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RE: [sct-user] Re: Speaking of Focal Reducers

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  • John Mahony
    Not true. Adding the reducer does not change what light rays make it through the rear port, except for some small change due to the fact that the primary has
    Message 1 of 10 , Aug 3, 2004
      Not true. Adding the reducer does not change what light rays make it
      through the rear port, except for some small change due to the fact that the
      primary has to be moved forward to reach focus with the reducer.
      With the reducer, if a star image in the focal plane is more than 12mm from
      the center (ie, outside a 24mm diameter circle), then if you remove the
      reducer, the image plane goes back to its original scale, and this star
      image is now more than 12/0.63 = 19mm from the center, so it is outside a
      38mm diameter circle. But the rear port has about the same diameter as the
      lens aperture of the reducer, 38mm, so some of the rays forming that star
      image will be vignetted.
      Reversing that argument shows that vignetting with the reducer starts at
      24mm diameter at the focal plane, at a minimum. In an 8" SCT, considering
      the forward end of the baffle tube reduces the unvignetted field even
      further.
      -John


      >From: "pensack1" <Pensack1@...>
      >Reply-To: sct-user@yahoogroups.com
      >To: sct-user@yahoogroups.com
      >Subject: [sct-user] Re: Speaking of Focal Reducers
      >Date: Tue, 03 Aug 2004 23:55:22 -0000
      >
      >Actually, the focal plane of the telescope does not change in diameter as a
      >focal
      >reducer is added, only the image scale changes, and that is why the field
      >of view is
      >larger (both true and apparent). A 35mm Panoptic, with a 38.7mm field stop
      >vignettes slightly at f/10, but the light dropoff is not noticeable. At
      >f/6.3, the baffle tube
      >effectively limits the field of view and causes vignetting, but even then,
      >it's not
      >severe--merely noticeable.
      >But, in both cases, the focal plane of the scope is the same size.
      >If the illuminated focal plane shrank with the reducer, the image would
      >shrink, but the
      >true field of view would stay the same. The apparent field of view would
      >have to
      >shrink because the magnification went down(remember, TFOV=AFOV/M).
      >Since it does not , but magnification goes down, true field goes up, which
      >means that
      >all that is happening is that the image scale has changed.
      >The true picture is more complicated, since the cause of vignetting in an
      >SCT can
      >be too narrow a primary mirror, too small a secondary, too long baffles, or
      >an
      >eyepiece with a field stop larger than the fully illuminated focal plane.
      >But the focal reducer does not reduced the size of the focal plane (if it
      >did, no one
      >would use one).
      >Don Pensack
      >
      >
      >
      >--- In sct-user@yahoogroups.com, "John Mahony" <jmmahony@h...> wrote:
      > > >From: "David" <Ginahoy@a...>
      > > >
      > > >--- In sct-user@yahoogroups.com, "ianhewitt2001" wrote:
      > > >[snip]
      > > > > I have a C11 (10yrs old) and a Meade 6.3 RC ...
      > > >
      > > >Ian,
      > > >... keep in mind
      > > >that any 2" eyepiece with a field stop larger than the clear aperture
      > > >of the R/C (about 38mm) will experience vignetting. (Note: full
      > > >aperture 2" eyepieces like the 30mm WideScan and 41mm Panoptic have
      > > >an effective field stop of ~47mm).
      > >
      > > It's worse than that: since the reducer reducers the size of the focused
      > > image, any EP with a field stop larger than 38*0.63 = 24mm will
      >experience
      > > some vignetting. Fortunately, the eye is pretty forgiving of mild
      > > vignetting.
      > > -John
      > >
      > > _________________________________________________________________
      > > Overwhelmed by debt? Find out how to `Dig Yourself Out of Debt' from MSN
      > > Money. http://special.msn.com/money/0407debt.armx
      >
      >
      >
      >
      >Visit the sct-user home page at:
      >
      >
      >
      >http://members.aol.com/RMOLLISE/index4.html
      >Yahoo! Groups Links
      >
      >
      >
      >
      >

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    • Chuck
      What about a C11 with the larger rear opening? I use a Starlight crayford which screws right onto the larger opening with a Starsweeper .5x focal reducer in
      Message 2 of 10 , Aug 3, 2004
        What about a C11 with the larger rear opening? I use a Starlight crayford
        which screws right onto the larger opening with a Starsweeper .5x focal
        reducer in my AP 2" Diagonal. Any idea what the amount of vignetting is
        with a 35mm Panoptic?

        Clear Skies,

        Chuck

        ----- Original Message -----
        From: "John Mahony" <jmmahony@...>
        To: <sct-user@yahoogroups.com>
        Sent: Tuesday, August 03, 2004 6:22 PM
        Subject: RE: [sct-user] Re: Speaking of Focal Reducers


        > Not true. Adding the reducer does not change what light rays make it
        > through the rear port, except for some small change due to the fact that
        the
        > primary has to be moved forward to reach focus with the reducer.
        > With the reducer, if a star image in the focal plane is more than 12mm
        from
        > the center (ie, outside a 24mm diameter circle), then if you remove the
        > reducer, the image plane goes back to its original scale, and this star
        > image is now more than 12/0.63 = 19mm from the center, so it is outside a
        > 38mm diameter circle. But the rear port has about the same diameter as
        the
        > lens aperture of the reducer, 38mm, so some of the rays forming that star
        > image will be vignetted.
        > Reversing that argument shows that vignetting with the reducer starts at
        > 24mm diameter at the focal plane, at a minimum. In an 8" SCT, considering
        > the forward end of the baffle tube reduces the unvignetted field even
        > further.
        > -John
        >
        >
        > >From: "pensack1" <Pensack1@...>
        > >Reply-To: sct-user@yahoogroups.com
        > >To: sct-user@yahoogroups.com
        > >Subject: [sct-user] Re: Speaking of Focal Reducers
        > >Date: Tue, 03 Aug 2004 23:55:22 -0000
        > >
        > >Actually, the focal plane of the telescope does not change in diameter as
        a
        > >focal
        > >reducer is added, only the image scale changes, and that is why the field
        > >of view is
        > >larger (both true and apparent). A 35mm Panoptic, with a 38.7mm field
        stop
        > >vignettes slightly at f/10, but the light dropoff is not noticeable. At
        > >f/6.3, the baffle tube
        > >effectively limits the field of view and causes vignetting, but even
        then,
        > >it's not
        > >severe--merely noticeable.
        > >But, in both cases, the focal plane of the scope is the same size.
        > >If the illuminated focal plane shrank with the reducer, the image would
        > >shrink, but the
        > >true field of view would stay the same. The apparent field of view would
        > >have to
        > >shrink because the magnification went down(remember, TFOV=AFOV/M).
        > >Since it does not , but magnification goes down, true field goes up,
        which
        > >means that
        > >all that is happening is that the image scale has changed.
        > >The true picture is more complicated, since the cause of vignetting in an
        > >SCT can
        > >be too narrow a primary mirror, too small a secondary, too long baffles,
        or
        > >an
        > >eyepiece with a field stop larger than the fully illuminated focal plane.
        > >But the focal reducer does not reduced the size of the focal plane (if it
        > >did, no one
        > >would use one).
        > >Don Pensack
        > >
        > >
        > >
        > >--- In sct-user@yahoogroups.com, "John Mahony" <jmmahony@h...> wrote:
        > > > >From: "David" <Ginahoy@a...>
        > > > >
        > > > >--- In sct-user@yahoogroups.com, "ianhewitt2001" wrote:
        > > > >[snip]
        > > > > > I have a C11 (10yrs old) and a Meade 6.3 RC ...
        > > > >
        > > > >Ian,
        > > > >... keep in mind
        > > > >that any 2" eyepiece with a field stop larger than the clear aperture
        > > > >of the R/C (about 38mm) will experience vignetting. (Note: full
        > > > >aperture 2" eyepieces like the 30mm WideScan and 41mm Panoptic have
        > > > >an effective field stop of ~47mm).
        > > >
        > > > It's worse than that: since the reducer reducers the size of the
        focused
        > > > image, any EP with a field stop larger than 38*0.63 = 24mm will
        > >experience
        > > > some vignetting. Fortunately, the eye is pretty forgiving of mild
        > > > vignetting.
        > > > -John
        > > >
        > > > _________________________________________________________________
        > > > Overwhelmed by debt? Find out how to `Dig Yourself Out of Debt' from
        MSN
        > > > Money. http://special.msn.com/money/0407debt.armx
        > >
        > >
        > >
        > >
        > >Visit the sct-user home page at:
        > >
        > >
        > >
        > >http://members.aol.com/RMOLLISE/index4.html
        > >Yahoo! Groups Links
        > >
        > >
        > >
        > >
        > >
        >
        > _________________________________________________________________
        > MSN Toolbar provides one-click access to Hotmail from any Web page - FREE
        > download! http://toolbar.msn.click-url.com/go/onm00200413ave/direct/01/
        >
        >
        >
        >
        > Visit the sct-user home page at:
        >
        >
        >
        > http://members.aol.com/RMOLLISE/index4.html
        > Yahoo! Groups Links
        >
        >
        >
        >
        >
      • John Mahony
        What s the lens diameter of the reducer? All you really have to do is look at the vignetting situation for an aperture that large at the rear of the scope,
        Message 3 of 10 , Aug 3, 2004
          What's the lens diameter of the reducer? All you really have to do is look
          at the vignetting situation for an aperture that large at the rear of the
          scope, and then "reduce" the picture by half.
          Since the lens is very likely less than 2", your unvignetted field will be
          no larger than 1", while a 35 Pan would have a field stop around 1.6". That
          doesn't meant the results will look terrible, since the eye is so forgiving
          of vignetting, but with such a wide TFOV EP, I think you'll notice the
          vignetting.
          -John

          >From: "Chuck" <cscappaticci@...>
          >Reply-To: sct-user@yahoogroups.com
          >To: <sct-user@yahoogroups.com>
          >Subject: Re: [sct-user] Re: Speaking of Focal Reducers
          >Date: Tue, 3 Aug 2004 19:09:21 -0600
          >
          >What about a C11 with the larger rear opening? I use a Starlight crayford
          >which screws right onto the larger opening with a Starsweeper .5x focal
          >reducer in my AP 2" Diagonal. Any idea what the amount of vignetting is
          >with a 35mm Panoptic?
          >
          >Clear Skies,
          >
          > Chuck
          >
          >----- Original Message -----
          >From: "John Mahony" <jmmahony@...>
          >To: <sct-user@yahoogroups.com>
          >Sent: Tuesday, August 03, 2004 6:22 PM
          >Subject: RE: [sct-user] Re: Speaking of Focal Reducers
          >
          >
          > > Not true. Adding the reducer does not change what light rays make it
          > > through the rear port, except for some small change due to the fact that
          >the
          > > primary has to be moved forward to reach focus with the reducer.
          > > With the reducer, if a star image in the focal plane is more than 12mm
          >from
          > > the center (ie, outside a 24mm diameter circle), then if you remove the
          > > reducer, the image plane goes back to its original scale, and this star
          > > image is now more than 12/0.63 = 19mm from the center, so it is outside
          >a
          > > 38mm diameter circle. But the rear port has about the same diameter as
          >the
          > > lens aperture of the reducer, 38mm, so some of the rays forming that
          >star
          > > image will be vignetted.
          > > Reversing that argument shows that vignetting with the reducer starts at
          > > 24mm diameter at the focal plane, at a minimum. In an 8" SCT,
          >considering
          > > the forward end of the baffle tube reduces the unvignetted field even
          > > further.
          > > -John
          > >
          > >
          > > >From: "pensack1" <Pensack1@...>
          > > >Reply-To: sct-user@yahoogroups.com
          > > >To: sct-user@yahoogroups.com
          > > >Subject: [sct-user] Re: Speaking of Focal Reducers
          > > >Date: Tue, 03 Aug 2004 23:55:22 -0000
          > > >
          > > >Actually, the focal plane of the telescope does not change in diameter
          >as
          >a
          > > >focal
          > > >reducer is added, only the image scale changes, and that is why the
          >field
          > > >of view is
          > > >larger (both true and apparent). A 35mm Panoptic, with a 38.7mm field
          >stop
          > > >vignettes slightly at f/10, but the light dropoff is not noticeable.
          >At
          > > >f/6.3, the baffle tube
          > > >effectively limits the field of view and causes vignetting, but even
          >then,
          > > >it's not
          > > >severe--merely noticeable.
          > > >But, in both cases, the focal plane of the scope is the same size.
          > > >If the illuminated focal plane shrank with the reducer, the image would
          > > >shrink, but the
          > > >true field of view would stay the same. The apparent field of view
          >would
          > > >have to
          > > >shrink because the magnification went down(remember, TFOV=AFOV/M).
          > > >Since it does not , but magnification goes down, true field goes up,
          >which
          > > >means that
          > > >all that is happening is that the image scale has changed.
          > > >The true picture is more complicated, since the cause of vignetting in
          >an
          > > >SCT can
          > > >be too narrow a primary mirror, too small a secondary, too long
          >baffles,
          >or
          > > >an
          > > >eyepiece with a field stop larger than the fully illuminated focal
          >plane.
          > > >But the focal reducer does not reduced the size of the focal plane (if
          >it
          > > >did, no one
          > > >would use one).
          > > >Don Pensack
          > > >
          > > >
          > > >
          > > >--- In sct-user@yahoogroups.com, "John Mahony" <jmmahony@h...> wrote:
          > > > > >From: "David" <Ginahoy@a...>
          > > > > >
          > > > > >--- In sct-user@yahoogroups.com, "ianhewitt2001" wrote:
          > > > > >[snip]
          > > > > > > I have a C11 (10yrs old) and a Meade 6.3 RC ...
          > > > > >
          > > > > >Ian,
          > > > > >... keep in mind
          > > > > >that any 2" eyepiece with a field stop larger than the clear
          >aperture
          > > > > >of the R/C (about 38mm) will experience vignetting. (Note: full
          > > > > >aperture 2" eyepieces like the 30mm WideScan and 41mm Panoptic have
          > > > > >an effective field stop of ~47mm).
          > > > >
          > > > > It's worse than that: since the reducer reducers the size of the
          >focused
          > > > > image, any EP with a field stop larger than 38*0.63 = 24mm will
          > > >experience
          > > > > some vignetting. Fortunately, the eye is pretty forgiving of mild
          > > > > vignetting.
          > > > > -John
          > > > >
          > > > > _________________________________________________________________
          > > > > Overwhelmed by debt? Find out how to `Dig Yourself Out of Debt' from
          >MSN
          > > > > Money. http://special.msn.com/money/0407debt.armx
          > > >
          > > >
          > > >
          > > >
          > > >Visit the sct-user home page at:
          > > >
          > > >
          > > >
          > > >http://members.aol.com/RMOLLISE/index4.html
          > > >Yahoo! Groups Links
          > > >
          > > >
          > > >
          > > >
          > > >
          > >
          > > _________________________________________________________________
          > > MSN Toolbar provides one-click access to Hotmail from any Web page -
          >FREE
          > > download! http://toolbar.msn.click-url.com/go/onm00200413ave/direct/01/
          > >
          > >
          > >
          > >
          > > Visit the sct-user home page at:
          > >
          > >
          > >
          > > http://members.aol.com/RMOLLISE/index4.html
          > > Yahoo! Groups Links
          > >
          > >
          > >
          > >
          > >
          >
          >
          >
          >
          >Visit the sct-user home page at:
          >
          >
          >
          >http://members.aol.com/RMOLLISE/index4.html
          >Yahoo! Groups Links
          >
          >
          >
          >
          >

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        • Chuck
          Thanks John, I think the lens is 2 , I m sure it vignettes but is definitely provides a wider field with the reducer than without ... Seems like it s worth
          Message 4 of 10 , Aug 4, 2004
            Thanks John, I think the lens is 2", I'm sure it vignettes but is
            definitely provides a wider field with the reducer than without ... Seems
            like it's worth the tradeoff for my C11

            Clear Skies,

            Chuck

            ----- Original Message -----
            From: "John Mahony" <jmmahony@...>
            To: <sct-user@yahoogroups.com>
            Sent: Tuesday, August 03, 2004 8:42 PM
            Subject: Re: [sct-user] Re: Speaking of Focal Reducers


            > What's the lens diameter of the reducer? All you really have to do is
            look
            > at the vignetting situation for an aperture that large at the rear of the
            > scope, and then "reduce" the picture by half.
            > Since the lens is very likely less than 2", your unvignetted field will be
            > no larger than 1", while a 35 Pan would have a field stop around 1.6".
            That
            > doesn't meant the results will look terrible, since the eye is so
            forgiving
            > of vignetting, but with such a wide TFOV EP, I think you'll notice the
            > vignetting.
            > -John
            >
            > >From: "Chuck" <cscappaticci@...>
            > >Reply-To: sct-user@yahoogroups.com
            > >To: <sct-user@yahoogroups.com>
            > >Subject: Re: [sct-user] Re: Speaking of Focal Reducers
            > >Date: Tue, 3 Aug 2004 19:09:21 -0600
            > >
            > >What about a C11 with the larger rear opening? I use a Starlight
            crayford
            > >which screws right onto the larger opening with a Starsweeper .5x focal
            > >reducer in my AP 2" Diagonal. Any idea what the amount of vignetting is
            > >with a 35mm Panoptic?
            > >
            > >Clear Skies,
            > >
            > > Chuck
            > >
            > >----- Original Message -----
            > >From: "John Mahony" <jmmahony@...>
            > >To: <sct-user@yahoogroups.com>
            > >Sent: Tuesday, August 03, 2004 6:22 PM
            > >Subject: RE: [sct-user] Re: Speaking of Focal Reducers
            > >
            > >
            > > > Not true. Adding the reducer does not change what light rays make it
            > > > through the rear port, except for some small change due to the fact
            that
            > >the
            > > > primary has to be moved forward to reach focus with the reducer.
            > > > With the reducer, if a star image in the focal plane is more than 12mm
            > >from
            > > > the center (ie, outside a 24mm diameter circle), then if you remove
            the
            > > > reducer, the image plane goes back to its original scale, and this
            star
            > > > image is now more than 12/0.63 = 19mm from the center, so it is
            outside
            > >a
            > > > 38mm diameter circle. But the rear port has about the same diameter
            as
            > >the
            > > > lens aperture of the reducer, 38mm, so some of the rays forming that
            > >star
            > > > image will be vignetted.
            > > > Reversing that argument shows that vignetting with the reducer starts
            at
            > > > 24mm diameter at the focal plane, at a minimum. In an 8" SCT,
            > >considering
            > > > the forward end of the baffle tube reduces the unvignetted field even
            > > > further.
            > > > -John
            > > >
            > > >
            > > > >From: "pensack1" <Pensack1@...>
            > > > >Reply-To: sct-user@yahoogroups.com
            > > > >To: sct-user@yahoogroups.com
            > > > >Subject: [sct-user] Re: Speaking of Focal Reducers
            > > > >Date: Tue, 03 Aug 2004 23:55:22 -0000
            > > > >
            > > > >Actually, the focal plane of the telescope does not change in
            diameter
            > >as
            > >a
            > > > >focal
            > > > >reducer is added, only the image scale changes, and that is why the
            > >field
            > > > >of view is
            > > > >larger (both true and apparent). A 35mm Panoptic, with a 38.7mm
            field
            > >stop
            > > > >vignettes slightly at f/10, but the light dropoff is not noticeable.
            > >At
            > > > >f/6.3, the baffle tube
            > > > >effectively limits the field of view and causes vignetting, but even
            > >then,
            > > > >it's not
            > > > >severe--merely noticeable.
            > > > >But, in both cases, the focal plane of the scope is the same size.
            > > > >If the illuminated focal plane shrank with the reducer, the image
            would
            > > > >shrink, but the
            > > > >true field of view would stay the same. The apparent field of view
            > >would
            > > > >have to
            > > > >shrink because the magnification went down(remember, TFOV=AFOV/M).
            > > > >Since it does not , but magnification goes down, true field goes up,
            > >which
            > > > >means that
            > > > >all that is happening is that the image scale has changed.
            > > > >The true picture is more complicated, since the cause of vignetting
            in
            > >an
            > > > >SCT can
            > > > >be too narrow a primary mirror, too small a secondary, too long
            > >baffles,
            > >or
            > > > >an
            > > > >eyepiece with a field stop larger than the fully illuminated focal
            > >plane.
            > > > >But the focal reducer does not reduced the size of the focal plane
            (if
            > >it
            > > > >did, no one
            > > > >would use one).
            > > > >Don Pensack
            > > > >
            > > > >
            > > > >
            > > > >--- In sct-user@yahoogroups.com, "John Mahony" <jmmahony@h...> wrote:
            > > > > > >From: "David" <Ginahoy@a...>
            > > > > > >
            > > > > > >--- In sct-user@yahoogroups.com, "ianhewitt2001" wrote:
            > > > > > >[snip]
            > > > > > > > I have a C11 (10yrs old) and a Meade 6.3 RC ...
            > > > > > >
            > > > > > >Ian,
            > > > > > >... keep in mind
            > > > > > >that any 2" eyepiece with a field stop larger than the clear
            > >aperture
            > > > > > >of the R/C (about 38mm) will experience vignetting. (Note: full
            > > > > > >aperture 2" eyepieces like the 30mm WideScan and 41mm Panoptic
            have
            > > > > > >an effective field stop of ~47mm).
            > > > > >
            > > > > > It's worse than that: since the reducer reducers the size of the
            > >focused
            > > > > > image, any EP with a field stop larger than 38*0.63 = 24mm will
            > > > >experience
            > > > > > some vignetting. Fortunately, the eye is pretty forgiving of mild
            > > > > > vignetting.
            > > > > > -John
            > > > > >
            > > > > > _________________________________________________________________
            > > > > > Overwhelmed by debt? Find out how to `Dig Yourself Out of Debt'
            from
            > >MSN
            > > > > > Money. http://special.msn.com/money/0407debt.armx
            > > > >
            > > > >
            > > > >
            > > > >
            > > > >Visit the sct-user home page at:
            > > > >
            > > > >
            > > > >
            > > > >http://members.aol.com/RMOLLISE/index4.html
            > > > >Yahoo! Groups Links
            > > > >
            > > > >
            > > > >
            > > > >
            > > > >
            > > >
            > > > _________________________________________________________________
            > > > MSN Toolbar provides one-click access to Hotmail from any Web page -
            > >FREE
            > > > download!
            http://toolbar.msn.click-url.com/go/onm00200413ave/direct/01/
            > > >
            > > >
            > > >
            > > >
            > > > Visit the sct-user home page at:
            > > >
            > > >
            > > >
            > > > http://members.aol.com/RMOLLISE/index4.html
            > > > Yahoo! Groups Links
            > > >
            > > >
            > > >
            > > >
            > > >
            > >
            > >
            > >
            > >
            > >Visit the sct-user home page at:
            > >
            > >
            > >
            > >http://members.aol.com/RMOLLISE/index4.html
            > >Yahoo! Groups Links
            > >
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            >
            > _________________________________________________________________
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            > Visit the sct-user home page at:
            >
            >
            >
            > http://members.aol.com/RMOLLISE/index4.html
            > Yahoo! Groups Links
            >
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