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Books+YouTube = Confusion (Exp times)

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#1 HoosH

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Posted 08 May 2021 - 11:40 PM

After a night with almost 2 hours on M106, my data looked terrible. Lacking a lot of detail and still with plenty of background noise. Included equipment below. I did 200 x 30" subs at unity gain. This was the recommended sub based on my equipment / sky (Bortle 7) using a spreadsheet I found here on the forums. So I started reading more and watching more tutorial videos. I tried SharpCap Smart Histo which wanted max gain and 90" subs. Seemed like too much. Read Jon Rista's posts about trying to get median background to an optimal level. Plugged my numbers into the formula and started taking test shots. To reach above minimum median value I need max gain and 180" subs. Does that sound right? I went ahead and let the plan run on APT with those numbers but it seems extreme (the gain that is). For reference with my camera, unity gain is 565 with a max of 2000.

 

I'm shooting OSC via Altair 269C cooled camera through Meade 8" f/10 ACF SCT. I am guiding with 0.1-0.2 pixel error and dithering every other shot. The recommended median ADU was 1100 by the formula with a minimum of 800. I'm aiming around 900-1k. I assume my scope's f/10 has something to do with it. I have a reducer on the way so f/6.3 should be easier to reach the ADU. Does this method sound right? Or should I go back to stacking a lot of <30 sec subs at unity gain? Feels like quality will suffer but I won't know until I have time to look at the data in a couple days.

 

Thanks in advance.



#2 ks__observer

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Posted 09 May 2021 - 05:02 AM

Sub exposure time will have minimal affect on quality.

If you have quality issues it is not subexposure time.

Look up "swamp read noise" to get a sense of exposure time.


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#3 robbieg147

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Posted 09 May 2021 - 05:09 AM

2 hours at F10 is only equal to roughly 30 minutes at F5, even at F5 you really need around 2 hours of data min so at F10 you need a lot longer total integration.

 

I agree with the above post sub exposure length does not make much difference, and if you are chucking subs due to tracking shorter subs could be better.


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#4 Midnight Dan

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Posted 09 May 2021 - 09:44 AM

Definitely not enough time at f/10.  Your focal reducer will help with that.

 

-Dan


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#5 HoosH

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Posted 09 May 2021 - 02:18 PM

Thanks all. Hopefully will have the reducer next week. So far it might be luck but I can guide up to 300s. I'm going to take gain back to unity and image there based on the histogram.

I did not think of f/10 in the sense of relative total integration time as you all point out. That's a great way to think of it.




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