5 seconds if they have a shutter.
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  #24  
Old 30-07-2015, 09:44 PM
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I'm experimenting at the moment. I think I may have found the root cause to be settings taken at the time. I'm redoing darks right now and already have a new set of flat and biases. So stay tuned....

I would agree with flats being about more than just dust. Watch this space and hopefully I can fist pump the air tomorrow.
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  #25  
Old 30-07-2015, 09:53 PM
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hopefully I can fist pump the air tomorrow.
Careful, that'll move the dust motes around.

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  #26  
Old 30-07-2015, 10:11 PM
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Careful, that'll move the dust motes around.

i am following this thread carefully
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  #27  
Old 31-07-2015, 12:10 PM
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Careful, that'll move the dust motes around.

Once the flats correct, I won't care anymore :-)
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  #28  
Old 31-07-2015, 12:20 PM
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AND THE WINNER IS:
Marc and Rick.

The problem turned out to be a screw up in image capturing. I blame upgrading my software but my Gain and Offset values for the camera were set at 14 and 133.

My dark library and bias frames had a gain and offset of 17 and 123 so the image calibration process was visually assaulting my frames quite badly. The crap bias frames would have introduced scaling issues.
I redid flats light night and then ran off 50 darks and 50 bias frames, and the result is below. Not sure where the gradient came from, but I don't care. DBE makes short work of that.

Funny story:

Actually I found the problem 2 days ago but when I ran the 50 darks and 50 bias frames I realised that I forgot to change the gain and offset values. I remembered 3 hours later, stopped the process, deleted the files and then noticed my telescope wasn't parked and was past the merridian, so I moved it parked it and then started taking darks and bias frames again.

Notice what I missed? 45minutes later I realised I still hadn't changed the gain and offset values. . So repeated the process set the gain and offset and then got 1 frame out before I decided it was bed time. There's really no helping me anymore is there


Anyway thanks all.
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  #29  
Old 31-07-2015, 12:56 PM
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Nice find. Sometimes you just have lousy nights eh? Is this a QHY camera?

Greg.
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  #30  
Old 31-07-2015, 01:07 PM
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Glad you got it resolved, Chris
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  #31  
Old 31-07-2015, 02:00 PM
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Nice find. Sometimes you just have lousy nights eh? Is this a QHY camera?
Yeah QHY10 why?
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  #32  
Old 31-07-2015, 02:29 PM
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Other brands I have used don't allow you to set gain and offset as far as I know.They are set by the manufacturer. I wonder what the advantage is for allowing it? It seems like an unnecessary variable and no doubt one setting puts the camera at optimum which would be something worked out by the engineers and tests.

Greg.
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  #33  
Old 31-07-2015, 05:19 PM
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Originally Posted by gregbradley View Post
Other brands I have used don't allow you to set gain and offset as far as I know.They are set by the manufacturer. I wonder what the advantage is for allowing it? It seems like an unnecessary variable and no doubt one setting puts the camera at optimum which would be something worked out by the engineers and tests.

Greg.
All you really need is a light box and 30-60 minutes so it can be done on a rainy day. The only reason I can think of for doing this is to allow the user to do the same thing you can do on a DSLR, increase the ISO if wanted.
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  #34  
Old 31-07-2015, 05:54 PM
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Or just not need to factory calibrate each unit :-)
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  #35  
Old 31-07-2015, 06:02 PM
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All you really need is a light box and 30-60 minutes so it can be done on a rainy day. The only reason I can think of for doing this is to allow the user to do the same thing you can do on a DSLR, increase the ISO if wanted.
I suppose you'd want to be careful about increasing the gain very much because I would expect you could lose dynamic range quickly or increase noise just like higher ISO's do.

ISO is a bit misleading in that you've got to capture the signal before you can amplify it. As I recall from school, 10 times zero is still zero!

Greg.
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  #36  
Old 31-07-2015, 08:04 PM
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I suppose you'd want to be careful about increasing the gain very much because I would expect you could lose dynamic range quickly or increase noise just like higher ISO's do.

ISO is a bit misleading in that you've got to capture the signal before you can amplify it. As I recall from school, 10 times zero is still zero!

Greg.
Have to be very careful, not something that I would personally recommend doing!
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  #37  
Old 03-08-2015, 08:12 AM
Garbz (Chris)
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I suppose you'd want to be careful about increasing the gain very much because I would expect you could lose dynamic range quickly or increase noise just like higher ISO's do.
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Have to be very careful, not something that I would personally recommend doing!
And yet there's an application that I use it for on a daily basis. Not everything requires the max dynamic range, specifically:

-Previewing: 2x binning + gain of around 40 allows me in 10 seconds to see if I have a faint outline of a nebula and I'm pointed in the right direction at the expense of horrendously clipped stars which are bleeding and drawing lines all over my picture.

-Focusing: doing a partial sensor download with the gain at almost as high as it will go makes short work of Bahtinov masks when focusing, even on a 1 second exposure.

Also lowest gain is not necessarily the highest dynamic range point on a sensor and binning has a big effect on gain and dynamic range so to maximise your dynamic range you want to run at a different gain when binning.
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  #38  
Old 03-08-2015, 09:03 AM
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And yet there's an application that I use it for on a daily basis. Not everything requires the max dynamic range, specifically:

-Previewing: 2x binning + gain of around 40 allows me in 10 seconds to see if I have a faint outline of a nebula and I'm pointed in the right direction at the expense of horrendously clipped stars which are bleeding and drawing lines all over my picture.

-Focusing: doing a partial sensor download with the gain at almost as high as it will go makes short work of Bahtinov masks when focusing, even on a 1 second exposure.

Also lowest gain is not necessarily the highest dynamic range point on a sensor and binning has a big effect on gain and dynamic range so to maximise your dynamic range you want to run at a different gain when binning.
I do totally agree with you, same thing I do on my DSLR. ISO up at Hi2 (~26,000) for focusing. It does have its uses, I was more referring to using it in capturing lights. You make a good point though, especially with the binning which is something that does need to be considered with cameras that allow you to set the values.
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  #39  
Old 05-08-2015, 03:08 PM
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An interesting use. I can see that for focusing etc. I usually use 3x3 or 2x2 binning for focusing and framing. 1x1 is too slow and too large an image. For critical focus 1x1 is good but slow.

Not 100% sure if it this applies to CCDs but I don't see why not. Dynamic Range on DSLRs is highest at lowest native ISO. Dynamic range is defined by well depth divided by read noise. As far as I am aware gain is applied during read out but before analogue to digital conversion so in effect it would be added to the read noise. That's why on DSLRs they have a native ISO range (amplifier gain before digital conversion).

So you lower gain when binning to prevent burn out bright areas?

Greg.


Also lowest gain is not necessarily the highest dynamic range point on a sensor and binning has a big effect on gain and dynamic range so to maximise your dynamic range you want to run at a different gain when binning.[/QUOTE]
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