The Occultation by Quaoar

July 7, 2018

This will no doubt be a big event for 2018, as Quaoar is one of the largest KBO's and we don't know much about it. RECON is planning to be out in force. I'll be just returned from the June Lake Triathlon in the Sierra that same morning. My plan is to observe from the corner of Bonny Doon Rd and Pine Flat Rd. If the fog layer comes in and is too deep, I will have to reconsider but I won't have much time to drive far. I might be able to get to Castle Rock, a thousand feet higher, if I hustle.

So for planning, I am wanting to know if I can even GET this event. The target star is magnitude 14.6. On the night of the Leukothea occultation Dec 17, 2017 I got data from Cabrillo Observatory under near perfect conditions - no moon, clean skies, dry. Probably much like a good night in the Sierra where we'll be. I put the Watec 910hx on maximum integration, which is 64x or 32 frames or 1 second integration. The maximum event length is predicted to be about 39 seconds, so this should be good.

Preston prediction page but notice as of June 29 it still has only the April prediction, w/o latest astrometry from New Horizons spacecraft.
Much better is the RECON webpage which does have the latest update, and is the one to use.

Latest astrometry shows this predicted path. From RECON web page


New Horizons astrometry helped get the rank up to a good level, as shown on the RECON page at least. 30% moon 129 degrees away and below the horizon for west coast.. Note the 1-sigma error in time is fully 156s or 2.5 minutes! Central occultation predicted on west coast for 5:34:30 UT or 10:34:30pm PDT, lasting about 35-40sec depending on albedo.

Test Frames

This field is at 23 degrees altitude in the south. I did a simple screen capture of the LiMovie view of this frame, and then stretched it in Photoshop. However, each of the stars labelled is seen on the original in LiMovie.

This field, same night, was at 62 degrees altitude. Dew eventually settled on front. Not sure if that affects here. Also, the stars are clearly not focused, limiting mag should be better if focused.


Finder Charts. For Celestron 8SE and for Dobsonians

Finder chart for Dobsonians. The circle is 1.2 degrees across. Note that the target star is not shown except as a small cross partially obscured by the very nearby V=10.6 star at center.

The circle is the field of view of the Q70 32mm eyepiece in the 2" diagonal at back of the Celestron 8SE . Same circle as at left, but note the field is left-right reversed since there are 3 (odd number) of mirrors in the system.

The square here matches the size and view of the fold-out LCD screen for the Canon ZR45mc camcorder, connected to the Celestron 8SE scope with f/3.3 reducer, or the f/6.3 reducer + Highpoint 0.5x, on Watech 910hx


My Observations

I set up at the Presbyterian Church on the corner of Bonny Doon Rd and Pine Flat. Dark and people-free and perfect for the event. Dry, warm (75F?) weather. Got a good recording of a tough challenge what with the 14.9 star so close to a star 100x brighter. Took several trys to get LiMovie to give me photometry on the target w/o interference from the 10.2 neighbor star

LiMovie settings

Target star right underneath the bright 10.2, used "meteor/Lunar Limb" aperature, oriented to not include 10.2 star, and nudged it extra away from that star. Very careful to center the tracking star which is right underneath the target before centering and track/linking the target very carefully positioned. Chose a comparison at lower right that was clearly not saturated and reasonably isolated in this rich Milky Way field.

The predicted event center was 5:34:30 +- 156 seconds (1 sigma). The target is in blue, comparison in pink. There's no consecutive zero readings on the target, and I interpret the drops as just noise.

Zooming in on the predicted event time.

Here's the light curve from 5:14UT to 5:28:36 UT, just to search for any drops due to possible satellites. Only once was there two consecutive integrations at zero, but that's not statistically significant i believe given the number of single integrations that were at ~zero.

Light dropped to zero for 2 seconds here, but unlikely to be real. It's only 2 integrations, and the star was so dim it's consistent with noise, given the long record and number of other 1-integrations drops to zero.

4 second integration shows target well, and a confounding faint star about 1.3 mag dimmer I'd guess. This star (arrowed), was in the sky annulus for perhaps most of the LiMovie photometry. If in the sky annulus, it would make the photometry of the target artificially lower by roughly 30%. Yet, the curve shows only 1 single second, well outside the predicted time period, of negative intensity (indicating occultation). Conclusion's the same - a miss from my site.


I've saved to .csv the entire light curve in two sections: 5:14UT to 5:28UT, and 5:28UT to 5:50UT. Available on request, files called "quaoar1" (6.1MB) and quaoar2 (11 MB), saved in the folder with the other /events files for the Quaoar event. Doesn't look like further analysis is warranted at this time.

Long 122 08 41.772
lat 37 03 53.436
elev 550m

I sent a report of my 'miss' to Brad Timerson on July 9, 2018. Excel file is, as usual, in AstOcc. I did not include the .csv photometry of the light curve because it is so large