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exposure time question


Tadakun
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excuse my apparent noobness to this whole thing, but is one longer sub better than multiple shorter ones? I would think yes, but I prefer not to assume.

say would 3x 5 minutes subs be better than 7x 2 minute dubs?

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The faintness of the features you can get is determined by the length of the sub (longer is better). However, noise reduction is proportional to the square of the number of subs combined (more is better).

Of course, the real answer is to take lots of long exposures (7x 5minute subs :))

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If your light pollution allows it - longer subs are always better.

And the detector. If you're in a S/N regime where read noise is negligible then you may want to split integrations to stop the detector saturating, especially with NABG CCDs, or to be less affected by satellite trails, etc.

Under very dark skies (Paranal) I still tend to split exposures - e.g. for some high-resolution spectroscopy i'm doing (R~67,000 at S/N~200) i'm using 5x600s rather than a single 50-minute integration. Adds overheads, but prevents strong emission lines saturating, cosmic ray/sky line removal is easier and you can co-add spectra later for your final S/N.

Edited by Ben Ritchie
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A 1 x16 min exposure is less noisey than a stack of 16 x 1min, or 8 x 2min, or even 2 x 8 min.

If you have any light pollution then the difference between these various options may be utterly miniscule and invisible to the eye.

NigelM

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The faintness of the features you can get is determined by the length of the sub (longer is better). However, noise reduction is proportional to the square of the number of subs combined (more is better).

Not really. The faintness of the features you can get is mainly determined by the total exposure time. If read noise in you camera is not an issue (e.g you have light pollution), then there is no real difference between 16x1min or 1x16min.

NigelM

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A piece of advice I was offered was to do a test exposure of around 3 - 5 minutes and examine the background sky glow. For my Atik 16HR it should be no more than about 3,000 ADU, so reduce or increase the exposure time accordingly.

Mike

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The photon shot noise should be the same in both (as it only depends on the total number of photons collected).

The read noise is larger the more times you read the sensor.

Quantisation error will dominate at very short subs (I think!).

The other big variable is can you trust your tracking not to ruin a long sub... Last night, at 7min subs, I had only one badly trailed sub so that was close to optimal.

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Not really. The faintness of the features you can get is mainly determined by the total exposure time.

I thought that depended on how the software combined the pictures? So with DSS (which uses some sort of averaging rather than summing algorithm?) you won't get anything fainter with more exposures, but just less noise?

Really not sure (hence the '?'s), just seeking clarification.

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Averaging and summing is the same thing (provided you have enough headroom in the numbers you use).

You will get fainter stuff as you add exposures and this is why. Suppose you have a source that sends one photon every 20 minutes, on average. You won't pick it up on every 5 min exposure but if you get enough 5min exposures it will show up on every 4th, on average. If you sum tens of exposures, the invisible source will show up.

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Well that is what I originally thought, and said more or less the same thing in another thread: http://stargazerslounge.com/imaging-discussion/117576-exposure-time-target.html. However, the response I got was "The star/nebula image remains the same brightness regardless of the number of subs. The number of subs only influences (reduces) the background noise."

Now I am totally confused ...

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However, the response I got was "The star/nebula image remains the same brightness regardless of the number of subs. The number of subs only influences (reduces) the background noise."
Oh this is because people keep insisting that there is some magical difference between averaging and adding (which there isn't). If you average then the brightness remains the same and the noise goes down, if you add, both go up but by different amounts, however the ratio of signal to noise remains the same in either case (which is what matters).

(and adding your exposures is much the same as taking one long one - read noise permitting)

NigelM

Edited by dph1nm
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