Collimation issues (GS Dob)

BLiTZWiNG (Trent) · #1 26-01-2005, 12:26 AM

I seem to be able to line the spider up in the center of the primary mirror, but I can't seem to get the image of the primary in the center of the secondary mirror.

This has led me to thinking that the secondary mirror is off somehow, but the adjustment screws don't seem to move it..

Any ideas?

BLiTZWiNG (Trent) · #2 26-01-2005, 12:47 AM

I think I managed to get it (I think), but gee it's tough when you can't actually see stuff moving when you turn screws...

janoskiss (Steve H) · #3 26-01-2005, 01:07 AM

Yes, it's fiddly. You need to loosen one screw a little and then tighten the other two before you see the change.

BTW. Silly me just noticed there is a pinhole on the focuser cap that came with my Dob. Are you using it for the collimation?

Also, as gaa_ian pointed out there is a fairly comprehensive user manual for a very similar scope, the Orion XT8. Get it here. It's got very good instructions on collimation! --- the best around for the GS Dob.

gaa_ian (Ian) · #4 26-01-2005, 01:07 AM

It takes a bit of practice

Especially when things dont move the way you expect when you adjust the primary mirror.
The aligment of the secondary, is a matter of getting the reflection of your eyeball in the middle of the secondary mirror.
Then do your star test

BLiTZWiNG (Trent) · #5 26-01-2005, 01:30 AM

thanks guys, I really, really appreciate the help. I think I got it lined up because I finally noticed my eyeball in the secondary mirror!

I haven't done a star test yet, but saturn is looking... well, as good as it gets I guess... I think I can see a darker ring in the middle.. I need more magnification though... 183x is not enough!

Janos, I didn't notice the hole either! and no I wasn't using it...

janoskiss (Steve H) · #6 26-01-2005, 01:52 AM

Quote:

Originally posted by BLiTZWiNG
I think I got it lined up because I finally noticed my eyeball in the secondary mirror!
...
Janos, I didn't notice the hole either! and no I wasn't using it...

See your eye is not enough. I could see my eye in the focuser when I got my dob, but the collimation was still way out.

Use the dustcap with the pinhole in it (in a brightly lit room or outside in the day time). It'll help keep your line of site centred in the focuser.

BLiTZWiNG (Trent) · #7 26-01-2005, 01:53 AM

I just used it and it all looked alright...

rumples riot · #8 26-01-2005, 06:46 AM

You guys should try to collimate an SCT. Took me two hours last night just to get it almost perfect, and I do mean almost. There is always room for improvement. Also unfortunately laser collimators are no help, thats because the secondary is usually meant to be a little off for great collimation. If I wanted to collimate even better then I could collimate with a barlow in and get a lot better collimation.

Anyway, collimation does seem daunting but once done it will hold for a little while. You might like to check it each time you take the scope out, that what you will only need to do small adjustments each time.

BLiTZWiNG (Trent) · #9 26-01-2005, 10:04 AM

It took me about 2 hours to get it done, because I wasn't sure what I was doing even after reading about it!

In the end, it was just time spent doing it that helped me see things and how in or out they were. Experience cannot be bought here.

As far as I can tell now it's pretty good. The wierd bit is now when I do star tests I don't see rings... all I see is the star go blurry and get bigger, and then I finally see the spider and secondary.

wavelandscott (Scott) · #10 26-01-2005, 10:35 AM

Improved speed and confidence will come over time...

Depending on how you handle your scope and how big your mirror is will help determine how often you will need to collimate and which adjustmetns you need to make.

Generally speaking, the bigger the mirror the more often it will need to be collimated...like wise the rougher you handle it the more often you will likely need realignment.

For me, not being very mechanically minded and not having much confidence in my knowledge I was very glad that I purchased my scope at Bintel...as I mentioned earlier, they walked me through the process in their shop with a scope similar to mine...

To me this was well worth the diference in price...

While I can not claim to be an "expert" I am gaining proficiency...it just takes time.

Another tool that I think you find helpful in collimation is a cheshire collimating eyepiece...there are several brands...I have the Orion

Some people are advocates of laser collimation and I am sure that this has advantages too but, from what I have read (I do not have a laser collimator) they can some times be out of collimation themselves! So simply using the laser may not be sufficient...

In all cases with a little patience over time as you get more familiar with the task your speed and accuracy will improve...

Starkler (Geoff) · #11 27-01-2005, 07:40 AM

Found this article on collimation which explains the basic concepts of collimation in an easy to understand way.

Worth a link in the resources section Mike?

janoskiss (Steve H) · #12 27-01-2005, 05:07 PM

Quote:

Originally posted by Starkler
Found this article on collimation which explains the basic concepts of collimation in an easy to understand way.

Good article, except for that horrifying picture of two plastic rulers scraping against the delicate surface of the main mirror. I'm still too much of a whimp to remove and centre spot my mirror.

The follow-up article by the same author about the Barlowed laser method is worth a look too: Sky & Tel., Jan 2003, pp. 121--124. Online copy at http://gmpexpress.net/~tomhole/blaser.pdf. Don't know if Sky & Telescope approves of this article being redistributed though. So here is an uninfringing alternative: http://www.globaldialog.com/~obsessi...llimation.html.

Starkler (Geoff) · #13 27-01-2005, 09:32 PM

Quote:

Originally posted by janoskiss
Good article, except for that horrifying picture of two plastic rulers scraping against the delicate surface of the main mirror. I'm still too much of a whimp to remove and centre spot my mirror.

Yes I don't like that either !

The better solution is to make a paper circle the same diameter as the mirror.

Fold it in half twice so it looks like a quarter of a circle. You then cut a millimetre or two of the point off and unfold the paper.

The result is a paper template with a small hole in the exact centre. Get a felt tip marker and jam it in the hole

Centre spotting really is easy, but dont do it until you have collimation tools.

janoskiss (Steve H) · #14 27-01-2005, 10:00 PM

Interesting idea about the template paper circle. How do you actually do the "spotting" without letting the paper touch the mirror surface? Also, I'd prefer a "ring" shape (spot with a hole in it) instead of a filled circle, so that I could still use my return-beam laser collimator (which itself needs collimation!).

When collimating with the laser I noticed I could see the light from the laser on the back of the mirror. That should work the other way too. So maybe I could stick the paper template on the back of the mirror and shine the laser through the hole in the paper to show me where the centre is on the coated side.

Starkler (Geoff) · #15 27-01-2005, 10:08 PM

Just be gentle with the paper.
If you gently push the centre down with your finger onto the mirror all will ok. Even if the paper was to harm the mirrors coating at this point it wouldnt be critical being in the shadow of the secondary.

I used my centre spot as a guide for placing a paper reinforcing ring.

** Moderator**

Shouldnt this be in the equipment forum ?

janoskiss (Steve H) · #16 27-01-2005, 10:16 PM

Quote:

** Moderator**
Shouldnt this be in the equipment forum ?

"Talk about anything and everything in here!"

--- IceInSpace Forums: General Chat

mch62 (Mark) · #17 28-01-2005, 07:55 AM

Make sure you check the focuser alignment as well on the GSO's.
I have seen 2 GSO's now where the focuser required some shimming on one side to get it square.
A 10" and a 12" which was way out.
This was not focuser slop either.

It's not a real problem on slow F ratios but important on an f5.

If you use a laser check to see it is in the middle of the diagonal from short side to short side , not front to rear that is.
I should be off centre from front to rear slight for diagonal off set , but it should be centre the other way.

If not , just lossen the focuser mount screws and shim one side with something non compressable.
Have fun!
Mark