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20 Aug 2017
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TOPIC: Estimating exposure time using surface brightness

Estimating exposure time using surface brightness 6 months 2 days ago #17251

  • oopfan
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Thanks to Klaus for leading me to this paper by Jose Torres where I learned how to calculate Surface Brightness given the size of the DSO and its magnitude. Now, I am wondering if I can use Surface Brightness to estimate the exposure needed to capture it according to the following formula:

m1 - mref = -2.5 * LOG (I1 / Iref)

rearranging terms yields:

I1 / Iref = 10 ** ((mref - m1) / 2.5)

In order to put this into practice I need a reference object and exposure. Let's say that I recently captured M57 with a 30-second exposure and that I am happy with the density of nebulosity on each subframe. Now I want to capture M27.

The surface brightness of M57 is 19.6.
The surface brightness of M27 is 20.2.

According to the above formula:
I1 / Iref = 10 ** ((19.6 - 20.2) / 2.5) = 0.57544

In other words it says that M27 is 57.544% of the intensity of M57.
That makes sense when I compare a 30-second exposure of M27 to a 30-second exposure of M57. The Dumbbell Nebula appears to have half the density on the subframe.

Then, can I safely estimate that I would need an exposure of 52 seconds in order to achieve the same density?
(i.e. 30 sec / 0.57544 = 52.134 sec).

If yes, can I safely use my reference exposure of M57 (a planetary nebula) as a basis for estimating the exposure time for other types of DSO's? Or should I endeavor to establish a different reference for each DSO type and only make intra-object-type exposure estimates?

Thanks to everyone for their help.

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Estimating exposure time using surface brightness 4 months 3 weeks ago #17894

Your reasoning is reasonable, and in theory you could just use one single reference object. But in practice DSOs are different from each other and have different brightness profiles, so I'd build a library of sample objects as you suggest.
(what I usually do instead is try with my "mmmh this should be fine" settings and see what I need to correct for that specific field I'm capturing at the moment)

And also, as usually we try to make pretty pictures, you have to consider field stars to avoid pixels blooming: stars are brighter and smaller than diffuse objects (so, for example, I ramp up exposure time until stars get worse). See en.wikipedia.org/wiki/Surface_brightness#General_description

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Estimating exposure time using surface brightness 4 months 3 weeks ago #17896

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Thanks for your reply, riccio. Yes, these are ballpark estimates just intended to get a wisp of nebulousity to imprint on the subframe. I decided that I desperately needed a tool like this when I was flummoxed as to why I couldn't get a print of the Fireworks Galaxy (NGC 6946) with a 30 sec exposure after I just got a print of the Ring Nebula at 30 sec -- after all, the catalog says that they are both magnitude 8.8. After that is when I discovered Surface Brightness.

The weather here has been terrible. I am lucky to get one night per week. Since then I did verify that a 52 sec exposure of the Dumbbell Nebula is roughly equivalent to a 30 sec exposure of the Ring Nebula, that according to my program. I have more testing to do. Right now I am hacking my 50-year old telescope and mount to enable auto-guiding. I have been pretty much limited to 1 minute exposures due to periodic error. I have now replaced the original synchronous motor with a stepper motor of my design, and putting the final touches on periodic error correction. I would be happy if I can do 5 minutes unguided with only minimal star elongation. The next step will be to write an INDI driver and do auto-guiding. First, though, I will try to go unguided.

Thanks for your input!

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Estimating exposure time using surface brightness 4 months 3 weeks ago #17906

Great!
I wish you clear skies, and don't forget to post your setup here when you'll be happy with it: indilib.org/forum/general/1014-post-your-indi-setup.html :)
The following user(s) said Thank You: oopfan

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