I have done some research and asked some questions about (f/6.3) reducers
recently, and have been interested to see the same questions coming up
(yet?) again. My conclusions were:

Visual: why bother? Use a longer f.l. eyepiece instead if a wider f.o.v.
is what is desired. An 8" scope gathers the same light whether at f/10 or
f/6.3, no amount of extra optics will make it brighter. A smaller visual
image of an extended object will appear brighter regardless of how it is
achieved, as the same amount of light will occupy a smaller area.

Film: at f/6.3, film exposures will (theoretically) be approximately 1.25
stops faster, i.e. about 60% shorter. With reciprocity failure taken into
account, the saving in exposure time may be even greater. As Ric Ecker
says, you will get more exposures in a night. However, this is not the
only effect. The image size will be smaller due to the shorter f.l. of the
focal reduced scope: if an 8" f/10 SCT has a 35mm field (angular) of
approx. one degree, then at f/6.3 the field will be 2000/1260 times larger,
i.e. approx. 1.6 degrees. No magic here. The image will need enlarging
more, by a linear factor of 1.6, and show much more grain. Bearing in mind
that many deep sky objects are very small (e.g. M51, 10' x 5': M27, 8' x
4'), you are only using a tiny portion of the 35mm frame anyway and
pushing the acceptability of an enlargement to or beyond the limits of
emulsions, so why make things worse? With the bigger objects (e.g. Rosette
Nebula, 1.2 x 1.0 deg) you will fit them on the frame with the reducer, but
suffer the inevitable vignetting described by Philip Perkins and Graham
Warellow among others. With intermediate sized objects you may get
acceptable results as the peripheral parts of the final print will be stars
on a dark background. Basically there is no substitute for the right f.l.
(and hence field) for a given subject. This is what you would normally do
with a terrestrial camera ... use different lenses.

CCD: in this case the vignetting is irrelevant as you are using only a
small central portion of the light cone onto a tiny chip. The effect of
shortening exposure time is much less important as the exposure is so short
anyway, measured in minutes only. The main point is that some objects are
simply too large to fit on the chip at a (to the viewer) acceptable
resolution, and that for the best images it is necessary to optimise the
field size per pixel, the exact value being controversial. It seems that
f/3.3 reducers are most popular for this application, but are much more
expensive so the f/6.3 could be a good start for a minimal investment.

Now, I am still very new to this and have taken few astrophotos and have no
CCD imager (but am playing with an ST4 for guiding). I was initially
interested in a reducer, and had one (Meade) on trial briefly. On my OM1
focussing screen, in daylight, there was appreciable darkening at the
edges, and as you looked away from the centre of the field, the image
became noticeably soft. I had been told it would "flatten the field", the
curvature of which I understand is also an inevitable optical consequence
of the SCT design: to me it appeared worse with the reducer in place. I
thought if it looked like that in daylight it would have no chance of
producing acceptable images on film of point sources! I sent it back.

Most of this is predictable because you are trying to cover a wider field
with an optical system that is only just able to cover 35mm in the first
place, and so vignetting and peripheral aberrations are inevitable. It is
likely that most of the vignetting is due to the exit pupil of the primary
baffle tube rather than the optical characeristics of the reducer itself:
the effect would be similar if instead you tried to use a larger film
format with no reducer. Conventional photographic lenses don't use focal
reducers, for the same good reasons, though I understand it is possible to
design a fully corrected reducer for some scopes, usually refractors that
can cover larger formats.

I now believe that for good results on film, it will be better if I choose
subjects appropriate to my native f/10 8" LX200, and perfect my technique
with that. I dread long periods of guiding so have bought an autoguider.
A piggyback lens or small scope (300mm f/4.5, 480mm f/6.8) will give wider
fields. Only when I have all these pegged shall I think about instruments
of other focal lengths. These are my conclusions, and my logic, with help
from this group ... others may differ!

Wishing you all dark skies and great photos in '98
Nigel
N.Yorkshire UK

View index by [date] [author] [subject]
Previous message: [M]: RE: Focal Reducer, Email address hidden
Next message: [M]: OT: Image Processing at Adler Planatarium, Stars00000
Next message in thread: [M]: Re: [M] Focal reducers, Ric L Ecker