Why Didn’t My SNR Improve Much from 3h to 5.5h on M81/M82?

Processing Acquisition Deep Sky
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Dante Hunter avatar
Dante Hunter avatar

Hi AstroBin community,

I have a question about adding more integration time to images.

Two nights ago, I imaged M81 and M82 and collected 3 hours of data. Last night, I had another (rare) stretch of clear skies and added an additional 2.5 hours, bringing my total to 5.5 hours.

Based on the square root SNR relationship, I was expecting a noticeable improvement (roughly ~25% increase in SNR). However, when comparing my 3-hour stack to the 5.5-hour stack, the difference wasn’t as significant as I anticipated.

Does the square root SNR rule still hold under light-polluted skies, or are there other limiting factors—such as sky brightness, gradients, or processing—that could explain the smaller-than-expected improvement?

I’ve attached my raw stacked images for comparison.

Thanks in advance for any insights!

3 hours: https://drive.google.com/file/d/1qnl-3TKeupuk92e1oBaP_XL0NL4SLmME/view?usp=sharing

5.5 hours: https://drive.google.com/file/d/1nzcYn-6ww6fyd_k4R_c8DS17EaDLOfGy/view?usp=sharing

📷 M81 and M82M81 and M82

https://app.astrobin.com/i/mk4s4c/

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Tony Gondola avatar
Tony Gondola avatar

It’s hard to tell as one of the images I downloaded is in .fits and the other is in .tif. Also seems that there’s a lot of frame movement in the stacking. What I’d like to see is the raw stacked result in fits for both, no processing.

As an aside, since you’re under B7 skies I think 5 min. sub-exposures are too long. When shooting in heavy LP you really want your subs to be just long enough to swamp the read noise. Anymore just makes the LP and associated gradients a bigger issue to have to deal with. Under B8, I rarely go longer then 60 sec. even for narrowband.

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Dante Hunter avatar
Dante Hunter avatar

Tony Gondola · Apr 9, 2026, 06:28 PM

When shooting in heavy LP you really want your subs to be just long enough to swamp the read noise. Anymore just makes the LP and associated gradients a bigger issue to have to deal with. Under B8, I rarely go longer then 60 sec. even for narrowband.

I’m sorry about the filetype difference. I’ve changed it so both of them will be in TIFF. I’ve never heard about this. Is there an accurate way to calculate/see the optimal exposure time to ensure you’re swamping the read noise?

Armin Lukas avatar
Armin Lukas avatar

My suggestions:

  • Shoot at gain 100-150

  • 60-180 seconds exposure time

  • Stack only with pixnsight

  • Dither every or every second frame (at least)

With higher light pollution shorter sub exposure times are needed. With very little read noise long exposure times are not needed at all. So try something like 60-180 seconds and make sure you do not clip the left side of the histogram.

I see your stars are bloated and the core of the galaxy is blown out, that’s because of the long exposure time and or processing.

From my experience stacking with pixinsight gives better quality of the stack.

Oh and yes, the light pollution (and exposure time) is the reason for lower SNR increase.

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Tony Gondola avatar
Tony Gondola avatar

Dante Hunter · Apr 9, 2026, 06:38 PM

Tony Gondola · Apr 9, 2026, 06:28 PM

When shooting in heavy LP you really want your subs to be just long enough to swamp the read noise. Anymore just makes the LP and associated gradients a bigger issue to have to deal with. Under B8, I rarely go longer then 60 sec. even for narrowband.

I’m sorry about the filetype difference. I’ve changed it so both of them will be in TIFF. I’ve never heard about this. Is there an accurate way to calculate/see the optimal exposure time to ensure you’re swamping the read noise?

Here’s a talk by Dr Robin Glover (the creator of SharpCap) that goes into all that.

https://www.youtube.com/watch?v=3RH93UvP358

Andrew avatar
Andrew avatar

to double your signal to noise ratio you need to quadruple your integration

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