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11-24-2009, 12:05 AM   #46
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QuoteOriginally posted by PentaxPoke Quote
Why are you still using very long exposures for astrophotography? That's so "film era" isn't it? Many DSLR astrophotographers prefer to use "stacking." .......snip

See, no need to get angry or borrow a different camera, just open your mind to new methods.
With all due respect, on a camera that forces a DFS on any exposure greater than 8 seconds (my K20D), it is still going to take twice as long to get my exposure as most any other camera.

And let's not confuse with the 'magic' of stacking. If you want to record very faint objects, the SNR of very short 30 second (or in my case with the K20D.. 8 second) exposures is not enough.... for example, you could take a 1000 - 8 sec exposures and you still won't come close to what 100 - 2 minute exposures will capture.

And if you are going to take 100 - 2 minute exposures with either the K7 or the K20D, be prepared to wait over 6 hours to complete an exposure that should only take less than 3 hours.

>Here is a beautiful shot of M42 from Andy Weeks website. It was done with 14, 30-second exposures. That is approximately
>equivalent to a 7min single exposure:

That is a nice photo of the Orion Nebula but that is a very easy photo to take because it is very bright. A few dozen 8 second exposures would suffice for such a photo.


Last edited by pentaxmz; 11-24-2009 at 12:29 AM.
11-24-2009, 12:24 AM   #47
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QuoteOriginally posted by slowdive101 Quote
So, if a shot requires a 2-minute exposure, I could instead take four 30-second exposures, and stack them with one of these programs? And this would yield a perfectly-exposed result?
Theoretically yes. But it is no free lunch. To obtain reason SNR (Signal to Noise Ratios), you must balance length of each individual exposure against the total number of exposures. Let's say, if one single exposure is not enough to grab enough (or ANY) photons from the subject, a thousand more stacked, won't make any difference. Each exposure needs to be long enough to grab some of those weak photons.

As an extreme example.... If your subject is the galaxy M51 (the elusive Whirlpool Galaxy), it won't matter how many 1/2 second exposures you stack... 100 or 10000... you're not going to capture it. But a single 8 second exposure might grab about 1/2 percent of all the practically available photons... and stacking 100 will allow you to see some of its form. On the other hand, stacking 10 - 2 minute exposures will still capture much more detail, even though the total time is equivalent.
11-24-2009, 08:33 AM   #48
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Just FYI -- with updated firmware, you can turn off DFS on the K20 on exposures of 30 seconds or less, just like on the K7.
11-25-2009, 08:46 AM   #49
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QuoteOriginally posted by Rondec Quote
Just FYI -- with updated firmware, you can turn off DFS on the K20 on exposures of 30 seconds or less, just like on the K7.
Which version of firmware? I was not aware of any changes to DFS for the K20D.

Anyhow, my earlier point is that 30 seconds is not usually enough time to capture photons from deep sky objects. If the first frame doesn't capture a single photon, stacking 100 more similar frames isn't going to help.

Deep exposures require long enough exposures to allow the sensor to capture at least a few photons. For deep sky objects, I prefer to take minimum exposures of 1-2 minutes each time about 10 - 20 exposures. Of course, it depends on atmospheric conditions, light pollution, and what I am trying to capture.

11-25-2009, 08:51 AM   #50
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QuoteOriginally posted by Rondec Quote
Just FYI -- with updated firmware, you can turn off DFS on the K20 on exposures of 30 seconds or less, just like on the K7.
Are you referring to v1.03?

Latest K20D Firmware Update : Software Downloads : PENTAX
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