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Re: [atm_free] Re: Other sources of astig in the Bath

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  • gregfurtman
    Mark, Don’t forget that when you mount it in a scope the backside of it is supported uniformly by the flotation arms in the mirror cell. So I would think
    Message 1 of 39 , Feb 25, 2013
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      Mark,
       
      Don’t forget that when you mount it in a scope the backside of it is supported uniformly by the flotation arms in the mirror cell. So I would think that the pressure would be distributed and so the astig won’t be as bad as when in a test stand. I also plan on making my mirror cell with the 90º contacts adjustable front to back so I can tweek where they contact the mirror
       

      Greg Furtman
      27426 County Rd H
      Webster, WI 54893

      http://home.centurytel.net/starguy/

      "My goal in life is to become the person my dog thinks I am.
       
      Sent: Monday, February 25, 2013 9:15 AM
      Subject: Re: [atm_free] Re: Other sources of astig in the Bath
       
       

      Greg and Vladimir,

      Yes, I concede that I must pay attention to this issue.  It is dawning on me that I did not see the astigmatism as readily when the mirror was still well undercorrected because its value was small in comparison to other errors.  That plus I have a nasty habit of turning off the primary astigmatism zernike terms.

      'llI have to look closely at the test rail, test environment and test stand before moving forward.

      Yes, I thought a thick mirror like this would be more immune to mounting issues and that temperature stabilization issues would be my major concern.  Now, I am worried about what I might encounter when I mount it in a telescope.

      Mark



       

      From: Greg Furtman <starguy@...>
      To: atm_free@yahoogroups.com
      Sent: Monday, February 25, 2013 4:51 AM
      Subject: RE: [atm_free] Re: Other sources of astig in the Bath
       
       
      Mark,
       
      I had the same problem with working on my 18” mirror. Dale pointed out that the contact should be along the edge at the center of mass. So I made a couple of circular disks with a 1/8” wide ridge around them. I adjusted them so they would be at the center of mass and this got rid of the astig I was seeing. My test stand has 2 points 90 degrees apart that the mirror sits on for testing. After doing this & seeing the change I decided what must have been going on was that one support was touching near the back edge and the other near the front edge. If someone had told me that a piece of glass 1.5” thick could flex I would have doubted him, but now I know it can.
       
       
      Greg Furtman
      27426 County Rd H
      Webster, WI 54893
       
       
      "My goal in life is to become the person my dogs think I am."
       
      From: atm_free@yahoogroups.com [mailto:atm_free@yahoogroups.com] On Behalf Of Mark Whitaker
      Sent: Sunday, February 24, 2013 9:45 PM
      To: atm_free@yahoogroups.com
      Subject: Re: [atm_free] Re: Other sources of astig in the Bath
       
      The bearing surfaces are still 1/2 in wide so there was a wide area of balance, complicated by the fact that the stand probably had some amount of tilt.  So I should go ahead and explore this aspect to my satisfaction.

      I would think that air currents would be more random but I recently mounted the test rail on inner tubes.  The effect of this was less high frequency vibration and nearby road noise being passed to the rail. 

      Just thinking out loud, there were 4 longitudinal points of contact along the rail before switching to the tubes.  The tubes decreased this to just two longitudinal points which I tried to space such that there were neither long spans nor excessive overhangs.  But it could be that I changed the vibrational mode to lower fundamental and first few harmonics and that is expressing itself as astigmatism.  So maybe I need to add more support stands and sets of tubes.

      One thing at a time.

      Mark
       
       

      From: atmpob <atmpob@...>
      To: atm_free@yahoogroups.com
      Sent: Sunday, February 24, 2013 8:36 PM
      Subject: [atm_free] Re: Other sources of astig in the Bath
       
       
      Well,
      If it balanced and the supports were 90 deg apart then it may not be the stand. Because that configuration will produce the least amount of induced astig.

      So now I'm doubting the stand is the source.

      I hope you are taking enough to eliminate air currents.

      Dale

      --- In mailto:atm_free%40yahoogroups.com, Mark Whitaker wrote:
      >
      > I very cautiously balanced it on the
      supports.  If anything, it was worse.  But that at least tells me something.
      >
      > I'll defer to the insistence by so many that the
      test stand may be the issue and focus my attention there.
      >
      >
      Mark
      >
      >
      >
      >
      >
      ________________________________
      > From: atmpob
      > To:
      href="mailto:atm_free%40yahoogroups.com" rel=nofollow target=_blank ymailto="mailto:atm_free%40yahoogroups.com">mailto:atm_free%40yahoogroups.com
      > Sent: Sunday, February 24, 2013 5:37 PM
      > Subject: [atm_free] Re:
      Other sources of astig in the Bath
      >
      >
      >  
      >
      Mark,
      >
      > To know for sure if you are on the COG the mirror should
      balance there with no other support. Most people get a little nervous. The top support should not touch the mirror but be just in front in case a large vibrational bump moves the mirror. If it wants to go one way or the other you are not on the COG.
      >
      > Dale
      >
      > --- In
      href="mailto:atm_free%40yahoogroups.com" rel=nofollow target=_blank ymailto="mailto:atm_free%40yahoogroups.com">mailto:atm_free%40yahoogroups.com, Mark Whitaker wrote:
      > >
      > > This mirror has a machined edge
      and is very uniform. 
      > >
      > > I am seeing
      astigmatism up to 1/2 wave on occasion and it does not follow the mirror so I am convinced that it is external but not yet convinced it is test stand related.
      > >
      > > Mark
      > >
      > >
      > >
      > >
      > > ________________________________
      > >
      From: Jan van Gastel
      > > To:
      href="mailto:atm_free%40yahoogroups.com" rel=nofollow target=_blank ymailto="mailto:atm_free%40yahoogroups.com">mailto:atm_free%40yahoogroups.com
      > > Sent: Sunday, February 24, 2013 12:42 PM
      > > Subject: Re:
      [atm_free] Re: Other sources of astig in the Bath
      > >
      > >
      > >  
      > > Mark,
      > >
      > > With
      1/2"wide supports it is possible, that the mirror is not
      > > supported
      under its center of gravity and maybe even not on both
      > > suppots in
      the same plane, because the mirror's edge can be a
      > > little bit
      irregular or not everywhere  exactly square with the
      > > back .
      I made my supports from Delrin, of which I filed the edges
      > > 'sharp'.
      > >
      > >
      > > --
      > > Jan
      href="http://members.ziggo.nl/jhm.vangastel/Astronomy/" rel=nofollow target=_blank>http://members.ziggo.nl/jhm.vangastel/Astronomy/
      > >
      > > Op 24-2-2013 19:20, Mark Whitaker schreef:
      > >
      > > Actually, the mirror is 2.3" (I don't know why 2.7 sticks in my
      head).  I ran through the calculation through the site you provided.  I am pretty much dead on with the bearing supports.  I measure the bearing support just inside 1.125" and the calculator suggests 1.09".
      > > >
      > > >The bearing surfaces are 1/2"
      wide.
      > > >
      > > >Mark
      > > >
      > > >
      > > >
      > > >
      > > >
      > > >
      > > >________________________________
      > > > From:
      atmpob
      > > >To:
      rel=nofollow target=_blank ymailto="mailto:atm_free%40yahoogroups.com">mailto:atm_free%40yahoogroups.com
      > > >Sent: Sunday, February 24, 2013 10:33 AM
      > > >Subject: [atm_free] Re: Other sources of astig in the Bath
      > > >
      > > >
      > > > 
      > > >Mark,
      > > >That is a good start however the support points
      > > should not
      be on the middle of the edge but on the
      > > edge center of gravity.
      Those are not the same since
      > > the mirror has been carved out on the
      front surface.
      > > >
      > > >It does make a difference
      where the support is. You
      > > must be within about 1/16 of an inch of
      the COG.
      > > >
      > > >This link will compute were the cog
      is.
      > > >
      href="http://www.cruxis.com/scope/mirroredgecalculator.htm" rel=nofollow target=_blank>http://www.cruxis.com/scope/mirroredgecalculator.htm
      > > >
      > > >Dale Eason
      > > >
      > > >--- In
      mailto:atm_free%40yahoogroups.com, Mark Whitaker wrote:
      > > >>
      > > >> It is a 15.4" x
      2.7" mirror supported by a
      > > beefy stand with 2 roller bearings at
      90deg along
      > > the bottom.  The contact point is also at
      the
      > > midpoint of the blank relative to its thickness. 
      I
      > > have a photo in my tnut55 album.
      > > >>
      > > >> I can't discount stand induced astigmatism.ÂÂÂ
      > >
      This is the heaviest mirror I have had on this
      > > stand.
      > > >>
      > > >> Mark
      > > >>
      > > >>
      > > >>
      > > >>
      > > >>
      ________________________________
      > > >> From: Stephen Koehler
      > > >> To: "atm_free@...
      rel=nofollow target=_blank ymailto="mailto:_free%40yahoogroups.com">mailto:_free%40yahoogroups.com>
      > > >> Sent: Saturday, February 23, 2013 10:21 PM
      > > >> Subject: Re: [atm_free] Other sources of astig
      > > in the
      Bath
      > > >>
      > > >>
      > > >>
       
      > > >> Mark,
      > > >>
      > > >> I forget what size your mirror is.  Could this
      > > be simple test stand deformation?
      > > >>
      > > >>
      > > >>
      > > >>
      > > >> On
      Sat, Feb 23, 2013 at 2:24 PM, Mark Whitaker
      > > wrote:
      > > >>
      > > >>
      > > >> >
      > > >> >
      > > >> >I ran a series of igrams at each
      45deg
      > > mirror station without touching anything else such
      > > that tilt, fringe orientation, etc. remain the
      > >
      same.  The astigmatism is clearly fixed and does
      > > not
      follow the mirror.  That convinces me that that
      > > the
      *large* component of astig I am seeing is
      > > external.
      > > >> >
      > > >> >I checked beam alignment as suggested
      by
      > > Steven and while not completely explaining what I am
      > > seeing, some of the data has clearly improved.
      > > >> >
      > > >> >I think I just need to step back and look
      > > at my interferometer and its construction.
      > > >> >
      > > >> >Mark
      > > >> >
      > > >> >
      > > >> >
      > > >> >
      > > >> >
      > > >> >
      > > >> >________________________________
      > > >> > From: atmpob
      > > >> >To:
      rel=nofollow target=_blank ymailto="mailto:atm_free%40yahoogroups.com">mailto:atm_free%40yahoogroups.com
      > > >> >Sent: Saturday, February 23, 2013 11:09 AM
      > > >> >Subject: [atm_free] Other sources of astig
      > > in the
      Bath
      > > >> >
      > > >> >
      > > >> > 
      > > >> >Mark,
      > > >> >
      > > >> >Mike Peck worte this late last year in
      the
      > > interferometry group.
      > > >> >
      > > >> >There have been a couple well documented
      > > cases where
      the magnitude and direction of tilt has
      > > affected measured
      astigmatism (and other aberrations
      > > to a smaller extent). Moving the
      direction of tilt
      > > around while keeping the magnitude
      approximately
      > > constant should cancel this out, so I'd go
      with
      > > Dale's suggestion.
      > > >> >
      > > >> >Dave Schaack made a pretty convincing case
      > > that this
      is due to coma in the laser optics. A
      > > spatial filter on the source
      or better source optics
      > > might cure the problem.
      > > >> >
      > > >> >Dale Eason
      > > >> >
      > > >> >
      > > >> >--- In
      href="mailto:atm_free%40yahoogroups.com" rel=nofollow target=_blank ymailto="mailto:atm_free%40yahoogroups.com">mailto:atm_free%40yahoogroups.com, Mark Whitaker wrote:
      > > >> >>
      > > >> >> Are there other sources of instrument
      > > astigmatism besides
      the (small) beam astigmatism and
      > > test stand astigmatism that can
      become significant?
      > > >> >>
      > > >> >> I am seeing relatively large
      > > astigmatism that does not
      follow the mirror but
      > > instead does seem to have a fixed
      relationship to
      > > fringe orientation.ÃÆ'‚ 
      > > >> >>
      > > >> >> Mark
      > > >> >>
      > > >> >>
      > > >> >>
      > > >> >>
      > > >> >>
      ________________________________
      > > >> >> From: atmpob
      > > >> >
      > > >> >> To:
      href="mailto:atm_free%40yahoogroups.com" rel=nofollow target=_blank ymailto="mailto:atm_free%40yahoogroups.com">mailto:atm_free%40yahoogroups.com
      > > >> >> Sent: Friday, February 22, 2013 2:44
      > > PM
      > > >> >> Subject: [atm_free] Re: Bath
      > >
      astigmatism removal whinning
      > > >> >>
      > > >> >>
      > > >> >> ÃÆ'‚ 
      > > >> >> Jerry,
      > > >> >> Many
      people do not understand that the
      > > bath induced astig value is very
      very tiny for most
      > > of the mirrors we do with the beam seperation at
      or
      > > below the recommended value in the wiki.
      > > >> >>
      > > >> >> Please report the value of your
      setup
      > > as reported by open fringe.
      > > >> >>
      > > >> >> --- In
      href="mailto:atm_free%40yahoogroups.com" rel=nofollow target=_blank ymailto="mailto:atm_free%40yahoogroups.com">mailto:atm_free%40yahoogroups.com, "Jerry" wrote:
      > > >> >> >
      > > >> >> >
      > > >> >> >
      > > >> >> > I could rotate my igrams 90
      > > degrees and be ok. if the
      remove Bath
      > > >> >> > astigmatism function worked on
      my
      > > system. Apparently it work on some peoples
      > > >> >> > systems. When I really get in the
      > > groove here I will
      be taking several
      > > >> >> > igrams at different
      orientations
      > > and removing test stand and Bath
      > > >> >> > astigmatism.
      > > >> >> >
      > > >> >> > But being able to remove Bath
      > > astigmatism
      when analyzing single igrams would
      > > >> >> > be
      desirable also.
      > > >> >> >
      > > >> >> > Jerry
      > > >> >> >
      > > >> >> >
      > > >> >> >
      > > >> >> > From:
      target=_blank ymailto="mailto:atm_free%40yahoogroups.com">mailto:atm_free%40yahoogroups.com [mailto:mailto:atm_free%40yahoogroups.com] On Behalf
      > > >> >> > Of Vladimir
      > > >> >> > Sent: Friday, February 22, 2013
      > > 12:11 PM
      > > >> >> > To:
      rel=nofollow target=_blank ymailto="mailto:atm_free%40yahoogroups.com">mailto:atm_free%40yahoogroups.com
      > > >> >> > Subject: [atm_free] Bath
      > > astigmatism
      removal whinning
      > > >> >> >
      > > >> >> >
      > > >> >> >
      > > >> >> >
      > > >> >> >
      > > >> >> >
      > > >> >> >
      > > >> >> >
      > > >> >> > Dale,
      > > >> >> >
      > > >> >> >
      > > >> >> >
      > > >> >> > If and when you will
      finaly
      > > rewrite "Help", please enter statement that the
      > > >> >> >
      > > >> >> > present OF dialog
      assumes x axis
      > > and beam separation axis have the same
      > > >> >> >
      > > >> >> > direction. If this
      is not so the
      > > offered option will not work.
      > > >> >> >
      > > >> >> > By the way PSI programs
      of
      > > Michael Peck allow for such deviations
      > > >> >> >
      > > >> >> > and calculate properly by
      using
      > > the deviation angle.
      > > >> >> >
      > > >> >> >
      > > >> >> >
      > > >> >> > Vladimir.
      > > >> >> >
      > > >> >>
      > > >> >
      > > >> >
      > > >> >
      > > >> >
      > > >> >
      > > >> >
      > > >>
      > > >>
      > > >> --
      > > >> Steve Koehler
      > > >>
      > > >
      > > >
      > > >
      > > >
      > >
      > >
      > > --
      > > Jan
      href="http://members.ziggo.nl/jhm.vangastel/Astronomy/" rel=nofollow target=_blank>http://members.ziggo.nl/jhm.vangastel/Astronomy/
      > >
      >
       


    • atmpob
      The star test is very sensitive for astig. So it is easy to test for in a telescope with the mirror in its cell. From my 16 x 1 inch dob mirror the two point
      Message 39 of 39 , Feb 25, 2013
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        The star test is very sensitive for astig. So it is easy to test for in a telescope with the mirror in its cell.

        From my 16 x 1 inch dob mirror the two point edge support works on the bench and in the scope.

        Dale Eason

        --- In atm_free@yahoogroups.com, "Vladimir" <elizabeta.galogaza1@...> wrote:
        >
        > The point is that without knowing the (posible) test stand induced astigmatism,
        > actual, on glass astigmatism, can not be evaluated (and corrected).
        > It is true what Greg said, and if circumstances allow, testing the mirror
        > in its cell is prefered testing strategy.
        > The silo on Greg's farm is ideal (but so rare) oportunity for testing in vertical tower
        > configuration.
        > Vladimir.
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