#21  
Old 12-02-2015, 10:23 PM
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killswitch (Edison)
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Originally Posted by gregbradley View Post
Here is a shot I took a couple of years ago 2 x 60 minutes Nikon D800e with 14-24mm lens at F2.8 ISO400 (it also has some narrowband material added from my FLI Proline camera but the guts of the image is the Nikon).

http://www.pbase.com/gregbradley/image/152348475/large

Greg.
Superb photo Greg

Side question, what would you say the D800's sweetspot is for exposure time/ISO?
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Old 16-02-2015, 05:06 PM
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Superb photo Greg

Side question, what would you say the D800's sweetspot is for exposure time/ISO?
1 hour ISO 400! I've read by an engineer who analyses DSLRs that the D800/e ISO is linear up to ISO1600. Above ISO1600 though it applies a digital amplification. In other words there is no gain in image quality above ISO1600. On some cameras the argument is its better to not go above ISO800 and go longer in exposures.

If I were using that camera in a telescope it would depend on dark skies or light polluted skies. Light polluted skies cause exposure time to be limited and best used with a light pollution filter. IDAPS I think its called.

Dark site you would go longer as the background does not saturate the image like in light polluted skies. So perhaps ISO400-800 and 10 minutes?

I imagine its similar to a dedicated astro cam at a dark site and I usually use 10 minutes, longer if the tracking is good.

ISO is amplification of the voltage read from the pixels. You amplify too hard and you are simply amplifying the noise along with the signal. You don't actually gain anything from ISO except convenience for say a nightscape shot with no tracking where a 14mm F2.8 lens for 30 secs and ISO 6400 gets you a nice well exposed image. But a deep sky object ISO6400 x 30 seconds isn't receiving as much signal so too much ISO will increase noise.

So longer and lower ISO and cooler would be the general idea with DSLR imaging at dark sites and shorter and perhaps higher ISO for light polluted sites (not sure about the higher ISO there). The point is light pollution saturates the background and prevents any contrast very quickly so you have to watch the histogram. I believe common practice is to expose to a 1.3rd of a histogram.


Greg.
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