What does a focal reducer do?
gig50
a lot more. He also plans on doing CCD imaging.
Just what will a focal reducer do for him?
Phil Wheeler
From a Celestron site:
> Reducer/Corrector f/6.3 – #94175
>
> This combination focal reducer and field corrector lens accessory
> works with all Celestron C5, C8, C9¼, C11 and C14 telescopes. This
> clever accessory makes it possible to have a dual focal ratio
> instrument, without sacrificing image quality. The Reducer/Corrector
> is f/6.3 for C5, C8, C9¼ and C11 telescopes and f/7 for the C14
> telescope.
>
> If offers wide fields of view with any Schmidt-Cassegrain telescope.
> Used for astrophotography, it reduces exposure time by a factor of 3.
>
>
> Celestron doesn’t offer a f/6.3 Schmidt-Cassegrain telescope, because
> the design is impractical, with a large central obstruction, and
> resulting loss in contrast. This, added to the inability to utilize
> longer focal ratios for certain objects, led Celestron to design the
> four-element, fully multicoated f/6.3 Reducer/Corrector. Results are
> amazing both visually and photographically.
>
>
Chuck Taylor
The focal reducer makes the scope act like a shorter focal length scope. Of
course, it is not perfect and with the SCTs, there is some vignetting. And,
it adds more glass to the optical path. But at the same time, you can use
those extra elements to also flatten the field, giving you better focus
across a wider area of film.
Clear Skies
Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/
"gig50" <palladin@hatespam> wrote in message
news:QsucnQLuetB...@giganews.com...
Mike Ruskai
A focal reducer is just what the name says - effectively reduces the focal
length of the telescope. This means that the magnification for a given
eyepiece will be lowered. It will not allow your friend to see any more
than he can now, unless all of his eyepieces have a short focal length,
and therefore restrict the field of view to something too small for larger
DSO's.
It will, however, help with CCD imaging. The CCD chip is quite small, and
reducing the image scale will allow fitting a larger region of space into
a single exposure. For DSO imaging, of course. Planetary imaging would
be adversely affected by using a focal reducer.
--
- Mike
Remove 'spambegone.net' and reverse to send e-mail.
William Mc Hale
> On Tue, 3 Jun 2003 16:52:54 -0700, gig50 wrote:
>>A friend has a LX200 8in and he says if he gets a focal reducer he will see
>>a lot more. He also plans on doing CCD imaging.
>>
>>Just what will a focal reducer do for him?
> A focal reducer is just what the name says - effectively reduces the focal
> length of the telescope. This means that the magnification for a given
> eyepiece will be lowered. It will not allow your friend to see any more
> than he can now, unless all of his eyepieces have a short focal length,
> and therefore restrict the field of view to something too small for larger
> DSO's.
I am not sure, but I think the focal reducer will make use of the full 2"
back of the SCT so it will allow extra field even in low power 1.25" like
a 32mm Plossl. Still I prefer a 35mm Panoptic anyday :)
--
Bill
***************************************************************************
Sacred Cows Make The Best Hamburger.
---------------------------------------------------------------------------
Home page - http://www.gl.umbc.edu/~wmchal1
***************************************************************************
Brian Tung
> A focal reducer is just what the name says - effectively reduces the focal
> length of the telescope. This means that the magnification for a given
> eyepiece will be lowered. It will not allow your friend to see any more
> than he can now, unless all of his eyepieces have a short focal length,
> and therefore restrict the field of view to something too small for larger
> DSO's.
I don't see how that's the case. The true FOV in radians, barring any
vignetting, is roughly the field stop of the eyepiece, divided by the
focal length of the telescope. The former is bounded by whatever visual
back or diagonal you're using, so if you reduce the latter, you increase
the upper bound on true FOV. I certainly can see more true FOV in my C5+
with the focal reducer than I can without it.
Brian Tung <br...@isi.edu>
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
dwight elvey
Hi
I'm with you Brian. I use a focal reducer with a 50MM 2 inch
eyepiece and I do see a wider true field of view( more stars
brought in from the sides ). Of course, the magnification is
less but that is what "focal reducer" means.
It is typically used with a CCD to better match the pixel
size with the limit of the telescope. Optically, it has
some limits. Even with good night adapted eyes, I can still
see some kidney bean effect from the secondary on my 8" SCT,
and the 50mm e.p. It isn't so bad I not use it, just that
I do notice it.
The kidney bean effect is what it is called when the shadow
of the secondary becomes visible as a darkening area in the
field of view that is related to ones lateral head position.
This is not like normal vignetting that moves in from the
sides but a darkening that can be moved around the field
of view, including the center.
This effect is noticed when using an obstructed telescope
at low powers that increase the exit pupil size to larger
than the eye's pupil size.
Dwight
CeeBee
> I am not sure, but I think the focal reducer will make use of the full
> 2" back of the SCT so it will allow extra field even in low power
> 1.25" like a 32mm Plossl.
>
FYI that's correct. The focal reducer is fitted directly on the OTA and the
visual back for the eyepieces is fitted on the focal reducer itself.
--
CeeBee
CeeBee's Rant @ http://www.geocities.com/ceebee_2
Mike Ruskai
>Mike Ruskai wrote:
>> A focal reducer is just what the name says - effectively reduces the focal
>> length of the telescope. This means that the magnification for a given
>> eyepiece will be lowered. It will not allow your friend to see any more
>> than he can now, unless all of his eyepieces have a short focal length,
>> and therefore restrict the field of view to something too small for larger
>> DSO's.
>
>I don't see how that's the case. The true FOV in radians, barring any
>vignetting, is roughly the field stop of the eyepiece, divided by the
>focal length of the telescope. The former is bounded by whatever visual
>back or diagonal you're using, so if you reduce the latter, you increase
>the upper bound on true FOV. I certainly can see more true FOV in my C5+
>with the focal reducer than I can without it.
SCT focal reducers have vignetted fields on a 35mm film frame.
It's not clear to me that they'd increase the true field by any
significant amount, given the restrictions ultimately caused by the
baffles.
I suspect that a good long focal length 2" EP without a focal reducer will
provide a better view than a shorter length EP with the reducer.
A typical 8" f/10 SCT gives a maximum useful EP focal length of about
70mm. With a reducer to f/6.4, you bring that down to about 45mm.
Mike Ruskai
[snip]
> The kidney bean effect is what it is called when the shadow
>of the secondary becomes visible as a darkening area in the
>field of view that is related to ones lateral head position.
>This is not like normal vignetting that moves in from the
>sides but a darkening that can be moved around the field
>of view, including the center.
> This effect is noticed when using an obstructed telescope
>at low powers that increase the exit pupil size to larger
>than the eye's pupil size.
The kidney bean effect is caused by spherical aberration of the exit
pupil, which can prevent you from taking in the entire field at the same
distance from the EP. Secondary shadows are another problem altogether.
Brian Tung
> SCT focal reducers have vignetted fields on a 35mm film frame.
Vignetted, but where's the zero-percent illumination circle? Probably
outside the 35 mm frame.
> It's not clear to me that they'd increase the true field by any
> significant amount, given the restrictions ultimately caused by the
> baffles.
They certainly do on the smaller SCTs. They might not on the bigger
ones, where a 2-inch eyepiece can be used without objectionable
vignetting. That isn't the case with the C5+.
Stephen Paul
"Mike Ruskai" <spamten....@begonedynnaht.net> wrote in message
news:gunaalqrneguyvaxa...@newstest2.earthlink.net...
> I suspect that a good long focal length 2" EP without a focal reducer will
> provide a better view than a shorter length EP with the reducer.
>
> A typical 8" f/10 SCT gives a maximum useful EP focal length of about
> 70mm. With a reducer to f/6.4, you bring that down to about 45mm.
I don't understand. In both cases you get 28x. What difference does it make
if one is with an R/C and the other not? (And FWIW, I don't really believe
for a second that you can use either of these focal lengths in their
respective configurations without _serious_ vignetting.
The R/C on my C5 with a 24mm Panoptic figures to be 2 degrees of field.
Without the R/C it's 1.2 degrees. Can I see a difference?, you betcha. The
difference is huge, and I notice no vignetting at 2 degrees with the R/C.
Without the R/C, I'd need to use my 35mm Pan to get near the same field of
view, but I expect a 2" eyepiece is going to vignette, since the primary
baffle tube is now the limiting field stop.
I'd like to think that the R/C works on the light path as it exits the OTA,
so vignetting by the primary baffle doesn't apply. But I doubt it. More
likely, I expect, the primary has to move further toward the secondary to
push the light through the R/C to reach the eyepiece, and that causes the
light path to be wider at the primary baffle, hence cutting into the edges
making the outer edges of the eyepiece field, unable to see the entire
primary. And, if that's the case, what difference does it make if the
primary baffle is vignetting a 2" eyepiece, or a 1.25" eyepiece with R/C
ahead of it?
I've been trying to understand this for almost two years now. Basically, I
decided that I prefer 1.25" eyepieces simply because they are smaller. The
R/C effectively reduces the focal length without increasing the CO, and can
be left on permanently for interchangeable CCD and eyepiece use. As for the
shorter focal length, one merely needs to add an eyepiece of shorter focal
length to regain high power views.
Also, I get 1.2 degrees out of the 24mm Pan at 1260mm with R/C attached to
the C8. I can't complain of find fault with a 1.2 dgree field in a scope
that tracks.
Everyone talks about vignetting in an SCT in an off-hand way, but no one
seems to offer a decent explanation of exactly where it occurs and just how
much field is lost to it. Given the limited number of eyepiece designs, and
an understanding of what's going on, I should think that _someone_ can come
up with the maximum fully illuminated field stop for a 5", 8" and 10 SCT.
(Feel free to dispute any claims above. Like I need to say that on saa.)
-Steve Paul
L.C.
Gotta reply as a focal reducer convert:
I can attest that it's a lot easier to find objects by dead
reckoning with the setting circles on my EQ mount
when I have the focal reducer on. I attribute that effect
(rightly or wrongly) to enhancement of the true field of view.
One can certainly argue both sides at middle
magnifications. A focal reducer gives one access to a
lower minimum magnification, no matter what EP you choose.
My practical limit without a FR, for instance, is on the order
of 70X. With the FR, it's about 45 power. Yes, high
magnification suffers, but the FR is easily removed for such
applications.
As an example, without the FR, M31, M32, and M101 must
be viewed separately in a C8. With the FR, they are all in the
same view. That's a very big difference.
Yes, one can purchase exotic long folcal length EP's to get
the same effect. One can do the same to avoid the purchase
of a barlow for the opposite effect, but a barlow is more
versatile. So is an FR.
There is more glass in the light path, but large dim objects still
benefit from the lower image scale.
Finally, with SCT's the field is flatter with a standard FR.
That's important too, especially for broad F'sOV.
Regards,
-Larry (Likes low mag) C.
Mike Ruskai
>"Mike Ruskai" <spamten....@begonedynnaht.net> wrote in message
>news:gunaalqrneguyvaxa...@newstest2.earthlink.net...
>
>> I suspect that a good long focal length 2" EP without a focal reducer will
>> provide a better view than a shorter length EP with the reducer.
>>
>> A typical 8" f/10 SCT gives a maximum useful EP focal length of about
>> 70mm. With a reducer to f/6.4, you bring that down to about 45mm.
>
>I don't understand. In both cases you get 28x. What difference does it make
>if one is with an R/C and the other not? (And FWIW, I don't really believe
>for a second that you can use either of these focal lengths in their
>respective configurations without _serious_ vignetting.
The minimum useful magnification for an 8" SCT is about 28x. Lower than
that, and the exit pupil will be larger than the typical eye's
dark-adapted pupil size.
I'm just saying that a longer focal length EP for a given magnification
without a reducer is likely to be better than a shorter focal length EP
with a reducer, to achieve the same magnification. Anyone with an 8" SCT
(the telescope referred to in the original question) and a reducer can
feel free to correct me.
>The R/C on my C5 with a 24mm Panoptic figures to be 2 degrees of field.
>Without the R/C it's 1.2 degrees. Can I see a difference?, you betcha. The
>difference is huge, and I notice no vignetting at 2 degrees with the R/C.
>Without the R/C, I'd need to use my 35mm Pan to get near the same field of
>view, but I expect a 2" eyepiece is going to vignette, since the primary
>baffle tube is now the limiting field stop.
The baffle tube is always a true field stop. You're talking about the
same EP with and without the reducer. Clearly the true field will be
larger with it than without it, but that's irrelevant to what I was
saying.
>I'd like to think that the R/C works on the light path as it exits the OTA,
>so vignetting by the primary baffle doesn't apply. But I doubt it. More
>likely, I expect, the primary has to move further toward the secondary to
>push the light through the R/C to reach the eyepiece, and that causes the
>light path to be wider at the primary baffle, hence cutting into the edges
>making the outer edges of the eyepiece field, unable to see the entire
>primary. And, if that's the case, what difference does it make if the
>primary baffle is vignetting a 2" eyepiece, or a 1.25" eyepiece with R/C
>ahead of it?
Forget barrel size. Assume the barrel size is large enough for the
apparent FOV in question. Given two EP's, of such focal lengths that the
one with the reducer produces the same magnification as the other without
it, the difference is twofold.
First, with the shorter focal length EP, there's more glass for the light
to pass through, with more aberrations added. Since the focal reducers
have reversed field curvature, the effect is to flatten the SCT's curved
field. This is useful for film astrophotography, but will not be noticed
in visual observing, where eye accomodation makes the field curvature
unimportant. Any other aberrations are not likely to be opposite in sign
from those that exist in the SCT's primary/secondary combination.
Second, it's easier to figure a longer focal length lens then a shorter
focal length lens. By that virtue alone, the longer focal length EP is
likely to be somewhat better than the shorter focal length EP. There's no
guarantee, of course, but we're not discussing specific equipment here, so
generalities are the rule.
>I've been trying to understand this for almost two years now. Basically, I
>decided that I prefer 1.25" eyepieces simply because they are smaller. The
>R/C effectively reduces the focal length without increasing the CO, and can
>be left on permanently for interchangeable CCD and eyepiece use. As for the
>shorter focal length, one merely needs to add an eyepiece of shorter focal
>length to regain high power views.
>
>Also, I get 1.2 degrees out of the 24mm Pan at 1260mm with R/C attached to
>the C8. I can't complain of find fault with a 1.2 dgree field in a scope
>that tracks.
In your case, you've provided a definite reason to use a focal reducer
over simply a longer focal length EP - you prefer smaller EP barrels.
If the original question had included the fact that 2" EP's were to be
avoided, then my response would have been entirely different.
>Everyone talks about vignetting in an SCT in an off-hand way, but no one
>seems to offer a decent explanation of exactly where it occurs and just how
>much field is lost to it. Given the limited number of eyepiece designs, and
>an understanding of what's going on, I should think that _someone_ can come
>up with the maximum fully illuminated field stop for a 5", 8" and 10 SCT.
I don't have the measurements necessary to come up with a reasonably
accurate figure, else I would have provided it. Or at least attempted to,
since I don't know any formula off hand, and would have to fiddle around
with diagrams and some trig, something I'm a bit out of practice with.
Stephen Paul
news:gunaalqrneguyvaxa...@newstest2.earthlink.net...
> The minimum useful magnification for an 8" SCT is about 28x. Lower than
> that, and the exit pupil will be larger than the typical eye's
> dark-adapted pupil size.
Sure, but you reach the vignetting point well before that since the design
imposed field stop of an 8" SCT is at best 38mm (the inside diameter, as
well as the rear aperture of the central baffle, according to Rutten and van
Venrooij, page 237, section 19.4.
>
> I'm just saying that a longer focal length EP for a given magnification
> without a reducer is likely to be better than a shorter focal length EP
> with a reducer, to achieve the same magnification. Anyone with an 8" SCT
> (the telescope referred to in the original question) and a reducer can
> feel free to correct me.
Here's a concrete example of how there is little difference.
Given the 38mm field stop limitation of the 8" SCT central baffle, let's
consider the TV line of eyepieces (since they conveniently provide field
stop values).
The 35mm Panoptic has a field stop of 38.7mm, so let's take that as the
reasonably largest 2" eyepiece. 2000/35 = 57x, 38.7/2000 * 57.3 = 1.1
degrees (remember that there is .7mm of vignetting here as well).
Now add the R/C, reducing the *" SCT focal length to 1260mm. At 57x the
eyepiece focal length will be 22mm. So let's call it 24mm and use the 24mm
Panoptic which has a field stop of 27mm. This then give us a 52.5x and a
true field of 1.22 degrees (without vignetting).
Similar field, similar exit pupil. The limitation of the field is imposed by
the central baffle. The R/C can get you around that, unless there is a
further reduction in the design imposed limiting field stop, which I am
unsure of, and which I expressed in my previous response.
If the addition of the R/C doesn't further decrease the limiting field stop
of the system, take the 38.7mm field stop of the 35mm Panoptic and apply it
to the R/C configuration. 38.7/1260 * 57.3 = 1.7 degrees!!
And, here it is that I am left without a clear answer. Does the R/C impose a
further restriction on field stop?
Now, if you want to discuss the 10" and larger SCT's, we'd need to know the
inside diamerter of the central baffle. This would not be difficult to
measure if you have a vernier, or a caliper, or whichever it is you use to
measure the inside diameter of a tube. Just pull the visual back, and
measure it. As Brian Tung pointed out, for the larger SCTs, the R/C solution
is probably less meaningful, since there's a good chance that the inside
diameter of that baffle is at, or near the 46mm field stop limit of a 2"
eyepiece.
-Steve Paul
Roger Hamlett
2" eyepieces on an 11" SCT (55mm focal length), field curvature is
distinctly visible, with it becoming impossible to get good focus right
across the field. Substituting a 32mm eyepiece, with a RC, gives a slightly
smaller FOV, but the actual visual result, is more like that seen from some
APO's with field flatteners built in, and can be more pleasing, with the
quite noticeably flatter field. It is also worth realising, that the fully
illuminated field, really makes very little difference, the human eye, is
not a very good astrometric instrument, and telescopes with very significant
vignetting of the field can be used without this becoming noticeable, let
alone a problem. It is typical to see fully illuminated fields that are much
smaller than you think (I have one scope here, where this measurement is
less than 2mm across), yet this can happily be used with an eyepiece with a
20mm field stop, and the intensity 'drop' at the edges of the field, is less
than 25%, and normally invisible, unless using a CCD.
If you look at 'uncorrected' images from most 8" SCT's, when using R/C
lenses, with quite small CCD's (typically perhaps a maximum of about 15mm
diagonally on the CCD, and with the F/6.3 reducer), vignetting is allready
visible, which has to imply the fully illuminated field is less than 30mm
across. There is a balancing act with vignetting, in that it is perfectly
possible to make an 8" SCT, with a 50mm 'unvignetted' field, but doing so,
requires the corrector to grow, and the baffling to be larger, making the
scope likely to have more problems with bright objects close 'off axis' (and
also at the higher angles involved other defects in the optics will become
worse). I'd suspect that most commercial 8" SCT's are showing some
vignetting at a field diameter of perhaps 20 to 25mm, and this probably
becomes visibly noticeable at about 30 to 40mm.
Best Wishes