#61  
Old 31-10-2013, 12:20 PM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 17,877
Dan, I think that discussion was about DSLRs not astro CCDs. The main difference being 9 micron pixelled CCDs have 100,000 electron full well capacity (how many electrons the pixel can hold before spilling over).

DSLRs well capacity would be less. Also they are quite a bit less sensitive than the CCD's which are usually 50% at the low end and up to 77% or more at the high end for CCDs. Also having a colour filter array this makes them even less sensitive.

ISO simply is an amplifier of the signal. Its like turning the sound up on your stereo. It does not add anything merely magnify what is there.

The whole point there is getting the faint details above the noise of the camera. At what point that occurs is the question. From my experience of imaging faint objects hundreds of times, that will not occur in 45 second-4 minute subs with a basically not very sensitive DSLR.

The M101 example he posted proves the point. There is a lot more faint detail missing from that image. Its impressive from the standpoint of a DSLR captured image but not impressive compared to a reasonable CCD image of similar duration from an 8 inch scope. It looks black clipped as he mentions it probably was.

DSLRs are great for bright objects but the dim ones they are unsuitable because of the above.

I definitely see a gain in 15 minutes over 10 minute subs. Its subtle but its there. Tracking has to be good though as elongated stars are worse than not getting some tiny faint detail.

Greg.
Reply With Quote
  #62  
Old 31-10-2013, 12:46 PM
swannies1983 (Dan)
Registered User

swannies1983 is offline
 
Join Date: May 2006
Location: Adelaide
Posts: 781
Quote:
Originally Posted by gregbradley View Post
Dan, I think that discussion was about DSLRs not astro CCDs. The main difference being 9 micron pixelled CCDs have 100,000 electron full well capacity (how many electrons the pixel can hold before spilling over).

DSLRs well capacity would be less. Also they are quite a bit less sensitive than the CCD's which are usually 50% at the low end and up to 77% or more at the high end for CCDs. Also having a colour filter array this makes them even less sensitive.

ISO simply is an amplifier of the signal. Its like turning the sound up on your stereo. It does not add anything merely magnify what is there.

The whole point there is getting the faint details above the noise of the camera. At what point that occurs is the question. From my experience of imaging faint objects hundreds of times, that will not occur in 45 second-4 minute subs with a basically not very sensitive DSLR.

The M101 example he posted proves the point. There is a lot more faint detail missing from that image. Its impressive from the standpoint of a DSLR captured image but not impressive compared to a reasonable CCD image of similar duration from an 8 inch scope. It looks black clipped as he mentions it probably was.
All good points but the OP didn't specify this as a "CCD only" discussion . In fact, there's another thread on the first page purely discussing CCD and exposure length.

Obviously, if you compare CCD vs DSLR, CCD will win hands down. But the advantages of long subs over short subs is still up for debate.

Quote:
Originally Posted by gregbradley View Post
DSLRs are great for bright objects but the dim ones they are unsuitable because of the above.
I disagree. You only have to look at Scott's work to appreciate what can be achieved with a modded dslr under dark skies. Just check out some of his work

M81, M82 and the Integrated Flux Nebula (ok, ten minute subs for this one).

Witch head (5min subs @ ISO800)

Leo Triplet, including tidal stream of NGC 3628 (only 3 min subs for this galaxy)

Whirlpool Galaxy (5 minute subs)

He has many more examples of what can be achieved using short subs with a dslr. Granted, the amount of hours he puts in to get these images is much more than one would do with a CCD camera, but some of his images easily surpass CCD images.
Reply With Quote
  #63  
Old 31-10-2013, 12:54 PM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 17,877
Yes that guy is hardcore. 49 hours exposure - wow.

I saw the name Roger Clark and I know he does a lot of DSLR testing and reports on their performance. I also read some of it and they were talking about the histogram which is more of a DSLR imaging technique.

I agree you can take some great images with a DSLR. Its just harder work on the dim objects and the difficult part with a DSLR is to maintain star colours. They often wash out to white in DSLR images. Even those you linked the star colours are off.

The basic theory is the signal has to get above the noise floor of the camera system to see anything. My point really is with a CCD camera with deep wells and low read noise that is rarely less than 15 minutes.

If the top guys are seeing a gain with 30 minute subs using these types of cameras given stable weather and good tracking its worth a try to see for yourself.

I think also it depends on the target object. Some are quite bright and require a different strategy. I wouldn't do 30 minute subs on the Orion Nebula for example, it would be a mess but on a faint spiral galaxy at a dark site with excellent tracking and a deep well camera I would.

This is all highly theoretical anyway because I imagine very few would be able to achieve round stars in 30 minute subs anyway. That's a whole long runway up to achieve that. Its gonna require a very good mount that works reliably, perfect polar alignment like only achieved with T-point modelling, a very good PEC (a whole world in itself), really well balanced, a good autoguiding system with no flexure so that means self guiding or an off axis guider or an AO unit ( it also means guide scopes are unlikely to be able to achieve it), no cable drag, a solid pier or tripod that is well supported and a reliable power supply. If you setup every night it would be a no go. If you have a permanent mount but not all the bells and whistles needed to set it up its a no go. Unless you have a very expensive mount it is probably unrealistic.

Accurate tracking is a large enough barrier to make greater than 10 minutes unreachable for most. It really is the big obstacle in astroimaging. 10 minutes is hard enough most of the time!


Greg.
Reply With Quote
  #64  
Old 31-10-2013, 03:10 PM
rmuhlack's Avatar
rmuhlack (Richard)
Professional Nerd

rmuhlack is offline
 
Join Date: Nov 2011
Location: Strathalbyn, SA
Posts: 916
I'll chime in here. All of my imaging is with a DSLR (i have a 1000D and a 400D). I don't have a permanent setup and have to setup prior to each session. I have two setups: a VC200L at f6.4 (FL 1278mm) on a EQ6, and a 130mm f5 newt (FL 650mm) on a HEQ5. All of my finished images to date have been with a guidescope, although I have recently purchased a Lacerta OAG to be used with my 1000D on the VC200L (although I don't have any complete projects with the OAG yet).

As my DSLRs do not have cooling, thermal noise is significant and will vary with sub length as well as ambient temperature. I have done a number of tests as per Roger Clark's methods here to determine the required sub length to optimise SNR. For example, see my image of NGC6744 here: http://www.iceinspace.com.au/forum/s...d.php?t=108354

So that takes care of sub length. I terms of total exposure, given that I am already behind the QE 8-ball by using a DSLR my strategy is basically to take as many subs as I can, however beyond about 200 and my PC is working pretty hard to integrate all the calibrated subs. So given that I typically use 3-4min subs during winter, that gives a total exposure of up to 10-14ish hours.
Reply With Quote
  #65  
Old 31-10-2013, 03:13 PM
rmuhlack's Avatar
rmuhlack (Richard)
Professional Nerd

rmuhlack is offline
 
Join Date: Nov 2011
Location: Strathalbyn, SA
Posts: 916
ps an added benefit of taking so many (dithered) subs is that minimal (if any) noise reduction is required during processing.
Reply With Quote
  #66  
Old 31-10-2013, 03:37 PM
Shiraz's Avatar
Shiraz (Ray)
Registered User

Shiraz is offline
 
Join Date: Apr 2010
Location: ardrossan south australia
Posts: 4,918
hi Richard. So your sub length is essentially limited by the thermal noise as the chip heats up during imaging - have i got that right? Dark calibration must be a pain if that is the case. Does the optimum sub length vary with ambient temp?
Reply With Quote
  #67  
Old 31-10-2013, 03:44 PM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 17,877
So that takes care of sub length. I terms of total exposure, given that I am already behind the QE 8-ball by using a DSLR my strategy is basically to take as many subs as I can, however beyond about 200 and my PC is working pretty hard to integrate all the calibrated subs. So given that I typically use 3-4min subs during winter, that gives a total exposure of up to 10-14ish hours.[/QUOTE]

You can go longer. You stack as many as your computer can handle and label it a submaster. Then stack the next load and so on. Then stack the resulting submasters. I do that all the time as my computer bogs down with 10 x 1x1 binned 16803 images at 32mb each. You may lose a bit of your statistical rejection of outliers but it you still get the gain in SNR as far as I know.

Greg.
Reply With Quote
  #68  
Old 31-10-2013, 03:50 PM
rmuhlack's Avatar
rmuhlack (Richard)
Professional Nerd

rmuhlack is offline
 
Join Date: Nov 2011
Location: Strathalbyn, SA
Posts: 916
Quote:
Originally Posted by Shiraz View Post
hi Richard. So your sub length is essentially limited by the thermal noise as the chip heats up during imaging - have i got that right? Dark calibration must be a pain if that is the case.
Based on my interpretation of the test results I have seen you are correct - thermal noise is the limiting factor. I have dark masters taken at various temperatures, and do my best to match sub frames with the appropriate master (within the limits of practicality). My dark masters are typically 50 subs or more. My bias master has 100 subs.

Quote:
Originally Posted by Shiraz View Post
Does the optimum sub length vary with ambient temp?
Yes it does. As a rough approximation, optimal sub settings for me would be 4 mins ISO1600 in winter, 3-4mins ISO1600 in autumn/spring and probably 1-2mins in summer. Admittedly the first time I used this method of sub calculation was in May of this year, so I haven't had the opportunity to test it at ambient nighttime temps >20C
Reply With Quote
  #69  
Old 31-10-2013, 03:52 PM
rmuhlack's Avatar
rmuhlack (Richard)
Professional Nerd

rmuhlack is offline
 
Join Date: Nov 2011
Location: Strathalbyn, SA
Posts: 916
Quote:
Originally Posted by gregbradley View Post
Quote:
Originally Posted by rmuhlack View Post
So that takes care of sub length. I terms of total exposure, given that I am already behind the QE 8-ball by using a DSLR my strategy is basically to take as many subs as I can, however beyond about 200 and my PC is working pretty hard to integrate all the calibrated subs. So given that I typically use 3-4min subs during winter, that gives a total exposure of up to 10-14ish hours.
You can go longer. You stack as many as your computer can handle and label it a submaster. Then stack the next load and so on. Then stack the resulting submasters. I do that all the time as my computer bogs down with 10 x 1x1 binned 16803 images at 32mb each. You may lose a bit of your statistical rejection of outliers but it you still get the gain in SNR as far as I know.

Greg.
Thanks Greg - that is good to know
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 07:14 AM.

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