Newtonian secondary collimation (question)

After centering the secondary using a site tube, I aligned it using a laser
and a center dot on the primary. I then collimated the primary by getting
the return beam to meet the source beam for the laser.

Now, when I look through the cheshire, the reflection of the illuminated
metal piece is offset in the secondary, away from the primary, at a postion
possibly two thirds length of the major axis.

In a star test the images are pretty good close to focus, but racking focus
in and out a far distance shows the shardow of the CO as shifted this same
distance. First on one side of focus it is to the top (away from primary)
and then on the other side of focus it is to the bottom (toward the
primary).

What's up with that?

I've tried moving the secondary both away from and toward the primary and I
can't seem to ever get this centered. Also, if I ignore the laser, and
center the reflection of the cheshire, the collimation in star testing is
way off. Putting the laser back in and aligning, immediately improves the
star images, even though the previous problem persists far outside and
inside focus.

With the cheshire collimation, (where the shifted reflection is also evident
in the the secondary) the laser and the cheshire agree that the mirrors are
lined up on the center axis of the primary (the center dot appears in the
center of the cheshire reflection, as well as returning the laser beam into
the beam at the source).

Where am I going wrong?

Is my secondary too far from the primary? Too close? Or is something else
awry?


--
-Stephen Paul

After centering the secondary using a site tube, I aligned it using a laser
and a center dot on the primary. I then collimated the primary by getting
the return beam to meet the source beam for the laser.

Now, when I look through the cheshire, the reflection of the illuminated
metal piece is offset in the secondary, away from the primary, at a postion
possibly two thirds length of the major axis.

Don't worry about getting the return beam back to the source. I
realize that most of the laser collimator instructions out there
direct the user to do this, but all it takes is a slight mis-rotation
of the secondary to throw the whole collimation off. IMO, this is the
big reason why lasers sometimes get a bad rep for not being reliable
collimators.

Rather, what you should do is get the two dots to meet up on the
secondary, as seen by reflection in the primary. This is essentially
what the Cheshire does. Just look down into the tube at your primary
and look at the reflected face of your secondary on it. If the scope
is mis-collimated you'll see two identical laser dots: one at the
center (the incident dot) and one somewhere else (the reflected dot).
Adjust the primary's collimation bolts until the reflected dot lines
up with the incident dot. You're done! Now if you look at the face
of your collimator, you may see that the returning beam is not going
back into the source. That's fine. All it means is that your
secondary's axial rotation is a bit off and you'll lose a bit of field
illumination. But image quality will be unaffected and the scope is
in collimation with respect to alignment of the mirrors. The Cheshire
should now agree with the laser.

Ideally, of course, you'll want to rotate the secondary properly. You
need to rotate it until the outline of the secondary as seen through
the sight tube looks perfectly circular. But this is not critical.

I've tried moving the secondary both away from and toward the primary and I
can't seem to ever get this centered. Also, if I ignore the laser, and
center the reflection of the cheshire, the collimation in star testing is
way off. Putting the laser back in and aligning, immediately improves the
star images, even though the previous problem persists far outside and
inside focus.

That's strange. If the Cheshire says it's collimated, it should be.
Maybe something's wrong with your Cheshire?

Ritesh

ritesh wrote :
. . Ideally, of course, you'll want to rotate the secondary properly. You
need to rotate it until the outline of the secondary as seen through
the sight tube looks perfectly circular. .

This adjustment can be done with greater precision using a single
beam collimator. Temporarily misadjust the secondary by tipping it
either towards or away from the focuser using its angular adjustment
screws. Then, rotate the secondary holder on the tube axis; the
reflected laser spot will trace an arc on the primary at the bottom of
the tube. Look at this spot from the front of the telescope with your
eye gazing through a line between the secondary and the focuser. You
can run a string across the tube opening's diameter as a reference, or
use your spider vane if it happens to be oriented that way. Rotate the
secondary so that the laser spot reaches a point in line with both the
secondary and focuser. Lock the secondary rotation adjustment at this
point. Now, reset the secondary angular alignment.

Howie

This adjustment can be done with greater precision using a single beam
collimator.

Good point Howie. I would only add that it's difficult, at least with
my Protostar assembly, to lock the secondary's rotation in the correct
position. I have to hold it in place by hand while tightening the
nut, otherwise the secondary will rotate as I'm tightening. It's hard
to tell if I held it perfectly still while tightening. Is there a way
around this problem? I'm sure many others have experienced it too.
"Getting it close" may be the best we can do for field illumination!

Ritesh

The Cheshire and the Laser both show the optical axis of the focuser, lined
up with the center dot on the primary. When I look into the focuser, the
reflection of my eye coincides with the primary center dot. The Laser and
the Cheshire all agree on this.

The problem is that _that_ reflection sits too far "up" the tube (away from
the primary) when you look through the open focuser draw tube. It's as if
the secondary were too far toward the primary, as it rests under the
focuser. Or stated another way, it's as if the focuser were too far up the
tube, away from the primary.

I have no doubt that the optical alignment is correct, but that damn out of
focus star image has me fussing with the primary collimation trying to
center the shadow of the CO. When I get it centered, the collimation is WAY
off. I have to put the laser back in and realign to get pinpoint star images
again.

The scope star tests fine. I just think there's something really awry with
the position of the secondary, and I don't seem to be able to get it
sensibly positioned.

Anyway, I'll figure it out eventually. It will probably be something obvious
that I'm doing wrong. In the meantime, the scope is working well as long as
I align with the laser, using the method that you indicated (which is
exactly what I do, it just came out wrong in the writing).

Thanks Ritesh, and Howie.....

Stephen

On 10/25/03 06:26 +0900, Stephen Paul wrote:

The scope star tests fine. I just think there's something really awry with
the position of the secondary, and I don't seem to be able to get it
sensibly positioned.

Have you read Nils Olof Carlin's articles on collimation?
"Telescope Techniques" in the June 2002 and January 2003 S&T. The
2002 article is a crash course; the 2003 article covers barlowed
lasers. They might be helpful to you.

trane
--
//------------------------------------------------------------
// Trane Francks Tokyo, Japan
// Practice random kindness and senseless acts of beauty.
// http://mp3.com/trane_francks/

The scope star tests fine. I just think there's something really awry with
the position of the secondary, and I don't seem to be able to get it
sensibly positioned.

Yes I think that's the problem. Do you have a sight tube? With this
in place it's pretty easy to position the secondary axially. Not only
does it center your eye over the focuser, it gives you a long narrow
tunnel to look through so that you're forced into looking down the
centerline of the focuser instead of at an angle.

Ritesh

"Axel" wrote in message
om...
The scope star tests fine. I just think there's something really awry
with
the position of the secondary, and I don't seem to be able to get it
sensibly positioned.

Yes I think that's the problem. Do you have a sight tube? With this
in place it's pretty easy to position the secondary axially. Not only
does it center your eye over the focuser, it gives you a long narrow
tunnel to look through so that you're forced into looking down the
centerline of the focuser instead of at an angle.

Ritesh

Thanks for taking the time to respond Ritesh. I'm all set. It took a really
long time to get things lined up. I could probably tweak it some more, but
it's time to take a break and settle for "good enough" for a while.

The position of the secondary was too far toward the primary as suspected. I
made use of Bryan Greer's "Adventures in Collimation" to get properly
oriented and then used both the site tube and laser collimator.

What helped me in the end was to adjust everything a little at a time. A
little on the secondary, a little on the primary, a little on the spider
vanes to change the position radially, and a few turns of the secondary
retaining screw to move axially.

Cripes... the secondary is pretty complex. Makes primary collimation seem
like child's play.

This must have happened way back when I removed the secondary and holder to
make some modifications. Clearly I didn't position the secondary properly.
It's interesting that this never caused a problem in "close focus" star
collimation.

Somewhere I read that in collimation the first step is to get the shadow of
the secondary centered in the out of focus image. Since the secondary was
_way_ to far toward the primary, that's what was getting me into trouble. On
the other hand, it's also what clued me in to the extent of the position
error.

Too bad it's cloudy. I'm now hankering to get out and see if I get better
overall results, optically.
-Stephen

Too bad it's cloudy. I'm now hankering to get out and see if I get better
overall results, optically.

Glad it worked out. Should make a difference in illumination of wide
field views, but I don't expect any improvement in on-axis sharpness.
I'd be interested to hear your findings.

Ritesh