#1  
Old 24-02-2016, 06:07 PM
blink138's Avatar
blink138 (Pat)
Registered User

blink138 is offline
 
Join Date: Aug 2009
Location: perth w.a.
Posts: 2,217
full well depth?

very new to ccd imaging, how do i tell when i have full well depth on my qhy10 ccd?
does it count during the exposure or do you have to wait til it finishes to find out?
i have read that it is best to aim for the 45k mark for depth and not the 65k?
if someone could explain these to me i would be grateful, i am not afraid of google but i will be buggered if i can find out exposure length and well depth anywhere
oh and should the "auto" (sliders) be ticked to check well depth?
pat
Reply With Quote
  #2  
Old 24-02-2016, 06:40 PM
Atmos's Avatar
Atmos (Colin)
Ultimate Noob

Atmos is offline
 
Join Date: Aug 2011
Location: Melbourne
Posts: 5,928
With the QHY products do you need to play around with the Gain and get that right. The aim is to have pixel full well at 65535 ADU or at least as close as possible to it. On my QHY9 via ASCOM this occurs at a Gain of 21 and Offset around 106 (gives a bias of 930 ADU).

If you have your gain and offset done correctly (mostly gain) you know that you have hit your full well capacity when you have 65535 ADU. The reason you want full well capacity to be at 65k ADU is that it gives you the largest dynamic possible which is your ability to capture both bright and faint parts of an image.

When referring to hitting 45k instead of 65k, this is for your target. Lets say that you're imaging a relatively bright nebula (not as bright as Orion), if you can keep the brightest part of that nebula around the 45-60k ADU mark that region will be bright but it will not be saturated.

When it comes to stars, if they are saturated, they will appear white as everything is at max value. So if you want star colours you do need to keep away from full saturation.

The "auto" sliders should just be do to with screen stretching (white and black point) for visually looking at an image and are not always helpful when trying to ascertain whether a part is saturated or not. An example of this, opening up a 5s sub in Maxim DL of M42, because it has such a large dynamic range, the whole core region will appear blown out under a Medium stretch even if it only has a maximum ADU value of 5,000. So screen stretches are not always good indicators of saturation, you do need to look at the individual pixels.
Reply With Quote
  #3  
Old 24-02-2016, 06:54 PM
blink138's Avatar
blink138 (Pat)
Registered User

blink138 is offline
 
Join Date: Aug 2009
Location: perth w.a.
Posts: 2,217
ok thanks a lot colin, i have done my offset and gain i was wondering how do i know when full well depth has happened, for instance can you see a count as the exposure is going along or do you phisically have to do the sub and look at to check, and if it is reading 65k do you then say cut the exposure in half timewise and check the well depth again............. in other words trial and error?
and how do you look at the individual pixels?
pat
Reply With Quote
  #4  
Old 24-02-2016, 07:19 PM
Atmos's Avatar
Atmos (Colin)
Ultimate Noob

Atmos is offline
 
Join Date: Aug 2011
Location: Melbourne
Posts: 5,928
You have to wait until readout, when the exposure has finished. Depends on the software package that you're using as to how you go about it. With MaxIM DL you can either put your curser over a star and it'll display the ADU count down towards the bottom right of the screen, or you can open the "Toggle Information" tab, put it on aperture and select a star.

In SGP you put your curser over an area and look at the 21x21 box information.

In Nebulosity there is a pixel information tab that can be used.
Reply With Quote
  #5  
Old 24-02-2016, 07:52 PM
blink138's Avatar
blink138 (Pat)
Registered User

blink138 is offline
 
Join Date: Aug 2009
Location: perth w.a.
Posts: 2,217
thanks for sticking with me colin, i have nebulosity so when i have downloaded my image i can open the pixel info tab and then i presume run my cursor to different parts? to check saturation?
mine is full at 45k not 65k, but i am sure the 65k is what it says in the right corner
pat
Reply With Quote
  #6  
Old 24-02-2016, 08:05 PM
Atmos's Avatar
Atmos (Colin)
Ultimate Noob

Atmos is offline
 
Join Date: Aug 2011
Location: Melbourne
Posts: 5,928
If you are full at 45k then you need to play with your gain a bit more
Reply With Quote
  #7  
Old 24-02-2016, 10:06 PM
blink138's Avatar
blink138 (Pat)
Registered User

blink138 is offline
 
Join Date: Aug 2009
Location: perth w.a.
Posts: 2,217
colin thanks for all of this, i am not sure if i get it all but i will now go over what you have told me and see if i can get my head around it on the nebulosity simulator
i hope you will be listening on here if i falter haha!
pat
Reply With Quote
  #8  
Old 25-02-2016, 02:52 AM
billdan's Avatar
billdan (Bill)
Registered User

billdan is offline
 
Join Date: Mar 2012
Location: Narangba, SE QLD
Posts: 1,215
Hi Pat,

If you are using the EZCAP software that came with the QHY10, then in capture mode you tick the box that says Noise analysis ( under the heading Image). It will give you 9 separate areas of the chip and report back the Peak and Mean values.

Cheers
Bill
Reply With Quote
  #9  
Old 25-02-2016, 11:07 AM
blink138's Avatar
blink138 (Pat)
Registered User

blink138 is offline
 
Join Date: Aug 2009
Location: perth w.a.
Posts: 2,217
thanks bill i have the nebulosity and gives a similar thing
it has a mean and maximum count, do you think that i should aim for the 45k as the maximum value for a sub?
pat
Reply With Quote
  #10  
Old 26-02-2016, 07:54 PM
billdan's Avatar
billdan (Bill)
Registered User

billdan is offline
 
Join Date: Mar 2012
Location: Narangba, SE QLD
Posts: 1,215
I'm not sure Pat, the old rule of thumb was to keep exposing until the background sky glow starts to take over. This would probably mean saturated stars, but good for faint DSO's.

Bill
Check this out

CCD Exposure theory
http://starizona.com/acb/ccd/advtheoryexp.aspx

Last edited by billdan; 26-02-2016 at 08:01 PM. Reason: More info
Reply With Quote
  #11  
Old 26-02-2016, 08:50 PM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 15,048
Full well depth is the same really as regular photography where you don't want to overexpose your images.

These days most CCDs have antiblooming features which means once the pixel is fully saturated more exposure simply gets bled off.

So in practice overexposure is hard to do on an astro target as they are usually very dim. Its more prominent on the brighter objects like M42. The core will blow out just like a daylight over exposed image.

There you simply take 2 sets of images, one short exposures so as to keep the core properly exposed and another longer to get the dimmer areas nicely exposed. Then combine them in Photoshop.

65535 is the fully saturated pixel level for a 16 bit CCD (they all are).

Greg.
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 06:05 PM.

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