#1  
Old 26-04-2013, 10:30 AM
peeb61's Avatar
peeb61 (Paul)
Always looking up

peeb61 is offline
 
Join Date: Jan 2007
Location: Wollongong
Posts: 716
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
Click for full-size image (MasterFlat_0.13.jpg)
163.6 KB37 views
Click for full-size image (Omega with Masterflat.jpg)
181.0 KB37 views
Click for full-size image (Masterflat_DSS.jpg)
192.3 KB32 views
Reply With Quote
  #2  
Old 26-04-2013, 12:46 PM
Terry B's Avatar
Terry B
Country living & viewing

Terry B is offline
 
Join Date: Mar 2006
Location: Armidale
Posts: 2,789
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.
Reply With Quote
  #3  
Old 26-04-2013, 01:25 PM
LewisM's Avatar
LewisM
Novichok test rabbit

LewisM is offline
 
Join Date: Aug 2012
Location: Somewhere in the cosmos...
Posts: 10,388
I get green flats a lot. They work, so I am not concerned with the colour I use one of EXFSO's light boxes.
Reply With Quote
  #4  
Old 26-04-2013, 01:31 PM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Sydney
Posts: 22,060
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.
Reply With Quote
  #5  
Old 26-04-2013, 04:22 PM
peeb61's Avatar
peeb61 (Paul)
Always looking up

peeb61 is offline
 
Join Date: Jan 2007
Location: Wollongong
Posts: 716
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.
Reply With Quote
  #6  
Old 27-04-2013, 12:39 PM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Sydney
Posts: 22,060
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.
Reply With Quote
  #7  
Old 27-04-2013, 02:14 PM
peeb61's Avatar
peeb61 (Paul)
Always looking up

peeb61 is offline
 
Join Date: Jan 2007
Location: Wollongong
Posts: 716
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
Reply With Quote
  #8  
Old 27-04-2013, 02:53 PM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Sydney
Posts: 22,060
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.
Reply With Quote
  #9  
Old 27-04-2013, 03:40 PM
peeb61's Avatar
peeb61 (Paul)
Always looking up

peeb61 is offline
 
Join Date: Jan 2007
Location: Wollongong
Posts: 716
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
Reply With Quote
  #10  
Old 27-04-2013, 04:13 PM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Sydney
Posts: 22,060
Quote:
Originally Posted by peeb61 View Post
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.
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 03:20 AM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Advertisement
Bintel
Advertisement
Testar
Advertisement