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Old 18-03-2013, 11:25 AM
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Shiraz (Ray)
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Location: ardrossan south australia
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Quote:
Originally Posted by naskies View Post
Thanks for writing that up, Ray - very interesting read. Along Rick's suggestions, I'd be interested in a theoretical optimal sub exposures for say the Orion nebula where you need (with most CCDs) to take multiple exposures to capture both the faint background dust and the core.
stars are not in the model Dave, but can do a quick and dirty back of the envelope on M42 -

Assuming that M42 is about mag 21 in the outer reaches – same as a fairly dark sky.

The optimum SNR calculation for the nebula calls for sub exposures of about 300 secs with the 254f4.75 8300, which yields a headroom of 9.4 mag. From available info on the distribution of surface brightness in M42, this headroom should cover the hot bits of the nebula, but not the brightest stars.

With a sky of mag 21 and mag 9.4 headroom, stars will saturate at mag 11.6 if all of the star energy is contained in 1 pixel. In 2 arc sec seeing, star energy will be distributed over about 4 pixels, so saturation will occur for stars of 4x mag 11.6, or mag 10.1 for 300 sec sub exposures.

Since the brightest star in the trapezium is mag 5.1, we have to reduce the exposure by the equivalent of mag 5, which is about 1/100 to keep that star from saturating. So, for broadband imaging, the trapezium exposure would need to be less than 3 seconds. The energy in each colour channel would be roughly 1/3 that in broadband – so the trapezium stars should remain unsaturated up to ~9 seconds exposure in the colour subs, depending on the star colours.

I haven’t got any of my old M42 data easily to hand to check if this is reasonable – is anyone able to comment on the basis of their data? Bear in mind though that this result is specific to the chosen equipment setup and sky/nebula brightness assumptions. and as a "quick and dirty" it might have glossed over an important factor and be just plain wrong

Your question also raises the prospect of doing some genuine hi res lucky imaging on the trapezium and similar - these stars could be imaged at well below the saturation level at high framerates (~10% of saturation in 0.3 sec for example). will have to follow this up - could be fun.

Last edited by Shiraz; 18-03-2013 at 11:23 PM.
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