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Stephane,
Sub lengths for DSLR’s 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 what’s the most appropriate sub length for a particular object under my Sky conditions , not what’s the longest sub I can push with the DSLR
Not one shoe fits all and there’s 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) I’m not a betting man but I reckon the image with 90 sec subs will win every time using a DSLR. Long subs don’t 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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