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27 Feb 2017
INDI development team is happy to announce the release of INDI Library v1.4.0. This new exciting release builds on the maturity of INDI Library and comes with many new supported devices and fixes for existing drivers. Several improvements and enhancements are included in this release including native support for Cygwin and MacOS platforms in addition to Linux, BSD, and Windows (Client only).
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TOPIC: Estimating exposure time using surface brightness

Estimating exposure time using surface brightness 2 weeks 22 hours 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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