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Is 5 minutes exposures too long for small galaxies ?

Optolong L-Pro Acquisition Beginner Deep Sky Galaxy
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Eric Gagne avatar
Eric Gagne avatar
I am tring my first galaxy tonight, I only have a Samyang 135mm so it's quite small but it's gonna be fun anyway, an a good experience.   Here's a preview from the Asiair.  It's 300 seconds with an Optolong L-Pro.  The core looks quite bright to me, I wonder if it will be burned when I stack 4 or 5 hours of this.  Should I take shorter exposures ?  Like maybe 120 seconds or even less ?

AstroRBA avatar
AstroRBA avatar
Can you provide a bit more info? Camera type, gain, Sky SQM (Bortle), etc. Are you shooting at F2?

I would say 120 sec (or lower) at very low gain would be more than enough for that Focal Length but other factors need to be considered too..
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Francois Theriault avatar
Francois Theriault avatar
Eric,

I typically image with 60 seconds for LRGB and 5 minutes for Narrowband.
As you can tell, I image with a mono camera. I assume it would be similar for OSC.

I also image from a Bortle 7-8 location.
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Eric Gagne avatar
Eric Gagne avatar
I should have given more details.  It’s a cooled osc, shooting at gain 100, under bortle 6 sky at f2.8
Jonathan Thatcher avatar
Jonathan Thatcher avatar
I would go by the information displayed on the histogram. Ive had success keeping the data 1/4 to 1/3 in from the left of the graph.

J
Francois Theriault avatar
Francois Theriault avatar
Éric,

moi aussi, mais en mono.
Durant l'hiver, je ne met pas de refroidissement - la caméra ne peut pas atteindre -10°C quand il fait déjà -20°C dehors….

D'habitude je prends mes images à f/8. L'instrument le plus vite que j'ai est un Newtonian à f/4.

Mon gain est 139 pour ma caméra mono. Comme j'ai dit, 60 secondes pour LRGB et 5 minutes pour NB. C'a me donne des assez bons résultats. J'ai moins de bruit de fonds de cette façon.

Même après plusieurs années, j'en apprends encore!
V avatar
V avatar
Purely depends on the focal length- At 2032mm like I shoot on, I take 5-10 minute exposures- minimum. On lower focal length (and quicker systems) I take only 1-5m exposures at a maximum.
Eric Gagne avatar
Eric Gagne avatar
Running at 1 minute, I figured I should stick to that since there is a little wind, not much, but I couldn't get guiding to start this time, would not calibrate and I did not feel like losing time on this.
Rob7980 avatar
Rob7980 avatar
Probably be better off at 30 seconds or less even maybe at f/2.8 at 135mm, 60 seconds would be about my max with my Sammy 125 under Bortle 3. With a dual narrow band I’ll go upto 120 seconds.
Mijo avatar
Mijo avatar
If it's not saturated in a sub, it won't saturate in a stack, right?
Stacking averages out signal which stays in the same place in the star field. It doesn't intensify it.
Stacking cancels out noise that moves around randomly.
So if a galaxy core is 80% of full well in a sub, it's 80% in the stack.
That's how I understand it.
(I'm no mathematician. D'Oh)
You want the sum of sky background and bright objects below 90% of saturation.
Your 5min sub doesn't look blown out so maybe you could go to 8 min if you want more faint detail.
But I think stacking twice as many 4 minute subs is almost the same end result. Just more work.
It's the stretching after the stacking that makes faint objects bright.
if you stretch a sub (before cancelling out noise) you amplify the noise more than the signal.
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Francois Theriault avatar
Francois Theriault avatar
Éric,

look at your histogram as you take the picture.
If the peak of the histogram is well separated from the black point (left side), then it is enough to process.
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Brian Puhl avatar
Brian Puhl avatar
Simply look at the linear image.   

Are the core details blown?     No?  Then you're good.