Focal reducer for visual observing??
Chunga
a focal reducer.
My question involves the kinds of changes I would see with visual observing.
From what I can gather I'd get a larger true fov and lower magnification
from each ep. Is this correct?
Will there be changes in contrast? clarity?
When do you use your focal reducer? Which objects/conditions do you find an
advantage visually using a reducer?
Reccomendations for a brand? (I haven't seen many other than the celestron
f6.3)
TIA,
-Tim
Jeff Lee
It will be one of the better investments you make. Flattens the field as
well as giving you a larger FOV (unless you go for 2" eps). Unless I'm
looking at planets, it stays on my SCT all of the time.
--
Jeff Lee, MBA
TekXam Certified
"Chunga" <Chu...@Revenge.bark> wrote in message
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Chunga
these with a f-reducer?
I am having a hard time visualising a "flattened" field, can you describe in
other terms?
Your advice is encouraging, which one do you have?
Thanks
-Tim
"Jeff Lee" <artw...@ix.netcom.com> wrote in message
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Gert Weber
i had one (the Alan Gee Telekompressor buy Baader Planetarium) and I
can highly recommend it.
You can change the compression rate buy inserting addional rings; I
usually had it f6.3.
>I have a C8 (f10 of course) and was wondering what changes I would see with
>a focal reducer.
>My question involves the kinds of changes I would see with visual observing.
>From what I can gather I'd get a larger true fov and lower magnification
>from each ep. Is this correct?
than without the Telkompressor.
> Will there be changes in contrast? clarity?
focal reducer, because it needs some time to change the optics.
But offhand I'd say the difference is not big.
>When do you use your focal reducer? Which objects/conditions do you find an
>advantage visually using a reducer?
wider field for deep sky.
>Reccomendations for a brand? (I haven't seen many other than the celestron
>f6.3)
--
Pardon my english -
it became rusty under our german clouds and in the rain!
Jan Owen
"Chunga" <Chu...@Revenge.bark> wrote in message
Snip!
> I do have a 2" diagonal and 38mm and 50mm 2" eps. Would I not want to use
> these with a f-reducer?
OK, probably not, and no. The 2" diagonal will be fine, if you have a 1.25"
adaptor for it. The 38mm may begin to show vignetting (depending on it's
apparent field), and the 50mm will almost certainly show vignetting
(darkening around the edge of the field). Other 2" eyepieces, shorter than
about 32 to 35mm, should perform just fine with a reducer/corrector.
>
> I am having a hard time visualising a "flattened" field, can you describe
in
> other terms?
>
Imagine focusing crisply on a star in the middle of the field, then looking
at one much further out toward the edge, and having it ALSO in focus at the
same time... A curved field means when you look at stars other than in the
immediate vicinity of the one you focused on, those other stars will be a
little bit out of focus, because the plane of sharp focus is curved outward
from the center. A reducer/corrector not only speeds up the focal ratio, it
also has correction built into it to lessen the curvature of field inherent
in the SCT's design. It won't make all the stars in the field precisely in
focus at the same time, but it will improve sharpness noticeably over a much
larger portion of the field.
Snip!
Stephen Paul
"Chunga" <Chu...@Revenge.bark> wrote in message
> I do have a 2" diagonal and 38mm and 50mm 2" eps. Would I not want to use
> these with a f-reducer?
Unless I am mistaken (which is not really all that uncommon <g>), the
maximum field of any eyepiece is limited largest opening through which the
light can pass. In this case, it's the primary baffle and the rear port
opening of the 8" SCT, which is 1.5".
Using the 2" diagonal and eyepieces alone, you can exceed the maximum field
width limitations of the 1.25" barrel eyepieces, but only until you hit the
1.5" limit of the rear port. (In fact, I believe your 50mm exceeds this
value and you should see darkening at the edge of the field when you look
through that eyepiece).
Once you add the "reducer", the shortened focal length of the telescope will
increase the "actual" field width of the largest eyepiece you could use with
the 2" diagonal alone. This then causes that eyepiece to exceed the 1.5"
stop of the rear port, causing vignetting in that eyepiece.
So the largest 2" eyepiece is now actually a shorter focal length than it
was previously. As it works out, you can not get any larger field with the
2" eyepieces, than you can with a barrel limit of 1.25" and the R/C. Or more
simply put, with the R/C you no longer _need_ the 2" eyepieces to achieve
the maximum field of view. A 32 mm plossl will get you there.
>
> I am having a hard time visualising a "flattened" field, can you describe
in
> other terms?
Since we can achieve the maximum field with 2" eyepieces, the real benefit
of the R/C is found in the flattening of the field. Field curvature results
from the extreme curve of the F2 primary mirror, and prevents the entire
field from being in focus all at once. With experience and eyepiece time you
will notice this more and more in wide field observations. When you sharp
focus the stars at the center of the field, the ones at the edge will be
just a bit out of focus. The "Corrector" part of R/C "flattens" the field
so that the affect of the mirror's curve is reduced or eliminated. A 32mm
Plossl will look much better and give a wider field of view than before, and
a high end, well corrected eyepiece will have pinpoint stars throughout the
field of view.
The R/C is to the SCT, what the Paracorr is to a fast reflector, with the
additional benefit of reducing the focal length without increasin the size
of the central obstruction.
L.C.
I discover I can find objects more easily, particularly
when I am searching using analog setting circles on the
mount. M31 and M32 are visible in the same FOV. The
whole double cluster fits too. The field is noticeably flatter
as well - sharp almost from edge to edge.
The fr is easily removed, and really does turn the scope into
two scopes - one for planetary viewing and another for
deep sky. Tell you the truth, though, I normally leave it
in for everything.
The Meade and Celestron are equivalent. Buy the cheaper one.
-Larry Curcio
Jan Owen
can be replaced by a larger one that can be had from JMI (and I believe
other sources as well) for use with their NGF-S Crayford electronic focuser.
This larger port allows for a wider light path through the system before
vignetting is encountered, though it does appear that under the right
circumstances this could also lead to some unwanted stray light entering the
light path... So a 32mm Plossl need not be the largest practical eyepiece
that can be used with that 8" and an r/c.
While there may be some vignetting with a 35mm Panoptic using a 2" deep sky
adapter and 2" diagonal (or just a screw-on 2" diagonal) attached to the r/c
mounted on the standard rear port, but the images will still be awesome. A
32mm SWA or older TV Wide Field will also deliver some pretty impressive
views.
"Stephen Paul" <SPau...@hotmail.com> wrote in message
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SNIP!