#1  
Old 13-10-2012, 07:15 PM
roughy (Mark)
Registered User

roughy is offline
 
Join Date: May 2011
Location: Hobart
Posts: 112
Flat Fields Integration Time

Is there a formula for calculating the integration time for a flat based on your chip and camera characteristics? I have been perusing the "Handbook of Astronomical Image Processing" but cannot seem to find a specific equation.

Cheers

Mark
Reply With Quote
  #2  
Old 13-10-2012, 08:08 PM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 15,476
Quote:
Originally Posted by roughy View Post
Is there a formula for calculating the integration time for a flat based on your chip and camera characteristics? I have been perusing the "Handbook of Astronomical Image Processing" but cannot seem to find a specific equation.

Cheers

Mark
There's a fair bit of data about this. Richard Crisp wrote a paper about this 4 months or so ago. You could google for it.

I generally shoot until I get about 20,000 ADU. His paper suggests to do more as CCD responses are linear.

On the other hand I have had flats that are too bright cause unwanted effects in my images. Not 100% sure why. So I tend to stick to around 20,000 ADU plus or minus a bit. I also take a dark and subtract it when doing the master dark.

Also using good exposure darks helps with the flat fielding. A poor dark seems to throw off the flat fielding as well.

Greg.
Reply With Quote
  #3  
Old 15-10-2012, 11:39 AM
solissydney (Ken)
Registered User

solissydney is offline
 
Join Date: Mar 2006
Location: Sydney
Posts: 660
Flats

ADU's ????????
What?
Reply With Quote
  #4  
Old 15-10-2012, 12:00 PM
[1ponders]'s Avatar
[1ponders] (Paul)
Retired, damn no pension

[1ponders] is offline
 
Join Date: Nov 2004
Location: Obi Obi, Qld
Posts: 18,778
I'm with Greg, though I go a bit higher with 25-27K ADU. I have my Flats settings in Maxim set to 0.3 sec and adjust the brightness of my flat box to achieve the 25-27K. Any longer than about 0.5 sec and I take flat darks. For me using the SX cameras 0.3sec seems to be fine without flat darks.

ADU = Analog to Digital Unit. A conversion factor that converts your analog camera signal to digital units. These days most seem to be in the range of around 0.3 - 0.9 conversion rate (= gain. thanks Rick ). Short and simple, if your camera has a full well depth of 20,000 electrons to convert that to 16 bit digital units you need to divide your 20,000 by approx 0.3 to get 65535 ADU (or 16 bit). The deeper the well depth the larger the ADU conversion (gain) can be.

Last edited by [1ponders]; 15-10-2012 at 12:24 PM.
Reply With Quote
  #5  
Old 15-10-2012, 12:09 PM
RickS's Avatar
RickS (Rick)
PI cult recruiter

RickS is offline
 
Join Date: Apr 2010
Location: Brisbane
Posts: 10,584
The stuff by Richard Crisp is here: http://www.narrowbandimaging.com/flat_fielding_page.htm

It's not easy reading for the new or faint-hearted

Quote:
Originally Posted by solissydney View Post
ADU's ????????
What?
ADU = analog-digital unit. When your image analysis program tells you that the value in a pixel is 10456 then that's measured in ADUs. The other measure that is used is the number of electrons captured. The number of electrons is scaled by a gain factor to give ADUs. The gain factor varies depending on the characteristics of the camera sensor.

Ideally you want your flats to have a high average ADU count without going so high that you stray into the non-linear area of your CCD's performance curve. There isn't a specific formula for this because it will depend on your sensor. The usual advice is to target around 20,000 to 30,000 ADU. Richard Crisp's stuff talks about how many flat frames are needed to reduce flat-fielding noise to an appropriate level.

Cheers,
Rick.
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 07:04 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Advertisement
OzScopes Authorised Dealer
Advertisement
Lunatico Astronomical
Advertisement
NexDome Observatories
Advertisement
Celestron Australia
Advertisement
SkyWatcher Australia
Advertisement
Meade Australia
Advertisement
Bintel
Advertisement
Astronomy and Electronics Centre
Advertisement