Collimation of refractors

[Moonshane]

Hi all I am very familiar with newtonian collimation but have never really thought about collimation in refractors until someone on another thread asked if the drawtube of a scope was collimated to the objective.

Therefore assuming I know how to do it and applying logic I took out the diagonal from my 80mm Vixen f11 and added my long cheshire to the drawtube to see what the score was. I presumed that I should see the objective central to the bottom of the cheshire and split into equal quarters by the cross hairs. What I saw was not quite that but not that far away. Using my 2" self centering adapter I carefull adjusted the fit of the 2-1.25" adapter so that the objective was positioned as above and this seems to now be repeatably correct in the usual approximate focus position. I am not sure if the views will be greatly affected but it was useful nonetheless. All this said, should I have adjusted the drawtube position rather than fiddling with the adapters? If so, how do people do this with a R&P?

Cheers

Shane

[cutepetgroomer]

Personally I have never needed to collimate any one of my refractors. My oldest one being over 6 years old or so still provides viewing even if it's altaz mount is practically no good

[RikM]

I did my ST80 more or less the same way. Actually I knife and forked it by loosening the screws holding the focuser assembly onto the back of the scope, flexing it a bit and then re-tightening. It made a very noticeable improvement to the performance of the scope but not a method I would suggest for something precious/expensive.

[Moonshane]

cheers both

don't get me wrong the views at about 50x with the Lunt wedge have been nothing short of exceptional but I was just curious. I'll report back if the views change at all (I doubt it to be honest).

[John]

I did my ST80 more or less the same way. Actually I knife and forked it by loosening the screws holding the focuser assembly onto the back of the scope, flexing it a bit and then re-tightening. It made a very noticeable improvement to the performance of the scope but not a method I would suggest for something precious/expensive.

Thats the method I use for aligning the focuser with the optical axis. I use a (collimated) laser collimator in the focuser (no diagonal) and check that the beam exits the objective in the centre of the lens. My Moonlite focuser in the ED120 does have a collimation facility but thats unusual for focusers.

Once I've done the above I check the tilt of the objecitve using a cheshire collimator.

An F/10 refractor is quite tolerant of a little mis-collimation though, as would an F/10 newtonian.

[cotterless45]

I check my collimation on refractors using a collimated laser. I just put in the diagonal , focus out to where I observe at x50 .Then standing out of the path of the laser, note where it hits the objective. With my old Vixens this is central. The 150 does have push / pull collimation. You can make a paper disc to cover the objective with the centre marked.

Once set , just leave. A quick test is to see if everything looks sharp !

It is possible to play with your diagonal to get a central beam. If you like taking diagonals apart .

You're welcome to have a poke around the real world of fracs at PSP !

Nick.

[LukeSkywatcher]

I know "technically" that refractors can be collimated (as can bins). Ive never found a need to do so on either. 

[John]

Virtually all the chinese made refractors I've owned needed a bit of collimation. The Japanese made Vixen was and still is spot on. When you look at the objective cell and focuser of the Vixen the manufacturing tolerances are a lot tighter. You can also tell that the Chinese copied the Vixen.

I'm not having a go a Chinese refractors though, I've enjoyed the ones I've owned and the Japanese could never put a 6" frac on the market for a few hundred £'s.

[nightfisher]

Im pretty sure my TAL is off a little, but it does not worry me one little bit, the views are still top notch