Collimation - how to square a focuser - split from eyepiece thread

[CoombellKid]

Originally Posted by rusty

Hey Dujon

Thanks for the tip. I read your thread about collimation. I would be interested to hear how your Chesire works out for you.

Cheers

I brought a Celestron Chesire eyepiece at Astrofest 06, It works a treat!!!!
I had a laser colliomation tool and a homemade cap. The Chesire is the
easiest to use the laser is just a quick checker. I also found my focuser
wasn't inline with the optical train once I fixed that and re-collmated there
was a marked improvement, You can ask 1ponders how far out everything
was as I showed him at Astrofest.

Regards, G' luck!

Rob

[Nordo]

Originally Posted by CoombellKid

I also found my focuser wasn't inline with the optical train once I fixed that and re-collmated there was a marked improvement, You can ask 1ponders how far out everything was as I showed him at Astrofest.

Regards, G' luck!

Rob

Rob
I've read a bit about mis-aligned focusers. How did you identify your focuser was out of alignment? And how did you align it? Sorry Rusty, I'm a bit OT.

[CoombellKid]

Hi John,

only really figured out a way of alining/tilting the focuser from primary
to secondary. Not left to right. I removed the secondary and spider. Sloted
the Chesire in and peered through the sight hole using a ruler I took a
measurement by lining up the sight hole, the crosshairs and the end of a
ruler. Measured the distance to the front end of the tube. Doing this while
holding the ruler inside the tube closest to and over the focuser. Then I took
a measurement on the opposite side and found after a couple of comfirmation
checks that my focuser wasn't square by 3-4mm cant remember now. So
I loosen the bolts on the side that needed to be raised and rolled up a 1mm
or there abouts stick of al-foil and shimmied the focuser. After a couple of
tries I got it close to bang on.


hope this helps

regards

Rob

[Nordo]

I haven't tried that. That method assumes that the top of the OTA has been cut truly square with the axis of the tube. I've been thinking of using a set square - either a small carpenters square, or one that a draftsman would use. Place it with it's longer leg down the OTA, with the short leg on the "axis" of the focuser tube, pointing across the OTA. Look through the collimator, and slide the square up and down. If the focuser is square to the axis of the OTA, you will be sighting exactly along the edge of the short leg. If not, you will never be able to sight parallel to the edge of the short leg of the square.

Of course there is also the other axis of the focuser. Is the focuser's line of sight a true diameter of the OTA?

[Nordo]

Originally Posted by CoombellKid

Hi John,


Then I took a measurement on the opposite side and found after a couple of comfirmation checks that my focuser wasn't square by 3-4mm cant remember now. So I loosen the bolts on the side that needed to be raised and rolled up a 1mm or there abouts stick of al-foil and shimmied the focuser. After a couple of tries I got it close to bang on.


hope this helps

regards

Rob

Rob
To sight the opposite wall of the tube, did you have to remove just the secondary, or the spider as well?
John

[EzyStyles (Eric)]

i rechecked the tilt of my focuser last week. the method i did was pretty much the same. take secondary mirror and focuser out. measure the diameter of the focusing hole and find the center point. do the same thing opposite side of the tube and put a small dot. wack focuser and place laser into the focuser. (make sure your laser is accurate first). if the laser dot doesn't hit the opposite side dot, then the focuser needs abit of tilt.

[Nordo]

But we're measuring millimetres here. What if the top of the OTA was not cut truly square, or if the trim around the top is not correctly seated all around?

[astro_nutt]

I used the paper ribbon wrapped around the tube and measure the exact middle of the EP holder in relation to the focuser..remove the paper ribbon...fold it flat...mark the crease..replace ribbon on tube in exactly the same spot...measure the distance from the edge of the ribbon to the centre of the EP holder in the focuser..mark this distance from the crease with a straight edge ruler to the spot on the tube that would be exactly opposite the centre of the EP holder..remove the secondary...drill a 3mm hole in the tube..be sure to put a few layers of masking tape on the inside to catch any filings..then use masking tape to pick up any stray bits...if all went well, look down the focuser and see the hole..put in your collimation piece either cheshire cross-hair or laser and it should align with the hole..if not..adjust to suit...plug the hole with a self-tapping screw and paint it with a black texta...replace secondary and complete collimation.
Cheers!!

[Nordo]

Parts of what you describe are not too clear to me. Does the top edge of your paper ribbon protrude above the top of your OTA? And did you take the ribbon off again when using the straight edge to mark the spot opposite the focuser? Another problem for me is how do you find the exact centre of the focuser inner sleeve?

[CoombellKid]

Originally Posted by Nordo

Rob
To sight the opposite wall of the tube, did you have to remove just the secondary, or the spider as well?
John

As I said above, I removed the secondary and spider. I had to, to be able
to sight and take a measurement on the opposite side of the OTA.

Basically I just used what ever I had, when I got the Chesire I came up with
that idea. I guess it doesn't really matter how you do it as long as you get a
result. I assummed the tube/OTA to be pretty square, massed produced
things or machanically cut steal usually is, however mass produced and hand
cut might be a lil different.

regards,CS

Rob

[CoombellKid]

Originally Posted by Nordo

Another problem for me is how do you find the exact centre of the focuser inner sleeve?

Use a Chesire eyepiece and your eyeball and your pretty close to center

regards,CS

Rob

[astro_nutt]

Hi John..my apologies...I should have mentioned that you place the paper ribbon around the circumference of the optical tube alongside the focuser...and there you can measure the base of the focuser as to ascertain the middle of where the EP holder..Looking at the focser on my Saxon Dob I'll label the sides top, bottom, left, right...I measured the bottom to find the middle of the EP holder..then measure the right side to find the middle of the EP holder..as the paper ribbon is against the right side..mark the middle of the EP holder on the paper ribbon..and then mark the optical tube so you can replace the ribbon to EXACTLY the same spot...when you remove the ribbon then fold it in half..you should have a creasemark on the mark from the middle of the EP holder of the focuser..and another creasemark which will be the exact opposite of the optical tube when you replace the ribbon...all you need to do is mark the spot that you had measured from the bottom of the focuser to find the middle of the EP holder by using a straight edge along the creasemark on the ribbon on the opposite side of the focuser..this should mark the spot of the middle of the EP holder of the focuser on the exact opposite side of the optical tube!!
Another way to find the middle of the EP holder is by using a carpenters square, a cheshire collimator and a ruler.
Place the collimator into the EP holder...place the square's handle in line with the tube with the number side against the right side edge of the focuser pointing away from the tube..all you need to do is measure from the centre of the collimator to the number edge of the square..
Once again..sorry for any confusion.
Cheers!

[Nordo]

OK, thanks for the extra explanation.
I'll have to remove the top rim off my OTA, but that will be easy to do. It will allow me to slip the paper ribbon off and on.
Also by taking the rim off the top of my OTA, I will have the raw top edge of the tube which, as Rob says, should be square to the tube's axis.
Thanks