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Image timings?


TheThing

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Something that has always puzzled me and I've not found a clear answer to it as yet - How do you DSO imagers know how long to take exposures for?  I suppose I could just go through the pictures that are posted and make a table to correlate them, but is there a hard and fast rule or is it just luck?

For example, say that I wanted to take some images of the Orion Nebula, how long should my exposures be and how many should I take?  Is it the same for the Pleiades and the Andromeda galaxy or do these differ?

Thanks in advance,

Puzzled of Epsom Spa  

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It is a bit of trial and error.  Different targets will require different exposures due to their differing brightnesses.  M42 is very bright and if you want detail in the centre you will need a very different exposure to that required for a dimmer target. As for how long to take exposures for, well as long as possible in general. Darker skies will require less exposure than brighter skies as faint details will be easier to capture. The camera you are using will make a difference as different cameras will require different settings.  With some experience you should get a rough idea of what camera settings to use for different targets.  Don't forget that the telescope/lens used will have an effect too.

You can calculate how long before you get diminishing returns if you want to - Google for details - based on sky brightness levels and other factors.

So far I have found with my setup that M42 needs exposures of different lengths to cope with the big brightness range so you get details in the bright core and the fainter outer parts.

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Sub duration will depend on your light pollution (or other noise source) and read noise level of your camera.

You want to swamp read noise with any other noise source (all other depend on time and read noise is constant per exposure). You want to swamp it by factor of at least x3, preferably x5.

Easiest way to estimate this is by examining single exposure you took in expected conditions and measure background levels and from that derive LP noise. If you use uncooled camera - you could also examine your darks to see if dark current is bigger factor than light pollution.

Total exposure time will depend on many factors and there are several calculators that will help you determine total exposure time needed. You will however need to estimate brightness of the faintest part of the target and your LP levels and set wanted signal to noise ratio that you'll be happy for faintest parts.

Most people don't bother with either point 1 or point 2 and settle for some sensible exposure length - that is long enough but not too long so they get issues from tracking/guiding (usually in rage from few minutes to 10-20 minutes). You will need longer exposures if you have CCD vs CMOS and you will need longer exposures if you do narrowband versus regular OSC/LRGB imaging. Both of these impact ratio of read noise to background noise.

Also - people shoot for some amount of time - like one night and examine their data. If they are happy - they make an image, if not - they add another night worth of imaging or even couple of nights.

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Depends mainly on

1) the speed of your system.  This is not simply determined by F ratio. It is determined by the amount of light you put on each pixel. If you put bigger pixels in the same scope, each pixel will  receive more light and you will gain speed and, possibly, lose resolution.

2) the sensitivity of your camera. That's an easy one.

3) your level of skyglow. At a dark site you can expose for longer than at a light polluted one whose ambient light will soon overwhelm your camera.

4) your camera read noise. You get one dose of read noise per exposure, so CCD cameras benefit from long exposure because they have high read noise. A CMOS camera has low read noise so it does no harm to take more and shorter exposures.

5) the brightness of your target. The faint stuff takes a lot more time to capture.

6) the quality you want to achieve. You need to increase your total exposure time by four in order to double your signal to noise ratio. Quality is, alas, subject to diminishing returns.

7) the precision of your tracking.

8 ) whether or not your camera is cooled.

I've been typing at the same time as Vlaiv, here, and I agree with his advice. Experiment, find a suitable sub exposure time and stick with it. Very, very occasionally there is a need to shoot short exposures for very bright parts of an image. M42 is the obvious example. You'll also need to find a way of combing them using a high dynamic range technique. (There are many.) Lots of beginners shoot a random assortment of different exposures on the same project. Don't, it is a bad idea.

Olly

 

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25 minutes ago, TheThing said:

Something that has always puzzled me and I've not found a clear answer to it as yet - How do you DSO imagers know how long to take exposures for?  I suppose I could just go through the pictures that are posted and make a table to correlate them, but is there a hard and fast rule or is it just luck?

For example, say that I wanted to take some images of the Orion Nebula, how long should my exposures be and how many should I take?  Is it the same for the Pleiades and the Andromeda galaxy or do these differ?

Thanks in advance,

Puzzled of Epsom Spa  

Generally, once you've determined an appropriate sub exposure length by the methods mentioned above, there is no need to adjust sub exposure time for different targets using the same equipment from the same location. The only real need to deviate from that would be on targets with a very high dynamic range of brightness (e.g. M42) - in this case, you may also shoot some short exposures specifically to capture the very brightest parts (these areas would most likely have been overexposed with a longer sub).

29 minutes ago, wongataa said:

As for how long to take exposures for, well as long as possible in general. Darker skies will require less exposure than brighter skies as faint details will be easier to capture.

It should be clarified that this true for total exposure (integration) time, not individual sub exposures. Darker skies would require longer subs to facilitate the swamping of the read noise, but the desired signal to noise ratio would be achieved in less total integration time.

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Regarding the need for short subs, simply look at your linear (unstretched)  long sub stack. If an object is not saturated (exposed to the camera' full brightness limit) you don't need shorter subs, you just need to stretch with care, possibly even blending two different stretches.

Olly

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Hey @TheThing , I asked a similar question here a few weeks back. Might be worthwhile having a read through to see what else you can pick up in addition to what's been said here. A number of the kind folks on SGL were kind enough to add their thoughts, plenty of great advice there which I will be putting in to practice as and when the opportunity arises 👍

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A potentially useful rule of thumb: If long exposures are not a problem for you in themselves, expose till you "blow out" a few dozen pixels or so at the top end, but no more. This ensures that each subexposure is as long as it can be, minimizing read noise. As Vlaiv is fond of saying, dynamic range isn't really a thing when you're stacking exposures -- you can still capture photons that come so seldom that some exposures miss them entirely! -- but this does use all of your camera's range.

Intermittent tracking satellites and aircraft, cosmic rays, and other gremlins argue for shorter exposures, so there is a wider statistical universe for outlier elimination. And there are significant benefits to standardizing on one or a very few subexposure lengths (fewer sets of darks e.g.).

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Everyone's system is different, and not everyone lives in the same place regarding light pollution, so if your camera/ and skies allow you to do 30 min subs but your mount can only track for 3 mins then the whole system is reliant on the mount... So its a balancing act of your equipment and sky conditions

In darker skies you can exposure for longer as your subs won't contain signal from as much light pollution, so the main rule of thumb are longer subs in darker skies and shorter ones in more light polluted ones

High dynamic range targets such as m42 and m31 will need 2 different exposure times, for the trapezium in m42 then you will need exposures of only a few secs, and then much longer for the rest( 1-10 mins, depending on filter)

Guiding opens up your whole exposure range

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