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Re: Secondary obstruction - how measured?

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  • a.johnw
    From memory there is a paper by Couder on the subject but pass on where and when I read it. Seems that anything affects the light in the central spot. It went
    Message 1 of 130 , Jul 1, 2008
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      From memory there is a paper by Couder on the subject but pass on
      where and when I read it. Seems that anything affects the light in the
      central spot. It went though a number of arrangements including
      twisting vanes, curved vanes and a mask. It may have just had a
      reference to Couder but it was definitely professional observatory
      base work.
      One aspect of vanes I don't get but proves that it's not intuitive is
      the offset vanes arrangement. Essentially looking across adjacent
      vanes they don't line up. It seems however that providing they are
      parallel the diffraction pattern will be the same as when they are in
      line. That too makes me think it's not just a matter of light blocking.

      I always use the word 'seems' or something like that when ever I have
      only read and not tried it by the way. So I should have used the same
      word that prompted Tonys comment but I think this was a reliable source.


      John

      --- In atm_free@yahoogroups.com, "Tony Gondola" <acgna@...> wrote:
      >
      > Well what Steve said on the subject backs up what I've read from other
      > sources. The effect of the vanes is proportional to the area obstructed.
      >
      > Tony
      >
      > ----- Original Message -----
      > From: "a.johnw" <a.johnw@...>
      > To: <atm_free@yahoogroups.com>
      > Sent: Monday, June 30, 2008 3:58 PM
      > Subject: [atm_free] Re: Secondary obstruction - how measured?
      >
      >
      > > --- In atm_free@yahoogroups.com, "Al Germaine" <algermaine@> wrote:
      > >>
      > >> --- In atm_free@yahoogroups.com, "Stephen Koehler" <s.c.koehler@>
      > >> wrote:
      > >> >
      > >> > It appears that spiders can be ignored, compared to obstruction.
      > >> > --
      > >> > Steve Koehler
      > >> >
      > >>
      > >> Yes, but diffraction spikes can't be ignored. That's what drives
      > >> people to curved spiders despite the added diffraction of the thicker
      > >> gaged material. Yet, from what you are saying, even the contribution
      > >> from a curved spider can be ignored. So, it's a win for curved
      spiders.
      > >>
      > >> Al
      > >>
      > > Think you will find that curved spider supports actually take more
      > > light out of the diffraction spot just as a central obstructions does.
      > > I other words it's detrimental to contrast.
      > > In fact I'm sure it does.
      > >
      > > John
      > >
      > >
      > >
      > > ------------------------------------
      > >
      > > Yahoo! Groups Links
      > >
      > >
      > >
      >
      >
      >
      --------------------------------------------------------------------------------
      >
      >
      >
      > No virus found in this incoming message.
      > Checked by AVG.
      > Version: 8.0.101 / Virus Database: 270.4.3/1526 - Release Date:
      6/30/2008
      > 8:43 AM
      >
    • Tony Gondola
      I don t know John, I guess I ve yet to be convinced about any TCT design...... Tony ... From: a.johnw To:
      Message 130 of 130 , Jul 4, 2008
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        I don't know John, I guess I've yet to be convinced about any TCT
        design......

        Tony

        ----- Original Message -----
        From: "a.johnw" <a.johnw@...>
        To: <atm_free@yahoogroups.com>
        Sent: Friday, July 04, 2008 2:35 PM
        Subject: [atm_free] Re: Thinner spiders: was Secondary obstruction - how
        measured?


        > There is one 3 mirror design that comes with winspot that is
        > absolutely amazing. It's truly diffraction limited over a huge field.
        > One problem though. A mirror gets in the way and what ever I've tried
        > to do with it just wont get round the problem so it's a virtually
        > perfect but small field of view. I there may be another catch. I think
        > that the tolerances will be difficult to hold in the scope.
        >
        > John
        >
        > --- In atm_free@yahoogroups.com, "Tony Gondola" <acgna@...> wrote:
        >>
        >> It should get a lot of attention. Oh Stellafane, I used to go when I
        > lived
        >> back east, a long time back now.........
        >>
        >> Tony
        >> BigEye Optics
        >>
        >> ----- Original Message -----
        >> From: "Ed Jones" <solarview@...>
        >> To: <atm_free@yahoogroups.com>
        >> Sent: Thursday, July 03, 2008 1:31 PM
        >> Subject: [atm_free] Re: Thinner spiders: was Secondary obstruction -
        > how
        >> measured?
        >>
        >>
        >> > Tony,
        >> > No I haven't published the optical design, I haven't decided if or
        >> > when I will.
        >> > I don't have a recent picture of the 12.5. I have a picture of how
        >> > it first looked on photos in "spiderless". The eyepiece was too hard
        >> > to reach so I remade the box and rocker box. I'm putting the last
        >> > coats of varnish on now so no photos. I hope to have it at Stellafane.
        >> > Ed
        >> >
        >> > --- In atm_free@yahoogroups.com, "Tony Gondola" <acgna@> wrote:
        >> >>
        >> >> Hi Ed,
        >> >>
        >> >> I don't mind that it's custom I was just curious about the numbers.
        >> > Are they
        >> >> available?
        >> >>
        >> >> Tony
        >> >> BigEye Optics
        >> >>
        >> >> ----- Original Message -----
        >> >> From: "Ed Jones" <solarview@>
        >> >> To: <atm_free@yahoogroups.com>
        >> >> Sent: Thursday, July 03, 2008 10:32 AM
        >> >> Subject: [atm_free] Re: Thinner spiders: was Secondary obstruction -
        >> > how
        >> >> measured?
        >> >>
        >> >>
        >> >> > Do you mean the optical design? No, it's my own and uses custom
        >> >> > lesnes. On my Yahoo group "spiderless" I have some designs
        > that you
        >> >> > can make with off-the-shelf lenses.
        >> >> > Ed
        >> >> >
        >> >> > --- In atm_free@yahoogroups.com, "Tony Gondola" <acgna@> wrote:
        >> >> >>
        >> >> >> Hi Ed,
        >> >> >>
        >> >> >> Do you have the design numbers posted anywhere?
        >> >> >>
        >> >> >> Tony
        >> >> >> BigEye Optics
        >> >> >>
        >> >> >> ----- Original Message -----
        >> >> >> From: "Ed Jones" <solarview@>
        >> >> >> To: <atm_free@yahoogroups.com>
        >> >> >> Sent: Thursday, July 03, 2008 6:13 AM
        >> >> >> Subject: [atm_free] Re: Thinner spiders: was Secondary
        > obstruction -
        >> >> > how
        >> >> >> measured?
        >> >> >>
        >> >> >>
        >> >> >> > Well none of those. My last two scopes are an 8 inch and
        > 12.5 inch
        >> >> >> > f/6.8 CHiefspiegler (Catadioptric Herschelian). I'll post a
        >> > picture in
        >> >> >> > photos of my 8 I don't have a pic yet of my 12.5.
        >> >> >> >
        >> >> >> > Ed
        >> >> >> >
        >> >> >> > --- In atm_free@yahoogroups.com, "Al Germaine" <algermaine@>
        > wrote:
        >> >> >> >>
        >> >> >> >> Well now, what do you have -a refractor, a Newtonian with an
        >> > off-axis
        >> >> >> >> mask or an off-axis scope (what's the functional difference
        >> > between
        >> >> >> >> these last two)?
        >> >> >> >>
        >> >> >> >> Al
        >> >> >> >>
        >> >> >> >> --- In atm_free@yahoogroups.com, "Ed Jones" <solarview@> wrote:
        >> >> >> >> >
        >> >> >> >> > I resolved the problem by not using obstructions or spiders
        >> > in my
        >> >> >> >> scopes.
        >> >> >> >> >
        >> >> >> >> > Ed
        >> >> >> >> >
        >> >> >> >> >
        >> >> >> >> > --- In atm_free@yahoogroups.com, "Al Germaine" <algermaine@>
        >> > wrote:
        >> >> >> >> > >
        >> >> >> >> > > Not that it causes more problems. It's that there's a
        >> > trade off.
        >> >> >> > Some
        >> >> >> >> > > people use them.
        >> >> >> >> > >
        >> >> >> >> > > Al
        >> >> >> >> > >
        >> >> >> >> > > --- In atm_free@yahoogroups.com, "Mick D" <mick@> wrote:
        >> >> >> >> > > >
        >> >> >> >> > > > So in short it causes more problems than it solves
        > and thats
        >> >> >> >> > > > without mentioning the cost. Well i just had to ask.
        >> >> >> >> > > > Cheers guys
        >> >> >> >> > > >
        >> >> >> >> > > > Mick
        >> >> >> >> > > >
        >> >> >> >> > > >
        >> >> >> >> > > > ----- Original Message -----
        >> >> >> >> > > > From: Al Germaine
        >> >> >> >> > > > To: atm_free@yahoogroups.com
        >> >> >> >> > > > Sent: Tuesday, July 01, 2008 11:43 PM
        >> >> >> >> > > > Subject: [atm_free] Re: Thinner spiders: was Secondary
        >> >> >> > obstruction
        >> >> >> >> > > - how measured?
        >> >> >> >> > > >
        >> >> >> >> > > >
        >> >> >> >> > > > Mark,
        >> >> >> >> > > >
        >> >> >> >> > > > That is not at all a dumb question. This is sometimes
        >> > done. A
        >> >> >> >> glass
        >> >> >> >> > > > plate holding a secondary is called an optical
        > window. An
        >> >> >> >> additional
        >> >> >> >> > > > benefit is that it can (plus some other mods) close
        >> > the tube
        >> >> >> > of a
        >> >> >> >> > > > Newtonian. An optical window is ground flat and
        >> > parallel and
        >> >> >> > then
        >> >> >> >> > > > multicoated against reflections. It's not prohibitively
        >> >> >> >> > expensive, but
        >> >> >> >> > > > not cheap. It cures spikes and spider diffraction. It
        >> >> > introduces
        >> >> >> >> > > > reflections and aberrations, if not well made.
        >> >> >> >> > > >
        >> >> >> >> > > > Al
        >> >> >> >> > > >
        >> >> >> >> > > > --- In atm_free@yahoogroups.com, "Mick D" <mick@>
        > wrote:
        >> >> >> >> > > > >
        >> >> >> >> > > > > Hi Guys I
        >> >> >> >> > > > > I feel a bit reluctant to join in this discussion
        > mainly
        >> >> >> >> > because all
        >> >> >> >> > > > you guys seem to really know your stuff,
        >> >> >> >> > > > > but there is one thing that i would like to ask and
        >> >> > thats why
        >> >> >> >> > > > doesn't any of the big company's mount the
        >> >> >> >> > > > > secondry mirror in a thin piece of glass(for the
        >> > newtonian)
        >> >> >> >> to do
        >> >> >> >> > > > away with the spider (like with mak's and
        >> >> >> >> > > > > sct) i just know there's a simple answer but i cant
        >> >> > think of a
        >> >> >> >> > > > better place to ask this question.
        >> >> >> >> > > > > I hope this isn't a dumb question
        >> >> >> >> > > > > Thanks Guys
        >> >> >> >> > > > >
        >> >> >> >> > > > > Mick
        >> >> >> >> > > > >
        >> >> >> >> > > > > ----- Original Message -----
        >> >> >> >> > > > > From: Stephen Koehler
        >> >> >> >> > > > > To: atm_free@yahoogroups.com
        >> >> >> >> > > > > Sent: Tuesday, July 01, 2008 9:06 PM
        >> >> >> >> > > > > Subject: Re: [atm_free] Thinner spiders: was
        > Secondary
        >> >> >> >> obstruction
        >> >> >> >> > > > - how measured?
        >> >> >> >> > > > >
        >> >> >> >> > > > >
        >> >> >> >> > > > > Mark,
        >> >> >> >> > > > >
        >> >> >> >> > > > > > As an aside to Steve, can you model vanes that
        > thin or
        >> >> >> > better?
        >> >> >> >> > > > > > Oversampling for FFT tends to require arrays at
        > least
        >> >> >> > 2048 or
        >> >> >> >> > > > 4096 on
        >> >> >> >> > > > > > a side... The MTF curve looks identical, but image
        >> >> >> > simulations
        >> >> >> >> > > > > > compared to thicker and particularly curved vanes
        >> >> > might be
        >> >> >> >> > > > instructive.
        >> >> >> >> > > > >
        >> >> >> >> > > > > I could probably push it to .001 of the primary
        >> > diameter.
        >> >> >> > The R
        >> >> >> >> > > > > implementation of FFT isn't constrained to powers of
        >> > 2 in
        >> >> >> >> > size, but
        >> >> >> >> > > > > the upper limit for me is something like 2048x2048.
        >> >> >> >> > > > >
        >> >> >> >> > > > > I'm somewhat doubtful that any spider effects for
        > normal
        >> >> >> > spiders
        >> >> >> >> > > will
        >> >> >> >> > > > > show up in image simulations that we can do. .12
        >> > waves of
        >> >> >> > SA was
        >> >> >> >> > > > > already pretty subtle, and that was equivalent to a
        >> > spider
        >> >> >> >> > width of
        >> >> >> >> > > > > 1.7mm (fairly thick), after scaling down from a 16"
        >> >> >> > objective to
        >> >> >> >> > > an 8"
        >> >> >> >> > > > > objective because of my factor of 2 error.
        >> >> >> >> > > > >
        >> >> >> >> > > > > Spider effects are easier to see in an in-focus star
        >> > test
        >> >> >> >> > > simulation,
        >> >> >> >> > > > > but that doesn't show contrast loss in an extended
        >> > image.
        >> >> >> >> > Perhaps it
        >> >> >> >> > > > > would be better to switch to a bright, contrasty
        > image
        >> >> > like a
        >> >> >> >> > lunar
        >> >> >> >> > > > > landscape. I think any effect from the spider would
        >> > be lost
        >> >> >> >> on the
        >> >> >> >> > > > > softer detail of the Ring nebula. I will try to
        >> > scare up a
        >> >> >> > clean
        >> >> >> >> > > > > lunar landscape that has image scale information.
        > I do
        >> >> > not yet
        >> >> >> >> > > have a
        >> >> >> >> > > > > curved spider simulation, but that shouldn't take too
        >> >> > long to
        >> >> >> >> > > > > construct.
        >> >> >> >> > > > >
        >> >> >> >> > > > > --
        >> >> >> >> > > > > Steve Koehler
        >> >> >> >> > > > >
        >> >> >> >> > > > >
        >> >> >> >> > > > >
        >> >> >> >> > > > >
        >> >> >> >> > > > > __________ NOD32 3232 (20080701) Information
        > __________
        >> >> >> >> > > > >
        >> >> >> >> > > > > This message was checked by NOD32 antivirus system.
        >> >> >> >> > > > > http://www.eset.com
        >> >> >> >> > > > >
        >> >> >> >> > > >
        >> >> >> >> > > >
        >> >> >> >> > > >
        >> >> >> >> > > >
        >> >> >> >> > > >
        >> >> >> >> > > > __________ NOD32 3232 (20080701) Information __________
        >> >> >> >> > > >
        >> >> >> >> > > > This message was checked by NOD32 antivirus system.
        >> >> >> >> > > > http://www.eset.com
        >> >> >> >> > > >
        >> >> >> >> > >
        >> >> >> >> >
        >> >> >> >>
        >> >> >> >
        >> >> >> >
        >> >> >> >
        >> >> >> > ------------------------------------
        >> >> >> >
        >> >> >> > Yahoo! Groups Links
        >> >> >> >
        >> >> >> >
        >> >> >> >
        >> >> >> >
        >> >> >> > No virus found in this incoming message.
        >> >> >> > Checked by AVG - http://www.avg.com
        >> >> >> > Version: 8.0.135 / Virus Database: 270.4.4/1530 - Release Date:
        >> >> > 7/2/2008
        >> >> >> > 8:05 AM
        >> >> >> >
        >> >> >> >
        >> >> >> >
        >> >> >>
        >> >> >
        >> >> >
        >> >> >
        >> >> > ------------------------------------
        >> >> >
        >> >> > Yahoo! Groups Links
        >> >> >
        >> >> >
        >> >> >
        >> >> >
        >> >> > No virus found in this incoming message.
        >> >> > Checked by AVG - http://www.avg.com
        >> >> > Version: 8.0.135 / Virus Database: 270.4.4/1532 - Release Date:
        >> > 7/3/2008
        >> >> > 8:32 AM
        >> >> >
        >> >> >
        >> >> >
        >> >>
        >> >
        >> >
        >> >
        >> > ------------------------------------
        >> >
        >> > Yahoo! Groups Links
        >> >
        >> >
        >> >
        >> >
        >> > No virus found in this incoming message.
        >> > Checked by AVG - http://www.avg.com
        >> > Version: 8.0.135 / Virus Database: 270.4.4/1532 - Release Date:
        > 7/3/2008
        >> > 8:32 AM
        >> >
        >> >
        >> >
        >>
        >
        >
        >
        > ------------------------------------
        >
        > Yahoo! Groups Links
        >
        >
        >
        >
        > No virus found in this incoming message.
        > Checked by AVG - http://www.avg.com
        > Version: 8.0.135 / Virus Database: 270.4.5/1533 - Release Date: 7/3/2008
        > 7:19 PM
        >
        >
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