From: "Dwight K. Elvey" 
To: atm{at}shore.net
cc: nilsolof.carlin{at}telia.com, bobmay{at}nethere.com
Reply-To: "Dwight K. Elvey" 


>From: "Jack Schmidling" 
>
>From: "Dwight K. Elvey" 
>>  This is why I wanted you to describe your method. The peep hole
>> should not be offset relative to anything other than the secondary's
>> reflection( which it should be offset ). It still sounds like you are
>> not aligning correctly......
>
>According to Suiter p.112, the peep hole is offset to the front of the scope
>with reference to the apparent position of the diagonal not the secondary's
>reflection.   The diagonal's  reflection is centered in the primary.

Hi Jack
 I have Suiter's book so I'll have to see what you are looking at(not with me).
The only off centered thing should be the reflection of the secondary.
EVERYTHING else should be exactly concentric! I believe that Suiter knows
what he is doing so the only thing I can think is that you are interpreting
something wrong.

>
>> 1. Center the secondary as seen through the focuser( not the reflection ).
>> 2. Tilt/rotate the secondary such that you can see the
>>  edges of the primary, evenly in the field of view.
>>  ( repeat to step 1 & 2 if needed, they do interact ).
>> 3. Do a preliminary adjustment of the primary such that
>>  the center spot of the primary matches the hole in a
>>  peep site.
>
>When all this is done correctly, the peep hole reflection is not centered
>but offset to the front.

 Maybe your center mark is incorrect? If not, it sounds like
you are not getting the secondary properly framed in front of the focuser.
As an alternate to step two, you can also adjust the secondary such that
the peep holes image hits the primary's center spot. Then check to see that
the secondary is still framed properly in the focuser view. Both framing
the primary or centering the peep hole by adjusting the secondary should
produce the same results. If not, the secondary is not framed correctly in
the focuser.
 It is even possible that your mirror has a parabola that is
not centered on the mechanical center of the mirror. This does not
necessarily mean that the mirror is useless, only that you are using it in
an off axis manor.

>
>
>> 4. On a star ( Polaris is good with no drive ), using a
>> medium power eyepiece, find where in the field of view
>> the diffraction rings are best concentric ( this is just
>> slightly out of focus. If you can see the secondary shadow
>> you have gone way too far ). Once you've found this location,
>> without moving the telescope, move the image of the star
>> to the center of the field of view. Repeat step 4 with
>> a high power eyepiece until satisfied.
>
>Several  problems here:
>
>First of all, moving the star to the center with the tilt adjustment does
>the same thing as moving the telescope to center the star after tilting but
>it does seem like a more intuitive method.

 I've used both and like moving the star back to center better. It
seems more intuitive and seems to speed things up.

>
>Secondly, I do not understand how you can see infocus diffraction rings when
>slightly out of focus.  Furthermore, they are very difficult to see except
>under excellent seeing and very high power.  Out of focus rings and diagonal
>shadow tell the same story and are easy to see under any conditions at
>moderate power.

 The problem with the secondary shadow is that it should be off center.
by the time you've moved the focus so far off that you see this, the
diffraction bands next to this shadow should be off center.
 It is true that you'll need good seeing to deal with the higher power
adjustments. I can't do anything about that. Still, even at medium power,
slight out of focus images of a star produce clear diffraction rings in all
but the worst seeing.

>
>Finally, if I follow your procedure, I have the same problem I started with.
>The difraction rings centered on one side of focus will not be centered on
>the other.

 You might try checking your focuser. Find a laser colimation tool
someplace. Remove the secondary. First rotate the laser in the focuser to
determine if the laser is aligned correctly. The spot shouldn't move. Now
run the focuser in and out over the travel that you'd normally move. You
should see no change in position. This checks that the travel is aligned
with the draw tube.
 Also, this check is only valid, close to focus. When outside
of focus far enough to see the secondary shadow, it should be off center on
both sides.

>
>>On fast telescopes, having the secondary's
>> image, as seen from the focuser, reflected in the primary,
>> be concentric is an indication of incorrect alignment!
>
>I find this confusing and possibly in conflict with what I cited above.
>Please look at Suiter's diagram and see how it fits into your explanation.
>
>If you don't have the book, I can scan the picture and post it.
>

 I have the book but it will be tomorrow before I can get back to
you. Like I said, I trust Suiter's book, I'm just not sure you are
interpreting it correctly. Ask Nils Olof, Bob May or any of the others on
this group that understand collimation. I'm sure they'll tell you the same
as me. Their methods may vary but the end result would be the same.
Everything should look concentric except the reflection of the secondary.
That is everything, including the peep hole, the secondary as seen directly
from the focuser, the center spot on the primary and the edges of the
primary. Dwight

>js
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