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Old 23-07-2018, 07:44 PM
LewisM
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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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Old 23-07-2018, 08:10 PM
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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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Old 23-07-2018, 08:24 PM
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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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Old 24-07-2018, 01:42 PM
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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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Old 24-07-2018, 02:01 PM
LewisM
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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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Old 24-07-2018, 06:00 PM
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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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Old 25-07-2018, 06:19 PM
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Quote:
Originally Posted by LewisM View Post
Thatís precisely what I am thinking Dunk. I am going to try say Leonís suggested 6 min and see. Iíll 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 )
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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Old 25-07-2018, 08:27 PM
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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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Old 25-07-2018, 08:50 PM
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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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Old 26-07-2018, 11:54 AM
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Originally Posted by luka View Post
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).
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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Old 26-07-2018, 12:54 PM
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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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Old 26-07-2018, 03:06 PM
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Doing the PI script, I get an Antsey Limit of 22seconds...yeah
Make sure you do the test during the night time
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Old 26-07-2018, 03:43 PM
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make sure you do the test during the night time
lolol
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