#41  
Old 20-10-2012, 08:40 PM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 15,213
I mainly use 5 and 10 minute exposures for LRGB and 20 and 30 minutes for narrowband with a FSQ-106ED (f/5) and a STL11K camera. I have a new camera with a deeper well depth (KAF16803 sensor) and I may play with some longer exposures. Even at 5 minutes I often need to add some shorter exposures with HDR to stop the stars from saturating and losing colour.

Cheers,
Rick.[/QUOTE]

Rick,

I think 5 minutes is too short for FSQ106 and STL11. 10 minutes would be minimum, 15 minutes better. SBIG or perhaps Wodaski has an exposure calculator that tells you the optimum exposure length for various cameras based on their characteristics. Of course that doesn't take into account tracking ability, clouds, wind, flex, narrowband or LRGB.

Marcus Davies noted an improvement in his SNR using an STL11 with 15 minute subs over 10 minutes. I used to use 15 minutes as my standard using STL11. Basically its a pretty noisy camera by todays standards (but still a lovely camera and so convenient) and you need to get the signal over the noise floor to register those really faint details.

If really faint details are not higher than the noise floor then I am not sure that many exposures will show that as they are not there to be noticed above the noise no matter what mathematics are employed.

To get above the noise floor for your camera (they vary) 15 minutes is probably the sweet spot for that camera in LRGB if your tracking and lack of cloud cover/wind allows it.

Greg.
Reply With Quote
  #42  
Old 20-10-2012, 08:45 PM
RickS's Avatar
RickS (Rick)
PI cult recruiter

RickS is offline
 
Join Date: Apr 2010
Location: Brisbane
Posts: 10,582
Thanks, Greg. I'll try 15 minutes and see if it's an improvement.
Reply With Quote
  #43  
Old 20-10-2012, 09:21 PM
naskies's Avatar
naskies (Dave)
Registered User

naskies is offline
 
Join Date: Jul 2011
Location: Brisbane
Posts: 1,865
As Rick mentioned, only the total integration time matters in statistical theory (i.e. sum of multiple Poisson random variables is another Poisson random variable with mean equal to the sum of the individual means).

In practice with CCDs/CMOS chips, you're limited by a whole range of factors, such as noise, non-linearity, quantisation, quantum efficiency, well depth, and chip defects. Noise is fairly straightforward, but the others aren't:

* Non-linearity: a perfectly linear CCD would measure photons/electrons at the same rate regardless of the exposure time (e.g. 10,000 ADUs for 30 sec vs 20,000 ADUs for 60 sec). Although most astro CCDs are very good in this respect, no CCD is perfectly linear.

* Quantisation: the number of electrons in a pixel is converted into a digital number that has a limited amount of precision - e.g. a number between 0 and 65,535 (for a 16 bit sensor). The consequence is that there is a minimum signal strength that you can capture for a given exposure duration - anything less is effectively lost.

* Quantum efficiency: similar to the previous point, imperfect QE means that you effectively "lose" photons and thus also raises the minimal signal strength needed to record *any* signal.

* Well depth: too long of an exposure will result in lots of blooming and huge loss of information in surrounding pixels. Shorter subs with a higher gain may give you a higher effective dynamic range than a single long sub.

* Chip defects: if you have a dead pixel/column, then it doesn't matter how long you exposure a single frame for - you'll never be able to recover the missing information. With multiple subs and dithering, you can "fill in the gaps".

Due to a combination of the above (and other factors as well), you end up with three zones of behaviour:

(1) Signal is too weak for short subs: if you don't clear the minimum signal hurdle, you'll never record any information - i.e. why we can't image mag 36 objects using lots of 1/1000 sec exposures.

(2) Multiple short subs = single long sub: there's a region where stacking short subs is similar to a single long sub (most of us will use exposures in this region). However, stacking multiple subs will be beneficial due to chip defects, tracking limitations, unwanted objects in the sky, etc.

(3) Signal is too strong for the long sub: the wells become saturated in the long sub, but not in the short subs. For example, in my light polluted back yard I can't see the Horsehead Nebula at all on the back of my DSLR with a single shot - it's overwhelmed by skyglow. However, by stacking multiple subs it easily pops out.

Interesting discussion (Please feel free to correct me on any inaccuracies.)
Reply With Quote
  #44  
Old 20-10-2012, 11:03 PM
RickS's Avatar
RickS (Rick)
PI cult recruiter

RickS is offline
 
Join Date: Apr 2010
Location: Brisbane
Posts: 10,582
Great summary of many of the issues with our current sensor technology, Dave.

In the past I have wondered about QE and why it shows such a complex relationship to frequency. After reading your post I actually got around to looking for an explanation and found one here: http://learn.hamamatsu.com/articles/...fficiency.html. It was an interesting read. If only I could afford a camera with a back-thinned CCD!

Cheers,
Rick.
Reply With Quote
  #45  
Old 20-10-2012, 11:53 PM
naskies's Avatar
naskies (Dave)
Registered User

naskies is offline
 
Join Date: Jul 2011
Location: Brisbane
Posts: 1,865
Thanks for the link, Rick - an interesting read.

Interestingly enough, our eyeballs are like back-thinned CCDs - the rods and cones actually point to the back of the retina (rather than towards the light).

The rods/cones in the peripheral vision and outer macula are effectively "binned" for greater light sensitivity, but the inner macula (fovea) is missing the neuronal connections for binning. This results in the fovea having much sharper - but less light sensitive - vision.
Reply With Quote
  #46  
Old 21-10-2012, 08:42 AM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 15,213
Good post Dave.

I think you summed it up nicely. One point though with small wells. Most chips these days are anitblooming so if the full well overflows it is simply channelled off in the side gutter so to speak. Like excess water going into an oveflow downpipe.

So loss of surrounding pixel numbers I think is more the fact that bright light sources are not perfectly bordered but drop off in brightness the further away you go from the centre - bell shaped curves.

So you get bright halos that wipe out nearby data. At least thats how I think it goes. Certainly its observable in some systems that overwhelming the small wells gives bright ugly haloed stars. I have seen that with my KAF8300 yet others have not had it happen. I think the difference was my system was faster optically and medium focal length and theirs was slower and longer so less light fills the wells.

Its something to watch as I notice almost all of the mega megapixel Kodak chips have quite small wells - KAF39000 and up and quite low QE.

I suspect the next monster astro CCD chip may have to come from someone else than True Sense imaging who have not released any new chips since they bought Kodak Imaging Sensor business from Kodak.

Either that or finally CMOS chips will become so good they will be the norm.

Greg.
Reply With Quote
  #47  
Old 21-10-2012, 05:20 PM
Poita (Peter)
Registered User

Poita is offline
 
Join Date: Jun 2011
Location: NSW Country
Posts: 3,585
Quote:
Originally Posted by alocky View Post
Nope - in a perfect world there would be no noise

Love it!
Reply With Quote
  #48  
Old 21-10-2012, 08:51 PM
RickS's Avatar
RickS (Rick)
PI cult recruiter

RickS is offline
 
Join Date: Apr 2010
Location: Brisbane
Posts: 10,582
In a perfect world Light would make up its mind whether it was a particle or a wave phenomenon

Quote:
Originally Posted by naskies View Post
Interestingly enough, our eyeballs are like back-thinned CCDs - the rods and cones actually point to the back of the retina (rather than towards the light).
I saw a short video a while back that explains how you can see the nerve cells in your eye which are, surprisingly, on the lens side of the rods and cones. Normally these are processed out by the optical system but if you make a small aperture with your thumb and index finger, hold it to your eye and jiggle it you can faintly see these structures. Unfortunately, I haven't been able to find the video again
Reply With Quote
  #49  
Old 22-10-2012, 12:30 AM
blink138's Avatar
blink138 (Pat)
Registered User

blink138 is offline
 
Join Date: Aug 2009
Location: perth w.a.
Posts: 2,221
er..............very nice discussion.......... so whats the answer to the question then?
pat
Reply With Quote
  #50  
Old 22-10-2012, 07:58 AM
Peter.M's Avatar
Peter.M
Registered User

Peter.M is offline
 
Join Date: Sep 2011
Location: Adelaide
Posts: 914
I have seen many threads like this and the answer is always the same. Lots of long exposures. If you can expose for 60 minutes and have good stars
(i doubt they are not over exposed) then why not do this 20 times and make a nicer image.
Reply With Quote
  #51  
Old 22-10-2012, 12:43 PM
alocky's Avatar
alocky (Andrew lockwood)
PI popular people's front

alocky is offline
 
Join Date: Aug 2010
Location: perth australia
Posts: 1,289
If I might attempt to summarise:
Step1. Take as long an exposure as you can, stars nice and tight, no planes or satellites, sensor not saturating, particularly on the interesting bits.
Step2. Repeat step 1 until you are bored, or the target is lost in the evening sky.
Step3. Stack images.
Have a look at the sort of hours Martin Pugh spends on image acquisition- not to diminish his considerable processing skills, but that is 90% of the reason his images are so exceptional. Good data with high signal:noise.
It would seem for the vast majority of objects, 10-15 min subs seems to be a good starting point, with the exception of globular clusters. Looks like you need a range of sub lengths to capture the full dynamic range. Of course if you are doing narrowband, your subs will need to be longer. Not a game to be played with a poor mount.
Cheers,
Andrew.
PS I am not suggesting for a second that if you take as much data as Martin Pugh that your images will be as good. That extra 10% (my opinion only, others have suggested 60, after all only Mr Pugh knows for sure) does not come easily, you need to develop the skills to push the data as far as it can be pushed yourself. Fortunately, that part is mostly effort and not dollars.

Last edited by alocky; 22-10-2012 at 03:33 PM. Reason: To avoid offending anybody else.
Reply With Quote
  #52  
Old 27-10-2012, 04:27 PM
Joshua Bunn's Avatar
Joshua Bunn (Joshua)
Registered User

Joshua Bunn is offline
 
Join Date: Oct 2011
Location: Albany, Western Australia
Posts: 1,086
Thankyou to everyones input into this thread. some interesting reading.

Josh
Reply With Quote
  #53  
Old 28-10-2012, 08:03 AM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 15,213
[QUOTE=alocky;907050]If I might attempt to summarise:
Step1. Take as long an exposure as you can, stars nice and tight, no planes or satellites, sensor not saturating, particularly on the interesting bits.



You don't have to worry too much about satellites if only a few subs have them. They then disappear or weaken greatly if you use a stacking algorithm that rejects "outliers" - data that is outside the statisical norm which is assumed to be an artifact, hot pixel or the like. If you use sum or add then they of course stay in. The more subs you have the more these artifacts fade because of the statistical nature of the stacking algorithms that determine what value should be reasonably expected in a particular pixel. That needs a wide enough sample to be more and more useful in getting rid of outliers.

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 12:03 AM.

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