Re: (ATM) Coma and Telescope Design

[Chuck Grant <grant@aretha.llnl.gov>]

I basically agree, in general, with all your points.  But I think that
there are two sides to each of them.  I think there is not one clear
black and white answer, but a lot of grey area which depends on the
specific observing application and design.  I think it is true that
longer focal ratio telescopes are usually better for visual
observing, but I think that it is not true that longer focal ratio is
ALWAYS better ALL types of visual observing.  Likewise for shorter
focal ratios and photography.

Pat is thinking of building his first telescope (for unspecified
types of visual observations only) and wants to know should he worry
about coma.  I think the most reasonable answer is no.  You have
plenty of other factors which also influence the perfomance and
affect the design to worry about.  The effect of coma, even for an
f/5 is quite small compared to the effect of many of the possible
mistakes that could occur.  If you get all these other things right,
you will have a great instrument, and will probably not even notice
the coma anyway.

> From: lensnut@tpoint.net (Steve Strickland)
> >> From: lensnut@tpoint.net (Steve Strickland)
> >> >Would a f/4.5 telescope with a coma corrector give an observable
> >> >improvement at the edge of wide fields than a f/6.5 telescope without
> >> >one
> >> >Patrick Martin
> >> The f/6.5 telescope will clearly outperform the f/4.5 one, no matter what
> >Why?  And just how clearly is "clearly"?  And how much of this effect is
> >due to coma?
> This remark was directed at visual observing, as stated, and is intended to

"Visual observing" is not a very specific description of the kind of
observing.  The requirements for "planetary" and "deep sky" observing
are quite different, yet both are visual.

> compare two telescopes of identical clear aperature. The defects one sees

Although this particular question at the end of the original post
mentions f/4.5 and f6.5, original poster is considering is two
telescopes of identical FOCAL LENGTH. A 10" f/5 and a 8" f/6.5.  So
this comparison is kind of irrelevant.  

We all remember the motto -- "There is no substitute for aperature."

Anyways, wouldn't identical clear aperature include identical
obstructions?

You can always stop down the faster scope, but you can't "stop up"
the slower one.

> are mostly caused by coma and astigmatism which are present in all
> paraboloidal reflectors. Both of these defects are exagerrated in the f/4.5

I disagree.  Unless you have spent a thousand dollars on a set of
Naglers then the largest defects you would see at these focal ratios
would most likely be due to eyepiece astigmatism.  Which of course is
also more exagerrated with an f/4.5.  Sure primary coma and
astigmatism are there, but you are lucky if you telescope is good
enough that the other problems are so small that you can see the
primary abberations.

> system compared to the f/6 system.  Another 'clear' advantage the longer
> focal length has is the ability to use longer focal lengths in your
> eyepieces to achieve the same magnification.

There are many advantages to long focal ratios, but that "advantage" really
has nothing to do with "clearly outperforming" the short focal ratio.

> >And for what kind of observing?  Clearly the 10" f/5 will be better for
> >faint fuzzies than an 8" f/6.5. The 10" will even be better for planetary
> >observing during brief periods of really good seeing.
> 
> A 10" of any focal length is capable of resolving smaller and dimmer
> objects than any 8" of equal quality. This is not to say that such
> performance is achieved in practice. Also, one cannot directly compare two
> such disparate optical systems. A telescope which is optimized for
> astrophotography makes a terrible visual instrument while one optimized for
> visual observing makes a poor astrophotography instrument. In the above
> comparison, the 8" f/6.5 optimized for visual will have a superior OTF
> curve than the 10" f/4.5 system optimized for astrophotography. This is due
> to the disturbances to the central diffraction spot caused by the
> secondaries.

Your use of the word "optimized" suggests to me that you are saying
that there is one particular design that is "optimum" for all "visual
observing" and another that is "optimum" for all "astrophotography"
(for a given size).  I know better than to think that is what you
meant to say,  but beginners reading this may take the word at face
value.  You never define what you mean by "optimized for visual
(resp. photographic) observing." In the above paragraph it seems that
all you mean is the diagonal mirror size.  Clearly a telescope of any
focal ratio can be a good visual instrument, although perhaps not for
all types of visual observations.

> >> kind of "corrector" you employ, as long as visual observing is the goal.
> >> The longer focal length is also very much more likely to turn out excellent
> >> than the short one is.
> >>
> >> For astrophotography, the f/4.5 system is superior, as long as you are able
> >
> >Why do you think so?  and by how much?  It would depend on what kind of
> >astrophotograph you were taking, I would imagine.

The longer focal ratio scope would have less coma (and for the same
aperature, less field curvature) making it easier to use large film
formats (not necessarily for a wider view).  Or suppose you like to
photograph planets, the larger image size lets you get more detail
onto the film grains (or CCD pixels).  So in some cases the f/4.5 is
not a superior photographic instrument to an f/6.5

> > No answers are that easy.

> No they're not, but the answers do exist. 

Unfortunatly, more than one set of "answers" exists and they are
all "right" in different ways.

Chuck