From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


>From: "Nils Olof Carlin" 
>
>Dwight Elvey:
>
>>  One should note that I don't believe Nils Olof was
>> saying that this would give you a finished collimation,
>> only that it helped to remove one of the biggest errors
>> when using laser collimators. If you really wanted to
>> properly mark a mirror, you could do a fine collimation
>> using a star or artificial star ( see note below ). You
>> could then place the donut right at the optical center
>> of the mirror.
>
>Collimation by any means against a center mark presumes it is at the
>true optical center. If it isn't, collimation will be off, as can be
>detected in a star test. I too would like to know if this is common.
>>
>> Note: Artificial Star:
>>  While one needs to be concerned about the distance and
>> size of an artificial star while using it to measure
>> such things as spherical aberration, for collimation
>> purposes, these are somewhat eased. As long as clear
>> diffraction rings can be seen it should work well
>> for collimating with.
>
>You have to consider the focuser out-travel - if an artificial star is
>so close that you must re-focus to where you have vignetting by the
>secondary, you do not test the full surface of the mirror. You may use
>a peephole to see how far out you can move the focuser and still see
>the primary's full edge in  the secondary, and from the out-travel
>calculate the minimum useful distance.

Hi Nils Olof
 Yes, I'd not thought about this factor. You do need to be
far enough that the outer parts of the primary still make it to the
eyepiece. This may be a problem for those that have marginal secondary
sizes. Another way to check this is to defocus a little longer and move the
telescope around. One can usually, clearly see the clipping effects of the
secondary by the bite taken out of the disk, as it move around the field of
view. If the center shows no clipping, you are OK. Dwight

>
>> You should not be able to
>> see a center that is much bigger than the diffraction limited
>> Airy disk. Having spherical aberration, by having the
>> source too close, can actually be used to advantage while
>> collimating. It is an aid and not a detriment.
>
>>From simulations, I believe the highest sensitivity is at focus, by
>judging the asymmetry of the (first) diffraction ring. But it may be
>practical to try different defocused positions.
>
>Nils Olof.
>
>

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