The Washingtonia Occultation

Apr 13, 2014

This Sunday evening event was in evening twilight. Walt Morgan emailed me around 6:30pm and said he was going to try it, and I had it on my list as well, since the path had decent rank and went right over Santa Cruz. But the star was magnitude 11.8, and the sun was only 10 degrees down, so if I'd paused I'd have realized that there was just no way I was going to get on target with my current scattered video gear not yet tested, and I'd have planned from the start to try for a star-trail timing. Instead, I spent valuable time setting up the 10" at Cabrillo and then when it was only 28 minutes till event time, noting how very bright the twilight still was... I gave up on trying to do it by video or even trying to find it visually. I went in to the 12" scope and almost gave up altogether... but decided I was already here and what the hell. I hurriedly popped open the dome, powered up Spock, the TV monitor, the dome motors, took off the optical caps, opened the software (CCDOPS, Gemini.Net, C2A, and NIST Time Widget in the proper order, got it all talking to each other, swung the scope over the Beta Tauri, calibrated the scope position... and noted/remembered the desiccant badly needed baking and I had not yet installed the new "o"-rings I'd just bought which would make the baking worth doing. So, ice on the chip window - bad, but not necessarily fatal. About this time, Chris Angelos showed up. I was SURE that it was the police driving up, because Facilities (?) had apparently been in the observatory and had shut the cover on the motion detector code-in unit, and after I got in, it took me more than 30 seconds to figure how how to open the code-in panel and dis-arm it. The alarm went off. Well, the police never did show up. Did they note that I keyed in the proper code eventually after triggering it? I'll find out at some point.

So, Chris watched over my shoulder and we both tried to ID the star patterns. The target was about a degree from the open cluster M36, in a very busy star field of the Milky Way. The severe icing and bright twilight and rapidly approaching pressure of time made it difficult. I got to the point that I thought the target was probably on the frame, and at 8:24:42 on my watch, I started a 25 second exposure... except that CCDOPS complained I was still on low-res mode and so I had to agree to hi-res and hit "start" again. That happened at 8:24:49.1 on my watch... except I realized after some seconds that the tracking was still on! Doh! I immediately turned off tracking, and got the last half of the 25 second interval. That's a lot of bogies to throw at this attempted science!

Let's list 'em:
-- strong twilight,
-- bad icing on the chip window,
-- faint 11.8 mag star in dense star field where
-- over-lapping of trails was a real danger,
-- not at all sure I've got the star field as I begin the exposure
-- CCD mode still in low-res, so start is not when I planned, but later
-- forgot in the rush to turn off tracking until mid-exposure

Fortunately, when I checked the calibration of my watch, it was ahead of real time by 2.65 seconds, giving me that bit of extra time to pin some hope on. The image that resulted was pretty ratty. But, after checking more carefully post-event, I and Chris confirmed the target star was indeed on the chip, and placed fairly well at that. And it does seem to have a gap at the right time.

Image start: 8:24:49.1 pm by my watch = 3:24:46.45 UT with the 2.65 second time offset (=my watch - UTC calibrated 20 minutes after the event)
Image end therefore = 3:25:11.45 UTC

Because I clicked off tracking at an unknown time, I need to get the velocity of the tracking from a separate exposure, which I did and show below. Then I can subtract from the relatively known time of the end of the exposure. Event Star trail, and untracked 25 second image afterward, for drift rate calculation.

A rough ruler-on-laptop screen estimate is 1.75" = 25 seconds, putting the center of the ~3.5 second occultation "gap" at 3:25:04.3, in good agreement with the predicted center of the event at 3:25:02.2 and predicted duration of 3.2 seconds. I'll annotate and post the photos when I get photoshop loaded on this new computer at some point soon.

The target star is the blue star just above center and just left of center, and has a reddish star above it whose star trail terminates just barely to the left of the bluish target star. The "gap" is roughly co-incident with a streak of less ice on the chip window.