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

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  #21  
Old 16-10-2017, 07:53 PM
dpastern (Dave Pastern)
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Originally Posted by RickS View Post
To get the best SNR in the shortest total time you need to take subs long enough that read noise is swamped by shot noise from the detected signal. If you have a sensor with low read noise, e.g. ASI1600, then you can take shorter subs and still be in this magical goldilocks zone. With a KAF-8300 you'll need to take longer subs which requires better guiding, results in more data loss if a cloud wanders through your FOV, and probably gives you detail that's a little less sharp. On the plus side, you'll need less CPU grunt to calibrate, register and integrate the data.

You can get good results with CMOS or with CCD. It's a matter of using either sensor to best advantage.

If terms like read noise, shot noise and quantum efficiency mean nothing to you, then you really should do a little reading. The Handbook of Astro Image Processing book by Berry and Burnell is very good or try the signal to noise articles by Craig Stark: http://www.stark-labs.com/craig/articles/articles.html

You'll hear a lot of opinions about this stuff but many of them are uninformed. It's a good idea to learn a little. The maths behind this stuff isn't terribly difficult and intuition will often lead you astray.

PS. Ray (Shiraz) is one of the folks that does understand the maths. You can generally believe what he says

Cheers,
Rick.
Rick - you are a very bad man! I had not heard of The Handbook of Astro Image Processing before and now you are making me want to buy it...when I am trying to stop buying stuff!

I read Craig's article and it is excellent. The maths isn't hard, but I have worsening concentration issues, so for me it was a difficult read. But yes, I would agree, it's well worth the OP reading Craig's insights on SNR etc.
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  #22  
Old 16-10-2017, 08:13 PM
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RickS (Rick)
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Rick - you are a very bad man! I had not heard of The Handbook of Astro Image Processing before and now you are making me want to buy it...when I am trying to stop buying stuff!

I read Craig's article and it is excellent. The maths isn't hard, but I have worsening concentration issues, so for me it was a difficult read. But yes, I would agree, it's well worth the OP reading Craig's insights on SNR etc.
Sorry, Dave, but it is a good book and not difficult to read I think every serious imager would benefit from an understanding of the basic concepts. It would save a lot of pointless arguments
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  #23  
Old 16-10-2017, 08:24 PM
dpastern (Dave Pastern)
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Sorry, Dave, but it is a good book and not difficult to read I think every serious imager would benefit from an understanding of the basic concepts. It would save a lot of pointless arguments
All good. I actually have always enjoyed reading, but as I've gotten older, my eyesight has degraded (just presbyopia, nothing serious) and my attention span (suspected adult ADHD amongst other things) has severely worsened. I hate it, cos it makes reading difficult for me, and I used to love reading well into my late 20s. Frustrating as hell...

All good re: book - will buy it cos it looks worthwhile. May take me a while to read through it and even longer to digest the information inside of it (that's another thing that's gotten severely worse for me as I've gotten older - information retainment).
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  #24  
Old 16-10-2017, 09:29 PM
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Shiraz (Ray)
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Originally Posted by Crushellon View Post
Thanks for the replies, there's some interesting points to consider there.
I don't understand the short exposures though, if the 1600 can do long exposures at neutral gain settings why are they commonly used for short subs and lots of them? What's the benefit to it? Surely longer exposures are better for NB imaging, or does it not matter so much because the background has a low noise level with the 1600?
Also, if used at low gain settings, are you just holding back the capabilities of the camera unnecessarily? As in, if I did 20x600subs at low gain and 20x90sec subs at a higher gain, would the resulting amount of signal captured be the same? Therefore making the longer subs redundant? Or would you have to do 130x90sec subs to match the amount of signal as the 20x600sec subs?
I'm just trying to get a better idea of why they are "generally" used for sort subs and lots of them to make up integration time rather than fewer long subs. The main reason I'm trying to figure this out specifically is because the idea of capturing and processing 100's of subs does not sound desirable to me at all. Measuring weights, registration, local normalisation, integration, then at the end something didn't work out right and you have to go back and find the issue within 100's of subs... uggh makes me shudder, lol.
But in saying that, if lots of short subs is the best way to get results with the camera (even if it is capable of longer subs), then that's just how it is.
there are some advantages to short subs - eg:
- better resolution and star shape due to ability to remove transient bursts of poor seeing by rejecting a small percentage of the subs
- easier guiding/better resolution by keeping exposures well within mount PE periods .. normal PA errors and guide scope drift are immaterial
- less data lost to transient events such as clouds
- more effective non-linear rejection of transient noise such as cosmic rays

however, if you don't want to take advantage of the above, then there is no major penalty to running the 1600 camera at zero gain and using "normal" long subs for broadband imaging. Provided you use long enough subs to overcome read noise, the total exposure time is what matters - 10x5 minutes = 50x1 minute. As far as I can tell, the CCD equivalent mode of the chip is at zero gain even though it seems to be widely assumed that "unity gain" has some inherent significance.

Narrowband imaging is a different kettle of fish altogether. The 8300 will be severely limited by read noise for all practical sub lengths under dark sky, whereas the 1600 at high gain can give genuine shot noise limited imaging - ie, the 1600 final result will not include ~any read noise, whereas the 8300 result will still have a significant read noise component. Thus, the 1600 will go deeper (possibly by quite a bit). Of course the 1600 will require quite long narrowband subs to get to sky-limited performance under dark sky, but at least it can do it. There are quantisation effects from the 12/16 bit conversion (not hidden by read noise), but FWIW, I have not found this to be an issue. Suggest that you ignore any advice that says that you should always use short subs for narrowband 1600 - sometimes worthwhile, but not always so. http://www.iceinspace.com.au/forum/s...d.php?t=155908 may possibly be useful if you haven't already read it.

The other thing that might be worth considering is the very fast download rate of the 1600. A full res download takes ~ 1 second or less and once you have done a 9 step full frame focus run in under 1 minute, it is very painful to go back to a CCD camera (I tried).

Last edited by Shiraz; 16-10-2017 at 11:14 PM.
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  #25  
Old 16-10-2017, 09:40 PM
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Crushellon (Tim)
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Thanks guys. All very clear now. I'll have to have a look at that book Rick

Anyhow, looks like the 1600/variant will be the go then.
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  #26  
Old 19-10-2017, 06:23 PM
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graham.hobart (Graham stevens)
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which mono camera

looks like my FLI 8300 mono is being overtaken in the slow lane !!.
Interesting thread folks, and as always, good information backed by experience and objective data. I personally think, mainly because of the members willingness to share and also to experiment, this is the best astro forum.
Every one pat yourselves on the back and buy a new filter!!

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  #27  
Old 20-10-2017, 01:47 PM
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I wouldn't say that at all...they're still mighty effective photon collectors, that hasn't changed.

The CMOS revolution has been coming for years...Sony announced several years back that development (but not production) of their CCD range had ceased, leaving the excellent 674/694 sensors as their last hurrah.

With the CMOS sensors, their primary purpose for production is something else, so we're just benefiting from the economies of scale, which in turn makes them a more accessible entry into astrophotography...which is a Good Thing(tm) IMO.
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