Re: [ATM] Collimating, the eccentric way

["Rob&DonnaF" <TheFifareks@cox.net>]

Wait a minute, I want to agree, that the precise alignment of the secondary 
90 degrees to the optical path is not necessary, but imagine the cone of 
light that is reflected, less or more than 90 degrees, even a small fraction 
would distort cone, would it not? And aren't we talking about the nano level 
here?
Rob in Ramona
----- Original Message ----- 
From: "Nils Olof Carlin" <nilsolof.carlin@telia.com>
To: "mbyorick" <mbyorick@starband.net>; "'ATM LIST'" <atm@atmlist.net>
Sent: Wednesday, March 17, 2004 4:52 AM
Subject: Re: [ATM] Collimating, the eccentric way


> Mike Byorick wrote about collimation with a string between spider and
> mirror cell:
>
>> At this point, you have a properly positioned spider (relative to the
>> mirror's optical axis and to the focuser), and a focuser that is
>> 'automatically' aligned in one plane relative to the optical axis, since
>> the string/line has established the 'left/right' focuser alignment by
>> default, leaving only the 'up/down' alignment (with respect to the
>> longitudinal axis of the OTA).
>
> This should work, but as an alternative (possibly easier and even more
> accurate) is to do a full collimation, then check the actual position of 
> the
> optical axis relative to the OTA opening. Just take a ruler, hold it over
> the edge, moving the end until you see, in a sight tube, its reflection
> enter the mirror's - measure the distance here from the edge at 4 places.
> Now you see if the optical axis is close enough to where you want it - if
> not, re-position the diagonal/spider and/or focuser by the required amount
> and repeat.
>
>
>> The 'up/down' focuser alignment can be
>> carried out simply by putting a square to the string and aligning the
>> focuser accordingly. You are now 'in the ballpark', as David Harbour
>> would say.
>
>> I think the above 'mirror-centric' technique may avoid a result where
>> you have a perfectly collimated system, but where the 'focuser-centric'
>> method has allowed the focuser to remain 'tilted' relative to the
>> tube/truss,
>
> The importance of the exact 90 deg flection has been repeatedly claimed 
> and
> also debated here - my view is that since the diagonal is flat, this is
> fairly unimportant, and I have yet to hear what harm a moderate error 
> could
> do to the image. Any takers? There has been made telescopes with 45 deg
> between incident and reflected axes, with AFAIK no ill effect (round
> diagonal - lower eyepiece position, but trickier baffling and larger
> secondary-to-EP distance).
>
>
>
>> and subsequent collimation relative to the focuser has
>> caused the mirror's optical axis to be skewed relative to the
>> tube/truss, such that vignetting (etc.?) may occur.
>
> The check above will also show the margins to vignetting - anyway, if any,
> it is easily seen if you look for it into the focuser, as a reflection in
> the primary of the outer part of the tube.
>
>
> regards,
>
> Nils Olof
>
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> 


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