#1  
Old 15-08-2012, 08:02 AM
DJ N
Registered User

DJ N is offline
 
Join Date: Jul 2006
Location: Adelaide, South Australia
Posts: 412
So how long do you go? CCD that is!

Last night a thought crossed my mind. Am I taking appropriate length sub exposures? I came across the Starizona Ideal Exposure calculator and decided to have a play around.

http://starizona.com/acb/ccd/calc_ideal.aspx

Now my QHY8L is not listed, but I assumed that I could use either the QHY8 or QHY8 Pro that is listed (both do give the same result).

Using a couple of recent images, I opened them up in Maxim and used the "minimum figure" as my background ADU (not sure if this is the correct method).

The images were captured with the same setup, just different locations.

The equipment is as follows,

Imaging scope : ED120 with WOIII 0.8x flattener/reducer = 720mm f6
Imaging camera : QHY8L

The subexposure details are as follows,

1. Metro Adelaide (Maxim information mode - area) 10minutes
2. Metro Adelaide (Maxim information mode - aperture) 10 minutes
3. Dark site (Clayton) (Maxim information mode - area) 20 minutes
4. Dark site (Clayton) (Maxim information mode - aperture) 20 minutes

When I input the data in to the "calculator", I get 2.34 minutes for metro and 4.09 minutes for dark sky site as the "ideal sub exposures".

Surely this cannot be right?

So in essence, how do you know when you are at a point in terms of exposure length, that you are at the optimum exposure length?

I think the next time we have a bit of a break in the weather, I will run some trials at various exposures and try and determine the differences.

Look forward to some thoughts and experiences.

Cheers,

Daniel
Attached Thumbnails
Click for full-size image (Metro Area.JPG)
58.4 KB37 views
Click for full-size image (Metro Aperture.JPG)
59.9 KB32 views
Click for full-size image (Dark Sky Area.JPG)
49.2 KB30 views
Click for full-size image (Dark Sky Aperture.JPG)
50.3 KB29 views
Reply With Quote
  #2  
Old 15-08-2012, 08:12 AM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 15,473
You want signal to be significantly higher than the various noises in the CCD.

So that means the emphasis is on the noise levels of the camera from various sources. Usually dark current (thermal noise) is the largest hence the need for strong cooling. Electronics quality also plays a large part.

I have used both 5, 10 and 15 minutes as standard sub lengths.

Another factor is how accurate your tracking is. No good doing 15 minute subs if they all have elongated stars. So that comes into it.

Another factor is weather. Again, its not productive doing 15 minute subs if there is patchy cloud around. 5 minutes may have a higher success rate.

So with those things in mind I usually settle on 10 minutes as a good compromise between tracking results, weather, camera noise, depth of signal.

If the tracking is good I use 15 minutes especially on faint targets like galaxies.

On cameras with small well depth and with fast scopes I use 5 minutes to control bright star sizes and that worked well for me.

So I think there are several factors that need to be balanced out when working out ideal exposure length.

What you don't want is 15 different exposure lengths you use as that makes managing a darks library impractical.

Greg.
Reply With Quote
  #3  
Old 15-08-2012, 09:25 AM
Poita (Peter)
Registered User

Poita is offline
 
Join Date: Jun 2011
Location: NSW Country
Posts: 3,585
I'll have to give that calculator a go with the hyperstar, I've been trying to work out what the best exposure times are with the QHY8, imaging at f2 is a different experience to say the least.
Reply With Quote
  #4  
Old 15-08-2012, 10:32 AM
SkyViking's Avatar
SkyViking (Rolf)
Registered User

SkyViking is offline
 
Join Date: Aug 2009
Location: Waitakere Ranges, New Zealand
Posts: 2,260
Great topic.
I have been wondering about the same. Most people seem to stick with 'standard' subs of say 10mins etc, but surely that's quite random and cannot be optimal. Given that time is our most precious resource in this hobby the goal must then be to capture the highest S/N in the shortest amount of time.
I did standard 10min exposures until doing those exact calculations for my setup recently. According to the calculations my ideal sub lengths turned out to be a lot shorter. Being still somewhat sceptical I settled on a compromise of 5min subs for my recent image of Barnards Galaxy. That reached a limiting magnitude of ~24 which is a good deal beyond what I have previously managed, so can't say I'm disappointed.

Currently I'm working on another deep image with even shorter subs, 2-3mins, and the data looks very promising so far.
Something to also bear in mind is that ideal sub lengths are not the same in each filter (L, R, G, B). So I now use four different sub lengths and RGB subs range from 2 to 6.5mins.

BTW, I use scaled darks so no need for different dark sub lengths and that has served me very well so far.
Reply With Quote
  #5  
Old 15-08-2012, 10:42 AM
Terry B's Avatar
Terry B
Country living & viewing

Terry B is offline
 
Join Date: Mar 2006
Location: Armidale
Posts: 2,761
I don't like wasting time to things that are out of your control like satelites, planes, cosmic ray hits. I use 10min max as a poorly placed cosmic ray hit is difficult to fix and you have to toss the exposure.
Wasting a 30 min exposure for this is frustrating.
Reply With Quote
  #6  
Old 15-08-2012, 11:02 AM
DJ N
Registered User

DJ N is offline
 
Join Date: Jul 2006
Location: Adelaide, South Australia
Posts: 412
Good points everyone.

I suppose until now, imaging from metro Adelaide, I find about 10 minutes is the max until I start getting sky glow. As for dark sites, I suppose 20 minute subs at f6 stems from previously doing 30 minutes subs with a DSLR at the native focal length of my ED120 (f7.5).

I just try to ensure I don't blow out any of the details or the star colours. A bit of trial and error.

I dare say there does come a point of diminishing returns in that going for the super long subs does not add much further detail.

Cheers,

Daniel
Reply With Quote
  #7  
Old 15-08-2012, 02:28 PM
Paul Haese's Avatar
Paul Haese
Registered User

Paul Haese is offline
 
Join Date: Jan 2009
Location: Adelaide
Posts: 9,430
Well for my setup it says 13.08 minutes. Which is about that I have been working with for a little while. Although I find that I get better results with longer subs still at around 20 minutes for the faint stuff I have been imaging.

The general rule of thumb that I work to is that back ground ADU has to be around 1000 to overcome noise in a certain amount of subs.
Reply With Quote
  #8  
Old 15-08-2012, 02:51 PM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 15,473
Hi Rolf,

Isn't the deep mag 24 a result of the long total hours rather than the number of subs?

Each time you do a sub the signal has to rise above the noise floor to be added to the signal. So faint details may in fact be lost as they are never recorded in the first place in short exposures. Adding exposures doesn't work unless the signal rises above the noise floor otherwise the faint signals are lost in the noise.

The limiting factors are more tracking accuracy, QE, mono versus one shot colour, weather, well depth, cooling power of the ccd to get noise lower and total exposure time.

In the end we need highest possible QE with lowest noise and signal recorded above the noise floor of the system to detect a signal.

Same chip in different cameras can perform slightly better with the correct supporting low noise electronics and more powerful cooling. Most of the modern camera makers are now quite close with their top of the line cameras for the same chip. It used to be a larger gap between some.

Level of seeing and long or short focal length comes into it but thats really a different topic of matching CCD camera to your optical system. Some work much better with some setups than others. Look at STL11/FSQ106 images for example. They match really well. So does the 16803 chip with just about anything. 8300 chip is also very good but more suited to faster focal lengths.

Greg.
Reply With Quote
  #9  
Old 15-08-2012, 07:49 PM
SkyViking's Avatar
SkyViking (Rolf)
Registered User

SkyViking is offline
 
Join Date: Aug 2009
Location: Waitakere Ranges, New Zealand
Posts: 2,260
Quote:
Originally Posted by Paul Haese View Post
The general rule of thumb that I work to is that back ground ADU has to be around 1000 to overcome noise in a certain amount of subs.
I agree, that's the basic principle behind the Starizona calculations. It then also follows that the optimal sub exposure length is different for each filter.
I calculated the optimal for my L and then experimented with the length of R,G and B until they displayed the same background ADU count as the optimal L. The result is four different sub exposure times and that's what I use now.

Quote:
Originally Posted by gregbradley View Post
Hi Rolf,
Isn't the deep mag 24 a result of the long total hours rather than the number of subs?
Yes the total time was indeed longer. 14 hours versus the 8-9 hours I had been doing on other images. But there is not an order of magnitude difference in those times.
Interestingly, I could clearly see from the raw data that stacking say 20x5mins gave superior S/N compared to 10x10mins so that's why I was convinced shorter subs was the way to go. It may even be better to stack 40x2.5mins, I just didn't try

Quote:
Originally Posted by gregbradley View Post
Each time you do a sub the signal has to rise above the noise floor to be added to the signal. So faint details may in fact be lost as they are never recorded in the first place in short exposures. Adding exposures doesn't work unless the signal rises above the noise floor otherwise the faint signals are lost in the noise.
I'll say the signal is always recorded, unless the exposure is so short that no photons actually reached the chip (When we are talking exposures of a minute or more then that's probably unlikely).
The value of each pixel is the signal plus the noise. In the final stacked image the faintest details are typically only a few ADU counts higher than the background. It's only when the signal itself approaches the square of the averaged noise background that it gets lost. But the signal is always there in every sub frame, it's just a matter of having enough subs to smooth the noise enough to reveal the signal.
Reply With Quote
  #10  
Old 16-08-2012, 09:14 AM
Dennis
Dazzled by the Cosmos.

Dennis is offline
 
Join Date: May 2005
Location: Brisbane
Posts: 10,902
Thanks for the interesting discussions. Here are the results from plugging in some historical data from ST2000XM/CFW9 LRGB exposures of 10 and 15 mins and what exposure the Starizona calculator then recommends.

Cheers

Dennis
Attached Thumbnails
Click for full-size image (ST2000XM Calculations.jpg)
120.5 KB49 views
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 03:39 AM.

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