Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Subexposure length calculations for my new ASI6200MC Pro

  • Please log in to reply
12 replies to this topic

#1 Coconuts

Coconuts

    Viking 1

  • *****
  • topic starter
  • Posts: 524
  • Joined: 23 Sep 2012

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

 

 


  • lynnelkriver, psandelle, R Botero and 3 others like this

#2 Gipht

Gipht

    Vanguard

  • *****
  • Posts: 2,311
  • Joined: 12 Nov 2016
  • Loc: Prescott Valley, AZ.

Posted 15 January 2020 - 09:52 AM

What a beautiful place  to set up a telescope!


  • ks__observer likes this

#3 bobzeq25

bobzeq25

    Hubble

  • *****
  • Posts: 19,791
  • Joined: 27 Oct 2014

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.

  • lynnelkriver likes this

#4 lynnelkriver

lynnelkriver

    Messenger

  • -----
  • Posts: 453
  • Joined: 14 May 2007
  • Loc: Elk River, Mn.

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

Coconuts

    Viking 1

  • *****
  • topic starter
  • Posts: 524
  • Joined: 23 Sep 2012

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

pbealo

    Apollo

  • *****
  • Posts: 1,041
  • Joined: 12 Oct 2006
  • Loc: New Hampshire

Posted 15 January 2020 - 05:53 PM

Kevin,

 

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


  • lynnelkriver and psandelle like this

#7 Coconuts

Coconuts

    Viking 1

  • *****
  • topic starter
  • Posts: 524
  • Joined: 23 Sep 2012

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


  • pbealo, cobestluc, psandelle and 1 other like this

#8 pbealo

pbealo

    Apollo

  • *****
  • Posts: 1,041
  • Joined: 12 Oct 2006
  • Loc: New Hampshire

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

 

Attached Thumbnails

  • 20200118_152305_resized_2.jpg


#9 Kasra

Kasra

    Lift Off

  • -----
  • Posts: 20
  • Joined: 09 Jan 2020
  • Loc: United Kingdom

Posted 23 February 2020 - 03:40 PM

Have you done a sensor analysis for your ASI6200MC Pro? I wonder what is the unity gain and best chip temperature? 



#10 Coconuts

Coconuts

    Viking 1

  • *****
  • topic starter
  • Posts: 524
  • Joined: 23 Sep 2012

Posted 23 February 2020 - 04:46 PM

No, but I'm hoping to find time to do that soon.  For my short exposures, 0C is plenty.  Less battery draw.  I am likely to use 0 gain setting to avoid ridiculous numbers of subexposures to process.  Largest full well, also highest dynamic range by a bit.  Offset of 5.

 

All the best,

 

Kevin


Edited by Coconuts, 23 February 2020 - 04:56 PM.


#11 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 9,516
  • Joined: 12 Oct 2007

Posted 23 February 2020 - 05:38 PM

What do you mean by imaging etendue?  It appears to be pixel etendue.  What are its units?

 

You don't mention star saturation here.  What exposure would you use for the double cluster?

 

What exposure would you use for a faint dwarf galaxy?

 

You seem to acknowledge the pain of accumulating many short exposures.  Wouldn't you rather go much longer - and far from "optimal" - if you can avoid saturation?

 

And if even 22 seconds still saturates the double cluster and you lose color, wouldn't you want to go much shorter than "optimal?"

 

Frank



#12 Coconuts

Coconuts

    Viking 1

  • *****
  • topic starter
  • Posts: 524
  • Joined: 23 Sep 2012

Posted 24 February 2020 - 07:09 AM

Frank:  A lot of questions.  Comments follow:

  • As solid angles are dimensionless, etendue has units of area.
  • To better compare different astrograph-astrocamera systems on extended source objects, I added a QE multiplier to create an "imaging etendue".
  • For the specific values given in my OP, "Imaging Etendue" equals 424.36 * QE * pixel size^2  / focal ratio^2
  • Pixel size is in um.  The normalized version (QE = 100, pixel size 1 um, focal ratio 1.0) has an "imaging etendue" of 42,436.
  • That constant was to align this with other etendue calculator results: https://www.cloudyni...ndue-calculator
  • Point sources are another thing altogether.  My Tak hyperbolic astrographs claim spot sizes of "10 um stars across the field".
  • That is still likely to split any given star over at least four pixels.  With a FWC of 51k e- per pixel at Gain 0, that is over 200k e-.
  • I don't yet know what exposure length will avoid saturation and color loss for the double cluster. 
  • For the faint dwarf galaxy, as well as nebular objects, I will use gain zero (highest dynamic range and FWC), and subexposures of 2-3 minutes.
  • I am not interested in going longer, because the "diminishing returns" of SNR are to my opinion outweighed by the added challenges to mount performance, having to guide or not, and lengthy images lost due to clouds and other artifacts.
  • I don't plan to bin in order to preserve pixel scale.  I will similarly avoid high gain, to avoid its dramatic reduction in FWC.
  • My comment regarding "ridiculous numbers of subexposures" was referring to the very short sub lengths possible when binning, and/or at high gain.
  • I am aware that you found Robin Glover's videos uninformative.  I am just getting started, and I found them to be very enlightening.

All the best,

 

Kevin



#13 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 9,516
  • Joined: 12 Oct 2007

Posted 25 February 2020 - 02:06 AM

Frank:  A lot of questions.  Comments follow:

  • As solid angles are dimensionless, etendue has units of area.
  • To better compare different astrograph-astrocamera systems on extended source objects, I added a QE multiplier to create an "imaging etendue".
  • For the specific values given in my OP, "Imaging Etendue" equals 424.36 * QE * pixel size^2  / focal ratio^2
  • Pixel size is in um.  The normalized version (QE = 100, pixel size 1 um, focal ratio 1.0) has an "imaging etendue" of 42,436.
  • That constant was to align this with other etendue calculator results: https://www.cloudyni...ndue-calculator
  • Point sources are another thing altogether.  My Tak hyperbolic astrographs claim spot sizes of "10 um stars across the field".
  • That is still likely to split any given star over at least four pixels.  With a FWC of 51k e- per pixel at Gain 0, that is over 200k e-.
  • I don't yet know what exposure length will avoid saturation and color loss for the double cluster. 
  • For the faint dwarf galaxy, as well as nebular objects, I will use gain zero (highest dynamic range and FWC), and subexposures of 2-3 minutes.
  • I am not interested in going longer, because the "diminishing returns" of SNR are to my opinion outweighed by the added challenges to mount performance, having to guide or not, and lengthy images lost due to clouds and other artifacts.
  • I don't plan to bin in order to preserve pixel scale.  I will similarly avoid high gain, to avoid its dramatic reduction in FWC.
  • My comment regarding "ridiculous numbers of subexposures" was referring to the very short sub lengths possible when binning, and/or at high gain.
  • I am aware that you found Robin Glover's videos uninformative.  I am just getting started, and I found them to be very enlightening.

All the best,

 

Kevin

Hi Kevin-

 

Thanks for the replies.  I normally use 'etendue' to refer to the entire system and 'pixel etendue' just for the pixel.  The units are spatial area times solid angle - but you need to specify the units on the solid angle and the area.  It could be steradians or square arc-sec.

 

The etendue of the system won't be affected by binning and just depends on sensor size and f/ratio - or aperture area and solid angle field of view.  But the pixel etendue will be affected by binning.  On the other hand, you can capture unbinned and then bin later as much as you want.

 

QE is normally from 0 to 1, so having it as 100% is a bit odd, but ok.

 

My point about exposure times is that for something bright like the double cluster you may need very short exposures to avoid saturation.  And for faint objects like a dwarf galaxy there is no point limiting yourself to short exposures based on sky glow.  You can go much longer as long as you don't get saturation and you can still guide ok.  My 'long side' recommendation is 5 minutes for a cmos camera - and if you look at astrobin you will find many images with that subexposure - and few that are much longer.  It's just a nice practical compromise.

 

It isn't that I find the Glover video 'uninformative.'  It describes the basic noise terms at work, but I think practical factors usually dominate and the word 'optimal' shouldn't be used.  And since you are already talking about the challenges of many short exposure frames - that suggests you should go longer.  As long as you don't saturate.

Frank




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics