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23-07-2018, 07:44 PM
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Novichok test rabbit
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Join Date: Aug 2012
Location: Somewhere in the cosmos...
Posts: 10,388
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10 or 5?
I've been doing 10 min subs in rather shocking LP for quite some time now. PixInsight takes care of the LP pretty well, but I've been thinking...
Would I - given the severity of LP - be better reducing my exposure time back to 5 min subs instead? Here is an example of my current setup (FSQ-106ED plus SXVR-M26C OSC CCD) - 600 sec subs (2 hrs total), with NO filtering whatsoever with merely PI LP/gradient removal: https://www.astrobin.com/full/352941/B/
I know this one has been hacked out to death, but I think I may actually gain by less exposure (but same or longer total time) to the deep red menace that is my LP.
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23-07-2018, 08:10 PM
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Registered User
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Join Date: Jan 2011
Location: Rylstone, NSW, Australia
Posts: 1,444
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I may have this wrong, but I'd reason this way. You want to expose long enough to just not over fill the camera wells. If you take a lot of shorter exposures I think the read noise will increase and I'm not sure the final result will be any different (other than noisier). The ratio of LP vs signal will always be the same so I cannot see how shorter exposure would "fool" the LP somehow to stay away from your camera wells. I'm sure others will correct me if I'm wrong!
Peter
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23-07-2018, 08:24 PM
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Registered User
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Join Date: Apr 2006
Location: Warrnambool
Posts: 12,650
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Lewis although it was some time ago and i also used a FSQ 106 in a heavily light polluted area, although not using the camera you are using presently I found the sweet spot at 6mins at 400 ISO
I did however use a Hutech LP Filter, attached to the FSQ in front of the Camera
I got pretty good results IMHO.
Leon
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24-07-2018, 01:42 PM
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Drifting from the pole
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Join Date: Feb 2013
Location: Brisbane
Posts: 5,460
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My understanding is that once you're sky-limited, anything longer is futile. You collect more signal, but also more LP.
The more subs you take, the better your chances are that you can overcome the "noise" == LP and thus bring the real signal out.
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24-07-2018, 02:01 PM
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Novichok test rabbit
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Join Date: Aug 2012
Location: Somewhere in the cosmos...
Posts: 10,388
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That’s precisely what I am thinking Dunk. I am going to try say Leon’s suggested 6 min and see. I’ll run the PI script too and see what it reports (it told me ideal for the FSQ85 was 14 mins which seems a touch enthusiastic )
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24-07-2018, 06:00 PM
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Registered User
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Join Date: Sep 2014
Location: North Queensland
Posts: 3,240
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Since the SX website states that the read noise for your camera is 'Less than 9 electrons RMS - typically ONLY 7 electrons! ', I suggest sticking with 10 minute exposures rather than shortening them to 5 minutes. Having said that, from my limited experience, the length of exposure may vary depending on the target - with brighter targets like the Carina shorter subs are the way to go, while for dimmer stuff longer subs work better.
In the end - the only way to know for sure what works in your location and with your imaging train is to do say 1 hr exposure of the same target and on the same night with 10m and 5m subs and compare - standard deviation of the background and areas of interest, SNR with PI script etc.
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25-07-2018, 06:19 PM
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Drifting from the pole
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Join Date: Feb 2013
Location: Brisbane
Posts: 5,460
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Quote:
Originally Posted by LewisM
That’s precisely what I am thinking Dunk. I am going to try say Leon’s suggested 6 min and see. I’ll run the PI script too and see what it reports (it told me ideal for the FSQ85 was 14 mins which seems a touch enthusiastic )
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If you have/take some calibration frames, it's not had to pin the tail on the donkey re: where sky limited background should be, and then you just need to expose long enough to hit it
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25-07-2018, 08:27 PM
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Registered User
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Join Date: Feb 2006
Location: Sydney
Posts: 18,032
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I agree shorter subs probably would do better and is the conventional wisdom on the subject.
There is a calculator on the CCDware website that helps calculate the ideal exposure lengths for your setup and light pollution. I'd check that out.
Greg.
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25-07-2018, 08:50 PM
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Unregistered User
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Join Date: Apr 2007
Location: Perth, Australia
Posts: 1,164
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Amongst other factors the optimum exposure time will depend on the amount of light pollution. Your optimum (sky-limited) exposure will not be the same as for someone else. My camera has a similar read noise like yours and 3min is the optimum sky limited exposure for my location (<5km from Perth CBD).
If you expose for longer than sky-limited you will not improve the SNR but you will lose dynamic range. You should not be guessing, the optimum exposure is relatively easy to calculate, see this great IIS thread by Ray.
There is also a PI script called CalculateSkyLimitedExposure (or something similar).
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26-07-2018, 11:54 AM
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Novichok test rabbit
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Join Date: Aug 2012
Location: Somewhere in the cosmos...
Posts: 10,388
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Quote:
Originally Posted by luka
Amongst other factors the optimum exposure time will depend on the amount of light pollution. Your optimum (sky-limited) exposure will not be the same as for someone else. My camera has a similar read noise like yours and 3min is the optimum sky limited exposure for my location (<5km from Perth CBD).
If you expose for longer than sky-limited you will not improve the SNR but you will lose dynamic range. You should not be guessing, the optimum exposure is relatively easy to calculate, see this great IIS thread by Ray.
There is also a PI script called CalculateSkyLimitedExposure (or something similar).
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Thanks - it is exactly the loss of dynamic range I have been concerned about, and hence why I am reading up around it.
Doing the PI script, I get an Antsey Limit of 22seconds...yeah Gonna do 4.5x10e77 exposures....(didn't matter if I used a starless of normal image - got the same result time and time again)
Thanks everyone - keep the topic alive
Last edited by LewisM; 26-07-2018 at 12:17 PM.
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26-07-2018, 12:54 PM
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Galaxy hitchhiking guide
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Join Date: Dec 2007
Location: The Shire
Posts: 8,335
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I have found longer subs to be better. I could show you an equation that supports this, but hey, it simply comes down to do everything you can to maximise signal (e.g more aperture, higher QE) , and everything you can to minimise noise (low noise camera, many subs, deep cooling, ultra clean optics). Is sky glow noise? Not really...just unwanted signal.
With my RC16 I typically grab 15 minute subs from my seriously bright skies...and if using NB filters 20-30 minutes.
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26-07-2018, 03:06 PM
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Unregistered User
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Join Date: Apr 2007
Location: Perth, Australia
Posts: 1,164
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Quote:
Originally Posted by LewisM
Doing the PI script, I get an Antsey Limit of 22seconds...yeah
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Make sure you do the test during the night time
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26-07-2018, 03:43 PM
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Novichok test rabbit
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Join Date: Aug 2012
Location: Somewhere in the cosmos...
Posts: 10,388
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Quote:
Originally Posted by luka
make sure you do the test during the night time
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lolol
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