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The principal telescope of the Cesco Observatory is a Double Astrograph consisting of two lenses each 51-cm in diameter, one optically corrected for blue light and the other for yellow light. The lenses focus the light separately onto two photographic plates with dimensions of 17 x 17 inches covering a field-of-view of 6.5 degrees on a side. Until their production was discontinued by Kodak in 1997, photographic plates were purchased in the US, shipped to Argentina, exposed in the telescope, developed and then shipped back to the US. In New Haven, Connecticut, the plates were measured with the PDS precision measuring machine and the analyses performed which allow us to determine the absolute proper motions of the stars. The photographic plate system was replaced by a CCD detector system in 1999 with the support of a grant from the NSF.
The telescope, designed and fabricated by Rademakers in Holland, consists of a fork-mounted tube with a diameter of approximately 1.5-meters. The upper end of the tube contains a heavy steel plate which holds both 50-cm lenses as well as two 25-cm lenses that serve as the auto- and manual guiders. Interior baffles prevent light from the four lenses reaching the wrong focal plane. The lenses are four-element Ross-type astrographic lenses, whose design was improved and then fabricated by the Perkin-Elmer Corp. in 1962 along with the yellow lens of the Lick Observatory Double Astrograph. As a consequence of its improved design, the YSO blue lenshas slightly better imaging quality than the Lick blue lens, which was constructed in 1947. The two yellow lenses are virtually identical. The fields of view on 17 x 17 - inch plates at the plate scale of 55.1"/mm is 6.3 x 6.3 degrees, or 40 square degrees. Photography for the SPM and the Lick NPM was therefore done on 5 degree centers so as to have a minimal amount of overlap. The blue photographs were taken unfiltered, since the four elements of the blue lens filter out most of the unwanted ultraviolet light. The yellow lenses use Schott optically polished filters, a GG14 for the Lick yellow lens, and an OG515 for the YSO yellow lens(see Vasilevskis in PASP 76,14, 1964 for a description of the Lick lens and an interesting chronology of the astrograph design). The effective passbands for the YSO lenses are then 410-480-nm for the blue and 520-570-nm for the yellow, with minimum foci at 435-nm and 535-nm, respectively.
The YSO astrograph has two guide telescopes, one for visually setting the telescope on the field, and another equipped with an image-dissector autoguider whose electronics were designed and constructed at Lick following the design of their autoguider. The opto-mechanical parts of the YSO autoguider were designed and fabricated in the instrument shops at Yale and the system was installed in 1979. The image-dissector autoguider works in the open-loop mode, integrating the error signal for a predetermined period and then it applies a correction that is proportional to the integrated error to the plate position. Controls on the electronics allow us to vary the voltage on the image-dissector to accomodate for guide stars of differing brightness, change the integration time depending on the quality of the tracking of the astrograph and alter the conversion factor that changes the integrated error-signal amplitude into a position correction that is applied to the plate position. The autoguider was installed in 1979. All of the first-epoch SPM plates were visually guided while the second-epoch plates were all taken with the autoguider in operation.
The double astrograph has been computer
controlled for a number of years and that capability has been
substantially improved this year by R. Meyer as we have brought the
new CCD camera
system into operation. The current
system allows the operator to input the desired coordinates manually
or automatically from an observing program and incorporates
corrections for atmospheric refraction, polar axis misalignment and
telescope flexure.
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Glass |
Index |
Zvertex |
Curvature |
Element |
Diameter |
|
type |
|
(mm) |
(1/R) |
|
(mm) |
|
|
|
|
|
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|
:0 |
+0.000900341229 |
lens |
571.5 |
|
SK-2 |
1.62074 |
64.48 |
-0.00010269829 |
lens |
571.5 |
|
|
|
450.12 |
-0.00075939370 |
lens |
419.1 |
|
F-9 |
1.64146 |
484.43 |
+0.000301391827 |
lens |
419.1 |
|
|
|
523.93 |
-0.00019729508 |
lens |
406.4 |
|
F-9 |
1.64146 |
558.24 |
+0.000907556314 (asphere) |
lens |
406.4 |
|
|
|
912.24 |
+0.000247157687 |
lens |
508 |
|
SK-2 |
1.62074 |
969.81 |
-0.00097354868 |
lens |
508 |
|
|
|
4253.11 |
+0.0 |
focal plane |
425 |
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|
Glass |
Index |
Zvertex |
Curvature |
Element |
Diameter |
|
type |
|
(mm) |
(1/R) |
|
(mm) |
|
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|
0 |
+0.000913367 |
lens |
571.5 |
|
SK-2 |
1.61127 |
64.79 |
-0.000107584 |
lens |
571.5 |
|
|
|
451.91 |
-0.000775404 |
lens |
419.1 |
|
F-7 |
1.63175 |
486.39 |
+0.000310993 |
lens |
419.1 |
|
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|
519.82 |
-0.000182381 |
lens |
406.4 |
|
F-7 |
1.63175 |
554.3 |
+0.000925146 (asphere) |
lens |
406.4 |
|
|
|
914.66 |
+0.000249464 |
lens |
508 |
|
SK-2 |
1.61127 |
972.51 |
-0.000980584 |
lens |
508 |
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|
4225. |
+0.0 |
lens |
425 |
|
OG530 |
1.519 |
4231. |
+0.0 |
lens |
425 |
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|
4252.17 |
+0.0 |
focal plane |
425 |
Link to: CCD Camera page; Astrometry Research page; Astro. Dept. Home page
