MBCAA Observatory

Occultation by 27 Euterpe

Observed: 2 Nov 2015

Michel Bonnardeau
3 Nov 2015

Abstract

The minor planet 27 Euterpe was observed occulting a 12.1 mag star: the size of the shadow flying over my location was 119 km.

Introduction

The occultation of the 12.1 mag star 2UCAC 39644999 by the 10.1 mag asteroid 21 Euterpe on Nov 2, 2015 was predicted on Steve Preston's Predictions Website. The details of the prediction are here.

Observation

The observation was carried out with a Meade LX200 203mm f/6.3 Schmidt-Cassegrain telescope, a Clear fiter and a SBIG ST7E camera (KAF401E CCD). A few seconds before the occultation, the camera obturator is opened, the telescope equatorial motor still running,
3 seconds after the begin of the exposure, the motor is shut down,
and 60 seconds after the begin of the exposure the obturator is closed.

The resulting image show the stars (the 3 seconds the motor was running) with trails (the 57 seconds the motor was not running). The occultation shows up as a decrease of the star trail intensity. Actually this drop in the trail is rather weak as the asteroid was quite bright.

The time was recorded by the computer, the clock of which was reset over an internet time server, owing to Dimension4 and net time. Futhermore, the computer clock was checked by listening over a wire telephone to the Horloge Parlante (Observatoire de Paris, France).

Analysis

A scan of the star trail is:


The time goes from right to left.

The end of the trace is at:
xE = 6 ± 1 pixels
tE = 04:44:63 UT ± 0.5 s

The trace begin is at:
xB = 585.5 ± 0.5 pixels
and the trace duration is:
D = 57 s

The ingress and the egress are at:
ingress = 430 ± 5 pixels
egress = 190 ± 5 pixels
that is, in time:
tin = tE-(ingress-xF)*D/(xB-xE) = 04:44:21.3 UT ± 1.2 s
teg = tE-(egress-xE)*D/(xB-xE) = 04:44:44.9 UT ± 1.2 s

The duration of the event was then:
teg-tin = 23.6 ± 1.7 s
and the time at the mid-event was:
(teg+tin)/2 = 04:44:33.1 UT ± 1.7 s

At the time of the observation, the angular velocity of the asteroid was v = 0.13349 /day and the geocentric distance was d = 1.2452579 AU. Then the size of the shadow flying over my location was:
(teg-tin)*v*d = 119 km
This is about the same as the diameter of the asteroid, 125 km, so I was in the middle of the path.

The report send to PLANOCCULT is here (may not be on-line).

Technical notes

Telescope and camera configuration.

Computer and software configuration.



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