This is a fragment of my recent photo of the pinwheel galaxy (M 101):

M 101 picture with strong red noise

As you can see in this enlarged (200%) detail, the image is full of red noise, which makes all red objects (M 101 has several emission nebulae) literally drown in the noise: Detail


  • Canon EOS 70D (astro-modified)
  • Canon EF 100-400 mm f/4.5-5.6 L IS
  • 400 mm
  • ISO 2000
  • f/5.6
  • 22 frames @ 2.5" each
  • 5 dark frames subtracted

During the observation, the sky was clear, the object was near the zenith, and temperature was around +10 °C (+ 50 °F).

So, what should I do in the future to make my astrophotography less noisy? Is this maybe thermal noise? Thank you.

UPDATE: After fiddling around with the RGB channels (and virtually forgetting about the red channel), I could at least finish the picture, so that it looks like this: processed image

  • 3
    \$\begingroup\$ You say 22 frames with 5 dark frames subtracted. Do you shoot JPEG or RAW ? Do you use long exposure noise reduction (which would do dark frame correction automatically) ? Do you use any particular application for processing ? Have you tried any astronomy specific software to do noise reduction processing ? \$\endgroup\$ Commented May 2, 2020 at 5:30
  • \$\begingroup\$ RAW without LENR. I used darktable to process the RAW images, exported them as TIFF and stacked them with Gimp. No specific software has been used. \$\endgroup\$
    – Neppomuk
    Commented May 2, 2020 at 20:14

2 Answers 2


I believe ISO 2000 is a bit high. I'd experiment with lower ISO, like 800. This is a good article explaining ISO in astrophoto.

Also, 2.5 secs per picture is a bit low for an object like that. That's why you only get the bright core, but no arms.

Can you expose more time, like 15 secs or more? Taking more frames will help you to increase signal-to-noise ratio, but I believe 2.5 secs is too little signal to start with.

Finally, if you want to use free/open source sofware for astro stacking, try Siril.

  • \$\begingroup\$ The issue was that I still have slight issues with the iOptron (see my previous post), and most of my 4" pictures showed trails and other problems. \$\endgroup\$
    – Neppomuk
    Commented May 4, 2020 at 19:12

Some links for you :

  • Software : Try Deep Sky Stacker. It's freeware and designed for astrography. The site has some useful tips on technique and method as well, so read those even if you don't use the software.

  • General purpose software :

    • I do use Darktable for general photography processing, but it's not designed for astrography. For very tough RAW files I'm trying to squeeze the most out of I sometimes use RawTherapee which is slightly more specialized and has more control for demosaicing algorithm choices, but Darktable is now very good. Experiment with the Noise Reduction options on Darktable as you can do channel and range specific adjustments with masks that will make this easier.
    • I use GIMP. If you're not already using the G'Mic plugin then get it. There are a lot of great processing options in there, including Fourier transform and a lot of general purpose noise reduction stuff. GIMP is fine for general image processing and finalizing images, but I would not use until you've done the "heavy lifting" in something like Deep Sky Stacker. I would definitely not use GIMP for stacking astrography images.
    • There's an older software package called Iris which is possibly worth checking out for astrophotography. It might be a useful complement to Deep Sky Stacker.

Regarding general approach you need to do some experimentation to get the best from your particular mix of technique and equipment and software. Everyone has different kit and expectations and so they need different approaches.

I'm not sure you need dark frame subtraction per se on a modern camera like the 70D if you're also doing stacking and follow the very detailed suggestions in Deep Sky Stacker. Modern sensors have a lot of noise suppression for this kind of noise going on at a chip level and I'm not sure dark frame subtraction is quite as effective as it once was "in ye old days". In any case, have a good read of the material on Deep Sky Stacker's site and form your own view.

There's a technique which is sometimes useful nowadays which means you shoot at low ISO (whatever is optimal for dynamic range for your camera) and raise ISO in post processing. This is sometimes called ISOless shooting. The idea with modern sensors is that some of them produce less noise doing this than raising ISO in-camera and you preserve a larger dynamic range. This does not work on all cameras and can be done by the camera "under the hood" anyway. I would not worry about this unless you have run out of other options.

  • \$\begingroup\$ Yes, I've already got G'MIC. I use it to calculate a median of several (3 to 4) independent stacks, but this, time red (thermal?) noise was simply too high. Darktable does not incorporate an astrophoto workflow, unfortunately. A feature request has been on the wishlist for a couple of years, but nobody has implemented it so far. \$\endgroup\$
    – Neppomuk
    Commented May 2, 2020 at 21:27

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