Re: ATM Troubleshooting double image from 10 " f6 Dob.

["Richard Schwartz" <richas@earthlink.net>]

Check your diagonal mirror.  It might be cracked!   Check it from the front
and from the back because the crack might not go all the way through.

Your diagonal may be strained.  Be sure that your diagonal is mounted
without strain: on three litttle tits of silicone that allow free air
circulaton behind the mirror; it must NOT be glued against any surface.

If one image is brighter than the other, then your diagonal might be mounted
backward, with the unsilvered side facing the focuser.  That would give you
two reflecting surfaces, and two images, but this seems to be in conflict
with your ability to select one image or the other by blocking out half of
the primary.

Check your eyepiece; a cracked lens there can create your problem.   To be
sure that the problem is in the scope and not in the eyepiece, try a knife
edge test on the mirror when it is in the telescope pointed at a star.  Make
a 1/2" hole in the bottom of a film can and mount a razor blade inside so as
to cut off half of the hole.   Make a 1/4" viewing hole in the top of the
film can.   Assemble and view.   Also, take care not to dull the razor by
repeated contact with your cornea.

Finally, check your primary mirror...   this is highly unlikely, but it
might be cracked.  You would have to remove it from the mirror cell and
check it from the back as well as from the front because the crack might not
go all the way through.

Sorry, that's all I can think of.  I ruled out the possibility of a double
star because you have the same problem with the moon.  The moon is double
only after consumption of too much brew...

----- Original Message -----
From: "ChrisR" <chrisr@es.co.nz>
To: <atm@shore.net>
Sent: Sunday, August 05, 2001 10:48 PM
Subject: ATM Troubleshooting double image from 10 " f6 Dob.


>
> Recently I completed my 10" f6 scope.  Its a dob design using a solid
> tube with a homemade
> Crayford focusser that works nicely.  The 10" mirror was made by a glass
>
> technican I know (30 years in the job) and only cost me $50 though I am
> not sure
> how precisely the surface is figured.  He seemed to think the suface was
>
> figured correctly but had only performed the Ronchie test on it (no
> knife
> edge test).  Star testing the mirror provided an OK image (if we look at
>
> just one reflection)...but I got more than I had expected...two stars!
> There occurs at the sharpest focus point.  The double image is only
> really noticable at magnifications over x
> 60 where the separation between the two becomes sufficient to resolve.
> When I point the scope at the moon at x150 I see echos of all the lunar
> features slightly offset.  The degree of offset can be descibed by
> looking
> at Mars at x150.  The double image is offset so that the second image
> has
> its limb more or less in the center of the first image...obviously
> destroying any chance of good viewing.
>
> The scope was aligned as follows.
> 1.  Square focusser and drawtube.  Center spider in tube.
> 2.  Center secondary mirror optically in draw tube (2.1 inch commercial
> mirror)
> 3.  A laser collimator was used to send a beam from the focusser
> drawtube
> to   the optical center of the secondard which was then aligned such
> that
> the reflected beam strikes the main mirror center.
> 4.  The main mirror was then aligned so that the laser beam was
> reflected
> back to the laser collimator aperture located in the drawtube.
>
> This proceedure aligns the scope well in my estimation.
>
> My first thought was that there might be some optical problem with the
> main
> mirror such as an astigmitism.  The person who made the mirror is not
> sure what to make of it.
>  The main mirror and cell is centered as well as possible in the main
> tube (12" inside diameter plastic drainage tube 5mm wall) as I
> could determine.  Certainly it cannot be far out (few mm tops?).
>
> I found that covering one half of the objective aperture with a piece of
>
> card removed one of the reflections.  Repositioning the card over the
> remaining half of the objective removed the other reflection.  I also
> found
> that a particular orientation/rotation of the card proved to be more
> effective at removing the double image.
>
> With this information does anyone have some suggested what might be
> wrong
> with this telescope?
>
> Thanks for any suggestions.
> Chris.
>
>
>