Attempted detection of periodic variations in brightness of the comet Encke is described. Viewing problems due to the position, faintness, and rate of motion of the comet are discussed. The failure of attempts to perform photoelectric photometry and CCD imaging is described. Photometric observations of the prototype Earth crosser, (1862) Apollo, are described and a photoelectric light curve of observations made during a four-hour period is presented

Mccrosky, R. E.

English

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(NASA-CF-16Lu78)	 PHOZOMETRIC AMD CCD DIRECT	 bel-3%56
IMAGE C35EFVA:ION CF COMET LNCKE final
Peport, 19 Jun. 1980 - 1 Auq. lyol
(Smithsonian A.;troph ysica. CbSerV Jtoc y)	 8 p	 UnC1as
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PHOTOMETRIC AND CCD DIRECT IMAGE OBSERVATION OF COMET ENCKE
GRANT BP-724892
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Final Report
For the period 19 June 1980 to 1 August 1981
Dr. Richard E. McCrosky
Principal Investigator
	
July 1981	
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Jet Propulsion Laboratory
Pasadena, California 91103
Smithsonian Institution
rstruphysical Observatory
Cambridge, MA 02138
The Smithsonian Astrophysical Observatory
and the Harvard College Observatory
are members of the
Center for Astrophysics
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PHOTOMETRIC AND CCD DIRECT IMAGE OBSERVATION OF COMET STICK$
GRANT BP-72489::
Final Report
For the period 19 June 1980 to 1 August 1981
Dr. Richard E. McCrosky
Principal Investigator
July 1981
Prepared for
Jet Propulsion Laboratory
Pasadena, California 91103
Smithsonian. Institution
Astrophysical Observatory
Cambridge, MA 02138
The Smithsonian Astrophysical Observatory
and the Harvard College Observatory
are members of the
Center for Astrophysics
x
".his report details our attempts to assist in the determination of
the rotation rate of P/Encke by detecting, photoelectrically, periodic
variations in brightness.
I. Photoelectric Photometry
Photometry of Encke was scheduled for the 61" telescope at Agassia
Station for the dates (UT) of October 5 and October 7-12. In addition,
short periods of ri nights during the interval September 24-October 3 were
employed to test various aspects of the special system, described below,
required for photometry of this rather fast moving obiect.
Encke presented three problems:
1) Its position was somewhat uncertain. The two available ephemerides
-- by Marsden and Yeomans -- differed by nearly 0.5 arcmin on October 10.
2) Encke was expected to be faint. Since seeing as poor as 6" occurs
about 25% of the time at Agassir, we needed to be prepared for an invisible
Encke.
i
3) The comet's motion would be of the order of 3 aresec/minute and
telescope tracking of the comet would be desirable for long integration
times.
We resolved the problem of the court- s position on October 8 by making
t
a photographic astrometric observation on the 61". This indicated that the
i	 1
Marsden ephemeris was tha better of the two but that a correction of about
10", principally due to parallax, was needed. With hindsight, we should
have made this observation earlier. 7n addition to resolving the question
of position, we also learned that the comet was much fainter than we ex-
pected and were alerted to the importance of the second problem.
An understanding of our approach to the last two problems requires a
t	 description of three special telescope and focal plane motion we regularly
employ when making astrometric observations of comets and asteroids. I&e
photometer was carried on the Newtonian double-slide (and rotatable) plate
holder assembly. The X and Y coordinates can be read to .01 inches with
micrometer and 6 to 1 0 by a scale. The X motion can also be driven in
steps of 1/8000 inch by a stepping motor at periods greater than about
.04 secs/step. Thus, to cause the photometer field to track Encke ;while
the telescope tracks stars), one rotates the double-side to the position
angle of Encke's motion and drives the X-axis (about 12
 
sec/step) at Encke's
rate.
An additional feature of the telescope permits one to make precision
offsets (±5 aresec over several degrees) from one object to another. In
`	 preparation for our observing run, we made astrometric observations of the
comparison stass recommended by A'Hearn and Millis and used these also as
offset standards.
Finally, the actual offsets required at any given time were provided
by a special addition to the computer program that handles the photoelectric
observations. The computer read-outwas available at the telescope.
2
The observing procedure is probably now obvious. Measurements were
made of one of the comparison stars in a standard, sidereal manner. The
observer then interogated the computer on the required offset to Encke.
At the same time this offset was initiated, the comet tracking motor was
started. Thus when the offset was complete after a minute or so, Encke
should have appeared in the photometer diaphragm and remained there.
J
But of course we experienced the same problem as all other observers
+	 — Encke was never bright enough to see. In fact, our disappointment may
have been greatest of all 	 ounce we had photometric skies and reasonable
seeing (t2-3") for almost all of the 6 day prime ran time — a sequence of
nights three times better than wauld be predicted on the basis of the
Agassiz statistics.
It would no~. have been impossible to perform some photometry on an
invisible Encke. Since we were not aware that we were not alone with our
problem, we continued each night to attempt to detect Encke photoelectri-
cally by performing manual roster scans, using the micrometer, in the region
of Encke's position. This was not a terribly satisfactory procedure. One
must watch the integrated counts as they appear on the monitor and try to
determine, in real time, whether or not significant variations have occurred.
But we attempted it several times each night. Occasinally a faint object
'	 would seem to s,ow in the counter but never anything with Encke's motion.
We concluded that Encke must have been fainter than 15th but this only con-
firmed our earlier photographic observation.
I	 3
W.
II. CCD Imaging
We were scheduled for frequent access to the Mt. Hopkins 24" on those
dark nights in October and November when the CCD camera was at focus. The
24" primary was removed from the telescope in July for re-coating by a
vendor. It was to be re-installed in early September, 1980. The vendor
-- in what must be considered a major blunder — ruined the surface of the
mirror while in the process of removing their first, and unsuccessful,
!	 coating attenpt. Several months passed befoi,^ the vendor accepted
reAponsibility. The repolished and coated mirror finally arrived at
Mt. Hopkins in February 1981.
we tried to salvage some of this part of our program by scheduling
tine on the Mt. Hopkins 60" in early December. Four partial nights of CCD
time, just before dawn, were made available but after two attempts it was
clear that the comet ' s position, its great rate of motion and its large
size all conspired to make the observations worthless. The killing problem
was the rate, the motions of this telescope could not be biased off of
sidereal enough to track on the comet.
III. The Good News
1
The proto-type earth crosser, (1862) Apollo, made a close approach to
Earth in November, 1980. (We maintain a certain fondness for this object
since this observatory was responsible for its recovery in 1973). Having
learned nothing about the rotation rate of Encke, we turned to Apollo in
the hopes of assuaging our disappointment. In many respects, photometry
of Apollo duplicated the problems encountered in the Encke atteupt. We
^i
4
were able to use almost all of the techniques and instrumertation developed
in the. previous months. Apollo — unlike a comet — was predictably bright
enough but unfortunately much of the close approach time fell during bright
moon. We made our first observation on November 16-17 after receiving an
improved ephemeris from Marsden. Limits placed by available o yserving time
and clouds permitted only 20 minutes worth of data but these were sufficient
to demonstrate that Apollo would show large variations and, probably, a
short periods Am % 0.1 in 15 minutes. On November 19-20 the moon was already
too bright for useful photometry with the large diaphragm needed to match
the poor seeing. The moon interfered for the next four nights and clouds for
the following two. On November 25-26 we obtained good data for nearly four
hours. The results are reproduced in Figure 1. The amplitude Ot 0.4 mag)
and period (ti 3 hours) are, respectively in the high and low, extremes of
the distribution of all asteroids.
An additional snippet of the light curve was observed during the next
available night, November 29-30 (Figure 2), but there is not sufficient
information to tie this to the preceedi.ng observation. No further observa-
tions were attempted.
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Photometric and CCD direct image observation of comet Encke

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