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Focal Reducers, how do they work?



 
 
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  #1  
Old August 14th 03, 12:58 PM
Stephen Paul
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Default Focal Reducers, how do they work?


"Richard Kenney" wrote in message
thlink.net...
How does a focal reducer work on a SCT? Does it shorten the efective
focal lenght of the scope?


In the case of the F6.3 reducer, a 2000mm focal length SCT becomes an
effective 1260. The reducer screws on to the SCT rear port, and the visual
back screws onto the reducer. And, as Roger points out, it also corrects
curvature.

Curvature is an anomaly in the focal plane, where focus is achieved at a
greater depth at center, than at the edge. If you imagine the focal plane
as a straight line drawn on paper, you can then imagine curvature by bending
that line into an arc. When an eyepiece has curvature, depending on your
eye's ability to accomodate (acommodation is our ability to focus both close
and far from the same location), you may find that you can't bring the
entire view into focus. Either the center or the edge is in focus, but not
both. The general method to reduce the effect of this anomaly (in an
eyepiece) is to focus the stars in the middle area, half way across the
radius.

For photography, there's no accomodation. A curved field results in the edge
stars being somewhat elongated. The R/C reduces that to an acceptable level.

The other issue with the R/C is that it will introduce vignetting.
Increasing the field of view with the R/C effectively puts the focal plane
further inward toward the primary mirror, and this must then be brought back
out to reach the film plane of the camera, or the focal point of an
eyepiece. To bring the focal plane out to where it is reachable, you crank
the SCT focuser and move the primary forward toward the secondary. Because
of the baffle design in the SCT, this results in the inability to see the
entire primary mirror from the edge of the focal plane. The result is that
the edges of the focal plane experience a loss of illumination, by
percentage of visible mirror.

With the R/C in place, the area of the focal plane that is available without
vignetting is large enough to accomodate a CCD, but is borderline for 35mm
film. Meade makes an F3.3 R/C which can be used for CCD without vignetting,
but unlike the F6.3 R/C, this one cannot be used with eyepieces. In
practice, the maximum focal length of the eyepiece you can use with the F6.3
before vignetting is a factor is 30mm. The main point of that last tidbit is
that large 2" eyepieces like the Pentax 40mm or the Panoptic 35mm and 41mm
will show vignetting with the R/C in place. The technical reason for that is
that the field stop of these eyepieces is large enough, and the focal length
puts the eye far enough back, to allow the primary mirror edges to hide
behind the SCTs internal baffles.

-Stephen


  #2  
Old August 14th 03, 11:07 PM
William Hamblen
external usenet poster
 
Posts: n/a
Default Focal Reducers, how do they work?

In article nk.net,
Richard Kenney wrote:

How does a focal reducer work on a SCT? Does it shorten the efective
focal lenght of the scope? I am not an optical theorist so if ypou
choose to respond please do so in terms any bum off the street can follow.


The focal reducer is a positive lens that reduces the effective focal
length of the telescope. "Positive" means the focal length is a positive
number (you can use the lens as a magnifying glass). The amount it
changes the effective focal length of the telescope depends on the
focal length of the focal reducer and the spacing between the focal
reducing lens and the eyepiece or film. Focal reducers have been
around a long time. Lumicon included a focal reducer in its Cassegrain
Easy Guiders for years. Celestron used to sell a "rich field adapter"
that screwed into an eyepiece and was a focal reducer by another name.
Research telescopes have had them for many years.

  #3  
Old August 15th 03, 02:26 AM
Stephen Paul
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Posts: n/a
Default Focal Reducers, how do they work?


"Richard Kenney" wrote in message
hlink.net...

I guess that a dew shield would worsen the vignetting?


The C8 lens shield "claims" to, "unlike others", not cause vignetting.
Whatever that means. I use one on my C8 and the "shield" effect (which
blocks stray light) is more important to me than the dew prevention, since I
also have heaters.



  #4  
Old August 15th 03, 10:57 AM
Roger Hamlett
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Posts: n/a
Default Focal Reducers, how do they work?


"Richard Kenney" wrote in message
hlink.net...
Thanks for the good answers. Believe it or not I could understand them.
I guess that a dew shield would worsen the vignetting? I do have a
SAC ccd camera and intend to try it out tonight (first night in 2 week
without clouds). I only have 1 eyepiece over 30 mm, so I guess I will
be ok in that respect.

Richard Kenney
Your local bum off the street

The effect of the dew shield will probably be insignificant/non-existent
(thankfully!), on vignetting. Think of the light as a 'cone', that is
tapering (in front of the corrector), at 1/2 the FOV of your scope (so if
with the particular eyepiece, you have a 1 degree FOV, the cone widens on
each side by 0.5 degree). Now most SCT scopes have the edge of the
corrector, perhaps half an inch 'inside' the edge of the tube. In the length
of the dew shield (perhaps one foot), the cone will 'grow', by just 0.2", so
the dew shield shouldn't cause a problem _unless_ it is too soft, and 'sags'
into the light cone. :-)
Few SCT's will give a FOV, much beyond 1.5 degrees, which still leaves
clearance inside the dew shield.
It is worth also remembering, that though vignetting is annoying, it is
probably less so, than the opposite. If a scope has no baffle tube, dew
shield, or darkening of the internal tubes, the effect is for light well
outside the FOV, to 'pollute' the image. Mild vignetting, is rarely visible
(needing a camera to be detected).

Best Wishes

Stephen Paul wrote:
"Richard Kenney" wrote in message
thlink.net...

How does a focal reducer work on a SCT? Does it shorten the efective
focal lenght of the scope?



In the case of the F6.3 reducer, a 2000mm focal length SCT becomes an
effective 1260. The reducer screws on to the SCT rear port, and the

visual
back screws onto the reducer. And, as Roger points out, it also corrects
curvature.

Curvature is an anomaly in the focal plane, where focus is achieved at a
greater depth at center, than at the edge. If you imagine the focal

plane
as a straight line drawn on paper, you can then imagine curvature by

bending
that line into an arc. When an eyepiece has curvature, depending on your
eye's ability to accomodate (acommodation is our ability to focus both

close
and far from the same location), you may find that you can't bring the
entire view into focus. Either the center or the edge is in focus, but

not
both. The general method to reduce the effect of this anomaly (in an
eyepiece) is to focus the stars in the middle area, half way across the
radius.

For photography, there's no accomodation. A curved field results in the

edge
stars being somewhat elongated. The R/C reduces that to an acceptable

level.

The other issue with the R/C is that it will introduce vignetting.
Increasing the field of view with the R/C effectively puts the focal

plane
further inward toward the primary mirror, and this must then be brought

back
out to reach the film plane of the camera, or the focal point of an
eyepiece. To bring the focal plane out to where it is reachable, you

crank
the SCT focuser and move the primary forward toward the secondary.

Because
of the baffle design in the SCT, this results in the inability to see

the
entire primary mirror from the edge of the focal plane. The result is

that
the edges of the focal plane experience a loss of illumination, by
percentage of visible mirror.

With the R/C in place, the area of the focal plane that is available

without
vignetting is large enough to accomodate a CCD, but is borderline for

35mm
film. Meade makes an F3.3 R/C which can be used for CCD without

vignetting,
but unlike the F6.3 R/C, this one cannot be used with eyepieces. In
practice, the maximum focal length of the eyepiece you can use with the

F6.3
before vignetting is a factor is 30mm. The main point of that last

tidbit is
that large 2" eyepieces like the Pentax 40mm or the Panoptic 35mm and

41mm
will show vignetting with the R/C in place. The technical reason for

that is
that the field stop of these eyepieces is large enough, and the focal

length
puts the eye far enough back, to allow the primary mirror edges to hide
behind the SCTs internal baffles.

-Stephen





 




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