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Observation by Mike Greenhill-Hooper: ISS evening pass

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Mike Greenhill-Hooper


Mike Greenhill-Hooper


2021 May 27 - 21:54


2021 Aug 08 - 16:02



  • Telescope: 20" f/4 Obsession Dobsonian; 2x TeleVue barlow
  • ZWO ASI224MC camera with uv/IR cut filter

SharpCap gain 300, exposure 0.295ms


Miradoux, S.W. France

Target name



ISS evening pass

About this image

I am finding it very hard to get the best exposure time for imaging the ISS.  It is still too long in the video sequence used to obtain this stacked image.  The radiators are easily over-exposed but if the exposure time is reduced too much the solar panels are difficult to pick out as they seem to be much dimmer. Any advice on how to achieve this would be welcome.  Focusing needs to be very precise.  I am using a large dobsonian that I need to manually move during the 2-3 minute passage.  This requires me to do a couple of dress rehersals before hand to ensure that my laptop and camera cable can reach throughout the pass and that I don't have any trip hazards.  I estimate that my success rate is less than 50%.  I  use a Telrad finder to follow the ISS and typically only 50 frames out of as many as 15000 actually show ISS, most only part of it and even ones that show it all often suffer from some blurring.  A real challenge and quite stressful!     

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Alan Thomas
Alan Thomas, 2021 Aug 09 - 09:48 UTC

Well done! Perrsistence pays off.


Martin Lewis
Martin Lewis, 2021 Aug 09 - 11:36 UTC

Hi Mike,

I normally use ~0.7msec but with a lower gain (240) so that when you get an image there is less shot noise in it. I image at f14 whereas you are f8 so your image will be a lot brighter but your gain is 60db higher so that makes it 2x as bright. I would drop the gain a bit to maybe 240 as you really don't want to overexpose.

I also use an ASI174MM which is a very fast mono camera with global rather than rolling shutter- it allows 120fps at full frame which with the much larger chip size will mean you get far more successful frames. With a mono camera you can also add a red filter or a shorter wavelength IR filter to help steady the seeing.

I moved to a 9x50 Celestron RACI finder with illuminated double ruled cross-wires so you have a box at the centre of the finder to try and 'trap' the ISS inside. It sounds silly but it is much easier to do that rather than try and centre on cross-wires. Another tip is to use Firecapture and use the audible histogram switched on. When you have it on the frame you get an audible 'congratulations' which really incentivisess you.

For focussing you can't beat using a Bahtinov mask on Polaris but don't forget to remove it before moving to the ISS - been there!

Good luck,


Mike Greenhill-Hooper
Mike Greenhill-Hooper, 2021 Aug 10 - 17:22 UTC

Hi Alan,

thanks for your thumbs up!

Hi Martin,

thank you for your very helpful and detailed reply.  Your images of the ISS are superb, something for me to aim for.

I will reduce the gain and exposure time taking account of yours and our f ratio differences.  I have thought that if I had a bigger sensor and smaller pixels that might help capture more useful and more detailed images.  I have tried using a 3x barlow with my set up, but it's a real lottery to capture anything useful.  I like the idea of using a more suitable finder.  Maybe I can place this lower down on my very long tube so I can reach to view into it comfortably at right angles (my telrad is near the secondary mirror at the top of the tube and is ok for straight through viewing, although I have to do this on tip toes when the ISS is overhead).  Focusing on Polaris is a brilliant idea and obvious now you mention it.  With the ISS passes in twilight I don't have time to align and track with the goto capabilities on other stars before the event.

Thank you again!

Martin Lewis
Martin Lewis, 2021 Aug 11 - 12:11 UTC

Hi Mike,

Yes you have to be physically comfortable when chasing the ISS with hand guiding. Little improvements with this aspect can make a big difference to the number of useful frames. I always find passes at about 60-70deg the best as with overhead passes you are in 'Dobson's Hole' when the ISS is at its closest, and it is very hard to follow.

Good luck,


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