collimation help

Chaps, what does this mean?

I've spent some time fiddling with my skywatcher 200 dob collimation screws,
to gain confidence that they *can be* fiddled with amongst other things. I
first of all followed the instructions in the book, then did some of my own
intuitive adjustments. I got pretty good results with the book approach, but
was left with an "odd thing" (which I'll describe later). I have managed to
improve the "odd thing" with my fiddling, but can't get rid of it. I want to
know what it is :-).

I've put a picture here temporarily:
http://homepage.ntlworld.com/robinso...orld/temp1.jpg
...showing what I see when using a home-made collimation cap with a
translucent lid, and the scope pointing at the white garage wall. Working in
from the outside (please correct me if I'm not looking at what I think I'm
looking at):

1) Primary clips correctly visible and symmetric within limits of secondary
mirror edge (dotted).
2) "Odd thing"; shadow of secondary mirror(?) not quite aligned, and not
quite circular(?).
3) Reflected image of translucent cap of home-made pinhole collimation cap,
centered correctly
4) Primary mirror centre dot (factory inscribed 'thick circle'), correctly
centred
5) Reflected image of collimator pinhole, correctly centred

Adjusting the primary screws up the alignment between 4) and 5), and moves
2) off centre
Adjusting the secondary screws up the alignment of 1)

What should I do to bring everything into alignment? Is it a case of moving
the secondary up or down the tube, or rotating it, or something else?

Finally, how much margin is there in the adjusting screws? I don't want to
over-loosen anything and have it fall off :-).

Cheers

Alan

Alan

It seems that your secondary is probably not correctly adjusted in the first
place.

Stage 1/
It is important first to ensure that the outline of the secondary mirror is
centred
as seen through the collimation cap in both an up/down and a sideways sense
i.e. in its position axially along the length of the optical tube assembly
(OTA).
Be careful when adjusting the position of the secondary axially not to let
the secondary fall off the end of the bolt that it is held by! Get a feel
for the length of this bolt.
Depending on how the secondary is mounted, there may be some adjustment
screws on the periphery of the OTA that allow the secondary "spider" to be
moved in an up/down sense as seen through the collimation eyepiece. After
adjustment, the secondary should also appear as a circle to your eye rather
than an ellipse, as it will be inclined at 45 degrees or thereabouts.

If the focuser itself is not square with the OTA then it may not be easy to
get this intial alignment process spot on. Once this is correct, you should
not need to adjust the secondary unless you disassemble things.

Stage 2/
Now adjust the tilt of the secondary mirror to get the centre of the primary
centred in your view. If a lot of adjustment is requried, you may need to go
back to stage 1.

Stage 3/
Now adjust the primary to bring the reflected view of the pinhole centred.

It is worth noting that the initial alignment of the secondary basically
affects the light gathering aspect of the scope and should not affect the
crispness of the images you get so long as the stages 2 and 3 are correct.
That's not to say it's not worth getting it correct!

Your alignment looks pretty good from the drawing, although I accept that
the secondary is not quite correct.
Hope this helps

-Russell Healey
p.s.
Stage 4/ Don't spend too long worrying about this if the star images are
fine. If you are free to obsereve, enjoy the clear night and don't tinker
too much!!

1) Primary clips correctly visible and symmetric within limits of
secondary
mirror edge (dotted).
2) "Odd thing"; shadow of secondary mirror(?) not quite aligned, and not
quite circular(?).
3) Reflected image of translucent cap of home-made pinhole collimation
cap,
centered correctly
4) Primary mirror centre dot (factory inscribed 'thick circle'), correctly
centred
5) Reflected image of collimator pinhole, correctly centred

Adjusting the primary screws up the alignment between 4) and 5), and moves
2) off centre
Adjusting the secondary screws up the alignment of 1)

What should I do to bring everything into alignment? Is it a case of
moving
the secondary up or down the tube, or rotating it, or something else?

Finally, how much margin is there in the adjusting screws? I don't want to
over-loosen anything and have it fall off :-).

Cheers

Alan

Cool ....... thanks for the detailed response. In stage 1, are you talking
about observing the secondary directly throught the focusser tube? (I.e. not
the shadow reflected in the primary?)


"Russell Healey" wrote in
message ...
Alan

It seems that your secondary is probably not correctly adjusted in the
first
place.

Stage 1/
It is important first to ensure that the outline of the secondary mirror
is
centred
as seen through the collimation cap in both an up/down and a sideways
sense
i.e. in its position axially along the length of the optical tube assembly
(OTA).
Be careful when adjusting the position of the secondary axially not to let
the secondary fall off the end of the bolt that it is held by! Get a feel
for the length of this bolt.
Depending on how the secondary is mounted, there may be some adjustment
screws on the periphery of the OTA that allow the secondary "spider" to be
moved in an up/down sense as seen through the collimation eyepiece. After
adjustment, the secondary should also appear as a circle to your eye
rather
than an ellipse, as it will be inclined at 45 degrees or thereabouts.

If the focuser itself is not square with the OTA then it may not be easy
to
get this intial alignment process spot on. Once this is correct, you
should
not need to adjust the secondary unless you disassemble things.

Stage 2/
Now adjust the tilt of the secondary mirror to get the centre of the
primary
centred in your view. If a lot of adjustment is requried, you may need to
go
back to stage 1.

Stage 3/
Now adjust the primary to bring the reflected view of the pinhole centred.

It is worth noting that the initial alignment of the secondary basically
affects the light gathering aspect of the scope and should not affect the
crispness of the images you get so long as the stages 2 and 3 are correct.
That's not to say it's not worth getting it correct!

Your alignment looks pretty good from the drawing, although I accept that
the secondary is not quite correct.
Hope this helps

-Russell Healey
p.s.
Stage 4/ Don't spend too long worrying about this if the star images are
fine. If you are free to obsereve, enjoy the clear night and don't tinker
too much!!

1) Primary clips correctly visible and symmetric within limits of
secondary
mirror edge (dotted).
2) "Odd thing"; shadow of secondary mirror(?) not quite aligned, and not
quite circular(?).
3) Reflected image of translucent cap of home-made pinhole collimation
cap,
centered correctly
4) Primary mirror centre dot (factory inscribed 'thick circle'),
correctly
centred
5) Reflected image of collimator pinhole, correctly centred

Adjusting the primary screws up the alignment between 4) and 5), and
moves
2) off centre
Adjusting the secondary screws up the alignment of 1)

What should I do to bring everything into alignment? Is it a case of
moving
the secondary up or down the tube, or rotating it, or something else?

Finally, how much margin is there in the adjusting screws? I don't want
to
over-loosen anything and have it fall off :-).

Cheers

Alan

Yes - observing the _real_ outline of the secondary, through the collimating
eyepiece that you have made.

-Russell

"Alan" wrote in message
...
Cool ....... thanks for the detailed response. In stage 1, are you talking
about observing the secondary directly throught the focusser tube? (I.e.
not
the shadow reflected in the primary?)

Thanks. I checked again and it looks pretty good. I think I had to move the
secondary UP the tube, which meant tightening the bolt (no danger of it
falling off :-)). I have just seen the Cassini division through thin cloud
at 120x and 240x with the barlow, so things must be pretty good. Concentric
diffraction circles too either side of focus on stars, although there is
some astigmatism (could be my eyes though).

Things I'm left wondering ........

1) How do you know if the secondary is twisted around the axis?

2) Is collimation more important at low or high powers, and in 2" or 1 1/4 "
eyepieces?

Cheers

Alan


"Russell Healey" wrote in
message ...
Yes - observing the _real_ outline of the secondary, through the
collimating
eyepiece that you have made.

-Russell

"Alan" wrote in message
...
Cool ....... thanks for the detailed response. In stage 1, are you
talking
about observing the secondary directly throught the focusser tube? (I.e.
not
the shadow reflected in the primary?)

Good - well done. DOes the overall picture through the collimaing eyepiece
look better too?

The astigmatism should not be so noticeable at higher magnifications.
One way to check whether its your own eyes' or the eyepiece optics' problem
is to rotate yourself so that you are viewing through the eyepiece from a
different angle
(if you see what I mean). i.e. from a different side of the focus tube.
If it's your own eyes then the astigmatism will follow you. If the problem
seems to point in the same direction then its probably the eyepiece.

With low magnifications, you own eyes' astigmatism is more noticeable as its
easier to catch off-axis rays.

I'm not sure quite what you are referring to in Q1 below? Which axis?

Collimation is generally more important at higher powers, particularly when
seeking to bring out lunar and planetary detail. However, that doesn't mean
it's unimportant for deepsky observation. You don't really want stars with
comets tail's which is what you will get with coma, the problem that is
manifested when optics are miscollimated.

I'm very pleased you have some clear skies!!!

-Russell



Alan" wrote in message
...
Thanks. I checked again and it looks pretty good. I think I had to move
the
secondary UP the tube, which meant tightening the bolt (no danger of it
falling off :-)). I have just seen the Cassini division through thin cloud
at 120x and 240x with the barlow, so things must be pretty good.
Concentric
diffraction circles too either side of focus on stars, although there is
some astigmatism (could be my eyes though).

Things I'm left wondering ........

1) How do you know if the secondary is twisted around the axis?

2) Is collimation more important at low or high powers, and in 2" or 1 1/4
"
eyepieces?

Cheers

Alan

Thanks again. Not sure what you mean about the overall picture through the
collimating ep? It is just a film-can-with-hole clone made from a pill
case - everything looks pretty well aligned through it if that's what you
mean.

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



"Russell Healey" wrote in
message ...
Good - well done. DOes the overall picture through the collimaing eyepiece
look better too?

The astigmatism should not be so noticeable at higher magnifications.
One way to check whether its your own eyes' or the eyepiece optics'
problem
is to rotate yourself so that you are viewing through the eyepiece from a
different angle
(if you see what I mean). i.e. from a different side of the focus tube.
If it's your own eyes then the astigmatism will follow you. If the problem
seems to point in the same direction then its probably the eyepiece.

With low magnifications, you own eyes' astigmatism is more noticeable as
its
easier to catch off-axis rays.

I'm not sure quite what you are referring to in Q1 below? Which axis?

Collimation is generally more important at higher powers, particularly
when
seeking to bring out lunar and planetary detail. However, that doesn't
mean
it's unimportant for deepsky observation. You don't really want stars with
comets tail's which is what you will get with coma, the problem that is
manifested when optics are miscollimated.

I'm very pleased you have some clear skies!!!

-Russell



Alan" wrote in message
...
Thanks. I checked again and it looks pretty good. I think I had to move
the
secondary UP the tube, which meant tightening the bolt (no danger of it
falling off :-)). I have just seen the Cassini division through thin
cloud
at 120x and 240x with the barlow, so things must be pretty good.
Concentric
diffraction circles too either side of focus on stars, although there is
some astigmatism (could be my eyes though).

Things I'm left wondering ........

1) How do you know if the secondary is twisted around the axis?

2) Is collimation more important at low or high powers, and in 2" or 1
1/4
"
eyepieces?

Cheers

Alan

The "collimating eyepiece" is what you have made essentially out of the film
can.
Anyway, as you say the mirrors seem aligned now.

As far as Q1 goes (now I understand), if the secondary is twisted then it
won't appear circular. It will seem elliptical with the "vertical" axis
smaller than the "horizontal" axis. As you progressively rotate the
secondary about the axis of the OTA, the "squashed circle" effect will
become more pronounced.
The tilt of the secondary mirror (using the adjustment screws) will affect
things a little but not very much as the amount of adjustment in tilt is
relatively small, provided that the secondary mirror is about right and the
primary mirror is central in the main OTA. Technically, I do agree that one
part of the process can affect the other, but if you iterate and go back and
readjust things again then it is usual to converge on an aligned solution.
It sounds like you've got things correct now, Alan. Well done! It does take
a while to get used to how things appear. I've never had a look through one
of the Skywatcher series. They have good optics I hear. Are you pleased with
yours?

By the way, some excellent web page references are
http://zebu.uoregon.edu/~mbartels/kolli/kolli.html
http://w1.411.telia.com/~u41105032/myths/myths.htm
The second of these is quite reassuring!

-Russell
P.S. It cleared up for a while here and I had some quite nice views of
Saturn and M35




"Alan" wrote in message
...
Thanks again. Not sure what you mean about the overall picture through the
collimating ep? It is just a film-can-with-hole clone made from a pill
case - everything looks pretty well aligned through it if that's what you
mean.

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

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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.......... 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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