Fred Ward and Questar -- OFF-TOPIC

[Marc James Small <msmall@roanoke.infi.net>]

I found Fred Ward's posting most interesting and have forwarded it, without
comment, to the Questar List.

There are several points which should be borne in mind:

First, Fred:  WHICH Questar are you speaking of?  In the '70's, Questar
manufactured a 700mm lens, the 3.5" in field, standard, and duplex models,
the 7" in standard and duplex, and a 12" behemoth.  The latter three were
available with regular or broad-band coatings (a bit similar to
multi-coatings) and with either Pyrex or low-expansion mirrors.  I suspect
what you had was a 3.5" standard.

Second, Fred appropriately mentioned Questar's fine advertising.  Actually,
though Questar's glory days were in the '50's, when it WAS a stunningly good
design and quite a bit better than anything else on the market.  But times
change, and a lot of photographic objectives came on the market in the '60's
and '70's which could challenge it as a PHOTOGRAPHIC objective, though the
Questar, as a visual performer, is still unsurpassed to this day (though
Quantum, a modification of the basic design, probably equals it).

Third, 'f-stops' and cat designs are not completely compatible.  The central
obstruction  -- the reflex mirror -- on a Maksutov or Schmidt design makes
the lens a tad slower than the listed f/ratio.  The 3.5" Questar is
oestensibly f/14 but, due to the reflex mirror, the actual speed is closer
to f/15 -- I don't have mine here to measure it for a more precise answer.
The result -- the f-stop adjusted for the central obstruction -- is known as
a 't-stop' and was a feature of 1950's photography.  (The f/stop is a
mathematical calculation, the t-stop is a real-world factor derived by
subtracting ANYTHING which affects the full aperture of the lens.)

Fourth, the Questar would have come with an off-axis solar filter.  The
filter Fred ordered was probably the full-aperture filter.

Fifth, all solar filters can cause a double image whether completely
parallel or not, especially with off-axis images.  

Sixth, I suspect that what the fellow at Questar stated was that the
instrument, including the full-aperture filter, was intended for visual and
not photographic use.  This is quite true.  As the full-aperture filter was
originally contracted for by NASA, I would be rather surprised if it wasn't
parallel to the limits of testing -- the ones sold today certainly are.
However, VISUAL and photographic optics require different parameters for
design and construction, and the Questar has always been aimed at a visual
(astronomical) market and its photographic uses as a secondary benefit.
Questar DOES make purely photographic optics -- the Ed Kaprelian-designed
700mm lens being a fine example -- but the standard telescopes are
TELESCOPES which have a photographic capability.  (In the same regard, the
drive installed from the '50's to the '80's wasn't precise enough, by
several factors, for the hours of tracking required for most astronomical
photographs, though the current drive certainly is.)

In any event, Fred, thanks for a most interesting note.

Marc

msmall@roanoke.infi.net  FAX:  +540/343-7315
Cha robh bas fir gun ghras fir!