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Old 07-02-2021, 12:03 PM
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Andrew Pearce
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Longer exposures versus stacking

Hi All

A question from a newbie! In a relatively light polluted location using a DSLR camera, am I better off stacking shorter exposures (say 2, 3 or 6 x 10s exposures) or using a tracking mount and taking the equivalent length exposure? I would be looking at no more than a total integration time of 1 minute. In terms of minimising sky background brightness and maximising faint star detection which would be the preferred method?

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Andrew
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Old 07-02-2021, 12:54 PM
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Rerouter (Ryan)
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If your after faint things, you need the exposure length, generally the longer the exposure the higher SNR for the same total integration time, (within the limits of what your gear is capable of)

For sky brightness (I am in an orange sky zone) you can play with ISO to move the dynamic range however even on 30s exposures, at 800 iso my sky background is only about 3%, so easy to clean out.

Without tracking, use this tool, https://www.lonelyspeck.com/advanced...me-calculator/ It actually corrects for how high in the sky your target is, however the higher your focal length, the shorter the exposure you can get away with, for tools like depp sky staker, you need to aim for 3 pixels or less, or it will fail detecting them because your stars are not round enough (also found out the hard way)
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Old 07-02-2021, 01:20 PM
glend (Glen)
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Last edited by glend; 07-02-2021 at 02:31 PM.
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Old 07-02-2021, 01:48 PM
Zuts
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One of the staff at Bintel suggested that they are not the same. With very short exposures if the signal is not there (equipment & setup) then it doesn't matter how many you stack. If you want to get faint detail his suggestion was to go as long as possible.

It's not relevant to this post but for dso's i aim for between 5 and 10 minute subs. That's with a cooled asi2600 mc, a short focal length and guiding.

Attached is a 360 sec fit frame directly off the camera. It has not been debayered. The quality is extremely low as to get a jpeg i used the screen snipping tool and the resulting jpeg is only 47kb. I am not convinced you would get anywhere near the same detail if you stacked 100 3 second exposures?

If this was true and I could live with the stacking time i would do it every time, stack a few thousand 3 second images and avoid guiding errors, seeing and get wonderfully round stars.

YMMV this is only my opinion
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Last edited by Zuts; 07-02-2021 at 05:18 PM.
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Old 07-02-2021, 02:30 PM
glend (Glen)
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Quote:
Originally Posted by Zuts View Post
One of the staff at Bintel suggested that they are not the same. With very short exposures if the signal is not there (equipment & setup) then it doesn't matter how many you stack. If you want to get faint detail his suggestion was to go as long as possible.

It's not relevant to this post but for dso's i aim for between 5 and 10 minute subs. That's with a cooled asi2600 mc, a short focal length and guiding.

Attached is a 360 sec fit frame directly off the camera. It has not been debayered. The quality is extremely low as to get a jpeg i used the screen snipping tool and the resulting jpeg is only 47kb. I am not convinced you would get anywhere near the same detail if you stacked 100 3 second exposures?

If this was true and I could live with the stacking time i would do it every time, stack a few thousand 3 second images and avoid guiding errors, seeing and get wonderfully round stars.

YMMV this is only my opinion
This is exactly why I try to not get involved in these sort of discussions. Please delete your quote of my now deleted post. I want nothing to do with this.
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Old 07-02-2021, 02:58 PM
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Using a DLSR in the 'burbs can give good, but not great results.
This was taken with a Canon EOS Ra

Living in Sydney's south, I found the sky glow would really flood any faint stuff after a minute. The lack of well depth, lack of cooling and lower QE compared say mono cameras all conspire to limit what you can do.

Many problems go away if you use a cooled, low read noise, high QE camera...particularly with narrow band. See here for example.

But to answer your DSLR question. 1-2 minute subs tops. Stacking gives limited signal depth improvement after 30 or so exposures.

Under a dark sky the faint stuff is very easy to capture. 5 minute subs would be totally reasonable.

Last edited by Peter Ward; 07-02-2021 at 03:14 PM.
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