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Problem doing flats

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#1 mbushell

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Posted 20 June 2013 - 05:28 PM

I am imaging with an FSQ106 and FLI16803 Camera. The FSQ has a Starlight replacement focuser. I have done flats using a Flatman and they all show about a 15% drop off from center to corner. Pretty much expected I guess. However, the actual exposures don't show this. And when I apply the flats in MaximDL the result is unusable. The central part of the image becomes very dim and the outer parts very bright. I have tried long and short flat exposures ranging from 1 sec to 30 seconds with flatman turned down as low as it will go. Some advice from experienced imagers would be appreciated. Thanks!

#2 Madratter

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Posted 20 June 2013 - 07:12 PM

One thing I found when doing flats is that it made a big difference how I combine them if they were not all illuminated exactly the same. I needed to do a multiplicative normalization. Since you are using a Flatman I'm sure that isn't your problem. But if others are experiencing this, that might be part of their problem. I still get this to a minor degree simply because remaining noise is raised up and harder to then get rid of in the corners.

#3 zerro1

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Posted 20 June 2013 - 07:38 PM

I am imaging with an FSQ106 and FLI16803 Camera. The FSQ has a Starlight replacement focuser. I have done flats using a Flatman and they all show about a 15% drop off from center to corner. Pretty much expected I guess. However, the actual exposures don't show this. And when I apply the flats in MaximDL the result is unusable. The central part of the image becomes very dim and the outer parts very bright. I have tried long and short flat exposures ranging from 1 sec to 30 seconds with flatman turned down as low as it will go. Some advice from experienced imagers would be appreciated. Thanks!


It sounds to me like you're not getting the correct exposure for your flats so when they are used to calibrate, it's over correcting. On my KAF6303E camera, in MaximDL Pro, I shoot about "I = 1500" in the center field to get correct illumination for flats.

Have you tried using a different program to calibrate and stack just to see if it's related to maximDL or not? I personally use Images Plus to calibrate and stack. (only got maximDL because it supports the camera for capture)

#4 Mike7Mak

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Posted 20 June 2013 - 10:12 PM

Are you bias or dark subtracting the flats? You should.

#5 JJK

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Posted 21 June 2013 - 05:55 AM

Are you bias or dark subtracting the flats? You should.


Why subtract bias frames from flats?

#6 Madratter

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Posted 21 June 2013 - 06:52 AM

Are you bias or dark subtracting the flats? You should.


Why subtract bias frames from flats?


Generally speaking, Flat exposures are relatively short so there isn't much time for dark current to build up. As such, bias frames and dark frames aren't typically much different. So you can use one or the other. Personally, I still shoot darks for them.

#7 mbushell

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Posted 22 June 2013 - 10:32 AM

Are you bias or dark subtracting the flats? You should.

I did the flats in MAximDL with automatic dark correction turned on. That should take care of it shouldn't it?

#8 Mike7Mak

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Posted 22 June 2013 - 03:52 PM

Are you bias or dark subtracting the flats? You should.

I did the flats in MAximDL with automatic dark correction turned on. That should take care of it shouldn't it?

Somebody who knows Maxim will have to answer that. If they are getting dark subtracted then it's probably exposure as Robert mentioned. The histogram peak should be near the middle of the graph between 25k to 40k ADU.

#9 mbushell

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Posted 22 June 2013 - 07:09 PM

Are you bias or dark subtracting the flats? You should.

I did the flats in MAximDL with automatic dark correction turned on. That should take care of it shouldn't it?

Somebody who knows Maxim will have to answer that. If they are getting dark subtracted then it's probably exposure as Robert mentioned. The histogram peak should be near the middle of the graph between 25k to 40k ADU.


Yep, that's where I had the histogram peak. I just noticed that I had the readout mode set to 8 mhz with NO RBI flood. Could that distort the flats? I had the mode set to 1 mhz with flood but Maxim changed it back and I didn't notice.

#10 CounterWeight

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Posted 22 June 2013 - 09:02 PM

I'm a bit confused.

I've used MaxImDL and a Flatman on several scopes for some time now without issue. Most recently with the new FSQ85. I usually have two sets initially - I make one I call 'Weak' and the other 'Strong'. I then decide which to keep after getting some images done.

I think it's important to note which filters you are using - but then too in ways that shouldn't matter - as far as currently I've no issues with the Baaders or the Astrodons.

My confusion is this - what do you mean by the rest of what you are doing. OK - you get flats. I take mine all at the same exposure time 2.5 and 3.0 seconds (this due to the shutter lag in the SBIG). I usually keep my illumination at around 23 for the 'weak' and about 35 for the 'strong. With my gear I can't imagine a need to go outside that range.

I take at least 24 (according to the MaxIm authors that was considered good). As I keep the time and temps same across board I take my 'flats4darrks (also 24) with the Flatman removed and the cap on the scope.

I take these and put in two folders. Strong flats in one with the darks and then repeat for the weak.

In MaxIm I use one of the two methods, manual or wizard - and create the calibrations and if need 'replace with masters' - I forget which one does it automatically but I think it's the cal wizard.

Things that can make it weird is not using consistent names for filter designation (though you'd have to do some work to make that happen) and the program doesn't actually complete the process - though you get notifications when this happens.

My CCD doesn't have the RBI stuff so I cant comment there. Each time I change scopes or filters or whatever I always do some experimentation on flats just because I don't know it all.

so that is flats. Here is an example of what works for me. (this example for the Baader 7nm Ha)

Attached Files



#11 CounterWeight

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Posted 22 June 2013 - 09:12 PM

here is what I am using 'today' (that is when the clouds let me) Astrodon 5nm Ha.

Attached Files



#12 CounterWeight

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Posted 22 June 2013 - 09:18 PM

Not certain I understand the problem statement correctly. If you are having issues getting good flats of if not understanding how to apply depending on the path you take through calibration and stacking. MaxIm is absolutely KATN when it comes to this arena so barring something like a driver issue you should be able to get it sorted quickly.

Hope any of that above helps, if I'm in left field I apologize and will try another shot at it.

EDIT - that is sure some fantastic gear you have! :bow:

#13 CounterWeight

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Posted 22 June 2013 - 09:33 PM

And here is an example of using that flat on an actual Ha stack. Nothing done outside calibration and some contrast / brighten to make it show up better.

Attached Files








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