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Old 28-09-2020, 06:35 AM
tc1
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Newbie Image Question

Newbie question Ė All things being equal (tracking etc), will ten integrated 30 second images result in the same looking image as one five minute exposure? Thanks in advance.
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Tony
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Old 28-09-2020, 07:13 AM
RyanJones
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Hi Tony,

The simple answer is no. There are a lot of contributing factors to the answer though. If we are talking purely about signal ( the bits of the image you want ) then technically they will be pretty close as we are simply talking about the number of photons hitting the sensor over a given amount of time. However longer exposures by rights with have more signal vs noise. Noise is the bit we donít want. Then thereís things like amp glow that depending on the camera you are using can increase over long subs so you wonít get that in shorter subs. Then thereís the dynamic range of your camera. If you have a small well depth, over the space of 5mins you will loose star colour because the pixel will just read white. Shorter subs can be better for star colour. As you can see there are a lot of contributing factors. I try to keep it fairly simple to start with but you could talk for hours about the effects of different sub lengths, itís all a balancing act and there is no straight forward answer.

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Old 28-09-2020, 07:16 AM
glend (Glen)
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It depends. Each image that you take will contain some noise elements. In theory you will capture the same amount of signal data, and so cumulative signal will be equal. Some extremely low noise cameras are very good at building up images using short sub exposures, others not very good, so it depends on your camera choice and especially if the camera has effective cooling to minimise thermal noise. As an example, a very low noise camera, like the ASI1600 MM-C, works very well shooting lots of short sub exposures, and this is a valid way of over coming mount or guiding issues as well.
Just a comment on the post below, AMP glow is (at least for the 1600) easily processed out, and short sub shooting can lessen reliance on having to capture and process Darks. Cameras like the 1600 do not require Dark processing for sub exposures at or below 30 seconds, simply applying Bias frames will take care of that. As far as well depth is concerned, it can be somewhat irrelevant in short sub capture, as you are not exceeding available depth in many cases; but obviously sensitivity is important. At the risk of setting off a major debate here, let's just say it can be done if you pick the right camera.
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Old 28-09-2020, 04:03 PM
tc1
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Quote:
Originally Posted by RyanJones View Post
Hi Tony,

The simple answer is no. There are a lot of contributing factors to the answer though. If we are talking purely about signal ( the bits of the image you want ) then technically they will be pretty close as we are simply talking about the number of photons hitting the sensor over a given amount of time. However longer exposures by rights with have more signal vs noise. Noise is the bit we donít want. Then thereís things like amp glow that depending on the camera you are using can increase over long subs so you wonít get that in shorter subs. Then thereís the dynamic range of your camera. If you have a small well depth, over the space of 5mins you will loose star colour because the pixel will just read white. Shorter subs can be better for star colour. As you can see there are a lot of contributing factors. I try to keep it fairly simple to start with but you could talk for hours about the effects of different sub lengths, itís all a balancing act and there is no straight forward answer.

Cheers Ryan
Quote:
Originally Posted by glend View Post
It depends. Each image that you take will contain some noise elements. In theory you will capture the same amount of signal data, and so cumulative signal will be equal. Some extremely low noise cameras are very good at building up images using short sub exposures, others not very good, so it depends on your camera choice and especially if the camera has effective cooling to minimise thermal noise. As an example, a very low noise camera, like the ASI1600 MM-C, works very well shooting lots of short sub exposures, and this is a valid way of over coming mount or guiding issues as well.
Just a comment on the post below, AMP glow is (at least for the 1600) easily processed out, and short sub shooting can lessen reliance on having to capture and process Darks. Cameras like the 1600 do not require Dark processing for sub exposures at or below 30 seconds, simply applying Bias frames will take care of that. As far as well depth is concerned, it can be somewhat irrelevant in short sub capture, as you are not exceeding available depth in many cases; but obviously sensitivity is important. At the risk of setting off a major debate here, let's just say it can be done if you pick the right camera.
Thanks Ryan / Glen for taking the time to answer. Much Appreciated.
You have answered my question.
Cheers
Tony
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