Go Back   IceInSpace > Equipment > Astrophotography and Imaging Equipment and Discussions

Reply
 
Thread Tools Rate Thread
  #1  
Old 17-05-2011, 01:14 PM
troypiggo's Avatar
troypiggo (Troy)
Bust Duster

troypiggo is offline
 
Join Date: Nov 2008
Location: Brisbane, Australia
Posts: 4,792
captured FITS not saturated, apply flats saturated!

Capturing and calibrating images from my QSI583 with Nebulosity. The captured light frames have a max pixel value of 64000, which I assume is not saturated as it's less than 65535.

It seems a very nice, round number that 64000 and it's consistent among all light frames. Could be stars, or since this image is uncalibrated, it could be noise in the form of hot pixels I suppose.

When I apply darks to it, the max value drops to, say, 63636. This makes sense as darks are subtracted.

Now if I apply my flats to it, this is where I'm puzzled. I have an image that I think is unsaturated. I've ensured my flats have their histogram around 1/3 to 1/2 of max, and there's no blackpoints or whitepoints reached in them. Flats are divided into the light frames. After this process, somehow my calibrated image has a max pixel value of 65535 and a min of 0! I've tried variations with blurring the flats in case there's hot pixels or black pixels in there, same result though.

Anyone know/understand what's going on here? Do you reckon it's Nebulosity specific or is it common/normal? Am I worrying over nothing?
Reply With Quote
  #2  
Old 17-05-2011, 01:36 PM
Terry B's Avatar
Terry B
Country living & viewing

Terry B is offline
 
Join Date: Mar 2006
Location: Armidale
Posts: 2,758
A few points
You probably do have saturated images if you have pixels at 64000. I assume that the camera has antiblooming and probably this is stopping the pixels reaching ~65000.
Flat framing will change these levels and can make some increase. A flat is divided into the light frame pixel by pixel.
For the median value of your flat, those pixels will be divided by 1 resulting in no change. In dimmer areas of the flat the pixels will be divided by a number less than 1 resulting in an increase in the count of those pixels in the light frame.
The reverse occurs in the highlit areas on your flat.
Think about what happens when there is dust on your CCD. The counts under the dust mote will be lower than expected resulting in a dark dust bunny on your image. You take a flat frame and this shows up the dust bunny. When you apply flat it brings the count of the pixels under the dust bunny up to the same as the surrounding region therefore making the dust bunny disappear.
As to saturated pixels. Does it really matter? For me it does if I am doing photometry but for pretty pics it is pretty hard to not saturate bright stars and still get enough signal in extended objects to see them.
Reply With Quote
  #3  
Old 17-05-2011, 07:35 PM
peter_4059's Avatar
peter_4059 (Peter)
Big Scopes are Cool

peter_4059 is offline
 
Join Date: Jun 2007
Location: Brisbane
Posts: 3,986
Troy,

Are you using bias frames with the flats?
Reply With Quote
  #4  
Old 17-05-2011, 10:24 PM
troypiggo's Avatar
troypiggo (Troy)
Bust Duster

troypiggo is offline
 
Join Date: Nov 2008
Location: Brisbane, Australia
Posts: 4,792
Not using bias frames with the flats. They're only 5s exposures. I am using a master flat dark.

I checked some of the stars, and even though the pixel value is 64k, they do appear saturated (flat top on the stellar profile).
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 07:18 PM.

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