Collimating an old Celestron C8?
Bert Hyman
a question I first posted some years ago, which was never answered.
I have a 30+ year old C8 that's seen a lot of travel and abuse and has
been collimated a number of times. Of course I have no record or
recollection of how far or in what direction the individual
collimation screws have been turned over that time.
Are there travel limits on the screws?
Is it possible to damage or disconnect the C8's secondary by turning
the collimation screws too far one way or the other?
Could I drop the secondary into the primary, or have the secondary
flapping in the wind (so to speak) if I somehow managed to get one or
more of the screws out all the way?
Is there a way to get the secondary back to some sort of starting
position before doing another collimation?
--
Bert Hyman | St. Paul, MN | be...@iphouse.com
Dennis Woos
> flapping in the wind (so to speak) if I somehow managed to get one or
> more of the screws out all the way?
>
> Is there a way to get the secondary back to some sort of starting
> position before doing another collimation?
>
I think it is possible to loosen the screws too much. How about making sure
that they are all snug, and then only adjust two out of the three in the
future?
Dennis
Dennis Woos
Bert Hyman
news:wvSdnejuLv-ZtnPV...@posted.greenmountainaccess:
I guess that's a reasonable approach for starting from scratch.
And thanks for the link in your other post; it does tell me that the
collimation screws aren't the only things holding the secondary to the
corrector.
Dennis Woos
>> making sure that they are all snug, and then only adjust two out of
>> the three in the future?
>
> I guess that's a reasonable approach for starting from scratch.
>
> And thanks for the link in your other post; it does tell me that the
> collimation screws aren't the only things holding the secondary to the
> corrector.
>
When collimating our Newtonians, we use a laser collimator which gets us
pretty close. I know when I need to do an additional star test, as I can
tell when focusing that stars don't "collapse" with perfect symmetry in the
center of the fov. Many nights I don't even get to this point, as the seeing
can be too rough to detect this amount of miscollimation. However, when the
seeing is good then one can notice the miscollimation in the way stars
focus, especially when trying to split close doubles. A subsequent star test
and adjustment quickly corrects the problem for the rest of the night.
Dennis
jerry warner
dke...@hotmail.com
> try this:
>
> http://ngc1514.com/Celestron/disassem.htm
>
> good luck-
>
Hi
This page is good at telling you what needs to be done but like most
little help when actually doing it.
The notes about adjusting the screws are correct. It is possible
to over tighten the screws and either bend the mount for the secondary
or in some cases pop the secondary of its mount into the primary.
The trick here is to just not over tighten. If this rule is followed,
you should be able to readjust collimation every night for as long
as you live an not worry about damage to the secondary mount
( I'm sure you'd wear out the threads long before then ).
If you adjust 1 side, 1 turn in, the other two screw would need
1/2 turn out. One should still trust ones feel. You know when you
are tightening too much so just don't do it.
I have a trick that helps a lot when doing the fine adjustment.
Most just use the method of "tweak it and see if it gets better".
This works but can be very frustrating.
Follow my steps and it'll be much easier.
Find a nice bright star. Use Polaris if you don't have a clock drive.
Step 1. slightly defocus until you can see some diffraction rings but
not so much that your looking at the secondary or primary edge
shadow.
Step 2. Find the location in the field of view that seem to give the
most
uniform diffraction rings. Do this by moving the telescope. If you
can't
find a location in the field of view, use a lower power eyepiece.
If it seems close to the center, use a higher power eyepiece.
Step 3. Lock down the controls of the scope so it doesn't change the
position of the star.
Step 4. Draw an imaginary line between the star in the field of view
and the center of the field of view.
Step 5 ( Note this is for SCT's not Newtons ) Using the secondary
adjustment
screws, move the star along that imaginary line towards the center
but only 1/3 of the way.
Step 6 Repeat to step 2 until satisfied that no more improvement.
Do note that this general method works for Newtons except that for
Newtons, you adjust the primary and instead of only going 1/3
of the way, you adjust all the way to the center.
The 1/3 has to do with the fact that you are adjusting the secondary
of the SCT that magnifies the movement by about 3X. If one moved
the star all the way to the center, the collimation would diverge
instead
of converging as the 1/3 movements does.
Because the 1/3 location is a little hard to hit exactly any movement
towards that location will eventually get you there, just more
repeats.
I use this method on my SCT and it only takes me a minute or so
to fine adjust my collimation.
Dwight
William Hamblen
> If you adjust 1 side, 1 turn in, the other two screw would need
>1/2 turn out.
If you turn a C8 collimating screw a whole turn you are overdoing it
unless the telescope was extremely out of alignment to begin with.
Just the barest tweak makes a noticeable difference when the telescope
is closely collimated.
Bud