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Tracking down astigmatism (was: Sanding down secondary mount...)

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  • Chris Nicholl
    ... Joe- Astigmatism seems to be an error born more of the way an optic is held in place, rather than the figure of the optic itself. Not always, but usually
    Message 1 of 17 , Dec 1 4:20 AM
      --- Joe Huber wrote:
      > How do you tell if the astigmatism in from the secondary?

      Joe-

      Astigmatism seems to be an error born more of the way an optic is
      held in place, rather than the figure of the optic itself. Not
      always, but usually (an example of the exception: I believe Suiter
      talks about secondary mirrors being made using non-flat surfaces, and
      that astigmatism can then be manufacturered right into the mirror
      itself, but I believe that's rare). So, the good news is that it's
      often correctable.

      So, there are basically four potential sources of astigmatism:
      Your eyes - presumably you would know if your vision is astigmatic,
      so most of us can rule this one out right away.

      The eyepiece - I've heard that eyepieces can be the source, but I
      don't think I've ever heard anyway say "I found such and such an
      eyepiece to demonstrate astigmatism". It could be that we're usually
      dealing with higher-quality eyepieces these days, so this is largely
      an artifact of older eyepieces? In any event, you can rotate an
      eyepiece in the barrel; if the astigmatism doesn't rotate in a
      corresponding manner, then you can rule out the eyepiece.

      Primary mirror - I can't remember if astigmatism is the same (or has
      similar manifestations to) pinched optics. In any event, if the
      clips on the primary are too tight, it will pinch the mirror and
      deform the image. So, the clips on the primary should be one of the
      first things checked (probably *the* first thing) if you notice some
      evidence of astigmatism/pinched optics.

      Secondary mirror - Most of us go through the prior list, and still
      have some remaining astigmatism. This leaves the only remaining
      source - the secondary mirror/mount. Assuming it's not the mirror
      itself (which may or may not be a valid assumption, but it's probably
      the best starting point), you should examine the secondary mount to
      make sure that there is no pressure on the optic. Phil or someone
      else can talk about the Synta holders, but the Guan Sheng holders are
      notorious for imparting some astigmatism. That's one of the reasons
      that I'm upgrading my holder, as I've reached the point where I can't
      rid the system of astigmatism (I haven't tried Phil's latest
      suggestion, but probably won't, since there are other reasons that I
      want to replace the secondary mount as well).

      Good luck - it sounds more daunting in writing than it really is.
      Just proceed step by step, and I'm sure you'll be able to isolate it,
      then figure out the best way to rememdy it (if need be).

      Chris Nicholl
    • Chris Nicholl
      ... Ah, I forgot about that lip, and didn t actually realize that it came into play (or *could* come into play, given the presence of the foam). In any event,
      Message 2 of 17 , Dec 1 4:31 AM
        --- Phil Lefever wrote:
        > Your holder is the same as mine (a bit smaller though) and RTV would
        > work just fine! The mirror rests against a flat lip all around the
        > rear edge. There is a large opening into the holder body but ignore
        > that. All you need is 3 pea sized dabs of RTV on the lip around the
        > edge. Clean everything with alcohol first and you will be swearing
        > when you need to remove the mirror!

        Ah, I forgot about that lip, and didn't actually realize that it came
        into play (or *could* come into play, given the presence of the
        foam). In any event, sanding down is one thing, but that level of
        modification, given that I'm very likely replacing the secondary
        anyway, is probably overkill for me. I really wanted to do something
        to ensure that the secondary mirror was ok, but I think I'll do that
        by simply ordering the mount first, using the older mirror, and
        checking the image.

        Incidentally, I don't think I have a way to measure the minor axis on
        my secondary. Do you (or anyone else) know whether the secondary on
        the older (Guan Sheng) XT8 is 1.83" or 1.85" - someone a few posts
        back alluded to the fact that it might be 1.85". I can probably call
        Bryan at Protostar too, to see if he has any knowledge about this,
        since I'm sure he has a reasonable amount of experience with
        secondary upgrades of these scopes.

        > Also if you ever thought of a dew heater on the secondary install
        > it before you glue the mirror in place. A DewGuard from
        > Astrosystems is a excellent and simple choice!

        Thanks, although I'm doing my very best to stay caveman (non-
        electric) on my dob!

        Chris Nicholl
      • g2baraff
        ... astigmatic, so most of us can rule this one out right away. ************** And you CAN test for it, to find out if the amount you have is the cause of the
        Message 3 of 17 , Dec 1 4:38 AM
          --- In skyquest-telescopes@yahoogroups.com, "Chris Nicholl"
          <nicholl.cj@m...> wrote:
          > --- Joe Huber wrote:
          >
          > So, there are basically four potential sources of astigmatism:
          > Your eyes - presumably you would know if your vision is
          astigmatic, so most of us can rule this one out right away.

          **************
          And you CAN test for it, to find out if the amount you have is the
          cause of the troubel, or you still have to work with the optics of
          the scope: simply rotate your head while looking through the
          eyepiece, and see if the astigmatic image rotates too.
          *************

          > Primary mirror - I can't remember if astigmatism is the same (or
          has similar manifestations to) pinched optics.
          ********

          Physically, it is the same phenomenon, but because the mirror is
          usually held down by three clips, the out of focus image is
          triangular in symmetry and the triangle flips as you pass through
          focus, whereas in astigmatism, it is the long axis of the ellipse
          that flips as you pass through focus. If the hold down clips bend
          the axis of distortion along one direction, you experience it as
          astigmatism. If they set up three axes of distortion, you
          experinece it as "pinched optics".

          Gene
        • Phil Lefever
          ... Chris, I think your best upgrade route it to cut off the lip of your holder and RTV the mirror in place. It might completely solve your problem. The
          Message 4 of 17 , Dec 1 10:20 AM
            >Ah, I forgot about that lip, and didn't actually realize that it came
            >into play (or *could* come into play, given the presence of the
            >foam). In any event, sanding down is one thing, but that level of
            >modification, given that I'm very likely replacing the secondary
            >anyway, is probably overkill for me. I really wanted to do something
            >to ensure that the secondary mirror was ok, but I think I'll do that
            >by simply ordering the mount first, using the older mirror, and
            >checking the image.

            Chris,

            I think your best upgrade route it to cut off the lip of your holder
            and RTV the mirror in place. It might completely solve your problem.
            The question will lie in how you would attach your oversized GS
            mirror to the new holder, you will likely have to glue it to the new
            one as well as the shroud may not slip over it.

            If your specific goal is to eliminate astigmatism RTVing the mirror
            to your holder would give you a good enough test to tell if the error
            was in the glass. This would allow you to know if you need a new mirror.
            Heck it might even keep you from spending $100+....

            I do understand about being squeamish about it though, but you survived
            the 2" focuser swap so it should be easy!

            Clear Skies,

            PL

            --------------------------------------------------------------------
            Phil Lefever - Burnsville, MN
            Amateur Radio Callsign - KB0NES EN34jt
            C8-SP XT-10 C102 80WV
            MNAA - Minnesota Amateur Astronomers
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