Re: f-stop question
- From: Margo Schulter <mschulter@xxxxxxxxxxxxxxx>
- Date: 03 Jul 2007 03:26:38 GMT
Llanzlan Klazmon the 15th <Klazmon@xxxxxxxxxxxxxxxx> wrote:
"goodnigh" <goodnigh1@xxxxxxxxxxxxxx> wrote in news:DRYhi.3032$tj6.44
@newsread4.news.pas.earthlink.net:
Will be buying a 10 inch 1200mm f4.5 Dob.
After experimenting with inexpensive GoTo scopes,
I have decided to "GoTo" basic star hopping.
Somewhere I read there is an issue with fast scopes.
Just can't remember what they are.
mike
In the case of a newtonian like this a faster f ratio implies more off axis
coma and greater sensitivity to accurate collimation. Exit pupil might be a
problem if you plan to use 30mm or greater fl eyepieces.
Klazmon
Hi, there, Mike, and as a beginner in serious observational astronomy who's
looking forward to first light for my new 20cm f/6 Dob, I might enlarge a bit
on some of the points Klazmon has offered -- with the caution that I'm more
familiar with theory than with practice! I'd be interested to learn myself
how what I discuss below compares with people's experience, and would also
invite any corrections.
First, one of the consequences with fast scopes is that eyepiece imperfections
tend to be more obvious or significant in their impact on observing than with
slower optics. For moderately priced eyepieces with wide-angle fields, the kind
that you might well want to use to get the most out of the "light bucket"
qualities of the scope for diffuse deep-sky objects (DSO's), faster optics may
thus mean greater visual compromises of the kind Klazmon has mentioned: for
example, coma as one moves out from the central area of the field.
With an f/6 scope, for example, I've gotten a couple of ultra-wide angle
eyepieces: 30mm with an 82deg apparent field of view (AFOV); and 20mm with
an 80deg AFOV. These fields of view are identical or almost so to the 82deg
of the famous Nagler design: but obviously the quality will not be quite
so high, for example when it comes to edge sharpness and correction!
If I had an Schmidt-Cassegrain telescope (SCT) at f/10 or f/12, for example,
the visual performance of these eyepieces might not be too far from that of
the more precise Naglers which they emulate: slow optics are more forgiving.
With an f/6, it's a calculated compromise: a reasonable result might be a
measure of sharpness for the inner 2/3 of the field, say, with things
"softening" outward from there, and "space warp" effects in the outermost
10% or so of the field, with stars beginning to look like comets.
At around f/4.5 -- precisely 10" (254mm) aperature and 1200mm focal length
would be close but slightly slower, around f/4.7 -- I'd suspect that the
compromise would become less desirable, and one of the vendors in fact
recommends something like f/6 as a lower limit where things should be
reasonably satisfactory. In part it can be a matter of taste: someone tried
one of these eyepieces in a Dob at f/5 or so as a kind of extreme test, and
liked it -- _despite lots of distortion_, however.
Again, I'm using the example of a specific and demanding application: other
types of very useful eyepieces might not have these problems, and for really
wide-angle eyepieces there are some that do well even in quite fast scopes.
One bit of conventional wisdom has it that if you choose the advantages of
a faster Dob, then, all things being equal, you may need higher-quality
eyepieces in certain applications to get comparable visual results.
However, it's important to evaluate each eyepiece: reading reviews is one
approach, and another is trying out eyepieces of interest, if made available
by others, at a star party, for example ("try before you buy").
Also, TeleVue makes available a device called Paracorr to alleviate some
of the imperfections of fast scopes around f/4.5 -- a considerable
investment, but something that some users who can afford it have reported
can indeed improve performance with these scopes. Like TeleVue eyepieces,
the Paracorr seems designed for exacting observers; maybe others here
have used it and can comment.
A curious point is that your planned Dob has the same focal length as mine,
1200mm, so the same eyepieces would yield the same magnification. However,
as Klazmon has mentioned, the exit pupil would differ with the f-ratio.
Let's suppose, for example, you find a good 30mm eyepiece which works well
with your f/4.5 or f/4.7 scope.
One way of finding the exit pupil, the diameter of the light cone that
meets the eye, is to divide the focal length of the eyepiece by the
focal ratio of the scope. The concern to which Klazmon evidently refers
is that if the exit pupil is larger than the actual size of the viewer's
pupil, then some light from the image will be lost -- effectively
diminishing, from the viewer's perspective, the light-gathering ability
of the telescope as if its aperture had been reduced.
For a young adult, the pupil of the eye might dilate to around 7mm when
fully dark-adapted under dark-sky conditions. Here a 30mm eyepiece would
yield an exit pupil of around 6.67mm at f/4.5, 6.35mm at f/4.72 (a 10"
scope at 1200mm focal length), and 5mm at f/6. All of these should be
fine.
For an older adult like myself, the eye might dilate only to around 5 mm.
Thus we might have _some_ effective light loss with a 30mm eyepiece at
f/4.5 or f/4.7 -- but I'd say not too much. The manual for my telescope,
at any rate, suggests that an exit pupil around 6.4mm would likely be
acceptable for just about any observer -- but get into longer focal
length eyepieces where the exit pupil increases to 7mm or more, and
older observers, and eventually younger ones also, are going to
experience significant virtual aperture reduction, as it might be called.
Anyway, more experienced people may be able to give practical advice
and correct these newbie remarks, but I want to wish you the best with
your coming scope and with amateur astronomy.
Most appreciatively,
Margo Schulter
mschulter@xxxxxxxxxx
.
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- f-stop question
- From: goodnigh
- Re: f-stop question
- From: Llanzlan Klazmon the 15th
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