Lens Designs for 35mm film and electronic astrophotography
1. Introduction
Modern 35mm SLR camera lenses
from the very best names such as Leica, Nikon,
Canon and Olympus seem at first glance to
offer the possibility of high quality astrophotographs,
given a good quality tracking mount and fast colour film. But any perusal of
the opinions of users who've tried such lenses shows them to be generally
less than satisfied with the results that they achieve. There are a
number of reasons for this.
Firstly, it is not actually possible to design a lens to other than a trivial
specification which can perform to the limits set by diffraction.
Secondly, all real lenses designed and produced by the camera industry are a
trade-off between what is theoretically possible and what is economically
attractive to the purchaser. A considerable price premium must be paid
for any extreme care taken with the manufacture of the lens elements and the metalwork, particularly the mounting
flanges, sealing faces, the focus mechanism and the remotely controlled
diaphragm.
Thirdly, the camera lens designer can allow the performances away from the
centre of the field to fall. For astrophotography this relaxation is
hardly tolerable. Sky photographs from such lenses can be very
unattractive.
In this note I offer a number of designs which I prepared while looking for
an 800mm F/8 telescope objective lens. Although one seems able to buy
such a telescope, quantitative performance claims are rarely offered.
While doing this work, I thought it might be interesting to see what is
possible from a 400mm F/6 and a 200mm F/4 when used with a camera.
There are a number of ways of specifying the performance of lenses.
'Resolving power' is very much an arbitrary and subjective measure, and a bit
old-fashioned in these days of powerful design tools and measuring
equipment. In this paper we adopt the polychromatic modulation transfer
function, PCMTF, as the criterion. A value of PCMTF greater than 30% at
a spatial frequency of 100cycles/mm at any point in the field is
desirable. This value can be compared with typical camera lenses
where the typical PCMTF has fallen to 10% at a spatial frequency of 50cycles/mm.
Notice however that this type of lens is considerably more complex than a
simple telescope objective lens. This follows from the very much larger
field.
It's possible these lenses already exist as real camera lenses - but I doubt
it. Anyway, there may be some enterprising
ATMs who might be able to make them, or even a specialist manufacturer who
could tackle it. Judging by the FAQ on the WWW, there does seem
to be a large number of skywatchers
who could make good use of such lenses if they were available and affordable.
2. A design for an 800mm F/8 lens
This is an especially interesting design. It is
almost perfectly corrected for chromatism between
0.45µm and 1.1µm.
3. A design for a 400mm F/6
lens
This lens has an image field diameter of 6.2degrees
4. A design for a 200mm F/4 lens
This lens has a field of view greater than 12deg. diagonal
