Primary Mirror Dot

[rodroger (Rodney)]

Just wandering if anybody has any advice on primary mirror dots.

What colour? Red, White, black?

What size? is the best? or does this depend on the scope?
I have read that you can use a paper ring binder circle? isn't this too big?

Logic suggests that the smaller the dot the more accurate the collimation? such as the tip of a fine felt pen? (as long as you can see the dot!)

An another query is if the dot is too big would it not cause abberrations or a mark in the image quality?

[bird (Anthony Wesley)]

Best advice - buy reflective *red* triangles from catseyecollimation.com and orient them so that the corners point at your collimation screws.

I'm using them, and haven't seen anything better,

cheers, Bird

[Starkler (Geoff)]

Originally Posted by rodroger

ts that the smaller the dot the more accurate the collimation? such as the tip of a fine felt pen? (as long as you can see the dot!)

An another query is if the dot is too big would it not cause abberrations or a mark in the image quality?

If you are using a cheshire collimator the best size centre is one that matches the inner diameter of the illuminated ring, or just slightly smaller. That way small errors can be easily seen.

The "dot" lies in the shadow of the secondary mirror, so it wont affect the image quality at all other than how it improves your collimation

[rumples riot]

I concur with Bird. They are the best. Cat's eye collimation kits.

[erick (Eric)]

Pretty basic newbie question here, sorry

When I finally get around to washing my 202mm GSO mirror and most probably wash off the ring binder circle (unintentionally), is the "bottom" (centre?) of the paraboloid close enough to the geometric centre of the mirrror, so I can relocate it and re-attach a new centre spot. I'm guessing these mirrors are designed so it surely must be within a mm or so of the geometric centre otherwise things would start to go wrong in the image path?

[bird (Anthony Wesley)]

Eric, the manufacturing techniques used to make these mirrors pretty much guarantees that the physical centre will also be the optical centre, so I think you're safe to put the dot in the middle of the mirror.

cheers, Bird

[erick (Eric)]

Thanks Bird, I can wash with confidence now!

[CoombellKid]

Eric,

My scope came with the same ring binder type circle sticker. I've washed
the mirror probably a half dozen times over the years. And it's still looks
like the day it was stuck on.

regards,CS

[rodroger (Rodney)]

Thanks guys for all your help on this.

I have a Celestron Combination Cheshire/sightube which if I recall, can get the reflected circle from the cheshire, down to nearly zero apature.

I have already have been in contact with "Jim Fly" via E-mail as I have purchased one of his Catperch pro chair kits (plans and fittings only- arrived Monday morning - very quick) and has been asking him about his catseye autocollimators for my short tube 150mm and have just discovered that his collimators come supplied with the primary mirror dots of your choice, cool. Looks like I will be buying one.

I now understand how you don't see dot in your field of view, Thanks Starkler, this makes more sense now, don't now how I missed the logical reason behind this fact, brain cells must be getting a bit fuzzy in my ageing years.