Shor Focal Length AND Aperture?

Is there a scope that combines the wide-filed views of a short tube
(400mm or so) and light-gathering ability (8" or so). Or is the best
of both worlds too much to ask for?

Thanks


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On 09 Oct 2004 20:12:10 EDT, (RS) wrote:

Is there a scope that combines the wide-filed views of a short tube
(400mm or so) and light-gathering ability (8" or so). Or is the best
of both worlds too much to ask for?

Thanks

Sorry, you are out of luck.
An 8" f4 and a 31mm Nagler eyepiece will give you
2.5 degrees of sky. You are at the practical limit
of lower power, wide field with an 8" scope. .
A 400mm will obviously give you much more sky

On Sat, 09 Oct 2004 20:28:29 -0400, rander3127
wrote:

On 09 Oct 2004 20:12:10 EDT, (RS) wrote:

Is there a scope that combines the wide-filed views of a short tube
(400mm or so) and light-gathering ability (8" or so). Or is the best
of both worlds too much to ask for?

Thanks

Sorry, you are out of luck.
An 8" f4 and a 31mm Nagler eyepiece will give you
2.5 degrees of sky. You are at the practical limit
of lower power, wide field with an 8" scope. .
A 400mm will obviously give you much more sky


8" f4 sounds reasonable. Who makes such a scope?


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RS wrote:


8" f4 sounds reasonable. Who makes such a scope?


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There are plenty aroud, from various manufacturers. Just bare in mind
that coma is lightly to be an issue at this kind of focal ratio...

--
Steve Maddison
Den Haag, The Netherlands
http://www.cosam.org/

On 09 Oct 2004 20:12:10 EDT, (RS) wrote:

Is there a scope that combines the wide-filed views of a short tube
(400mm or so) and light-gathering ability (8" or so). Or is the best
of both worlds too much to ask for?

A 13cm f/6 apochromat refractor would quite possibly come closest to
meeting your criteria -- and then some!

You could get a 3 degree field at 20x (without a central obstruction
and without coma). The low magnification performance is truly
'uncompromised'.

Aperture for aperture nothing could beat it at high magnifications for
planetary work, etc. Again, there's no compromise.

A 5.1-inch apo doesn't have an 8-inch aperture, but it's more
efficient in light transmission than most other designs. An apo
concentrates a greater percentage of the collected light into actual
image detail than any obstructed telescope. Performance-wise, in the
real world, many 8-inch telescopes fall behind that of a 5.1-inch apo.

Cool-down time is better with a refractor than it is with most other
telescope designs. Tube currents are much less of a problem than they
are with Newtonians, etc. Collimation tends to be 'on the money' when
the scope leaves the manufacturer; and for most practical purposes it
stays that way.

I guess there are reasons why the demand for such telescopes exceeds
the supply.

OTOH, an 8-inch f/4 Newtonian can be had for a *much* lower price; but
you never mentioned price as a concern ;-)

(Unrelated note: my server lost at least a days worth of saa postings
recently -- first time that's happend in a long time.)

Sketcher
To sketch is to see.

Is there a scope that combines the wide-filed views of a short tube
(400mm or so) and light-gathering ability (8" or so). Or is the best
of both worlds too much to ask for?

Thanks

The limitation here is on the human eye, it only opens so far. In young people
this seems to be a maximum of about 7mm and reduces gradually as you grow
older.

So what this means is that there is a practical limitation on the lowest
magnification for any "effective" aperture.

The "exit" pupil of the telescope/eyepiece is calculated by dividing the
aperture of the telescope by the magnification. A 200mm (8 inch) scope at 20X
will produce an exit pupil, the beam of light leaving the eyepiece, that is
200mm/20=10mm in diameter.

Since you eye is only open 7mm maximum, all the light from the telescope does
not enter your eye and a significant amount is unused.

For an 8 inch telescope, the minumum magnification that will allow you to use
all the light is 200mm/7mm=29X.

As far as the 8 inch F4 scope goes, I don't recommend it. At F4, you will
reach that 29X with a 28mm eyepiece which is OK but the problem with an F4
Newtonian (the only thing made at F4) is that the stars are only sharp in the
center of the Field of view. With some extra money and some high quality
eyepieces, this can be improved but those big wide fields of view on wants will
never be sharp across the field of view.

In an 8 inch scope, one is better of IMHO with F5 or F6, at F6, things are
pretty nice with normal eyepieces and additional correctors ($300) are not
needed.

Bottom line: Best of both worlds is too much to ask for.

Jon

"rander3127" wrote in message
...
Yes, a coma corrector from TeleVue will be needed.

....or an 8" F/4 SNT.

John C

"John" wrote in message . ..
"rander3127" wrote in message
...
Yes, a coma corrector from TeleVue will be needed.

...or an 8" F/4 SNT.

And you still might need a coma corrector. 8" F/4 SNT has similar coma
to an 8" F/5 Newt.


John C

Mitch