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Old 16-12-2021, 05:41 PM
Stephane
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Longest sub exposure for DSLR

Hi all,

With my Nikon D7000, I have gone from 30Ē subs to 2í to 3í with results getting considerably better the longer the individual subs. Tonight I plan to go 5í. Too long for a DSLR?? What is the maximum exposure for a sub before running into problems? (Is the main problem bloating of stars?)

Also, should I be decreasing the ISO if I increase sub length?

Thanks for all you help,
Stťphane
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Old 16-12-2021, 05:44 PM
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Wilsil (Wilco)
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The main issue will be the thermal noise you will be generating with the longer exposures.
You might gain more signal, but you introduce thermal noise.
I think you have to play with it and find the right balance.

That's where the dedicated camera's are coming in with their cooling.
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Old 16-12-2021, 06:55 PM
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rustigsmed (Russell)
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well i tried a 30min sub (with a Ha filter) back in 2015 - it wasn't pretty
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Old 16-12-2021, 07:56 PM
Startrek (Martin)
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Stephane,
Sub lengths for DSLRís are limited by internal thermal noise from the sensor electronics , Sky conditions ( seeing and transparency) Sky glow or LP levels , humidity, evening ambient temperature, mounts tracking performance , guiding performance , various filters etc.........

A better question would be whatís the most appropriate sub length for a particular object under my Sky conditions , not whatís the longest sub I can push with the DSLR

Not one shoe fits all and thereís no magical number but commonsense tells us that pushing 5 min to 10 min subs or longer with a DSLR is a fruitless exercise.

You have to experiment with your Nikon and it will take a few months to get an understanding of it, whether your imaging in LRGB broadband or narrowband

The DSLR gurus advise that the 7000D at ISO 100 and ISO 200 ( probably leaning towards ISO 200 ) provides the best dynamic range at 13.6 Bit for the lowest read noise

Example
To me a bright object like M42 at magnitude 4.0 could successfully be imaged with good detail with 90 sec or 2 minute subs max with your DSLR provided you capture quite a few hours.
In saying that you need to mitigate your noise using calibration frames like darks , dither your subs , use a good stacking software and use processing techniques to reduce your noise

Canon 600D ( unmodded )
Over the 3 years I used my Canon 600D my sub lengths varied depending on where I imaged from , but never above 5 mins
In Sydney Bortle 8 my subs ranged from 60 sec to 90 sec depending on object
At my dark site south coast NSW sub lengths ranged from 60 sec to 5 minutes depending on object

So to answer your question I would experiment on a bright object like M42 with short subs say 90 sec and then long subs at 5 mins ( at least 3 hours of integration time for both sub lengths) Iím not a betting man but I reckon the image with 90 sec subs will win every time using a DSLR. Long subs donít necessarily provide more detail in an image using a DSLR , the noise floor will be very very high and the signal compromised ( SNR )

Hope the above is helpful

Cheers
Martin
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Old 16-12-2021, 08:16 PM
Stephane
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Thanks all for your replies.

Martin, thanks for sharing your DSLR experience. My camera is already quite noisy, so I might not push to 5í. Iíll stick to a max of 3í for fainter targets and less for brighter ones. I think though that experimentation will be key as you say. I might reduce ISO also in coming weeks.

Thanks again,
Stťphane
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Old 17-12-2021, 06:55 AM
RyanJones
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One thing not covered here is well depth too. DSLRs tend to have a lower well depth than dedicated astrophotography cameras ( unless itís a high end DSLR or a low end astro camera ). This means that after a certain amount of time the pixels picking up bright objects, eg. stars, will be ď full ď. This means they appear to be pure white and contain no colour definition. This is called white clipping. You can obviously extend this time by reducing your ISO but then as has been stated by the others, your thermal noise will take over your signal. Itís all a balancing act but I think one that makes this hobby so interesting. We all have our different ways of tackling the challenges.

Cheers

Ryan
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Old 17-12-2021, 01:22 PM
Stephane
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Quote:
Originally Posted by RyanJones View Post
One thing not covered here is well depth too. DSLRs tend to have a lower well depth than dedicated astrophotography cameras ( unless itís a high end DSLR or a low end astro camera ). This means that after a certain amount of time the pixels picking up bright objects, eg. stars, will be ď full ď. This means they appear to be pure white and contain no colour definition. This is called white clipping. You can obviously extend this time by reducing your ISO but then as has been stated by the others, your thermal noise will take over your signal. Itís all a balancing act but I think one that makes this hobby so interesting. We all have our different ways of tackling the challenges.

Cheers

Ryan
Thanks Ryan. If I want to progress in this hobby, I realize I will eventually need a dedicated astro camera and probably will end up buying one! Iíll stick to the DSLR for now though and gain some experience first.
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Old 17-12-2021, 02:58 PM
AdamJL
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I did 4 hours once in 2009.
Canon 40D
Image turned out well.
A few months later, the 40D died. Related? Undoubtedly
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Old 17-12-2021, 07:11 PM
RyanJones
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Quote:
Originally Posted by Stephane View Post
Thanks Ryan. If I want to progress in this hobby, I realize I will eventually need a dedicated astro camera and probably will end up buying one! Iíll stick to the DSLR for now though and gain some experience first.
Absolutely the advice I would give. My statement certainly wasnít to say that DSLR imaging is no good and in fact I think itís a great way to cut your teeth and develop a good understanding of what youíre doing. Finding the limitations of DSLR imaging will help you learn a lot about noise ( thermal and read ). It will help you learn a lot about calibration frames and how much impact they make. Dedicated cameras can have such low noise that you may skip over doing darks for example thinking theyíre not necessary. Making them a part of your work flow now when you need them will give you good imaging processes down the track when you do make the leap. What you are doing right now by asking questions and developing a proper understanding of the nuances of astrophotography as you go is fantastic so keep it up 👍
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