In this paper we propose a new method for photographic astrometric observations of asteroids. We discuss its advantages and disadvantages and compare them to the advantages and disadvantages of the classical photographic methods. The new method is best suited for observations on a spot where no CCD cameras, blink or stereo comparators are available and when a fast detection of unknown objects is required

Pauwels, Thierry

English

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Asteroids, Comets, Meteors1991, pp. 455_58 _- (_?Lunar and Planetary Institute, Houston, 1992 _) 7_ 455
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N - 9220
A NEW METHOD FOR ASTROMETRIC OBSERVATIONS OF ASTEROIDS
Thierry PAUWELS
Koninklijke SLerrenwacht van Belgie, Ringlaan 3, B-1180 Brussel, Belgium
Abstract. In this paper we propose a new method for photographic astrometric observations of asteroids. We
discuss its advantages and disadvantages and compare them to the advantages and disadvantages of the classical
photographic methods. The new method is best suited for observations on a spot where no CCD cameras, blink or
stereo comparators are available and when a fast detection of unknown objects is required.
Key words: asteroids - astrometry
1. INTRODUCTION
When no accurate positions are known, on astrographic plates asteroids reveal themselves among the
stars by their motion only. Therefore astrographic observations of asteroids are always done in such a way
that moving objects can easily be detected on plates. Nowadays, plate scanners or CCD cameras have
made this detection easier. However, CCD's still have a small field and are not available everywhere. Plate
scanners are often not available close to the observing site. So, in many cases observers still have to use
the old photographic methods, and afterwards scan the plates with their eyes. Here, we propose a new
photographic method and compare its advantages and disadvantages with those of the well-known classical
photographic methods. Since the proposed method is of no use in CCD observations, we shall not compare
it to CCD observations.
2. SIX METHODS
First method. One long exposure: moving objects show trails.
DISADVANTAGES
• Much sky background on plate.
• Very difficult and uncertain detection of objects,
especially when the exposure time is not too long:
double stars can look exactly like moving objects.
• No indication of the sense of the motion of moving
objects.
• Large reference stars, difficult to measure.
• Elongated moving objects, difficult to measure.
ADVANTAGES
• Detection of moving objects is reasonably fast.
• Very good plates for archival purposes (historical
record for possible later use in other astronomical
applications).
Second method. Tr_pied-Metcalf exposure: guided on the expected (mean) motion of the target
object(s). Objects with this motion show circular images. Objects with slightly different motions show
slightly elongated images.
DISADVANTAGES
• Detection of moving objects is very difficult, and
almost impossible when no position is known in
advance. Therefore this method should be used
in combination with another method.
• Elongated reference stars, difficult to measure.
• Bad for archival purposes.
• Not possible when different target objects have
very different motions.
s Target objects can be wiped out by star trails.
ADVANTAGES
• Moving objects will give very good measure-
ments.
• Ideal for very faint objects.
https://ntrs.nasa.gov/search.jsp?R=19930010031 2020-03-17T07:28:46+00:00Z
456 Asteroids, Comets, Meteors 1991
Third method. Gated exposure: several exposures on the same plate without any shift between
the exposures. The interruptions must be long enough to give detached images for objects moving at the
expected speed. Usual_ly the gating is asymmetric _ _rrder to give the sense of the motion.
DISADVANTAGES : ADVA_A_] i_S
....... == ....... _: detectlon of moving objects.
• Much sky background on plate. • Very fast and secure
• L_rge reference stars,_dimc_t...................to measure. • =The sense of the motion o_ngobjectsc_n be
• Loss of time during the interruption(s). = =_ : derived,:_ = provided= that the gating_ =is asymmetric.
• Good plates for archival purposes. .
• Images of object can be elongated, difficult to
measure.
• Several images of the objects to be measured for
only one poeitionf:i _
• Problem With e_chdetermination of pos!tio n,
especially if the shortest exposure(s) give images
too falnLto he measured.
-7 : !
Fourth method. Two plates. These are taken with a certain time interval, so that moving objects
have different positions. Moving objects are detected by blinking the plates or by stereo comparison. This
method can be combined with the Tr@ied-Metcalf technique.
DISADVANTAGES
• Detection of moving Objects impossible without
a stereo or (expensive) blink comparator.
• Very slow detection of moving objects.
ADVANTAG'E!U
• Very secure detection of moving objects.
• The sense of the motion of moving objects can be
derived.
• No loss of time between the two plates (except
when target objects are very slow).
• Good plates for archival purposes.
• Two independent positions can be derived.
• Possible even for extremely slow objects.
Fifth method. _ -Several exposures on the same plate. Between the exposures there is a shift,so that
allobjects show several images. Moving objects, however, will show a different pattern. This method can
be combined with the Tr_pied-Metcalf technique_ _= i = !i!_ _ _ _ _ _:_ :i
DISADVANTAGES
• Much sky background on plate.
• Very bad platesfor archivalpurposes.
• Difficultdetection of moving objects in crowd_
fields. _ ..
ADVANTAGES
• Reasonably easy detection of moving objects in
low-density fields.
• The sen_ of the motion can be derived.
• No loss of time between- t]_eexposures (except
when target objects _r_v_ry_s_low ._
• Several independent posltlons can be derived.
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458 Asteroids, Comets, Meteors 1991
Proposed method. Several superimposed Tr@ied-Metcalf exposures. A first exposure is started with
the telescope guiding at the average motion of the target objects. After some time, the telescope is brought
back to the starting position, and a new exposure is staxted. This is done as many times as the number
of exposures (and asteroid images) wanted, with all exposures of the same lengt-h: The duration of these
exposures should correspond to a motion that is _-_ast :t_e=d[ameter of the images of the target objects
on the plate, in order to get detached images for the targei objects. Stars will show only one (elongated)
image, while moving objects will show several-_mages. For objects w_th exactl-y-the introduced motion, the
images are circular; for objects with slightly differen_ motions, the images will be slightly elongated.
DISADVANTAGES == ADVANTAGES
• Much sky background on plate. " o_ery _ast an_ecure detection of moving objects.
• Reference stars are somewhat elongated, and dif-
ficult to measure.
• Only moderately good plates for archival pur-
poses.
• Sever_ i_ages o_the moving objects tobe mea-
surecl for only one pps!t!on: .............
• The minimal exposure times must be long enough
to detac h the d!ff_rent images of the moving ob-
jects.
* Not poss_b[ew_ndifferent targetobjects___ave
very different motiofis. :: :: : _ :
• Not possible when target objectsar¢ too slow.
All methods described here have their own advantages and disadvantages. The specific conditions in
which observations are done will determine which of the advantages and disadvantages are dominant, less
important or irrelevant. On a very good location with a very dark sky or with an instrument with long focal
length the quantity of sky background is less important. When observing well-known objects the @_tecAipn
speed is irrelevant. Large stars axe difficult 1:o measure in the classical _ay, but this is no longer the-case
when using a Schmidt telescope (giving sharp spikes around b_ght stars ,_or_=dig[t_zlng t_tes.
Combining two methods can also remove some of their disadvantages.
The proposed method seems to be best suited for observing on a site where no CCD cameras are available,
where no blink or stereo compaxator is at hand and where a fast detection of unknown moving objects is
required. The observer can also use the prop_ method for the discovery of objects, and us_ o_e Of the
classical methods (or small field CCD observations) for the follow-up. ::: : ::::: ::::
• No loss of time between the exposures.
• High accuracy measurements thanks to several
' :: almost circular images of moving objects.
• Good for faint objects. _
• The sense of the motion can be derived.
Aeknowiedgements. The author: Would like to thank Dr. E. Boweii for his useful comments and for
bringing the last disadvantage of the second method to his attention. -- 2 _: _:- _:)_YI_ _


A new method for astrometric observations of asteroids

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