Having been encouraged by imaging the ISS transitting the Sun I have been playing with Calsky looking for some interesting satellite transits of Solar System objects near to my home. Tonight at 5:36pm afforded an opportunity to attempt to see a transit of Saturn by the Hubble Space telescope just 6km away.
I made sure I got on the predicted centreline exactly, which was only 1km wide. I loaded the 12" Dobsonian into the car and got on site with 40min to spare. Setting up on top of a conveniently placed (and level) manhole cover in a park in Capalaba, dead on the centreline, I had plenty of time to collimate, align, test exposures, and chat to passersby who were very curious as to what the Dobsonian was. One young bloke stayed with me to see the HST pass over. He was also fascinated by the Moon.
With the time approaching I had everything ready. I had tried to balance the exposure and ISO to capture both Saturn and the HST whilst stopping any motion blur. Saturn was mag 0.9 and the HST was predicted to be mag 2.0 at transit. I knew, though, that the HST would appear brighter than Saturn so I exposed to that Saturn was as faint as possible to ensure I didn't blow out the highlights on the HST. the HST was going to be about 4.6 arcseconds in size. I ended up choosing ISO800 at 1/1250th second at prime focus (1500mm focal length at f/5) and a frame rate of 60fps.
I started recording the video 3 minutes before the time was due, before I could sight the HST. As the satellite approached it looked like it was going to completely miss the area of Saturn and I was tempted to adjust my aim. I held firm though and the HST passed below Saturn only by 143 arcseconds and it appeared on 4 frames on the video.
Attached is a single frame unprocessed (showing the relative brightnesses of the objects as they really appeared), another being a composite showing the 4 frames of the HST passing by, and another attachment comparing what was observed with what was predicted by Calsky.
Lessons learned from today:
-1/1250th sec exposure was too long to freeze the HST motion and also resulted in overexposure of the HST. 1/2000s would have worked better at that focal length and ISO.
-Calsky in this case was about 2 seconds out in time and 143 arcseconds out in angular displacement in its prediction. The angle difference translates into an error in the predicted centreline location being about 480m too far south. I will email my results to Calsky. This doesn't discourage me in using Calsky to keep trying for a planetary transit shot. I think there will always be necessarily chance involved with angles and time intervals so small. Lunar and Solar transit predictions won't need such level of accuracy and Calsky and will work fine in planning for those.
-It was a more enjoyable experience having more time to prepare and set up!
Thanks Jennifer, Dennis and Michael. I wasn't too disappointed that the HST missed Saturn and was glad that it at least came into my field of view! I get the feeling that to successfully capture a planetary transit it may involve coordinated observers situated either side of the centreline.
Well executed and reported. A commercial GPS unit should put you within 10m of the stated coordinates. I'm pretty impressed that CalSky can generate a ground track of a satellite and planet to just over 2 arc minutes, with all the variables involved including accurate solution of the geoid and projection. You are probably right about needing several people to try and capture the pass across the centreline as is done with occultation observations.
Thanks Rolf, John, Peter, Tony and Rob.
Tony I'm impressed by CalSky too. That is one of the reasons I chased this transit, to see how accurate the predictions are for this kind of thing.
Also, a heads up for this Monday morning for north of Brisbane: The ISS (and possibly the Atlantis if it launches on time) with be transitting (or eclipsing more like) Jupiter's disc somewhere near the centreline shown on the attached map. I am thinking of going for it near Morayfield. In any case, the ISS will be a fine sight from Brisbane.
I decided to try for the ISS transit of Jupiter his morning, going to a spot on Buchanan Rd just off the Bruce Highway, where CalSky placed the centreline. I set up the 12" Dobsonian, and the EOS 550D and on cue the ISS appeared, looking very bright.
This time the ISS/Atlantis didn't even enter the field of view which was a shame but it did look good to the eye! As soon as it passed I started chasing it in the finderscope and only managed one frame when it was fading off into the distance. I have attached a stacked Jupiter image from this morning, and the ISS, which wasn't very clear due poor seeing. I'll keep at it!
