James E. Gunn, Stephen M. Kent, and Ed Mannery
As pointed out in the design report for the Swope and Irenee DuPont Telescopes by I.S. Bowen and A.H. Vaughan (1973, Applied Optics, 12, 1430), it is possible to design a Ritchey-Cretien Telescope with a flat field by making the curvatures of the primary and secondary mirrors the same, which yields zero Petzval curvature in the focal plane. Since a Gascoigne astigmatism corrector is required, and since this element introduces a bit of positive field curvature, the design needs to deviate a little from this prescription, but not very much. This results in a final focal ration of just under twice the primary ratio, depending a little on the back focal distance, and a very large secondary, about half the primary diameter for the field sizes obtainable with f/4 primaries, (about 3 degrees). For large telescopes one would like to get around the limitation of slow primaries and large secondaries. These desiderata were met in the design of the 2.5-meter DuPont telescope with the introduction of only moderate field curvature.
The design described here uses the same philosophy taken to even faster primary and overall f/ratio. Our requirements are rather unusual for an astronomical telescope, since we want to do time-delay-and-integrate (TDI) or scanning mode imaging over a large field. This requires that we control distortion carefully, since either a change of scale or a differential deviation from conformality of the mapping of the sky onto the focal plane across a chip translates immediately into image degradation. It is easy to show, in fact, that for the pixel sizes and CCD sizes of interest, no axially symmetric optical design is satisfactory for large enough field angles. For a three degree field, troubles with mapping a sphere onto the focal plane cause image degradation of the order of 0.14 arcsecond, which with 0.4 arcsecond images is negligible, but the errors grow like the square of the field diameter. The desired map (onto a flat focal plane) is one which creates a Mercator-like projection of the sky onto the focal plane, with both parallels of �latitude�, (i.e. curves of constant angular separation from a great circle through the center of the field in the scan direction) and meridians mapped into straight lines, but no such map is axially symmetric, and we have not investigated the anamorphic optics which would be required for such a map. The errors for a zero-distortion design are in any case not excessive with our field. Conventional Ritchey designs have two orders of magnitude too much distortion for our application.
In additions, conventional Ritchey designs with Gascoigne correctors this fast have unacceptably large lateral color, both for our imaging application and (especially) for the fiber spectroscopy. We must therefore go to somewhat more complex systems, and have evolved a design with a two-element refracting corrector which has excellent performance. It makes use of the fact that the astigmatism correction of a Gascoigne plate goes as the square of the distance from the focal surface for a given strength, while the lateral color and distortion only go linearly. Thus a pair of plates, one of the usual form and of weak power placed some distance from the focal plan, and another the negative of the usual form of n times the strength of the first placed 1/n times the distance of the first from the focus, can correct astigmatism while introducing no lateral color or distortion. Distortion remains at a level (4 microns) set by the order of the aspheric used for the second corrector element and can in principle be removed (or specified) exactly; the lateral color is less than 4 microns RMS over the hole field over the spectral range of the spectrograph, and is a negligible contribution to the image diameter for any filter or field location in the camera.
Surfaces : 9 Stop : 1 System Aperture :Entrance Pupil Diameter Ray aiming : Off Apodization :Uniform, factor = 0.000000 Eff. Focal Len. : 12428.7 Total Track : 4405.59 Image Space F/# : 4.97147 Working F/# : 4.9701 Obj. Space N.A. : 1.25e-007 Stop Radius : 1250 Parax. Ima. Hgt.: 325.493 Parax. Mag. : 0 Entr. Pup. Dia. : 2500 Entr. Pup. Pos. : 0 Exit Pupil Dia. : 512.73 Exit Pupil Pos. : -2544.61 Maximum Field : 1.50017 Primary Wave : 0.476000 Lens Units : Millimeters Angular Mag. : 4.87586
Fields : 7 Field Type: Angle in degrees # X-Value Y-Value Weight 1 0.000000 0.000000 1.000000 2 0.000000 0.500010 1.000000 3 0.000000 0.750020 1.000000 4 0.000000 1.000050 1.000000 5 0.000000 1.216760 1.000000 6 0.000000 1.366800 1.000000 7 0.000000 1.500170 1.000000
Wavelengths : 5 Units: Microns # Value Weight 1 0.354000 1.000000 2 0.476000 1.000000 3 0.628000 1.000000 4 0.769000 1.000000 5 0.925000 1.000000
Surf Type Radius Thickness Glass Diameter Conic OBJ STANDARD Infinity Infinity 0 0 STO EVENASPH -11249.86 -3646.14 MIRROR 2503.645 -1.285 2 EVENASPH -7194.245 3621.59 MIRROR 1079.861 -11.968 3 EVENASPH Infinity 12 SILICA 721.8408 0 4 STANDARD Infinity 714 721.1468 0 5 EVENASPH Infinity 45 SILICA 748.2 0 6 STANDARD Infinity 5 BSL7 651.9637 0 7 STANDARD Infinity 8 651.5528 0 8 STANDARD Infinity -4.23364 651.4093 0 IMA STANDARD Infinity 0 651.0808 0
Surface OBJ : STANDARD Surface STO : EVENASPH Coeff on r 2 : 0 Coeff on r 4 : 0 Coeff on r 6 : 3.814e-022 Coeff on r 8 : -1.518e-029 Coeff on r 10 : 0 Coeff on r 12 : 0 Coeff on r 14 : 0 Coeff on r 16 : 0 Surface 2 : EVENASPH Coeff on r 2 : 0 Coeff on r 4 : 0 Coeff on r 6 : 1.785e-019 Coeff on r 8 : 0 Coeff on r 10 : 0 Coeff on r 12 : 0 Coeff on r 14 : 0 Coeff on r 16 : 0 Surface 3 : EVENASPH Coeff on r 2 : 2.3206e-005 Coeff on r 4 : -1.1731e-010 Coeff on r 6 : -7.873e-017 Coeff on r 8 : 1.5919e-022 Coeff on r 10 : 0 Coeff on r 12 : 0 Coeff on r 14 : 0 Coeff on r 16 : 0 Surface 4 : STANDARD Surface 5 : EVENASPH Coeff on r 2 : -0.00027324 Coeff on r 4 : 2.0562e-009 Coeff on r 6 : -5.809e-015 Coeff on r 8 : 1.7464e-020 Coeff on r 10 : 0 Coeff on r 12 : 0 Coeff on r 14 : 0 Coeff on r 16 : 0 Surface 6 : STANDARD Surface 7 : STANDARD Surface 8 : STANDARD Surface IMA : STANDARD
Thickness of 8 : Variable
Surf Glass 0.354000 0.476000 0.628000 0.769000 0.925000 0 1.00000000 1.00000000 1.00000000 1.00000000 1.00000000 1 MIRROR 1.00000000 1.00000000 1.00000000 1.00000000 1.00000000 2 MIRROR 1.00000000 1.00000000 1.00000000 1.00000000 1.00000000 3 SILICA 1.47623275 1.46375498 1.45715865 1.45387348 1.45140360 4 1.00000000 1.00000000 1.00000000 1.00000000 1.00000000 5 SILICA 1.47623275 1.46375498 1.45715865 1.45387348 1.45140360 6 BSL7 1.53784092 1.52266028 1.51478667 1.51095428 1.50815398 7 1.00000000 1.00000000 1.00000000 1.00000000 1.00000000 8 1.00000000 1.00000000 1.00000000 1.00000000 1.00000000 9 1.00000000 1.00000000 1.00000000 1.00000000 1.00000000
Wavelength: 0.354000 0.476000 0.628000 # Field Tan Sag Tan Sag Tan Sag 1 0.0000 deg: 4.9691 4.9691 4.9701 4.9701 4.9706 4.9706 2 0.5000 deg: 4.9664 4.9699 4.9672 4.9708 4.9676 4.9712 3 0.7500 deg: 4.9641 4.9703 4.9648 4.9710 4.9652 4.9714 4 1.0001 deg: 4.9647 4.9705 4.9653 4.9711 4.9656 4.9714 5 1.2168 deg: 4.9698 4.9708 4.9705 4.9713 4.9709 4.9716 6 1.3668 deg: 4.9756 4.9713 4.9766 4.9717 4.9771 4.9720 7 1.5002 deg: 4.9814 4.9719 4.9830 4.9723 4.9837 4.9725
Wavelength: 0.769000 0.925000 # Field Tan Sag Tan Sag 1 0.0000 deg: 4.9709 4.9709 4.9711 4.9711 2 0.5000 deg: 4.9679 4.9714 4.9680 4.9716 3 0.7500 deg: 4.9654 4.9716 4.9655 4.9718 4 1.0001 deg: 4.9658 4.9716 4.9659 4.9717 5 1.2168 deg: 4.9711 4.9717 4.9712 4.9718 6 1.3668 deg: 4.9773 4.9721 4.9775 4.9722 7 1.5002 deg: 4.9841 4.9726 4.9844 4.9727
Surf X coord Y coord Z coord X direc Y direc Z direc 1 0.000000 0.000000 0.000000 0.000000 0.000000 1.000000 2 0.000000 0.000000 -3646.140000 0.000000 0.000000 1.000000 3 0.000000 0.000000 -24.550000 0.000000 0.000000 1.000000 4 0.000000 0.000000 -12.550000 0.000000 0.000000 1.000000 5 0.000000 0.000000 701.450000 0.000000 0.000000 1.000000 6 0.000000 0.000000 746.450000 0.000000 0.000000 1.000000 7 0.000000 0.000000 751.450000 0.000000 0.000000 1.000000 8 0.000000 0.000000 759.450000 0.000000 0.000000 1.000000 9 0.000000 0.000000 755.216360 0.000000 0.000000 1.000000
Date created: 3/22/95 Last modified: 3/22/95
