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How much LP limit the exposure time?

astrophotography beginner dslr dso filters imaging LP
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#26 sbharrat

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Posted 04 December 2020 - 02:32 PM

I am in the same boat as you so to speak. Bortle 8 here in my backyard. Your results match mine, more or less.

 

With short exposure times, you will get enough photons to work with-- it's the read noise which could be an issue. I don't think its as big of a deal with current CMOS cameras as it used to be, FWIW. YMMV.

 

At f/4.9 (with reducer), on m31 I'm doing 20-30 second subs using an un-modded DSLR @800 ISO. This lands my histogram at around 1/3-1/2 peaked. (Edit: I'm not using any LP filter.)

 

I'm getting pretty decent results with this, considering.

 

FWIW, even 10 seconds on M42 with these settings blows out the core.

 

On a positive note, at 20 seconds, no guiding is necessary and my stars are perfectly round. lol.gif

This actually lines up with my plan! I ignored the tracking error part in my question but I am also unguided (Sirius EQ-G) and stars were just starting to trail at 30s. It sounds like you are getting good results at 20s so looking forward to getting a night of exposures at that. (Only had one night since I got the mount and those 2m exposures were all completely white!)

 

Appreciate the info...


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#27 sn2006gy

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Posted 04 December 2020 - 02:44 PM

Read Noise on modern sensors is pretty low/non existent and dithering seems to be a way to overcome any apparent cost of it. 1.5e read noise on a CMOS is nothing compared to all the other goofy signal you deal with.

 

Don't forget that integration is *additive* (average combination) it doesn't matter if you don't get that photon in every image - photons don't really work that way :)

 

Even if you image for 5 or 10 minutes - it doesn't mean you get that faint signal once every 5 or 10 minutes... Otherwise we'd have infinitely long subs without additive average combination.

 

Integration time is what matters the most. Whether 350 45 second subs or  for 4.4 hours or  18 15 minute subs. (as long as whatever you're imaging is correctly sampled and you have overcome the read noise of the sensor in your sub length)

 

In the end, the dithered & integrated 350 subs may be more SNR than just 18 15 minute subs even if the 15 minute subs look much better than the 45 second subs.  Mathematically the 18 15 minute subs may be 3-5% more "SNR" but dithering totally throws that mathematically curiosity out the door in Image Quality if you ask me.

 

obviously NB or filters change the sub exposure length(s) but the math still applies


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#28 PollAirUs

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Posted 04 December 2020 - 09:08 PM

Am I going to do 10 hours on the Pleiades (or M31 or M33) with, and then 10 hours without, just to test it out? That surely would be the best approach, but depending on which session came out ahead, the other 10 hours would be wasted (since they could have been 20 with the best scenario).

That would be a nice experiment, but I don't think you need to do 10 hours of each session to get the answer. As you said, depending on which session came out ahead, the time spent on the other one would be wasted.




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