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SCT Focal Reducers and Vignetting (explained)



 
 
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  #1  
Old January 23rd 04, 04:34 PM
Stephen Paul
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Posts: n/a
Default SCT Focal Reducers and Vignetting (explained)

I was trying to find some information on the workings of the Meade
F/3.3 Reducer for CCD imaging and I came across the web page linked
below. Interestingly, as a side effect of the infomration, I finally
understood the vignetting problem with R/C's. It was good to see a
graphic that depicts the light cones exiting the rear cell of the SCT
at F/10, F/6.3 and F/3.3. The key thing to note is the reduction in
the diameter of the light cone that hits the focal plane.

Consider that the inside diameter of the primary baffle tube of a C8
is 38mm. This is the maximum diameter that the light cone exiting the
back end of the C8 can reach (25mm in a C5, 47mm in a C9.25, and 52mm
in C11/C14's, IIRC).

Conveniently, 38mm matches closely the field stop diameter of the 35mm
Panoptic eyepiece, which in turn, at 2000mm focal length and F10
yields (for all intents and purposes) a fully illuminated field of
1.1 degrees.

Now, keep in mind that the field of view will never be larger than
what the field stop of the eyepiece will allow. BUT, you can surely
stuff a 55mm Plossl with a 47mm field stop into your 2" diagonal. The
result? You will still only have 1.1 degrees of full illumination,
because the light cone coming out of the back end of the C8 at F10, is
at best 38mm in diameter.

Exceeding the diameter of the light cone at the focal plane by using
an eyepiece with a field stop that is larger than the 38mm, means that
less to no light is reaching the edge of the eyepiece field. That's
vignetting.

Introducing a reducer, doesn't do anything to improve the situation.
All it does is reduce the diameter of the light cone at the focal
plane to something less than 38mm, so that all the light of the
objective fits within a _smaller_ field stop. As a result, increasing
the field of view within that smaller field stop.

Hence, introducing a reducer before an eyepiece with a field stop of
38mm (or greater, given the C8), will cause vignetting, where it
didn't exist before. You _will_ still get 1.1 degrees of fully
illuminated field, but that 1.1 degrees now fits in a smaller circle
at the center of the eyepiece, beyond which there is less and less
light striking the field lens of the eyepiece, going out to the edge.

So, just like stuffing the 55mm Plossl into the scope at F10, stuffing
a 35mm Panoptic into the scope at F6.3 will give you a wider field,
but the 100% illumination of the field will be something less than the
field stop of the eyepiece will allow, and no greater than the central
baffle will allow at F10.

It has been presented that beyond 75% illumination, one really begins
to notice the dimming of the field. So, the maximum useful gain of the
F/6.3 R/C with a 35mm Panoptic (38mm field stop), is that point
approaching the field stop at which only 75% of the objective is
visible through the 38mm primary baffle.

This might be calculated if all the values are known, or it might be
carefully measured by projecting light through the OTA, but in the end
the best way is probably by experiencing the effect in the eyepiece,
and doing some timings of objects as they pass through the acceptably
illuminated area (a subjective measure for sure). Since I just so
happen to have a 35mm Pan and an F/6.3 R/C, guess I need to get a 2"
Diagonal and do some timings. g

Seems so simple and obvious, now. I definitely got my picture's worth.
I hope you do too.

http://benedict.isomedia.com/homes/c...l_reducers.htm

-Stephen ("Steve") Paul
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  #2  
Old January 24th 04, 07:35 AM
Michael
external usenet poster
 
Posts: n/a
Default SCT Focal Reducers and Vignetting (explained)


"Stephen Paul" wrote in message
om...
I was trying to find some information on the workings of the Meade
F/3.3 Reducer for CCD imaging and I came across the web page linked
below. Interestingly, as a side effect of the infomration, I finally
understood the vignetting problem with R/C's. It was good to see a
graphic that depicts the light cones exiting the rear cell of the SCT
at F/10, F/6.3 and F/3.3. The key thing to note is the reduction in
the diameter of the light cone that hits the focal plane.

Consider that the inside diameter of the primary baffle tube of a C8
is 38mm. This is the maximum diameter that the light cone exiting the
back end of the C8 can reach (25mm in a C5, 47mm in a C9.25, and 52mm
in C11/C14's, IIRC).

Conveniently, 38mm matches closely the field stop diameter of the 35mm
Panoptic eyepiece, which in turn, at 2000mm focal length and F10
yields (for all intents and purposes) a fully illuminated field of
1.1 degrees.

an eyepiece with a field stop that is larger than the 38mm, means that
less to no light is reaching the edge of the eyepiece field. That's
vignetting.

Introducing a reducer, doesn't do anything to improve the situation.
All it does is reduce the diameter of the light cone at the focal
plane to something less than 38mm, so that all the light of the
objective fits within a _smaller_ field stop. As a result, increasing
the field of view within that smaller field stop.

Hence, introducing a reducer before an eyepiece with a field stop of
38mm (or greater, given the C8), will cause vignetting, where it
didn't exist before. You _will_ still get 1.1 degrees of fully
illuminated field, but that 1.1 degrees now fits in a smaller circle
at the center of the eyepiece, beyond which there is less and less
light striking the field lens of the eyepiece, going out to the edge.



Ya got me baffled


  #3  
Old January 24th 04, 07:35 AM
Michael
external usenet poster
 
Posts: n/a
Default SCT Focal Reducers and Vignetting (explained)


"Stephen Paul" wrote in message
om...
I was trying to find some information on the workings of the Meade
F/3.3 Reducer for CCD imaging and I came across the web page linked
below. Interestingly, as a side effect of the infomration, I finally
understood the vignetting problem with R/C's. It was good to see a
graphic that depicts the light cones exiting the rear cell of the SCT
at F/10, F/6.3 and F/3.3. The key thing to note is the reduction in
the diameter of the light cone that hits the focal plane.

Consider that the inside diameter of the primary baffle tube of a C8
is 38mm. This is the maximum diameter that the light cone exiting the
back end of the C8 can reach (25mm in a C5, 47mm in a C9.25, and 52mm
in C11/C14's, IIRC).

Conveniently, 38mm matches closely the field stop diameter of the 35mm
Panoptic eyepiece, which in turn, at 2000mm focal length and F10
yields (for all intents and purposes) a fully illuminated field of
1.1 degrees.

an eyepiece with a field stop that is larger than the 38mm, means that
less to no light is reaching the edge of the eyepiece field. That's
vignetting.

Introducing a reducer, doesn't do anything to improve the situation.
All it does is reduce the diameter of the light cone at the focal
plane to something less than 38mm, so that all the light of the
objective fits within a _smaller_ field stop. As a result, increasing
the field of view within that smaller field stop.

Hence, introducing a reducer before an eyepiece with a field stop of
38mm (or greater, given the C8), will cause vignetting, where it
didn't exist before. You _will_ still get 1.1 degrees of fully
illuminated field, but that 1.1 degrees now fits in a smaller circle
at the center of the eyepiece, beyond which there is less and less
light striking the field lens of the eyepiece, going out to the edge.



Ya got me baffled


  #4  
Old January 24th 04, 07:35 AM
Michael
external usenet poster
 
Posts: n/a
Default SCT Focal Reducers and Vignetting (explained)


"Stephen Paul" wrote in message
om...
I was trying to find some information on the workings of the Meade
F/3.3 Reducer for CCD imaging and I came across the web page linked
below. Interestingly, as a side effect of the infomration, I finally
understood the vignetting problem with R/C's. It was good to see a
graphic that depicts the light cones exiting the rear cell of the SCT
at F/10, F/6.3 and F/3.3. The key thing to note is the reduction in
the diameter of the light cone that hits the focal plane.

Consider that the inside diameter of the primary baffle tube of a C8
is 38mm. This is the maximum diameter that the light cone exiting the
back end of the C8 can reach (25mm in a C5, 47mm in a C9.25, and 52mm
in C11/C14's, IIRC).

Conveniently, 38mm matches closely the field stop diameter of the 35mm
Panoptic eyepiece, which in turn, at 2000mm focal length and F10
yields (for all intents and purposes) a fully illuminated field of
1.1 degrees.

an eyepiece with a field stop that is larger than the 38mm, means that
less to no light is reaching the edge of the eyepiece field. That's
vignetting.

Introducing a reducer, doesn't do anything to improve the situation.
All it does is reduce the diameter of the light cone at the focal
plane to something less than 38mm, so that all the light of the
objective fits within a _smaller_ field stop. As a result, increasing
the field of view within that smaller field stop.

Hence, introducing a reducer before an eyepiece with a field stop of
38mm (or greater, given the C8), will cause vignetting, where it
didn't exist before. You _will_ still get 1.1 degrees of fully
illuminated field, but that 1.1 degrees now fits in a smaller circle
at the center of the eyepiece, beyond which there is less and less
light striking the field lens of the eyepiece, going out to the edge.



Ya got me baffled


  #5  
Old January 24th 04, 02:38 PM
Michael A. Covington
external usenet poster
 
Posts: n/a
Default SCT Focal Reducers and Vignetting (explained)

To be precise -- A focal reducer takes the image and makes it *smaller* and
brighter.

So an image that would have been 2 inches in diameter gets shrunk down to
something smaller.

And it was 2 inches in diameter because of the size of the internal tubes in
the telescope.

I find it puzzling that people don't expect vignetting. Of course there's
vignetting. If you take a full-page document to Kinko's and get it reduced
to 4x6 inches, the adjacent pages don't magically appear next to it!


  #6  
Old January 24th 04, 02:38 PM
Michael A. Covington
external usenet poster
 
Posts: n/a
Default SCT Focal Reducers and Vignetting (explained)

To be precise -- A focal reducer takes the image and makes it *smaller* and
brighter.

So an image that would have been 2 inches in diameter gets shrunk down to
something smaller.

And it was 2 inches in diameter because of the size of the internal tubes in
the telescope.

I find it puzzling that people don't expect vignetting. Of course there's
vignetting. If you take a full-page document to Kinko's and get it reduced
to 4x6 inches, the adjacent pages don't magically appear next to it!


  #7  
Old January 24th 04, 02:38 PM
Michael A. Covington
external usenet poster
 
Posts: n/a
Default SCT Focal Reducers and Vignetting (explained)

To be precise -- A focal reducer takes the image and makes it *smaller* and
brighter.

So an image that would have been 2 inches in diameter gets shrunk down to
something smaller.

And it was 2 inches in diameter because of the size of the internal tubes in
the telescope.

I find it puzzling that people don't expect vignetting. Of course there's
vignetting. If you take a full-page document to Kinko's and get it reduced
to 4x6 inches, the adjacent pages don't magically appear next to it!


  #8  
Old January 24th 04, 03:22 PM
Rod Mollise
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Posts: n/a
Default SCT Focal Reducers and Vignetting (explained)


I find it puzzling that people don't expect vignetting. Of course there's
vignetting.


Hi:

Well, of course there is. HOWEVER...with a C8, it doesn't become objectionable
or even noticeable, really until you get to longer focal length eyepieces. The
good part is that with the r/c, you may not need longer focal length eyepieces
(I'm talking the f/6.3 reducer/corrector, the 3.3 is not useable visually).
Even then...well...there's obvious vignetting of the field of the 35mm
Panoptic, but it provides such a nice-looking wide field that I can easily
overlook that.

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html
  #9  
Old January 24th 04, 03:22 PM
Rod Mollise
external usenet poster
 
Posts: n/a
Default SCT Focal Reducers and Vignetting (explained)


I find it puzzling that people don't expect vignetting. Of course there's
vignetting.


Hi:

Well, of course there is. HOWEVER...with a C8, it doesn't become objectionable
or even noticeable, really until you get to longer focal length eyepieces. The
good part is that with the r/c, you may not need longer focal length eyepieces
(I'm talking the f/6.3 reducer/corrector, the 3.3 is not useable visually).
Even then...well...there's obvious vignetting of the field of the 35mm
Panoptic, but it provides such a nice-looking wide field that I can easily
overlook that.

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html
  #10  
Old January 24th 04, 03:22 PM
Rod Mollise
external usenet poster
 
Posts: n/a
Default SCT Focal Reducers and Vignetting (explained)


I find it puzzling that people don't expect vignetting. Of course there's
vignetting.


Hi:

Well, of course there is. HOWEVER...with a C8, it doesn't become objectionable
or even noticeable, really until you get to longer focal length eyepieces. The
good part is that with the r/c, you may not need longer focal length eyepieces
(I'm talking the f/6.3 reducer/corrector, the 3.3 is not useable visually).
Even then...well...there's obvious vignetting of the field of the 35mm
Panoptic, but it provides such a nice-looking wide field that I can easily
overlook that.

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html
 




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