ComatsRC J2

The first term in the square bracket is that corresponding to a classical Cassegrain (parabolic primary) and the second term is the RC supplement. Converting to arcsec with

(Comat) arcsec = (Comat)iat f , we have in Table 5.1 the decentering coma values in arcsec corresponding to 5 = 1 mm for the case of the 60-inch Mt. Wilson, 80-foot focal length Cassegrain focus (m2 = -3.2) and the case of Ritchey's RC telescope. We see from this table that the increase in coma due to the optical geometry of the Ritchey 40-inch as a classical telescope is a factor 2.16 whereas the increase due to its (extreme) RC form is a further factor of 1.88. As illustrated in Chap. 3, the latter factor is only about 1.05 for modern RC telescopes with much higher m2 and lower Ra-

3 The original Ritchey optics were lent to the Vienna Observatory in 1971 and were subsequently (1978) used in a 1 m telescope named after Doppler. I am deeply grateful to Dr. D. Osterbrock of Lick Observatory and Dr. R. Walker of the US Naval Observatory for the above information concerning the motivation for replacing the original Ritchey optics and the nature of the change. The thermal sensitivity of the original optics was probably mainly due to Ritchey's use of a "low-expansion glass" from St. Gobain with expansion "about one-half that of crown glass". This was a useful improvement in expansion compared with the 60-inch and 100-inch plate-glass blanks, but was inferior to the Pyrex of the time. The strong vignetting of Ritchey's secondary was probably due to his desire to reduce the central obstruction to a minimum and the fact that photographic photometry only later developed its full potential.

Table 5.1. Comparison of the lateral decentering coma with S =1 mm of the 60-inch Mt. Wilson telescope and Ritchey's 40-inch RC telescope for the US Naval Observatory

Case

Transverse decentering coma (arcsec) S = 1 mm

60-inch Mt. Wilson 80-foot focus classical Cassegrain (m2 = -3.2; f/16)

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