Comet Neowise. My first attempt at astrophotography using a stacking technique.

Comet C/2020 F3 ‘Neowise’ was discovered on March 2020 by Nasa’s WISE telescope and has been visible with the naked eye since June. It is visible when looking northwest-north hours after sunset, close to the Big Dipper, and north-northeast in the early hours of the morning before sunrise. If you haven’t seen it yet you can still try with binoculars although its magnitude is becoming dimmer as it travels away from both the earth and the sun. On July 23rd it was at its nearest point to the earth, and the light from the crescent moon is already spoiling the view this week. It won’t come back for about 6800 years. 

Although I’ve taken lots of pictures of the moon and planets before I’ve never tried the photography techniques and software involved in what is called ´astrophotography´. So after reading a lot about astrophotography stacking methods in the last month I have given it a try with Neowise comet. From my garden I took a series of pictures the comet, 110 photos exactly, and stacked them using the darks/flats/bias/dark flats/light darks technique using Deep Sky Stacker software. 

If you don’t know what lights/darks/bias/flats/dark flats mean don’t worry, I didn’t know either two weeks ago. Just think it’s a way to calibrate your optical system (camera + lens) so you can reduce the different types of noise in your final image. For the stacking technique I used Deep Sky Stacker (DSS), a specific stacking software. You can use Photoshop or Affinity Photo but software like DSS give you more astrophotography focused options.

Anyway, long story short, the stacking technique is meant to increase the detail and reduce the noise of your single images by stacking them using average/median/kappa sigma clipping and other nerdy digital image processing algorithms. Taking lots of pictures is the way to accumulate the sparse photons being captured by your camera sensor in every single photo and thus improving the overall quality of the final stacked image.  

I don’t have a star tracker, so due to the focal length I was using even exposures slightly longer than 1 second would show stars trails. The specifics were:

  • Date: July 17th, 1h40m after sunset, comet altitude 14º
  • Location: 200m above sea level, Bortle class 4 with clear sky
  • Images: 110 lights / 20 darks / 20 bias / 20 flats / 20 dark flats
  • Camera settings: 1s, 2000 ISO, F2.8 200mm with a Nikon D800E
  • No tracking, tripod Manfrotto 055XPROB

I remember studying some of these techniques during my engineering degree and how boring they seemed to me. Oh, had I known you can apply them to astrophotography! Obviously, there is maths under the hood of all of this (how could it not be as maths is everywhere!) and I love it. All these techniques really improve the SNR (signal to noise ratio) of the images.

The photo is not great, you should see what experienced astrophotographers can achieve! But I’m happy with the outcome as it’s my first attempt at astrophotography.

Apart from the huge characteristic dust tail of the comet that leaves a trail in its orbit (and that’s why it’s curved) it’s also possible to appreciate a thinner tail. This tail is called the ion tail and is a stream made out of gas and always points in the opposite direction to the Sun. One of the great things of the stacking technique is that it shows details that would be extremely difficult to get in just one image.

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