Exposer Times

[valleyman]

Is there any info about what exposer times should be used with the magnitude of the subject you are shooting.

ie: Telescope size, Camera chip size, magnitude of subject, = Exposer time.

[pete_l]

Is there any info about what exposer times should be used with the magnitude of the subject you are shooting.

In theory all you need to know is that that photon flux from a Mag. 0 star at the top of the Earth's atmosphere is 8.79 x 10**9 per m² per second and atmospheric extinction is 0.28 Mag per air-mass.

All the rest is just arithmetic

[JamesF]

I'd think there are just too many variables to make that feasible. Perhaps the best guide to start with is to use a capture program that can show you the histogram for the captured image and set the exposure time to get the histogram reasonably full whilst avoiding trailing in the image. Of course that doesn't always work because on some targets such as the Andromeda Galaxy or the Orion Nebula you'll get some parts blowing out way before others have sufficient exposure. But it's a start.

James

[ollypenrice]

It isn't possible to use charts from theory. F ratio, sampling rate, chip sensitivity, chip noise (particularly thermal in uncooled cameras), transparency, light pollution, polar alignment and tracking precision all have their influences.

Expose for as long as your sky or one of the other limiting factors allows. You may end up saturating bright galaxy cores and stars. If this is so take some short subs to blend in with the long ones in processing. In reality this is rarely needed if you get the hang of doing special soft stretches for the brightest parts of an image and layering them in.

Olly

[davew]

There is a sub exposure calculator here -

http://www.ccdware.com/resources/subexposure.cfm

Dave.

[valleyman]

Thanks for the replys guys.

I think the one thing I am not doing is having long enough exposers.

[JamesF]

Do you have any particular targets in mind and which kit are you using for imaging?

James

[valleyman]

I will be trying for Galaxies, star clusters and nebuler.

my equpment is HEQ5 pro, Opticstar AR 90 f6. QHY8L CCD Colour. Lights will be in RAW format.

I tried for M15 and Andromeda over the weekend but did not get much detail,

[JamesF]

You could probably go for exposures of several minutes on M31 if you can guide accurately enough. If you're not guiding then the accuracy of your polar alignment will perhaps be the limiting factor in length of exposure. I'd expect M15 to show some reasonable detail with shorter exposures, though you'd probably still need to stretch the histogram a fair bit to bring out the detail.

James

[valleyman]

I am guiding with PHD, so should be able to do some long exposers.