Loading ...
Sorry, an error occurred while loading the content.

Re: [atm_free] Re: ATM Cassegrain?

Expand Messages
  • lance clarke
    It is only difficult if you try to match the ROC of the secondary to the figure of the primary.  It is much easier to finish the secondary, and then figure
    Message 1 of 31 , Jan 1, 2009
    • 0 Attachment
      It is only difficult if you try to match the ROC of the secondary to the figure of the primary.  It is much easier to finish the secondary, and then figure the primary to match. 
       
      Lance


      From: a.johnw <a.johnw@...>
      To: atm_free@yahoogroups.com
      Sent: Wednesday, December 31, 2008 12:09:10 PM
      Subject: [atm_free] Re: ATM Cassegrain?

      Comments about the ease of making a dk often don't point out that the
      radius of curvature of the 2ndry needs to be very closely matched to
      the figure of the primary. The only truly easy thing about them is the
      lack of the need for a complicated 2ndry mirror test. The primary can
      also be null figured if reqd. Some would advise making the 2ndry 1st
      and then a primary to match/

      John

      --- In atm_free@yahoogroup s.com, lance clarke <alaskawolfjoe@ ...> wrote:
      >
      > The DK is easier to make both optically and mechanically.  For the
      secondary, you just make the secondary tool into a spherical test
      plate (foucault null test this), and then use it to fringe test the
      secondary mirror.  As for figuring the primary of a DK, the elipsoid
      is actually easier to figure than a parabola, or at least that was my
      experience.
      >
      > Lance
      >
      >
      >
      >
      > ____________ _________ _________ __
      > From: albireo13 <albireo13@. ..>
      > To: atm_free@yahoogroup s.com
      > Sent: Thursday, December 4, 2008 1:31:55 PM
      > Subject: [atm_free] Re: ATM Cassegrain?
      >
      >
      > Hmmm ... I'd like to play around with a SW simulator to check
      > sensitivity of a design to variations in tilt and separation.
      > I'm not a precision machinest so the mechanical construction issues
      > may drive me to favor one design over another.
      >
      > Can anyone suggest a sw program for simulating Cass. optical
      > systems? Something relatively basic, with spot diagram output would
      > be nice. So far I've been using CassDes by Mike Jones. It comes
      > up with designs and generates spot diagrams ... quite nice.
      > However, it doesn't let you modify spacings, etc to test effects on
      > the focal plane spot diagram.
      >
      > Rob
      >
      > --- In atm_free@yahoogroup s.com, "albireo13" <albireo13@ ..> wrote:
      > >
      > > I guess it boils down between which is harder to make
      > successfully .... the ellipsoidal DK
      > > primary or the hyperbolopidal CC secondary.
      > >
      > > If I go DK, what techniques have folks used for testing the DK
      > primary?
      > >
      > > Thanks,
      > > Rob
      > >
      > > Rob--- In atm_free@yahoogroup s.com, "Tony Gondola" <acgna@> wrote:
      > > >
      > > > In that case it really sounds like a longer focus DK would be
      > the best
      > > > balance between performance and ease of construction.
      > > >
      > > > Tony
      > > >
      > > > ----- Original Message -----
      > > > From: "albireo13" <albireo13@>
      > > > To: <atm_free@yahoogrou p s.com>
      > > > Sent: Wednesday, December 03, 2008 4:11 PM
      > > > Subject: [atm_free] Re: ATM Cassegrain?
      > > >
      > > >
      > > > > This would strictly be for visual ....
      > lunar/planetary/ doubles.
      > > > > With bigger CO and longer tube, I don't think Gregorian is
      > worth it. I
      > > > > won't use it for
      > > > > terrestrial so a non-inverted view is not important.
      > > > > Hmmm ....
      > > > >
      > > > > Rob
      > > > >
      > > > >
      > > > > --- In atm_free@yahoogroup s.com, "Tony Gondola" <acgna@> wrote:
      > > > >>
      > > > >> Hi Rob,
      > > > >>
      > > > >> Since you're looking at a fairly long focal ratio system
      > (F/15-18) I
      > > > >> wouldn't think that the extended field performance
      > performance would
      > > > >> really
      > > > >> be an issue. Also, keep in mind that a DK primary is simply an
      > > > >> under-corrected Newtonian primary. As you go from a sphere to
      > a parabolic
      > > > >> surface you pass through the shape and correction required
      > for the DK.
      > > > >> It's
      > > > >> usually around a conic constant of -0.8 or 80% of the full
      > correction
      > > > >> you'd
      > > > >> usually apply for a Newtonian primary. You just have to know
      > when to
      > > > >> stop.
      > > > >> Just plug in the correct target conic constant into your test
      > data
      > > > >> reduction
      > > > >> software and you're good to go.
      > > > >>
      > > > >> To answer your question about Gregorian verses DK I ray
      > traced two
      > > > >> systems,
      > > > >> both 6" F/16. The DK gives a diffraction limited FOV of
      > 2.4' , the
      > > > >> Gregorian
      > > > >> gives 24'. Both systems were calculated with F 3.3 primaries.
      > At first
      > > > >> the
      > > > >> Gregorian field looks great but it comes at the expense of a
      > much larger
      > > > >> diameter secondary. Interestingly, it's diffraction limited
      > field
      > > > >> performance is slightly better then an equivalent classical
      > cassegrain
      > > > >> but
      > > > >> you'll pay for that obstruction percentage. I haven't played
      > with that
      > > > >> design enough to know how much that can be reduced but I
      > suspect that
      > > > >> because of the position of the secondary which is outside
      > focus, it will
      > > > >> always be larger then the other designs for an equal amount
      > of field
      > > > >> illumination.
      > > > >>
      > > > >> I think what this really comes down to is what is the scope
      > going to be
      > > > >> used
      > > > >> for? If it's lunar and planetary viewing and imaging you may
      > as well go
      > > > >> with
      > > > >> the DK and extend the f ratio out into the F/22-24 range.That
      > will give
      > > > >> you
      > > > >> enough EFL that you won't need a barlow and the secondary can
      > be kept
      > > > >> small
      > > > >> (sort of the whole point of a long focus cass.) At that F
      > ratio, the
      > > > >> diffraction limited field opens up to almost 4' (plenty for
      > the intended
      > > > >> use) and the image scale is just about perfect in terms of
      > over-sampling
      > > > >> for
      > > > >> diffraction limited imaging without a barlow.
      > > > >>
      > > > >> Tony
      > > > >>
      > > > >>
      > > > >>
      > > > >>
      > > > >>
      > > > >> ----- Original Message -----
      > > > >> From: "albireo13" <albireo13@>
      > > > >> To: <atm_free@yahoogrou p s.com>
      > > > >> Sent: Tuesday, December 02, 2008 5:37 PM
      > > > >> Subject: [atm_free] Re: ATM Cassegrain?
      > > > >>
      > > > >>
      > > > >> > Hi Tony,
      > > > >> > I was leaning towards a classical Cass for two reasons:
      > > > >> >
      > > > >> > 1. it has far better coma performance than a like-f-ratio
      > DK
      > > > >> >
      > > > >> > 2. the primary is a paraboloid. I've done a few newt
      > mirrors so doing
      > > > >> > a
      > > > >> > paraboloid is
      > > > >> > something I understand. Doing an ellipsoid (DK) would be a
      > leap of
      > > > >> > faith
      > > > >> >
      > > > >> > Of course the hyperboloid secondary would be the CC
      > challenge.
      > > > >> >
      > > > >> > I was always intruiged by a gregorian so that would be a
      > neat option
      > > > >> > too.
      > > > >> >
      > > > >> > Any thoughts on how a Greg. would compare to a DK under the
      > stars?
      > > > >> >
      > > > >> > Rob
      > > > >> >
      > > > >> >
      > > > >> > --- In atm_free@yahoogroup s.com, "Tony Gondola" <acgna@>
      > wrote:
      > > > >> >>
      > > > >> >> I make 10" and 12.5" F/24 DK cassegrains on a regular
      > basis. It would
      > > > >> >> be
      > > > >> >> by
      > > > >> >> far the easiest cassegrain type to make. As you know the
      > secondary is
      > > > >> >> spherical and can be easily tested just just polishing out
      > the tool
      > > > >> >> and
      > > > >> >> using that as a test plate. The primary is a bit easier
      > then a good
      > > > >> >> newtonian primary except that you really have to nail the
      > overall
      > > > >> >> correction
      > > > >> >> if you want the best system performance.
      > > > >> >>
      > > > >> >> If tube length isn't a problem you might also consider a
      > gregorian. It
      > > > >> >> also
      > > > >> >> uses a parabolic primary and slightly corrected secondary
      > which is
      > > > >> >> concave
      > > > >> >> so it's also very easy to make and test.
      > > > >> >>
      > > > >> >> Tony
      > > > >> >> BigEye Optics
      > > > >> >>
      > > > >> >>
      > > > >> >>
      > > > >> >> ----- Original Message -----
      > > > >> >> From: "albireo13" <albireo13@>
      > > > >> >> To: <atm_free@yahoogrou p s.com>
      > > > >> >> Sent: Tuesday, December 02, 2008 8:41 AM
      > > > >> >> Subject: [atm_free] ATM Cassegrain?
      > > > >> >>
      > > > >> >>
      > > > >> >> >I have always had the itch to do a ATM Cassegrain ..
      > either Dall-
      > > > >> >> > Kirkham or Classical Cassegrain. I've done a few
      > Newtonian mirrors
      > > > >> >> > so I
      > > > >> >> > can handle paraboloids. The fastest I've done is a
      > f/4.7. This is
      > > > >> >> > a
      > > > >> >> > modest size scope for visual use only, maybe 6-8"
      > aperture and
      > > > >> >> > f/15-18.
      > > > >> >> >
      > > > >> >> > Any thoughts on which would be easier to complete? Any
      > hidden
      > > > >> >> > pitfalls? I'd love to hear from others who have
      > caompleted one.
      > > > >> >> >
      > > > >> >> >
      > > > >> >> > Thanks,
      > > > >> >> > Rob
      > > > >> >> >
      > > > >> >> >
      > > > >> >> >
      > > > >> >> > ------------ --------- --------- ------
      > > > >> >> >
      > > > >> >> > Yahoo! Groups Links
      > > > >> >> >
      > > > >> >> >
      > > > >> >> >
      > > > >> >>
      > > > >> >>
      > > > >> >> ------------ --------- --------- --------- ---------
      --------- -
      > --
      > > ---
      > > > > ---
      > > > >> > ------------ -
      > > > >> >>
      > > > >> >>
      > > > >> >>
      > > > >> >> No virus found in this incoming message.
      > > > >> >> Checked by AVG - http://www.avg. com
      > > > >> >> Version: 8.0.176 / Virus Database: 270.9.12/1824 - Release
      > Date:
      > > > >> >> 12/2/2008
      > > > >> >> 9:31 AM
      > > > >> >>
      > > > >> >
      > > > >> >
      > > > >> >
      > > > >> >
      > > > >> > ------------ --------- --------- ------
      > > > >> >
      > > > >> > Yahoo! Groups Links
      > > > >> >
      > > > >> >
      > > > >> >
      > > > >>
      > > > >>
      > > > >> ------------ --------- --------- --------- --------- --------- -
      > --
      > > ---
      > > > > ------------ -
      > > > >>
      > > > >>
      > > > >>
      > > > >> No virus found in this incoming message.
      > > > >> Checked by AVG - http://www.avg. com
      > > > >> Version: 8.0.176 / Virus Database: 270.9.13/1825 - Release
      > Date:
      > > > >> 12/2/2008
      > > > >> 8:44 PM
      > > > >>
      > > > >
      > > > >
      > > > >
      > > > >
      > > > > ------------ --------- --------- ------
      > > > >
      > > > > Yahoo! Groups Links
      > > > >
      > > > >
      > > > >
      > > >
      > > >
      > > > ------------ --------- --------- --------- --------- --------- -
      > --
      > > ------------ -
      > > >
      > > >
      > > >
      > > > No virus found in this incoming message.
      > > > Checked by AVG - http://www.avg. com
      > > > Version: 8.0.176 / Virus Database: 270.9.13/1827 - Release Date:
      > 12/3/2008
      > > > 5:41 PM
      > > >
      > >
      >

    • a.johnw
      I seem to have lost post problem so if this comes up twice sorry. Hi Tony. I wonder if the obstruction comment will bring out your photo s again. Tricky area
      Message 31 of 31 , Jan 12, 2009
      • 0 Attachment
        I seem to have lost post problem so if this comes up twice sorry.

        Hi Tony. I wonder if the obstruction comment will bring out your
        photo's again. Tricky area some people still didn't see the difference
        even when it's pointed out to them.

        I have no worries about drilling holes. I favour the coude approach
        because I would change both mirrors for an f ratio change and the
        obstructions could then be different too. This way the hole doesn't
        have to be sized to suit the widest beam.

        On the dk field and aberrations. Many sources say
        1) The dk has n times worse coma that a classic cas.
        2) A classic cas has the same coma as newt of the same f ratio
        3) Coma goes up as f ratio's get faster.

        I want to see what this means on the basis of a dk and the short focus
        newt many many people use. In arc secs of course.

        For my purposes I suspect a classic cass will be needed. It's all part
        of a life long search for that perfect all purpose telescope that I
        know doesn't exist.

        On the astrograph post I think there is a bit of a rut there too.
        Professionals and one Oxford Prof in the UK have moved on somewhat.
        Some sky survey scopes use a schmidt with a ccd camera. The flattener
        is a simple near plano convex lens. The prof uses a plano convex lens.
        Also invar and differential expansion to maintain focus. For those in
        the US oxford is the uk's premier university. Could be cambridge too.
        The system was available commercially a year or so ago but not sure if
        it still is.

        John

        --- In atm_free@yahoogroups.com, "Tony Gondola" <acgna@...> wrote:
        >
        > > I wonder about designs like that and many others. Amateur work seems
        > > to be stuck in a general purpose focal ratio cul de sac or short and
        > > fast. On the one hand one wants a fast F ratio for wide field
        > > photography and taking ancient figures based on film something like
        > > F32 for high resolution photography with even a 250mms instrument.
        > > Plus something like F3 may be great for photography but is going to be
        > > a bit of a pain with eyepieces. It's also going to have significant
        > > coma at the sort of field sizes it's likely to be used over. The other
        > > point buried in all of this is the size of the central obstruction. It
        > > tends to be large on designs such as the above. For higher res work
        > > that really does matter. I also maintain that it's significant
        > > visually on wide field too. :) No point arguing with me - I've
        seen it.
        >
        > Actually I would agree with you on the last point. Obstruction always
        > degrades the image so it should always be minimized as long as your not
        > violating any other design constraints such as field illumination by
        doing
        > so. Small obstruction designs usually begin with that parameter as a
        major
        > design factor rather then as something you try to minimize at the
        end of the
        > process. Also, you may or may not have realized that the main
        driving factor
        > for the diameter of the secondary in the DK cass. is the speed of the
        > primary. with a system speed of F/24 and an F/3 primary you can
        drive the
        > secondary diameter down to 15%.
        >
        > On a slightly different slant and subject that I think you're
        touching on,
        > I'm really becoming a proponent of longer focal ratio systems for DSO
        > imagers. The work I see these days really seems to be stuck in a rut
        with
        > everyone shooting the same wide fields at the same image scales.
        Reaching
        > for focal reducer is almost a knee-jerk reaction. I really think
        it's high
        > time that imagers started to break out of the "fast and wide" mode and
        > explore what longer focal ratios can do. Modern cameras are
        sensitive enough
        > and processing techniques are advanced enough that it's perfectly
        possible
        > to image regularly in the F/8 to F/16 range and even higher
        depending on the
        > target.
        >
        > > I think amateur cas designs miss the opportunity to remove some of the
        > > compromises that have to be made. For instance an F30 cass with an F3
        > > primary could be used primary alone with the 2 types of coma corrector
        > > that are available (well they may still be available) one will more or
        > > less retain the F ratio, the other will extend it.
        >
        > That's true, it just takes more effort in the design and building
        stage to
        > fully exploit that. Newtonian/Cass conversions are certainly nothing
        new.
        >
        > > I'm not proposing that as a design. I went about it in a different
        > > manner starting on the basis that I wanted a full moon available at
        > > the cas end. That seems to be too much for a truly minimal central
        > > obstruction on a 215 or so instrument but it's not too bad. I happen
        > > to have a roughed out blank of that size. I also wondered if it is
        > > possible to produce a simple singlet corrector up front of the
        > > newtonian mirror to allow a dk primary to be used. I haven't done any
        > > work on that at all yet but it seems to me that this sort of
        > > arrangement would offer a more useful instrument,
        >
        > If you want simple it sounds like we're back to the Mak...........
        >
        > > The stumbling block for me is the full moon view. Ideally it needs to
        > > fill a dslr ccd but that's not strictly needed. The exercise has made
        > > me wonder about classic cass's with 2 secondary mirrors and a newt
        > > focus using a coude cass style to avoid having too big a hole in the
        > > primary. After all most of us use a star diagonal all of the time
        > > anyway. That arrangement allows the stray light shields and everything
        > > else to be changed with the mirrors. I've also wondered if a dk
        > > designed for the full moon field could also be used with a higher mag
        > > 2ndry. Maybe still spherical. After all it's only going to be used
        > > over extremely narrow fields.
        >
        > Forget about getting a 30' diameter diffraction limited field with a
        > classical DK, I don't think that's going to happen without the
        addition of a
        > sub-aperture corrector group. Also, why go to such lengths just to
        avoid a
        > hole in the primary. If making the central perf or working with the
        mirror
        > in that condition seems like a problem, it isn't. Go here:
        > http://www.technologylk.com/catalog/3/Diamond-Core-Drill-Bits/
        > and buy a diamond core bit that's gives the OD you need, they aren't
        very
        > expensive and cut glass like butter. Make a paper centering mask and
        find a
        > drill press. Drill from the back side to within no more then 1/4" of
        where
        > the front surface will be after grinding. Fill the kerf with plaster
        then
        > grid, polish and figure as you normally would. When the mirror is
        finished,
        > center the paper mask over the back cut by looking through the front
        face of
        > the mirror and core from the front side until you meet the back cut.
        Remove
        > the plaster with a dremel tool and long drill bit. Smooth the edges
        with a
        > small stone and you're done. It's really a very easy process.
        >
        > Tony
        >
      Your message has been successfully submitted and would be delivered to recipients shortly.