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Subexposure length calculations for my new ASI6200MC Pro

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#1 Coconuts

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Posted 15 January 2020 - 08:31 AM

These turn out to be a lot shorter than I would have thought; they become crazy short if I bin 2x2.  The following is for a specific astrograph at each of two imaging locations, and is based on the work of SharpCap's Dr. Robin Glover, as described in his excellent YouTube video:

 

https://www.youtube....h?v=3RH93UvP358

 

along with his great tool for calculating sky electron rates, here:

 

http://tools.sharpcap.co.uk

 

For anyone who has yet to watch Dr. Glover's video, find the time.  Modern CMOS is a game-changer!

 

The following is a summary of subexposure calculations at my two imaging locations:

 

Optimal subexposure length

 

Formula: Optimal subexposure length = 10 * Read-noise^2 / Sky electron rate

 

The factor of 10 is based on Dr. Glover's recommendation that we allow 5% noise into an otherwise perfect image.

 

Location 1:  Gifft Hill, St. John, USVI
OSC CMOS camera: ASI6200MC Pro
Astrograph: Takahashi e-130D
SQM:21.2
T/ratio: 3.5
Pixel size unbinned: 3.76 um
Pixel size binned 2x2: 7.52 um
Pixel scale, unbinned: 1.80 arc-sec/px
Pixel scale, binned 2x2: 3.60 arc-sec/px
QE: 80% (Green)
Sky electron rate, unbinned: 1.0 e-/px/sec
Sky electron rate, binned 2x2: 4.1 e-/px/sec
Read noise, unity gain: 3.6 e-
Read noise, gain 101: 1.5 e-
Full well, unity gain: 51,400 e-
Full well, gain 101: 19,000 e-

Imaging etendue, unbinned: 39,180

Imaging etendue, binned 2x2: 156,720
Optimum subexposure, unbinned, unity gain: 130 sec
Optimum subexposure unbinned, gain 101: 23 sec
Optimum subexposure, binned 2x2, unity gain: 32 sec
Optimum subexposure, binned 2x2, gain 101: 6 sec

 

 

Location 2: Dorchester, NH:
OSC CMOS camera: ASI6200MC Pro
Astrograph: Takahashi e-180ED
SQM:21.8
T/ratio: 3.1
Pixel size, unbinned: 3.76 um
Pixel size, binned 2x2: 7.52 um
Pixel scale, unbinned: 1.55 arc-sec/px
Pixel scale, binned 2x2: 3.10 arc-sec/px
QE: 80% (Green)
Sky electron rate, unbinned: 0.75 e-/px/sec
Sky electron rate, binned 2x2: 3.0 e-/px/sec
Read noise, unity gain: 3.6 e-
Read noise, gain 101: 1.5 e-
Full well, unity gain: 51,400 e-
Full well, gain 101: 19,000 e-

Imaging etendue, unbinned: 49,943

Imaging etendue, binned 2x2: 199,773
Optimum subexposure, unbinned, unity gain: 173 sec
Optimum subexposure unbinned, gain 101: 30 sec
Optimum subexposure, binned 2x2, unity gain: 43 sec
Optimum subexposure, binned 2x2, gain 101: 8 sec

 

Despite the four-fold gain in etendue, I am likely to avoid binning, so as to keep the pixel scale reasonable and not undersample. 

 

Depending on what's in the field, I can either use unity gain or a gain of 101 to avoid saturating the wells.  That puts my subs in the range of 23 to 130 seconds. 

 

One thing stands out: at 372 MB per image (62 Mpx * 3 colors * two bytes/px/color), with hopefully hours of integration, and subs this short, I will be needing one of those powerful new AMD Threadrippers to process it all! 

 

Here's a photo of the e-130D at our St. John residence (pre-ASI6200MC Pro):

 

Takahashi Epsilon-130D Hyperbolic Astrograph.jpg

 

All the best,

 

Kevin

 

 


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#2 Gipht

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Posted 15 January 2020 - 09:52 AM

What a beautiful place  to set up a telescope!


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#3 bobzeq25

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Posted 15 January 2020 - 10:41 AM

Welcome to the new world.  <grin>  In those dark skies your exposures are much longer than mine with mag per arc sec squared low 18s.  C8 RASA and 183 camera.  The image referenced below was 667X10 seconds, and just a bit over 10 X RN^2.  Gain 0, to avoid even shorter exposures.

 

1950X Threadripper, it's fun to see 32 subs at a time being preprocessed.  I have a total of over 15 TB of storage, and now don't keep the raw data or intermediate steps very long, just the stacks, in case I want to reprocess them.

 

The upsides are shorter total imaging time, and capturing some pretty faint stuff in those light polluted skies.  In your dark ones, you'll be able to go deeper, there's no need to think about binning with that short fast scope.  Even with narrowband, although then more gain becomes useful.

 

Finally, you may want to sacrifice a bit of signal to noise ratio on high dynamic range targets, for more dynamic range, by going closer to 5 X RN^2.  Good discussion in this book.

 

https://www.amazon.c.../dp/1138055360/

 

https://www.astrobin.com/t5173s/B/


Edited by bobzeq25, 15 January 2020 - 10:59 AM.

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#4 lynnelkriver

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Posted 15 January 2020 - 10:52 AM

These new cameras really have me thinking on how to deal with all the data storage requirements needed.  I need to start doing what your doing Bob and just save the stacked image for further processing.  Thanks Kevin for the link to the Glover video.  It is really good!  Scott



#5 Coconuts

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Posted 15 January 2020 - 11:56 AM

Bob:  Gorgeous image, but wow, 667 ten second subs; that really is a new world!!  It's a good thing that massive storage is keeping up.

 

All the best,

 

Kevin



#6 pbealo

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Posted 15 January 2020 - 05:53 PM

Kevin,

 

If someone rents your villa(s), do they get access to the astrograph?? LOL!!


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#7 Coconuts

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Posted 15 January 2020 - 07:29 PM

Peter:  Alas, no.  Too big a risk of collimation loss from guests mishandling things (plus they drink).  And with one exception, I doubt if any guest would know what to do with all of this arcane gear.  You're green lighted, though.

 

Awakening in the middle of the night a couple of weeks ago, I went out on deck with a pair of  binoculars.  At first, I thought I recognized the Southern Cross.  It was actually a larger diamond asterism, composed of stars from both Vela and Carina.  After moving to get a better view SE past our flamboyant, I spotted the actual Southern Cross rising.  What struck me through the binoculars were a number of really promising nebula between Crux and Vela/Carina.  They were compelling enough for me to go in and open Bracken's "The Astrography Sky Atlas".  Too funny; the most prominent of this northerner's nebular "discoveries" turned out to be the Eta Carinae Nebula!  Its a bit low in the sky, even at 18.3 degree latitude, and I will probably have to find a site with a less obstructed horizon, but I'm itching to have a go at it.

 

All the best,

 

Kevin


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#8 pbealo

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Posted 19 January 2020 - 11:49 AM

But it's not like Kevin leaves vising astronomers high and dry! The villa (Plumaria) we are presently renting comes with a 4" f7 refractor  with nice eyepieces for all visitors.

 

Peter

 

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