Imaging the International Space Station

The technique for closeups of the International Space Station is to point the telescope at a point the ISS is predicted to pass through. I set the camera on high burst rate (6.5 fps is the fastest on my Canon 40D and my 70D has 7fps) JPEG (not RAW as the buffer fills up too quickly. For short sequences RAW can be fine but make sure to leave plenty of time for the buffer to be written to the CF card before removing the card!) at 1/1000 or 1`/2000 of a second exposure at ISO 1600 or 2000. As the ISS enters the field of view of the finder, then set the burst going and stop when it leaves. There are typically 3 shots with the ISS in the frame when overhead but I got 4 on one pass as I (coincidentally) had the camera oriented diagonally to its path. Quite a bit of the structure such as the various modules, support trusses and solar panels are clearly visible. To find out where it is going to pass through, the approach I use is to use Heavens Above (www.heavens-above.com/) or Calsky (calsky.org/) to find a star the ISS is going very close to and tracking on that (HA has stars to about mag 6, Calsky much fainter). With a GOTO scope I don’t need to use a star but get coordinates from Calsky and go to that coordinate and wait. Clearly focussing and seeing are now the limiting factors in how good an image one can get but if you can focus on the moon, a planet or a bright close double star (eg Castor) to get a sharp focus. A Bhatinov mask is another good way of focusing accurately. With a GOTO scope one problem is aligning it whilst still twilight (which is usually the conditions for an evening pass) even if the pass itself is in the dark. Usually there are not enough bright stars visible in the twilight to accurately align and then ensure the scope is properly polar aligned: there often is not enough time. If you have enough time to get a good alignment, then you can enter the coordinates extracted from Calsky.org to get very close to the passthru point. Without properly polar aligning, there can still be sufficient drift that I had to re-enter the coordinates in the scope a couple of minutes before the pass just so I could be sure the scope would still be pointed at the right area. The types of result one gets are shown below which is a set of exposures merged into a sequence in Photoshop. ISS Closeups A closeup is obtained by cropping the best image obtained as the ISS, even when overhead, results in an image of around 100 pixels size. I use Photoshop Elements to sharpen the image and adjust the highlights. The amount of detail is amazing given how far away it is and the usually bad seeing and atmospheric turbulence. Closeup of the International Space Station This works fine for a DSLR like mine with a 30’x20′ FOV (on my EdgeHD 925 scope). I would like to try with my 640×480 CCD camera but that has such a narrow FOV that I can’t be confident the ISS will actually pass through. I’ll try again when there’s a prediction of a very close pass to a bright enough star AND I have time to actually align very accurately. Some manage to take reasonably long videos using such cameras and then use software to align and stack the images to get a much sharper and detailed picture than a single shot can obtain. For the shots of ISS passes across the Sun. Never observe the sun without the use of approved solar protection filters.For these, aligning isn’t the problem but knowing when it’s coming through when you can’t see it. For these use a GPS for accurate timing and start the burst about 1s before the predicted transit and stop about a second after (the transits are usually over in less than a second, maybe 2 seconds when the sun is low in the sky). Using RAW is possible for a sequence this short (buffer on the 40D and 70D hold about 16 shots so about 2.5s worth at 6.5 fps) but make sure you let the buffer flush before taking the card out! 1/1000s or 1/1250s at ISO 1600 is about right for these shots (with a solar filter on the scope). International Space Station Transiting the Sun When the transit is not visible from my home so I can’t use my telescope I use my 100-400mm lens with a 2x extender, so 800mm focal length and a Thousand Oaks solar filter. The same technique is used: ISS Transit Across the Sun

For the Moon (statistically as frequent as Solar transits but in fact much more difficult given the phases) the same technique as for the Sun but without needing a filter. This sequence was taken at 1/1600 and ISO 3200. In rare cases the ISS may be visible but a bright ISS against a bright Moon will be harder to get the exposure right. This particular shot was published in the June 2015 Sky and Telescope.

Sequence of the ISS Transiting the Moon - Updated Sometimes when planning Sun or Moon transits, Calsky and Heavens-Above don’t always agree and this could mean that you get all set up and the predicted transit doesn’t occur at all or not when you expect it. The usual reason for this is that some orbital change has happened and one or both have not updated correctly. If they agree, then you should be fine. If they disagree then one could start the sequence at the earliest time on one and stop on the latest time on the other. However, if they are 20-40 seconds different that could mean the camera’s buffer gets full before the end plus nearly 100 shots to look through to find the few on which the ISS may be visible. More shots of differing type are visible here: https://www.flickr.com/photos/ejwwest/sets/72157623878835436/

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