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# Calculate exposure time and integration time for 2 scopes

4 replies to this topic

### #1 acrh2

acrh2

Ranger 4

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Posted 25 July 2021 - 05:40 PM

Hi.

So I have two scopes in the signature. One has - f/4.8, 72mm aperture, pixel scale 2.25 arcsec/px, the other has f/11, 127mm aperture, 0.56 arcsec/px,

I already took an image of the Crescent nebula with the first one.

https://live.staticf...8e8da6691_o.jpg

Now I'd like to zoom in with the second one, so please confirm my calculations.

So if I understand this correctly, to achieve the same brightness of the nebula in a sub, I would need to expose longer by a factor of 11/4.8 = 2.3.

And to get the same SNR, I would need 2.3^2 = 5.3 longer integration time.

Does that sound right?

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If so, here's the second problem. My guiding has an average RMS error of about 0.6". My first image subs were 4 min long. That is going to be a problem with the second scope. I would prefer shorter subs.

What if I bin 2x2?

ZWO website states "Something you will notice when you bin 2×2 is your camera resolution is quartered as you effectively have 4 times less pixels. This also doubles the signal-to-noise ratio."

https://astronomy-im...ndamentals.html

So then, in principle, I can cut down the subs to 2 min and achieve about the same brightness and SNR in a single sub. And the total integration time would not need be longer.

Does this sound right?

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So, overall I should be able to increase the resolution of the nebula by a factor of 2 with about the same integration time and SNR - the width of the nebula is about 400 pixels in the first image, and it would be about 800 pixels in the second image.

What do you think?

### #2 acrh2

acrh2

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Posted 26 July 2021 - 09:59 AM

Just a gentle bump. I'm a total newb when it comes to optics, so I'm almost sure I made some conceptual errors.

Help me out, guys.

### #3 Tapio

Tapio

Aurora

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Posted 26 July 2021 - 10:04 AM

Take same sub exposures but make total exposure time longer. X5 could be right.
I'm not sure if binning helps a lot, with cmos cameras.
Besides you can do it in processing phase if you want.

### #4 acrh2

acrh2

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Posted 26 July 2021 - 10:58 AM

I just read more from ZWO. They said that hardware binning is exactly the same as software binning for their CMOS cameras, and it should not be used in hardware unless framing or focusing, because it can be done later in software with identical results.

Hmm.

What would happen to SNR if I were to raise gain from 100 to 200?

I know that this would raise the brightness of a sub at the expense of full well and dynamic range, by about a factor of 3.

Does anyone know?

### #5 bobzeq25

bobzeq25

ISS

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Posted 26 July 2021 - 11:14 AM

This is complicated.  The F stop scale works a bit differently than that.  Results depend on target.  But, roughly.

F11 is a bit more than 2 F stops slower.  Subs should be increased by about 5X, to gather the same number of photons in each sub.  Ditto total imaging time.

Binning 2X2 trades resolution for signal to noise ratio.  There usually is such a trade whenever you change "X", nothing comes for free.   Seeing plays a big role.  It's all too easy to go for more resolution, trade snr for it, and wind up with a net loss because seeing wipes out any theoretical gains in resolution.

This happens all the time.  It's one reason why big scopes may not be better.

Generally I'm usually happy to trade resolution for snr, in my light polluted skies with generally mediocre seeing.

This is an excellent example of some things.

Resolution and snr usually involve tradeoffs between them.  One good example where people tend to ignore that is drizzling.

Skies and targets matter a lot in such trades.

Part of the fun of astrophotography (at least for me <smile> ) is just how complex it is.  You will never, ever, run out of new things to learn, new challenges to address.

Key bottom line.  For most things, numerical analyses are of little value (something else that's frequently ignored here).  Too many factors.  You're _far_ better off just trying things, and see what works for you.  Recommendation.

Try shooting 5X longer subs, and the same number of subs (so you're also increasing total imaging time by 5X).  Did that do what you wanted it to?  Many factors are involved, which means that it may or may not work.  Just part of the fun.  <smile>

Beware of advice that says X is clearly better than Y.  You often see that here, and it's often (not always) incorrect, because some site specific factors have not been included in the analysis.

I will give you one piece of advice.  Unless your mount can't deal with the longer exposures at gain 100, going to gain 200 is unlikely to improve your images.  What counts (always) is the total number of photons you gather, and raising the gain does not improve that.

This is why I got an F2 RASA.  It gathers photons faster.  There are some real costs, it's a fussy scope to live with.

Edited by bobzeq25, 26 July 2021 - 11:22 AM.

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