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Calsky and Heavens Above accuracy


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Last night I planned to photograph the ISS transiting the near full moon from my back garden through my telescope - a very rare event and where I could use my telescope (2395mm focal length), properly focus, track the moon etc etc. The conditions were predicted to be clear. For events like this that aren't visible from my home, I use the 100-400mm zoom and 2x extender (800mm focal length) which is very hard to focus properly and on a tripod doesn't track the moon so have to continually refocus after realigning the shot.

The event was predicted on Calsky until late yesterday afternoon at 20:15:06 and for good measure I checked Heaven Above. That predicted the ISS transiting at 20:14:48. Which to believe? When I got home prior to setting up, Calsky had changed its predictions to agree with Heaven Above, so I took that as the best prediction. I set everything up. Sharp focus, tested the exposures and was tracking the moon. At the predicted time I took a burst of about 25 shots (7fps on the Canon 70D). But when I inspected them, nothing of the ISS was visible. Later that evening I checked   Heavens Above again and it was showing a path that agreed with the earlier prediction of Calsky! Had I checked before the event I may have then decided to take two bursts at the different times, 20 seconds apart. Now I'll have to wait probably several years before such an event occurs again.

I can't really complain about Calsky and Heavens Above as these are sites run by volunteers and funded by their own efforts and donations. However, I would be interested in the experiences of others on the reliability of either or both as this is the third event in the past year or so where the timings were out and I missed an event (2 transits of the moon and one of the sun). Calsky provides much better timing information and coordinates than Heveans Above but that's only useful if they can be relied on. I suspect that both update the orbital elements of the ISS at different times and that leads to inconsistencies and, in cases like this, unreliability.

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You might consider where Calsky/Heaven gets their data. Here's a hint:

http://spotthestation.nasa.gov/sightings/

Now tell me - if they can't find their (partially) own space-shots - who does know? :dontknow::eek:

Clear Skies - might not help...

Dave

I assumed that was where they got it, but my question really concerns how often each site refreshes the parameters and why, as one gets closer to the actual time, they diverged rather than converged. Which is the most trustworthy?

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Neither are really expected to be 100% accurate, they are predictions.

Also times tend to be when at a given height (amgle) above the horizon. Do they both quote the same angle?

In my experience H-E generally supplies a time that is a bit earlier then the actual event.

Since the ISS is pretty bright would not the easy option to have been to watch it approach the moon, then as it got close take the multiple shots? Especially as 25 frames at 7fps is only about 3.5 seconds, so just about any error in their timings would have meant it being missed.

Calsky may have be fractionally different as it tends to take the location from the IP address, my IP address puts me about 20-25 miles East of where I actually am. For the ISS no real difference but for Iridium flares it's predictions are often irrelevant owing to the distance. Will say  I have always had difficulty getting my Lat/Long into Calsky and getting it to use that instead.

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Did you actually see the ISS ? it's looks a bit like Jupiter sailing serenely across the sky, taking a couple of minutes to pass over, longer in the summer as the Sun is higher, in the winter it can disappear suddenly  when eclipsed by the Earth.

Also check the time is in BST, I think Heavens Above is, not sure about CalSky, as you could be an hour out.

It tends to come round every hour and a half or so so if you miss one orbit you may get the next.

Dave

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Neither are really expected to be 100% accurate, they are predictions.

Also times tend to be when at a given height (amgle) above the horizon. Do they both quote the same angle?

In my experience H-E generally supplies a time that is a bit earlier then the actual event.

Since the ISS is pretty bright would not the easy option to have been to watch it approach the moon, then as it got close take the multiple shots? Especially as 25 frames at 7fps is only about 3.5 seconds, so just about any error in their timings would have meant it being missed.

Calsky may have be fractionally different as it tends to take the location from the IP address, my IP address puts me about 20-25 miles East of where I actually am. For the ISS no real difference but for Iridium flares it's predictions are often irrelevant owing to the distance. Will say  I have always had difficulty getting my Lat/Long into Calsky and getting it to use that instead.

 The ISS was in shadow as it approached the moon, so I was reliant on the timings (as one usually is for Lunar and always for Solar transits). In both Calsky and H-A I have the precise latitude and longitude coded in (and the same in each).

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Did you actually see the ISS ? it's looks a bit like Jupiter sailing serenely across the sky, taking a couple of minutes to pass over, longer in the summer as the Sun is higher, in the winter it can disappear suddenly  when eclipsed by the Earth.

Also check the time is in BST, I think Heavens Above is, not sure about CalSky, as you could be an hour out.

It tends to come round every hour and a half or so so if you miss one orbit you may get the next.

Dave

Really. Thanks for that. I must have been incredibly lucky to get these https://www.flickr.com/photos/ejwwest/sets/72157623878835436/ 

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