collimation help

Stephen may have his comments on this but, here is what I do...

Rack the focus tube otuwards
until the point just beofre the periphery of the film cannister starts to
obscure your view
of the edge of the secondary mirror.

As you adjust the rotation of the secondary mirror you will see the gap
between the edge of the film cannister
and the edge of the secondary mirror increase on one side and decrease on
the other.

You need to adjust the rotation until there is an even amount of gap between
the edge
of the secondary and the film periphery of the film cannister and may also
need
to adjust the spider if this gap is uneven. This is easy to see if you have
the focus tube racked out just the correct amount.

This is where a proper sight tube comes in useful - it may be that the home
made cannister is insufficently long.

The links I mentioned yesterday have a good description of this part of the
process.


-Russell

"Alan" wrote in message
...
......... as I suspected, so how do you know that the secondary is
correctly
oriented around the scope axis? Or is it good enough to get a visual
circle
looking down the focusser tube? Seems to me that the rotation causes
bigger
errors than the adjusting screws .........so it ought to need adjusting
quite accurately.

"Stephen Tonkin" wrote in message
...
Alan wrote:
What I meant by Q1 was how do you know the secondary is not (wrongly)
rotated around the main scope axis. I cant make up my mind if twisting
it round can be compensated for by adjusting the screws

It can't.

or if they are two independant things to get right .........

Exactly so.

Best,
Stephen

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Cool. Thankyou - I'll try it.


"Russell Healey" wrote in
message ...
Stephen may have his comments on this but, here is what I do...

Rack the focus tube otuwards
until the point just beofre the periphery of the film cannister starts to
obscure your view
of the edge of the secondary mirror.

As you adjust the rotation of the secondary mirror you will see the gap
between the edge of the film cannister
and the edge of the secondary mirror increase on one side and decrease on
the other.

You need to adjust the rotation until there is an even amount of gap
between
the edge
of the secondary and the film periphery of the film cannister and may also
need
to adjust the spider if this gap is uneven. This is easy to see if you
have
the focus tube racked out just the correct amount.

This is where a proper sight tube comes in useful - it may be that the
home
made cannister is insufficently long.

The links I mentioned yesterday have a good description of this part of
the
process.


-Russell

"Alan" wrote in message
...
......... as I suspected, so how do you know that the secondary is
correctly
oriented around the scope axis? Or is it good enough to get a visual
circle
looking down the focusser tube? Seems to me that the rotation causes
bigger
errors than the adjusting screws .........so it ought to need adjusting
quite accurately.

"Stephen Tonkin" wrote in message
...
Alan wrote:
What I meant by Q1 was how do you know the secondary is not (wrongly)
rotated around the main scope axis. I cant make up my mind if
twisting
it round can be compensated for by adjusting the screws

It can't.

or if they are two independant things to get right .........

Exactly so.

Best,
Stephen

Remove footfrommouth to reply

--
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+ Stephen Tonkin | ATM Resources; Astro-Tutorials; Astro Books +
+ (N51.162 E0.995) | http://astunit.com +
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

Alan,
come onto this thread a little late maybe but I have found this site
to be a useful one for a 'basic' understanding of practical
collimation.
http://www.schlatter.org/Dad/Astronomy/collimate.htm
Take your time and work through the pages. I found it a great help
having the telescope next to the computer as I progressed. I used an
old cheap .965" Huygens 6mm eyepiece with the lens taken out as my
'peephole'. Maybe it is a little big peephole diameter but it works
fine. There are more detailed sites out there but for a primer it
works well.
Good luck.
Squirrel

Russell Healey wrote:
Stephen may have his comments on this but, here is what I do...

Looks good. Takes care of everything except offset (and offset is only
worth worrying about on short-focus stuff, and then, only if you want
even illumination whose absence is probably undetectable visually).

Best,
Stephen

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+ (N51.162 E0.995) | http://astunit.com +
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