From time to time a check for potential ISS transits. Of course I do not do this by myself, but using internet based tools. A pass of ISS in front of the moon was announced, visible from my place. That does not happen too often, so I wanted to take the chance, if weatrher would allow. The prediction showed, that ISS would just pass the lower edge of the moon, and due to the latest docking manuver of the dragon, ISS’s trajectoy might have changed in the meantime.
Anyway, it was announced for the 30th of April 2021, 4:16 CEST. So I checked again the prediction a day before.
Preperation for the transit
transit prediction
by iss-tracker.com
weather prediction
by clearoutside
Indeed, the trajectory changed, but would still overlap with moon. The weather forecast for the early morning did not look too promising, but 41% of cloud coverage is better than 100%. So I decided to try it.
Therefore, I needed to find a place, where the show would be visible. Using Stellarium, I found that moon would be at about 180° in azimut, and only 15° in elevation. Last one was a major problem, since the surrounding houses and trees would make it impossible from my garden. With an old-fashioned sextant, I figured out a position that would allow to watch the moon at 4:16 from the balcony. Not so nice actually, but possible. So I assembled the telescope, aligned it roughly to north (normally short time observations do not require a high accuracy) and slewed it to the estimated angle (not all telescopes allow all attitudes, so better try before!!).
Telescope in final position
a last rehearsal
Since I want to photograph the event, I also wanted to prepare the camera in advance. Typically focussing is the most time consuming thing, that should be avoided short prior to the transit. I took my well proven Altair gpcamar0130c planetary cam. But for the first time under Linux!
It is also wise, to set the alarm not too tight before the event. Time to dress warm, get a coffee, grap the laptop and connect.. all this has to be considered. There is just one try, and it would be over within just a second.
A few seconds prior to transit
Camera running, 20 frames per second, exposure 3ms, moon had a nice colour and was in focus. Now waiting. In parallel, I simulated the situation in Stellarium, to find the right moment to “pull the trigger”… conservatively a few seconds earlier, just to be on the safe side…
A tiny, black dot flashed through the live view of the camera at the expected time. That was really quick! Stop recording, open the file and have a look.
SLOMO forward, frame by frame… YES!!! the whole transit was captured. ISS and moon are more or less in focus, I was more than happy!
ISS Transit in realtime
A first view…
The full composite image