Sub Length

MikeP
11 Nov, 2016 12:16
Hello,

I was wondering if anyone has found a good calculator/tool to determine the ideal exposure time for subs? The problem I come up against is that even though I have tried different tools they all seem to suggest shortish sub exposures e.g. 2 to 3 minutes. However if I look at some the awesome pictures posted here most of them seem to have 15 to 20 minute sub exposures. I am aware of the many factors that influence this calculation but am not sure that the theory that has been used to make the calculators actually matched your experience? Any good tools or rules of thumb that work for you?

Thanks,

Mike
Astrobert
11 Nov, 2016 14:40
Hi Mike,

The best tool I think is trial and error.  As you have mentioned already it depends on so many factors like weather, moisture, light pollution, filters, etc.  If I use my f2.8, 3 to 5 minuten for RGB is enough. If I use my f10 I can go for 10 minutes easy if I want to.

Bert
Magellen
11 Nov, 2016 20:12
Hi Mike,

take a look here: http://www.gibastrosoc.org/sections/astrophotography/optimum-exposures-calculator

HTH,
Fritz
KuriousGeorge
12 Nov, 2016 15:31
I've seen lots of theories. In my experience, I get the best results when the left-most  peak of a single image histogram is well away from 0 and the brightest pixels don't fall off the right. This maximizes the dynamic range of your detector. With my 16-bit camera, I can do this with 2 minute subs or 10 minute subs. Often you can't tell the difference between the two in the final image for the same exposure time.

If you do the above, then you need to optimize for your mount, focus, satellites, seeing, wind, etc. On less than ideal nights I'll go for 2 minute subs. On really steady nights, I'll go for 5 or 10 minute subs. My mount can do either. Shorter subs take much longer because you have to wait for more download/dither/settle time.

I OAG (upcoming ONAG) every shot. Even with my G11 mount, 2-minute subs are pushing it for the sharpest image. I update guiding every 2 seconds.
MikeP
13 Nov, 2016 08:59
Thanks everyone for your input, it's much appreciated! I think you are right, taking test images on the day and analysing them is the way to go.

Fritz, do you find that spreadsheet works for you? I've used it for my last session and it suggested very short sub lengths (45s) which I thought was too short so I went for 2min instead.  That's not to say that the spreadsheet is wrong!

Thanks,

Mike
Magellen
13 Nov, 2016 09:11
Servus Michael,

If i use this spreadsheet, i do it just the way you did: I use it as starting point, adjusting the exposure time if needed.
bobzeq25
14 Nov, 2016 16:23
I always use the histogram, which shows the skyfog peak.  It's simple (much simpler than the spreadsheets) and it works.  You want the
jpg or "stretched" histogram.  It should be 25-40% over from the left edge.  Less than 25% makes processing difficult, you're not that far
from the bias.  More than 40% is usable, but costs you some dynamic range, more stars saturate, which washes out their color.

The reason you see different people use different exposures is mostly light pollution.  In dark skies one can shoot exposures far beyond
what those of us imaging from suburban backyards can.  F number and ISO also count, but light pollution is the big deal.  I've seen subs
of as much as 30 minutes in pristine skies, 2-5 is more usual for the suburban backyard.

The histogram approach takes that into account, and does it easily.
Edited 14 Nov, 2016 16:26
Intihuatana
18 Jan, 2017 03:27
Going through the same dillema….don't mean to complicate things, but how about narrowband?…not affected as much by light pollution. I like the idea of playing with the histogram. by trial and error I get the stretched histogram perfectly centered within the dinamyic range of my camera by doing 10min NB exposures (Ex Ha).

Online calculators from reputed sites suggested 2min exposures….I guess Amateur AP is still mostly an obscure science like chemistry was in the 1800's!!..
xordi
18 Jan, 2017 09:16
I'm also using the referred sheet, but there is this SNR field which I dont fully understand and depending on the field value entered (95-97%) there can be big differences in the recommended exposure length.

Anyway according to this sheet the most important value of the calculation is the background level of the sub in ADU. As far as I see, if this is over 1500-2000 or so than the expo length must be set to a lower value, to keep the background level around 1000-1500. This value is valid for every type of images inc. NB, independent from the used equipment.
Edited 18 Jan, 2017 10:19
bobzeq25
19 Jan, 2017 05:21
Miguel Garcia
Going through the same dillema….don't mean to complicate things, but how about narrowband?…not affected as much by light pollution. I like the idea of playing with the histogram. by trial and error I get the stretched histogram perfectly centered within the dinamyic range of my camera by doing 10min NB exposures (Ex Ha).Online calculators from reputed sites suggested 2min exposures….I guess Amateur AP is still mostly an obscure science like chemistry was in the 1800's!!..
One nice thing about the histogram is that it also works for narrowband.  You actually want the histogram off center to the left.  You just need a good separation from the bias.  More than that (maybe 1/4 over from the left edge for narrowband) and you're losing dynamic range for no gain.
Shiraz
19 Jan, 2017 07:49
try this. works for any system broadband or narrowband. you need to work out one number for your camera and thereafter,  you can use it to set/check sub length in any circumstances.

http://www.iceinspace.com.au/forum/showthread.php?t=117010

short subs are appropriate with low read noise and/or bright sky background. The guys that use long subs with LRGB have very dark skies and cameras with high read noise. Of course, long subs rule for narrowband.
Edited 19 Jan, 2017 08:34
Stathis
09 Mar, 2017 02:55
I actually have gotten my best photos with the ADU being more than 3000. I too have seen those calculators, but decided to try long exposures anyway, (10 and 15 min subs that yield 3k-5k ADU depending on the target's height in the sky) to bring out the faint stuff. It works. But you need very good processing. Of course after a point you wouldn't get that good results, but I haven't figured out yet what that point is, as my guiding won't allow for more exposure time. Take all this with a grain of salt. This is my experience, doesn't mean it's the best, but overall if you want to capture faint stuff I think you can go well beyond the 2000 ADU mark.
Edited 09 Mar, 2017 02:58
 
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