The Palomar 200-inch f/9 Cassegrain CCD camera

This article describes the design and construction of a CCD camera for the f/9 Cassegrain focus of the 200-inch (5 m) Hale Telescope at Palomar Observatory. This instrument utilizes a set of 18inch (0.45 m) diameter lenses to change the Cassegrain focal ratio from f/16 to f/9, and the optical design prescription of these lenses is reported. For CCDs having 24zm pixels, the f/9 focus gives a reciprocal plate scale of 9 pixels per arcsecond, which is well suited to precision photometry as well as future application to adaptive optics. In addition to the CCD and shutter, the instrument also incorporates an offset guider and an 8-position filter wheel. 1 SCIENTIFIC MOTIVATION The standard f/16 Cassegrain of the Palomar 200-inch Hale Telescope has a plate scale of 2.54 arcseconds/mm, or 0.061 arcseconds/pixel for CCD pixels measuring 24 tim. It is therefore poorly matched to CCD imaging observations in the absence of any reimaging optics. For a Tektronix CCD containing 10242 pixels, this 16 pixels/arcsecond reciprocal scale provides a total field of about 1arcminute. In order to maximize the imaged field size, most existing CCD imaging instruments at Palomar ( e.g., the PFUEI' and COSMIC2 Prime focus instruments, as well as the 4-SHOOTER3 at the f/16 Cassegrain focus) have been designed with reciprocal plate scales of approximately 3 pixels/arcsecond. For most applications these instruments quite adequately sample the typical 1-arcsecond seeing disk diameter. To facilitate an observing program4 in which very precise time series photometry was required over a minimum field size of 90 arcseconds with a target image scale of 0.2 arcseconds/pixel or less, a new CCD imaging instrument was designed and constructed for the f/9 Cassegrain focus of the Hale Telescope. With a 1024 x 1024 CCD array of 24 im pixels, the f/9 focus provides a nearly 2-arcminute field of view at an image scale of 0.11 arcsec/pixel (or 9 pixels/arcsec). An increased number of pixels per arcsecond permits CCD photometry to achieve a greater S/N ratio in a single O-81941493X1941$6.OO SPIE Vol. 2198 / 163 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/18/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx exposure prior to saturation, an indeed increases the photometric accuracy achieved for the same stellar signal in the high S/N regime — where CCD readnoise is insignificant — by increasing the number of pixels included in the flat field for each star. These two advantages were both crucial to the planned observing program, in which solar-type oscillations in the stars of M67 were to be investigated photometrically, requiring an accuracy per exposure of order 300 zmag and an ultimate sensitivity for a 7-night time series of 20 jmag or better. 2 THE OPTICAL DESIGN The f/9 Cassegrain focus is produced by an 18-inch (0.45 m) diameter doublet or triplet lens system (depending upon the wavelength range of the observation; work in the red requiring the triplet configuration) located above the Cassegrain focus in the converging beam from the f/16 secondary mirror. These lenses were designed in the early 1960s by Ira S. Bowen at Caltech, and were fabricated in 1963. Table 1 gives a complete optical prescription for these lenses, in the triplet configuration used in the red () 5500 A), from a Code V raytrace conducted in the course of this work. For work at shorter wavelengths, only the K7/F2 doublet is used: surfaces numbered 7 and 8 in Table 1 are eliminated, thickness 6 becomes 38.691 inches, and the telescope secondary mirror focus must be changed so that thicknesses 1 and 2 become — / + 538.836 inches. Table 1 : Optical Design Prescription for the 200-inch f/9 Cassegrain Surface No. Radius (in.) Thickness (in.) Material Notes 1 —1335.700 —539.165 Reflection Primary mirror (k = —1.0000) 2 —325.465 539.165 Reflection f/16 seconary (k = —2.3177) 3 0.000 10.219 Airspace f/9 lenses fixed below primary 4 58.583 2.283 Schott K7 5 —36.299 0.433 Schott F2 6 —120.078 0.197 Airspace 7 103.937 0.394 Schott F2 Optional (red) lens element 8 80.315 38.100 Airspace To underside of f/9 mounting plate 9 0.000 0.043 Airspace Dewar window recess 10 0.000 0.125 Fused Silica CCD Dewar window 11 0.000 0.535 Vacuum CCD depth inside Dewar 12 0.000 0.000 Focal Plane CCD Location A cross-sectional view of the three f/9 lenses is shown in Figure 1 ;note the manner in which the third element, the red meniscus lens, can be moved out of the beam for work at wavelengths less than 5500 A. Together with the 200-inch primary and f/16 secondary mirrors, the 18-inch diameter f/9 lenses produce an unvignetted field of view 7.5 inches (14 arcminutes) in diameter. The original use of the f/9 system was in conjunction with 8 x 10-inch photographic plates, and it was used photographically for several years in the mid-1960s. The new instrument described in this paper represents the first application of the f/9 Cassegrain optics to modern CCD imaging.