next previous
Up: The U.S. Naval Observatory


Subsections

2 Introduction

2.1 History

Plans for absolute observing programs concurrently covering both hemispheres using the USNO Six-inch and Seven-inch transit circles date back to the 1970's (Hughes [1978]). The result was to be an all-sky, absolute catalog tied to the dynamical reference frame. This required each transit circle to observe in the daytime and be located at a latitude such that a fundamental determination could be made of the azimuth using circumpolar stars. It was also planned that the bulk of the stars, the program stars, would be observed in declination zones of 15$^\circ$ along with suitable distributions of reference stars to allow differential reduction on a semi-nightly basis. At this same time, during the 1970's, the European Space Agency (ESA) was studying the feasibility of the high precision astrometric satellite, Hipparcos (Høg [1978]). Though the estimated accuracy of Hipparcos was a significant improvement over that of a transit circle, the plans were to reference the satellite's positions to FK5 (Fricke et al. [1988], [1991]). By the late 1980's it was evident that the FK5 contained systematic errors and an improved global catalog was desirable. The USNO undertook the pole-to-pole project to address this need. Renovation and testing of the Seven-inch transit circle delayed the start of the observing program until 1985. The launch of the Hipparcos satellite took place in August, 1989. Even with a revised mission made necessary by the failure of the apogee booster, the satellite was able to operate until August, 1993. The Hipparcos Catalogue (ESA [1997]) was released in mid-1997. In the end, the Hipparcos Catalogue was referenced to the International Celestial Reference Frame (ICRF, Ma et al. [1998]) and not to FK5. Observations for the pole-to-pole project were completed in April, 1995 by the Six-inch transit circle and in February, 1996 by the Seven-inch transit circle. Instead of following the traditional procedures to form an absolute catalog, the stellar positions were differentially reduced using stars from the Hipparcos catalog and the resulting catalog is on the system of the ICRF. This project is the latest and largest of a long series of transit circle catalogs produced by the U.S. Naval Observatory. It is also, because of advancing technologies, certainly the last.
   
Table 1: Observations made by the Six-inch transit circle located in Washington DC, U.S.A., during 1985-1995
 
    Solar System Objects
Stellar Objects   Sun and Planets   Minor Planets
  Number Number     Number     Number
Star Class of Obn's in Class   Object of Obn's   Object of Obn's
IRS 141870 21509   $\ast$Sun $\ast$1863   Ceres 481
Clocks 37486 230   $\ast$Mercury $\ast$596   Pallas 396
FK5 94762 3234   $\ast$Venus $\ast$1426   Juno 312
Refraction (LC) 5729 121   $\ast$Mars (day) $\ast$134   Vesta 511
Azimuth (UC) 7607 23   Mars (night) 588   Hebe 288
Azimuth (LC) 7210 23   Jupiter 727   Iris 276
$\ast$Day $\ast$9255 $\ast$84   Saturn 764   Flora 257
Radio 3176 106   Uranus 729   Metis 221
Miscellaneous 10349 874   Neptune 617   Eunomia 265
Totals 317444 26204   Totals 7444   Totals 3007
$\ast$ Observations not reduced.
UC = Upper culmination.
LC = Lower culmination.



 

 
Table 2: Observations made by the Seven-inch transit circle located near Blenheim, New Zealand, during 1987-1996
 
    Solar System Objects
Stellar Objects   Sun and Planets   Minor Planets
  Number Number     Number     Number
Star Class of Obn's in Class   Object of Obn's   Object of Obn's
IRS 174997 23552   $\ast$Sun $\ast$1166   Ceres 337
Clocks 29878 223   $\ast$Mercury $\ast$461   Pallas 361
FK5 102112 3078   $\ast$Venus $\ast$867   Juno 335
Refraction (LC) 9111 98   $\ast$Mars (day) $\ast$521   Vesta 386
Azimuth (UC) 12295 49   Mars (night) 402   Hebe 277
Azimuth (LC) 11812 49   Jupiter 391   Iris 324
$\ast$Day $\ast$38907 $\ast$348   Saturn 446   Flora 238
Radio 3687 117   Uranus 581   Metis 189
Miscellaneous 17856 672   Neptune 516   Eunomia 298
              Hygiea 284
              Melpomene 244
              Nemausa 68
              Amphitrite 269
Totals 400655 28186   Totals 5351   Totals 3610
$\ast$ Observations not reduced.
UC = Upper culmination.
LC = Lower culmination.



 

 
Table 3: Estimated standard deviations of the selected right ascension tour adjustments models. Nine different models were tested to adjust each tour's observations of a set of Hipparcos reference stars and the one providing the best fit was used to differentially adjust all the observations from that tour
 
Estimated standard deviations of the models
Right Ascensions (units = arcseconds)
Telescope Circle Median Minimum Maximum No. of Tours
Six-inch Circle One 0.186 0.022 0.469 1969
Six-inch Circle Two 0.182 0.010 0.430 2719
Seven-inch Circle One 0.194 0.052 0.383 1452
Seven-inch Circle Two 0.201 0.046 0.421 1853



 

 
Table 4: Estimated standard deviations of the selected declination tour adjustment models. Twenty one different models were tested to adjust each tour's observations of a set of Hipparcos reference stars and the one providing the best fit was used to differentially adjust all the observations from that tour
 
Estimated standard deviations of the models
Declinations (units = arcseconds)
Telescope Circle Median Minimum Maximum No. of Tours
Six-inch Circle One 0.221 0.017 0.596 1947
Six-inch Circle Two 0.216 0.010 0.644 2714
Seven-inch Circle One 0.274 0.060 0.757 1447
Seven-inch Circle Two 0.293 0.020 0.648 1854


2.2 Observing program

This catalog contains the combined results of observations made with the Six-inch Transit Circle in Washington, DC U.S.A. and the Seven-inch Transit Circle in New Zealand, between April 1985 and February 1996. This is the second USNO catalog to be referred to the Equinox of J2000.0 and will be named the W2 $_{{\rm J}00}$. The Six-inch transit circle was built by the Warner and Swasey Company and has operated from the U.S. Naval Observatory in Washington, DC since 1897. The visual, two axis micrometer was the same one used during the previous programs, the W550 (Hughes & Scott [1982]) and the W1 $_{{\rm J}00}$ (Holdenried & Rafferty [1999]). The Seven-inch transit circle was built by the USNO Instrument Shop in 1948. One of its previous program, the WL50 (Hughes et al. [1992]), was a visual catalog made from El Leoncito, Argentina. For the W2 $_{{\rm J}00}$, the telescope was located on the Black Birch ridge at an elevation of 1350 m, 20 km southwest of the city of Blenheim, New Zealand. The station, referred to as the Black Birch Astrometric Observatory, was at a latitude of -41$^\circ$ 44$^{\prime}$ 41 $\,{.}\!\!^{\prime\prime}$4 and a longitude of 173$^\circ$ 48$^{\prime}$ 11 $\,{.}\!\!^{\prime\prime}$99 East. For the W2 $_{{\rm J}00}$ it was decided to continue to make observations with the Six-inch transit circle visually because of the long and continuous series of excellent catalogs made in this mode (its observations formed the backbone for the FK3, FK4, and FK5 catalogs). The major changes made to the Six-inch just prior to the start of the W2 $_{{\rm J}00}$ observing program included; a second glass circle, two additional magnitude screens, and an upgrade to the photoelectric circle scanning system. Midway through the observing program, the photoelectric scanners were replaced with CCD devices (Rafferty & Klock [1986]). Major changes made to the Seven-inch transit circle after its completion of the WL50, and just prior to the beginning of the W2 $_{{\rm J}00}$ observing program, included the replacement of the visual micrometer with a new one using an image dissector as the detector (Hughes et al. [1986]), the installation of a new temperature compensating objective built by the Farrand Optical Corporation of New York, the installation of new graduated glass circles mounted on steel wheels fabricated by Heidenhain Corporation of Germany, and the installation of a new photoelectric system for scanning the graduated circles. As in the case of the Six-inch, the circle scanning system was upgraded to use CCDs (Rafferty & Klock [1986]) midway through the observing program. Both transit circles were equipped with clamping devices that prevented any motion of the telescope in altitude during an observation. These devices were located near one of the pivots of the instrument and provide a convenient way of referencing the orientation of the telescope; that is the telescope could be in either a "Clamp East" or "Clamp West" orientation. Both transit circles were reversed (rotated 180$^\circ$in azimuth), thus changing clamp, approximately every 30 days. This was done to mitigate any clamp-dependent systematics.


 

 
Table 5: Weights applied when combining observations from the two transit circles to form a single position
 
Weights for combined observations
Zenith Distance RA obs. Dec. obs.
80.0 0.00 0.00
75.0 0.16 0.03
70.0 0.29 0.14
65.0 0.42 0.24
60.0 0.56 0.34
55.0 0.68 0.43
50.0 0.78 0.53
45.0 0.87 0.62
40.0 0.93 0.72
35.0 0.97 0.82
30.0 0.98 0.89
25.0 0.99 0.94
20.0 1.00 0.97
15.0 1.00 0.99
10.0 1.00 1.00
$\;\:$5.0 1.00 1.00
$\;\:$0.0 1.00 1.00

   


The wheels of the graduated circles of each of the transit circles were rotated about 10$^\circ$ with respect to the tubes midway through the observing program. Observations taken before this circle rotation were referred to as from "Circle One" and after the rotation from "Circle Two".

Most of the celestial objects observed in this program fall into three categories; FK5 stars (Fricke et al. [1988]; [1991]), the program stars and solar system objects. Tables 1 and 2 give the number of stars in each category and the number of observations made. The program stars, primarily, consisted of International Reference Stars (IRS) (Corbin & Warren [1991]), but also included in this class were AGK3R stars and SRS that were not in the IRS. For brevity's sake we shall denote this entire class as IRS. The magnitude range of the majority of the stars in this catalog extends from the brightest FK5 stars down to, on the faint end, about 10th magnitude. The Seven-inch transit circle was able to reach as faint as 11th magnitude which allowed it to observe the fainter minor planets, SRS and radio stars.

Every effort was made to obtain 6 good observations of each star distributed equally between clamps and circles. However, some stars were added to the program too late to obtain that ideal distribution. For the late arrivals it was decided that a minimum of three good observations in right ascension and declination would be required. Of course, the FK5 stars that served as clock, azimuth, refraction, IRS reference, and day stars accrued many more than the minimum number of observations.


 

 
Table 6: Mean right ascension positional errors for each transit circle as well as the mean errors and epochs for the final positions
Right ascension errors
  Six-inch Seven-inch Total
Declination $\sigma$ $\bar{\sigma}$ n $\sigma$ $\bar{\sigma}$ n mean $\sigma$ $\bar{\sigma}$ n
Range mas mas stars mas mas stars epoch mas mas stars
+90 to +85 234 68 97       1990.80 234 68 97
+85 to +80 218 71 265       1990.73 218 71 265
+80 to +75 203 70 435       1990.70 203 70 435
+75 to +70 208 74 577       1990.73 208 74 577
+70 to +65 203 72 736       1990.76 203 72 736
+65 to +60 203 74 874       1990.80 203 74 874
+60 to +55 197 72 1018       1990.79 197 72 1018
+55 to +50 197 72 1156       1990.86 197 72 1156
+50 to +45 193 71 1277       1990.83 201 74 1277
+45 to +40 192 71 1416       1990.83 192 71 1416
+40 to +35 188 70 1533       1990.86 188 70 1533
+35 to +30 188 70 1579 128 94 16 1990.79 188 70 1579
+30 to +25 190 72 1765 165 63 191 1990.84 189 71 1765
+25 to +20 191 70 1748 181 55 236 1990.87 191 69 1748
+20 to +15 199 74 1782 187 48 228 1990.84 199 73 1782
+15 to +10 192 72 1811 191 45 210 1990.85 193 71 1811
+10 to $\;\:$+5 191 71 1827 193 44 244 1990.90 192 70 1827
$\;\:$+5 to $\;\;\:\:\;$0 193 73 1820 209 76 1792 1991.51 208 55 1822
$\;\;\;\:\:$0 to $\;\:-$5 194 74 1727 208 74 1796 1991.58 208 55 1802
$\;\:-$5 to -10 189 52 282 206 72 1825 1992.14 205 70 1822
-10 to -15 185 48 207 200 70 1829 1992.13 200 69 1831
-15 to -20 183 52 204 200 70 1842 1992.14 200 69 1841
-20 to -25 163 51 200 199 69 1701 1992.13 199 69 1701
-25 to -30 149 56 186 198 68 1596 1992.12 196 68 1596
-30 to -35 123 98 67 196 69 1787 1992.16 196 69 1787
-35 to -40       199 69 1832 1992.16 199 69 1832
-40 to -45       200 69 1657 1992.23 200 69 1657
-45 to -50       198 70 1644 1992.21 198 70 1644
-50 to -55       201 70 1329 1992.15 201 70 1329
-55 to -60       204 71 1184 1992.27 204 71 1184
-60 to -65       201 70 1034 1992.21 201 70 1034
-65 to -70       205 70 799 1992.24 205 70 799
-70 to -75       208 70 648 1992.22 208 70 648
-75 to -80       222 73 494 1992.25 222 73 494
-80 to -85       229 70 310 1992.30 229 70 310
-85 to -90       237 65 111 1992.26 237 65 111



next previous
Up: The U.S. Naval Observatory

Copyright The European Southern Observatory (ESO)