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Old 26-04-2013, 10:30 AM
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peeb61 (Paul)
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Weird Flats

Hi All,
Could someone please tell me where we are going wrong taking flats.

We have taken flats using a light box and averaged 28486 as a pixel count. We stacked 25 flats using Deep Sky Stacker with one light to give us a Masterflat. (See attachment)

The resulting Masterflat appears to be Green in Deep Sky Stacker and Grey in Nebulosity.

When we stack the flat with 60 lights and after a quick process using CS3, the resulting image is horrible. (See attached)

Any ideas where our problem lies??

Many thanks

Paul
Attached Thumbnails
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Click for full-size image (Omega with Masterflat.jpg)
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Click for full-size image (Masterflat_DSS.jpg)
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  #2  
Old 26-04-2013, 12:46 PM
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I don't know about the colour but presumably one program displays the data as colour and the other as a grey scale.

As for the stack.
Flats are "divided" into the light frames not stacked or added in. If you have just stacked them all together you will get a horrible image as you describe.
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Old 26-04-2013, 01:25 PM
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I get green flats a lot. They work, so I am not concerned with the colour I use one of EXFSO's light boxes.
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Old 26-04-2013, 01:31 PM
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Your flat looks alright but it's over correcting. If you remember the duration and the temprature you shot your flats at, try to make a series of darks of the same duration, make a master and use that as a bias and subtract it from your master flat. Then divide the master flat into your sub and you'll get the result you're after.
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Old 26-04-2013, 04:22 PM
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Quote:
Originally Posted by Terry B View Post
I don't know about the colour but presumably one program displays the data as colour and the other as a grey scale.

As for the stack.
Flats are "divided" into the light frames not stacked or added in. If you have just stacked them all together you will get a horrible image as you describe.
Hi Terry, The Masterflat was added as a Flat and Lights accordingly, is this what you mean?

Quote:
Originally Posted by multiweb View Post
Your flat looks alright but it's over correcting. If you remember the duration and the temprature you shot your flats at, try to make a series of darks of the same duration, make a master and use that as a bias and subtract it from your master flat. Then divide the master flat into your sub and you'll get the result you're after.
Thanks Marc, The flats were captured at -15deg and duration was 0.13 sec, does this sound right?

Quote:
Originally Posted by LewisM View Post
I get green flats a lot. They work, so I am not concerned with the colour I use one of EXFSO's light boxes.
Thanks Lewis, this makes me feel I'm on the right track.
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Old 27-04-2013, 12:39 PM
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Quote:
Originally Posted by peeb61 View Post
Thanks Marc, The flats were captured at -15deg and duration was 0.13 sec, does this sound right?
Yes that's fine. So now shoot some darks at -15c and 0.13s duration. Do 100 of them. Stack them and call it master bias.

If you're familiar with pixel/file maths. Get your master flat and subtract the master bias you've just done then save it as new master flat. Use that one to do your flat fielding and you'll have a nice even background with no dust motes or color gradients.
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Old 27-04-2013, 02:14 PM
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Quote:
Originally Posted by multiweb View Post
Yes that's fine. So now shoot some darks at -15c and 0.13s duration. Do 100 of them. Stack them and call it master bias.

If you're familiar with pixel/file maths. Get your master flat and subtract the master bias you've just done then save it as new master flat. Use that one to do your flat fielding and you'll have a nice even background with no dust motes or color gradients.
Thanks Marc for getting back on this, I'm sorry I'm not familiar with pixel/file maths??

It does sound like I'm getting close thanks to you!

Paul
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Old 27-04-2013, 02:53 PM
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Quote:
Originally Posted by peeb61 View Post
I'm sorry I'm not familiar with pixel/file maths??
No worries. You need to to do a bit of reading on the maths behind flat fielding. It's very simple arithmetic. Adding/subtracting and dividing.

Here's a good place to start. The bottom line is that subtracting the bias from your master flat will scale it correctly so when you divide it into your sub the correction will be properly weighted. Understanding calibration is critical to good imaging.
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Old 27-04-2013, 03:40 PM
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Quote:
Originally Posted by multiweb View Post
No worries. You need to to do a bit of reading on the maths behind flat fielding. It's very simple arithmetic. Adding/subtracting and dividing.

Here's a good place to start. The bottom line is that subtracting the bias from your master flat will scale it correctly so when you divide it into your sub the correction will be properly weighted. Understanding calibration is critical to good imaging.
Thanks Marc for your time, I will get stuck in and read what its all about.

Paul
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Old 27-04-2013, 04:13 PM
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Thanks Marc for your time, I will get stuck in and read what its all about.

Paul
Get a trial version of CCD Stack and play with it.
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