Crayford mounting radius

[BC]

I’m mucking around with all possible adjustments in the name of getting things just right. I’ve noticed that the Crayford focuser appears to have a mounting radius suited to an 8”, rather than my 10”. This means it only contacts the tube along 2 lines, rather than a surface contact. I don’t have a camera, so I’ve drawn a little pic. The reason I’m looking closely at this is that the focuser is not square to the tube, but is pointing over to one side (a bit). Lots of collimating documents talk about shimming the focuser to square it up, this is going to look pretty dodgy with a strip of something sticking out. Is this mounting arrangement normal on the 10” scope?

One tiny mod I did was to put a rubber band (still looking for the right sized O-ring) around the top of the main focuser shaft. This cushions the BANG when I’ve backed off the tension screw too much and the weight of a 2” EP is too much for it.

[slice of heaven]

They must have used all the focusers for the 10" scopes on the 12s :lol:
I've the same problem on my 12", its fitted with a focuser for a 10" ,I trimmed some foam weather strip and used that to seal the gap.

[iceman]

I've read an article somewhere that suggested that squaring up the focuser isn't 100% necessary to be able to collimate properly.

Here: http://web.telia.com/~u41105032/myths/myths.htm

[rmcpb]

You are right, the GS Crayford is set up for the 8". It fits perfectly on my OTA so some sort of "adaptor" may be necessary for the larger scopes.

[Starkler]

Its just cosmetics if there is a gap at the side :shrug:

[slice of heaven]

Depends if you want to totally seal the scope from dust. There's a fair gap on the 12s. The better its sealed the less often you need to clean the mirrors. :D

[asimov]

I used to concern myself with "squaring" the focuser too, until I read that article that Mike posted (as well as several others). I no longer worry about it.

My focuser is "fits all dia. tubes" setup. It's flat, with raised points at 2 edges for contact with the tube. I always cover the OTA when not in use anyway, The amount of dust that's going to get in through this gap between the focuser & tube as opposed to the open ends of the OTA (whether is use in the field, or covered when not in use) is not even a factor worth considering. (In my opinion anyway) :D

[slice of heaven]

John, I suppose if you dont cap each end of the scope and cap the focuser , a small gap like that wouldnt matter at all :)

[asimov]

It gets totally capped slice. Using a breathable zip-up bag...similar to the material used in a car cover.

[BC]

Thanks to all, just checking what's normal

[slice of heaven]

The keyword in that link is 'accurately'.
What he's saying is a focuser off square by a hair is not dramatic.
I had a page of his bookmarked (I cleaned up my links a while ago and have lost it) where he defines how close to square the focuser should be and the effects this has on the mechanical collimation.
He stated that if it is an 'obvious' error that can be seen by eye, it needs addressing.
The faster the scope the more critical it is.
The link below is for tilt 'across' the tube which is worse than tilt 'along' the tube. But both should be addressed.

http://www.atmsite.org/contrib/Kniffen/collimate/collimat.jpg

[asimov]

http://www.oarval.org/collimatE.htm

[asimov]

I prefer to think of collimation as simple if one simplifies his thinking: I "square" my mirrors up to suit the focuser...not the other way round. I added a mod to my primary mirror holder to take it "off-axis" if I chose to do so, or the necessity arose. Call it collimation in reverse if you want. lol. Imagine if the focuser was NON-MOVABLE....what do you do to "square" it to the mirrors? You "square" the mirrors to the focuser, yes?

I prefer the "caveman" days. :D :P

[slice of heaven]

It's not that simple if the focuser is way out ,unless you've a very oversized tube.
The lightpath will be obstructed by the tube if the main mirror is not parallell(or close to it) to the tube. That will cause vignetting of the view.
Nothings immovable.
I'm only following advice from more knowledgeable people than myself on this subject.
Nils Olaf Carlson,the author of the link Mike posted, even suggests making masks for the end of the tube to suit the mirrors to test the relationship of the entire components of the OTA. A lot of trouble for something some deem unnecessary.
Each to their own but a properly setup newt gives better views than one thats not.

[asimov]

I agree with all that you say Slice. I don't believe the half of what I read, so I like trying different ways by myself to see the results, even if it is thinking way outside the box briefly from an idea I've read somewhere. And that's why I throw stuff like this in this forum to see the reaction/opinions from others...At the risk of making myself look like a total dork lol...So yeah, everyone to their own I guess like you say.
TY for the discussion. :thumbsup:

PS I think I'll make my next project a collimatable focuser...perhaps push-pull screw design like on my refractor objective..:rolleyes:

[RAJAH235]

BC, See pic. of my Meade focuser. This is how they are able to mass produce them. As you can see, there's a considerable gap underneath it, & I've had to adjust to align & collimate the t'scope... No big deal. HTH. :D L.

[BC]

I've also read the articles saying the squaring isn't necessary, but like others, I like to tweak about and learn more about the whole thing. When I get some spare time, I'll square it up anyway, but I'll measure it all up first and find the exact point on the opposite side of the tube to aim for. That way I can quantify the error and see what diference (if any) it makes.

[slice of heaven]

If it gives you no more than peace of mind BC,it cant hurt.
If you gain a better image, then it was worthwhile.
I could post a few more links I've found on the negatives of an unsquare focuser but lets wait for your opinion if you get to do it.