A focal reducer or an expensive wide field lens?

Am I correct in assuming that a focal reducer, whilst effectively shortening
the focal length of a 'scope, also cuts
down the amount of light which eventually hits your eye? Wouldn't this
degrade the clarity and quality of image you will get?

I am assuming you are considering a focal reducer for an commercial SCT. These
function both as a focal reducer and a field flattener so that not only do you
get wider field of view for a given eyepiece, you also get a flatter, better
corrected field of view.

I think the bottom line is that the light loss from inserting a focal reducer
is very small, a couple of percent max (probably) and the advantages in a
wider, flatter (edges in focus) field of view are definitely big so it's
something that every SCT owner ought to have.

Jon

These are definitely worth having, they effectively turn an F10 scope into an
F6.3 scope but they also function

"Kaustav Bhattacharya" wrote in message
om...
I have been reading about focal reducers (FR) quite a bit, lately. It
seems like a cheaper alternative to buying a wide field or ultra wide
field lens which can run in to many hundreds of Pounds/Dollars. It
seems to me that a FR would be most useful when observing large
objects (such as the Moon and large deep space objects) or when using
a high power EP which effectively reduced your FOV.

Using the R/C will not increase the field of view per unit of power. FOV is
strictly a function of focal length of telescope and field stop of eyepiece,
(although the alternative method of dividing the eyepiece's apparent field
of view by the magnification it provides is pretty close to accurate
enough).

At high powers the R/C does nothing for you, except flatten the field. At
high powers, (to me) the field curvature is less noticeable.

Don't take this as a negative, but using the R/C permanently attached,
effectively robs you of your highest power eyepiece, since it reduces the
focal length of the telescope. That said, I use one, so I simply added one
more eyepiece at the short end (7.5mm) for use with the R/C in place.
Otherwise the shortest eyepiece focal length I generally use is 9mm. (At
that, I get to 9mm using an 18mm in a barlow g). The point being that
adding the R/C also can incur the cost of that additional eyepiece depending
on how you intend to use it.

Chances are, you will only want/need one wide field eyepiece for an SCT. If
you already have a 2" diagonal, pulling out the 1.25" adapter for a 2"
eyepiece is a lot easier than swapping in the R/C. (I've dropped my R/C
twice in the dark, and neither time was I too happy). I don't have a 2"
diagonal, but I do have a 35mm Panoptic (my only 2" eyepiece) that I use in
my Dob. I've often wondered if I got a 2" diagonal for my SCT, if I would
bother with the R/C for visual. My sense is, I would not, for exactly the
reason I just stated. I'd have to see just how much the SCT's field
curvature affects the view in the 35mm Pan.

As one last point of possible interest, I also have a 24mm Panoptic that I
use with the R/C to get the same field of view as I'd get with the 35mm
without it. That makes it kind of pointless to bother with the 35mm Pan in
my SCT, unless I was desparate for more than 1.2 degrees, which I am not. I
find that ample. If I really want a wider field, I'll pull out the 80F5
achromat, which also cost less than a premium eyepiece g.

I decided to get the R/C for widest possible field, with 1.25" eyepieces. If
I remember correctly, I got the R/C before I got into imaging.

-Stephen Paul

CeeBee wrote:
As indicated in the other responses, a great benefit of the focal reducer
is a flatter field of view, i.e. less distortion at the edges of the
field.
If you are using a commercial SCT it could have a reduction of say F/10 to
F/6.3, which will almost make up for the loss of brightness due to an
extra piece of glass.

I don't think that's right. A lower focal ratio doesn't mean the image
itself is any brighter at any given magnification. The extra glass means
probably no more than a few percent or so. It's basically a non-issue.

If you mean that it looks brighter because the magnification is lower
for any given eyepiece, the difference there is a factor of 2.5, which
will swamp the few percent from the air-glass interfaces.

The focal length of such an SCT is often more than 2000 millimeters, and
an example looking at a star cluster like M45 will give you only a part of
the field. The benefits for _visual_ observing are clear in this case. But
how many objects are that big? How often do you want a full moon disk in
your FOV?

So most use the focal reducer to minimalize distortion at the edge of the
field while imaging. And so my guess is you probably won't benefit much
vrom a focal reducer for visual observation.

I disagree considerably. I use my reducer/corrector visually, and I
find it quite beneficial. One advantage is the larger true field of view,
which means that star-hopping through the eyepiece--at the last stage,
when you are fine-tuning the pointing of your telescope at your target--
is much easier. Another advantage is that due to the field flattening,
stars at the edge of field are easier to see, again making finding the
right star patterns easier. What's more, not only stars are easier to
see, but the dim fuzzies are, too. If a small dim fuzzy is at the edge
of field without the R/C, you might miss it.

I get a nearly 2-degree TFOV with the R/C and my 32 mm Plossl on my C5+.
While it's true that most objects aren't anywhere near that large, you
mustn't underestimate the visual impact of surrounding a cluster with
enough of the comparatively sparse background that you can really *tell*
it's a cluster. Just fitting a cluster into the field of view doesn't
help enough.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

CeeBee wrote:
I agree, but I was referring to the situation of the OP; I didn't state
it wasn't beneficial for visual observing, I stated that it is largely
used for better imaging performance; and I might very well been mistaken
with that, but I found most people refer to focal reducers when they're
referring to imaging.

No, I think a pretty large fraction use them for visual use also or only.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

I get a nearly 2-degree TFOV with the R/C and my 32 mm Plossl on my C5+.

While it's true that most objects aren't anywhere near that large, you
mustn't underestimate the visual impact of surrounding a cluster with
enough of the comparatively sparse background that you can really *tell* it's
a cluster. Just fitting a cluster into the field of view doesn't
help enough.


Yep, I agree. Having a border frame a cluster is a nice thing, with no border
it does not stand alone as something special.

I have a R/C corrector for my C-5, get that same nearly 2 degree FOV as Brian.
Of course 'taint quite as impressive as the darn near 2 degree FOV I get with
my 10 inch F5 DOB....

Regarding the OP...

Well it turns out the guy has a Meade ETX-105 which seems to complicate the
whole R/C situation....

jon

If you are talking about spending several hundreds of dollars, perhaps
the answer is building a 10" to 12" f 4.5 Dobsonian. Plenty of
wide-field viewing pleasure for those who don't watn to pursue
photography.

"Awhite" wrote in message
om...
If you are talking about spending several hundreds of dollars, perhaps
the answer is building a 10" to 12" f 4.5 Dobsonian. Plenty of
wide-field viewing pleasure for those who don't watn to pursue
photography.

Interestingly enough, my departure into expensive wide-field eyepiece
designs that were well corrected for astigmatism, occurred only after I
purchased an F5 reflector. Until then, and even now, I find that the
Celestron Ultima eyepiece line is plenty sufficient at taller ratios,
especially in clock driven scopes.

I enjoy wide field views, but I really don't use more than a 1 degree field
all that often, even in the Dob (except as a finder).

Bottom line is I have more invested in my Dobsonian setup than I do in my
SCT setup; not that others have to, and not that I don't use those eyepieces
in all of my scopes, now that I have them, but short ratios bring out the
worst in 50 degree eyepieces, which may not be what you want in a wide field
(just a word of caution).

Stephen Paul

I enjoy wide field views, but I really don't use more than a 1 degree field
all that often, even in the Dob (except as a finder).

I enjoy using a 1.7 degree field of view in a 10 or 12 inch scope, especially
from a dark site. Its pretty amazing what one can see/find just scanning.
Also, some targets like the Veil cry out for that widefeild.

But even smaller targets like m93 are fun, those sharp tiny little stars show
up nicely even at low powers in a larger scope.

jon