Collimating an ST-80

All you refractor guru's:

Last night a friend was over with a Nexstar 80GT, this is a typical Synta 80mm F/5 OTA. At higher magnifications, it exhibited what I take to be rather severe coma, viewing Castor at 90x, I could see the airy disks but to one side, there as a large blur which under scrutiny looked to be coma.

A laser showed that the diagonal seemed to be collimated so I am guessing I need to adjust the objective cell which I believe is threaded on (I could be wrong on that.) Any thoughts on what I might do here to try to improve things.

Thanks

Jon

>>A laser showed that the diagonal seemed to be collimated so I am guessing I need to adjust the objective cell which I believe is threaded on>>

Generally a refractor lens is designed and constructed to be coma-free. This means no matter how you tilt the lens with respect to the optical path, you will not see coma. You will see astigmatism off-axis, but this is not coma.

Therefore, if you have coma in the image, it is not because the lens is tilted, or the lens cell is tilted with respect to the tube. The only way to get coma in a doublet is to shift one of the lens elements sideways with respect to the other element, or to have a wedge in the airspace that exists between the two elements.

The fact that you have coma means that you have one of the above two problems, but you do not have the first problem, which is a tilt of the objective lens or cell. Therefore tilting the cell in the other direction will not solve the coma problem.

Collimation for a doublet or triplet lens means lining up the elements (sideways shift) until the optical centers align, plus eliminating the wedge in the airspace. Both of these are critical. Wedge in the airspace must be less than .0001" or 2.5 microns, and the sideways tolerance is on the order of .0005" or 5 microns.

Rolando

I'll first say I've never worked on an ST-80, but have
worked on lots of other models. There are two things to
check -- that the lens is mounted square to the eyepiece,
and that the airspace is not wedged.

Check the first one using a Cheshire device either in the
diagonal (i.e. test that too) or directly in the focuser.
This will project a pattern of donuts onto the lens
surfaces, and if all is square, the donuts will all
line-up and be on top of each other. If not, you'll need
to tilt something to fix it. I expect these don't have
collimation screws on the lens cell. It may be easier to
tilt the focuser on the back of the scope -- maybe you'd
need to slightly elongate one or more screw holes to get
there. You can do some trial and error by taking out
one or two of the focuser screws where it attaches to the
scope, tilting it around, and taping it temporarily in place
with blue painter's tape.

It could also be caused by the airspace being wedged.
I don't know if these use three small spacer tabs or
a thin gasket all the way around between the elements.
First see if there is excessive rattle or looseness of
the elements. Maybe there is a retaining ring that is
too loose, so the elements are flopping around.
Or if you've got the spacer tabs -- maybe one
is missing or has moved.

Another trick is to check for
Newton's rings between R2 and R3. Get a big diffuse
fluorescent light behind your head, hold the lens at
arms length, and look at the reflection of the light
in the lens. You should be able to pick-up a faint pattern
of rings or arcs in the lens. If the airspace is OK,
there will be a nice pattern of concentric rings centered
near the lens center. But if its wedged, they will
smashed-up against one side, or even appear as parallel
lines (not arcs).

If you need to adjust the airspace,
it will be very sensitive -- like 0.01mm adjustments.
Sometimes adding a tiny piece of thin Saran wrap on one
of the spacers is enough to fix it. (Normal spacing on
most refractor lenses is around 0.1mm.) If one is missing
altogether, it may be easiest to replace all 3 spacers
with tiny bits of black plastic tape, postage stamps, etc.
If you've got the ring gasket all the way around -- sometimes
it gets damaged or slips out of place when the lens is assembled
-- you could take out the damaged gasket and replace with
three tabs of stuff.

If you need to get the lens apart -- here is the way to
do it: Get some tube that is about 60mm in diameter,
place it vertically on a table, put a piece of lens
tissue (or paper towel) over the end and tape securely
around the sides. Take the retaining ring off the
lens cell, and then slowly drop the lens cell over the
end of the tube. The elements should end-up resting
on the tissue / tube and the cell should drop to the
table. You want to take the lens cell off the assembled
elements, not the other way around. (Do not try to
take the elements out of the cell -- if they get cocked
and stuck you will have a bad problem.)

I did some write-up on adjusting the ED-80 some time ago:
http://tech.groups.yahoo.com/group/Orion_ED/files/Orion%20ED-80%20collimation/

Good luck!

<>

I had always thought that the Nexstar was just a ST80 on a hyped GOTO ALT-AZ mount. Is it really an upgraded and well built 80mm scope? That would be good news.

I had one of the original ST80's from Celestron and while it had a metal focuser and tube, the lens cell was plastic and the focuser was the worst of the rack and pinions. If the Nexstar is typical, I would think it is the same. Sliding the dew shield off revealed the three mounting screws for the lens cell. The tips of these screws were just slightly too long and were easily visible with a casual look into the objective. There is a possibility that it could be as much as a couple of degrees out of alignment if one of the mounting screws is in crooked. The lens cell on this scope was easily distorted so much with just hand pressure that it would be almost impossible to move the retaining ring if you gripped the cell tightly. I don't think there is any real hope of permanent adjustment on this type of cell.

I still have one of these lenses and the plastic cell. It is OK at low power but it is not sharp once you crank up the power.

dan

Jon Isaacs said:

All you refractor guru's:

Last night a friend was over with a Nexstar 80GT, this is a typical Synta 80mm F/5 OTA. At higher magnifications, it exhibited what I take to be rather severe coma, viewing Castor at 90x, I could see the airy disks but to one side, there as a large blur which under scrutiny looked to be coma.

A laser showed that the diagonal seemed to be collimated so I am guessing I need to adjust the objective cell which I believe is threaded on (I could be wrong on that.) Any thoughts on what I might do here to try to improve things.

Thanks

Jon

There's really no collimation that can be done on the objective end. What most of us do is loosen the screws on the tailpiece and collimate with that. There's usually enough play in the screw holes to allow you to tighten back down in the proper position.