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INSTRUMENT SCIENCE REPORT
FOC­081
TITLE: Cycle 4 Verification of the FOC f/96 Aperture Location
AUTHOR: W. Hack DATE: 17 October 1994
ABSTRACT
Observations were taken on 5 September 1994 to verify the location of the f/96 aperture.
The PDB was updated in early June to correct for an error in the f/96 aperture location
since SMOV. These observations were designed as a quick check to verify the accuracy
of the new PDB coordinates. Three snapshots centered on astrometric stars in the NGC­
188 cluster were taken. Despite minor errors in the target coordinates, the aperture's ref­
erence position was found to be (V2,V3) = (242.9329, 135.2735), compared to the PDB
value of (V2,V3) = (242.9714, 135.2052). The scatter in the observed positions of the
stars provides a mean positional error of 0.10'' in the aperture reference position.
DISTRIBUTION:
FOC Project:
IDT:
:
:
SIB/SOB:
:
SCARS:
SESD:
SPD:
USB:
RSB:
D. Eaton, B.G. Taylor, R. Thomas
R. Albrecht, C. Barbieri, A. Boksenberg, P. Crane, J.M. Deharveng,
M.J. Disney, P. Jakobsen, T. Kamperman, I.R. King, C. Mackay,
G. Weigelt
C. Cox, P. Greenfield, M. Lallo, A. Nota, S. Osmer, F. Paresce,
All Instrument Scientists
P. Hodge
W. Safley, M. Miebach
J.C. Blades, R. Jedrzejewski, F. Macchetto
A. Saha
D. Gilmore, D. Golombek

1. Introduction
The re­analysis of the SMOV aperture location data resulted in a new position for the f/96
aperture and was used to update the PDB (see ISR FOC­080). Proposal 5717 was writ­
ten as a quick check to verify the accuracy of the new aperture locations. The next sec­
tion describes what observations were taken with this proposal. Section 3 then discusses
the analysis of those observations followed by a summary of the results in section 4.
2. Data
Three observations were taken on 5 September 1994 using the 512zX1024 format, one
each centered on star 5, 51, and 54 in the NGC188 astrometric field. These images
relied on individual pointing to obtain the targets rather than POS TARG offsets from the
first image. The first image was centered on star 5 with stars 3, 4 and 17 also being visi­
ble in the image. This image was necessary not only for the aperture location itself, but
also to serve as positive identification of the targets since the last two images only con­
tained 1 star each. The second image was centered on star 54, and the third image had
star 51 as its target.
The coordinates given for the targets in the proposal were based on PASS's 1990 cata­
log, unfortunately the guide star system (GSSS) uses a 1985 catalog. This difference in
target coordinates resulted in the stars being located some small distance from the refer­
ence point in the images. The coordinates for the target stars and the guide stars are
given in Table 1 with the difference between the PASS and the GSSS catalog coordi­
nates listed in the last columns. Using the GSSS catalog coordinates for the guide stars
and the PASS coordinates for the target stars, it was expected that the stars would fall
about 1'' from the reference position in the image.
The images taken came out exactly as expected with the targets falling away from the
reference point by about an arcsecond. This can be clearly seen in Figure 1, the central
512x512 region of the sum of all three images. The observed positions for the stars were
measured from each image using `imcntr' and were provided in Table 2 . For compari­
son, the reference point corresponds to pixel (468,536).
Table 1: Coordinates for the Target Stars in the NGC­188 Field (Plate ZZZZ)
Star ID
GSSS Catalog PASS Catalog DRA
(``)
DDec
(``)
R.A. Dec. R.A. Dec.
54 0h 46m 40.526s 85š 14' 39.45'' 0h 46m 40.484s 85š 14' 40.46'' ­0.05 1.01
51 0 46 33.693 85 14 33.46 0 46 33.692 85 14 34.39'' 0.00 0.93
5 0 46 51.474 85 14 32.95 0 46 51.485 85 14 34.08 0.01 1.13
GS104 0 34 43.615 85 22 24.48 0 34 43.583 85 22 25.63 ­0.04 1.15
GS296 0 47 53.335 85 4 1.61 0 47 53.171 85 4 2.55 ­0.21 0.94

Figure 1: Central 512x512 region of the composite image of all three observations taken of the
NGC­188 astrometric field.
Reference Point
4
5
54
51
1''

3. Analysis
The V2­V3 positions of the stars were requested from PASS based on PASS catalog
coordinates for the target stars and also for the guide stars. The positions they calculated
for the stars are given in Table 2. The pixel positions and PASS V2­V3 positions were fit
using 'fitxy' to obtain a linear plate solution for the aperture. That solution was then eval­
uated using 'evalxy' to obtain a position for the reference point of (V2,V3) = (242.9329,
135.2735). The PDB value for the reference point was (V2,V3) = (242.9714, 135.2052).
This indicates a difference of only D(V2,V3) = (­0.0385'', 0.0683") from the PDB value.
The errors in this determination can be obtained by comparing the V2­V3 positions pro­
vided by PASS with those obtained from the star's positions in the image, given as the
Image Positions in Table 2. The PASS positions should correspond to the star position in
the image given that a consistent set of coordinates were used in the computations.
However, the images were taken using the old coordinates for the stars. Figure 2 shows
the V2­V3 offsets for both the PASS positions and the stellar image positions: both sets
are roughly scattered around the same positions, but the image positions have a mean
radial difference of 0.10" from the PASS positions. This scatter then represents the error
in the determination of the reference point, and therefore the aperture center.
Table 2: Target Positions
Star ID
Pixel Positions PASS Positions Image Positions
X Y V2 V3 V2 V3
First Image
3 621.62 198.38 242.055 140.520 242.124 140.459
4 702.18 389.34 244.499 138.975 244.598 138.813
5 462.39 595.45 243.353 129.473 243.376 134.452
17 443.37 1015.94 246.423 129.473 246.484 129.274
Second Image
54 469.98 585.28 243.338 134.601 243.386 134.633
Third Image
51 466.51 578.95 243.335 134.601 243.294 134.682

4. Summary
Based on astrometric observations of stars in NGC188, the position of the f/96 aperture
given in the PDB was found to be (V2,V3) = (242.9329, 135.2735) with errors of about
0.10". The analysis clearly demonstrated that the limiting factor in determining the aper­
Figure 2: This plot shows the positions of the stars relative to the reference point as determined
from the PASS coordinates and from the images.

ture center is now inaccuracy in the catalog coordinates. The larger scatter in the
images' positions reflects the larger errors of the guide star coordinates pulled from the
GSSS catalog. Monitoring of the same field will continue, using the GSSS Plate­ZZZZ
coordinates, to determine any shifts in the aperture position.