Refractor collimation

[Acrab67]

Hi,

I have found that the position of the Cheshire on the 2" to 1.25" adapter is very critical for the purpose of checking the collimation of a refractor. In the case of my doublet, a TS PhotoLine 125, if the adapter is fully tightened on the telescope's focuser, and the Cheshire is turned on the adapter, the reflections from the two component lenses are concentric with each other, but appear slightly displaced with respect to the silvery reflection of the Cheshire. If, on the contrary, I release the adapter and the Cheshire, and manually move the assembly, I achieve that the reflections of the lenses and the silver reflection of the Cheshire are perfectly concentric. This leads me to think that the ideal would be to be able to collimate the cell with the Cheshire perfectly aligned with the optical axis, but that is impossible given that with the simple tightening of the adapter to the focuser, there is already variation, and to that we must also add the tightening of Cheshire itself. Taking this into account and assuming that the focuser is square to the cell, does anyone know what pattern the major telescope dealers follow when they have to check/correct the collimation of a refractor?
Thanks

 

Víctor

[Peter Drew]

I was never a major dealer but during 35 years as a professional telescope maker I had an optical bench which utilised a 12" parabolic mirror and an artificial star at the focus, telescopes were collimated against this image.  Nowadays, I would use an ordinary artificial star at the appropriate distance.  To collimate my personal refractors I would first of all determine that the focuser was square to the objective by shining a laser collimator through the eyepiece holder and making sure it exited through the centre of the objective.  In order to avoid decentring due to slop in the fit of the eyepiece holder I made a 2" to 1.25" adapter that had a firm fit on both dimensions.     🙂  

[Acrab67]

Thanks

We agree, there would be no other option than to machine a specific adapter with the smallest possible tolerances so that the Cheshire would remain as centered to the optical axis. What I don't know is what tolerances in terms of collimation would be considered normal. In any case, for practical purposes when it is actually used, we would have a deviation in the optical axis, yes or yes.

 

 

 

[Franklin]

I found this same problem and the best I could do was to use a Baader 2" Clicklock screwed to the drawtube, which then held a 2"-1.25" self-centering adaptor to hold the cheshire. According to Suitors book, a slight deviation from perfection is unnoticeable in visual use.

[Acrab67]

Although to a lesser extent it seems that even Baader Click lock adapters also introduce some displacement in the optical axis

 

 

 

[Captain Scarlet]

The Glatter Parallizer is designed to solve that problem, I think. I have one, use it as my standard 2”-1.25” adapter, and think it’s a very clever design.

Magnus

[Acrab67]

Well yes, the Glatter Parallizer partly solves the problem from the point of view of the placement of the Cheshire, but there is still the deviation caused by tightening the adapter in the focuser. By the way, I don't understand why a Cheshire hasn't been made directly in 2"

[Second Time Around]

I have the excellent Farpoint 2 inch Cheshire.

I also have a  couple of Parallizers, plus I have a 1.25 inch Cheshire on the way from Farpoint,

[Acrab67]

Yes, but we are talking about the classic short Cheshire to collimate refractors. 2" Farpoint's model does not work.

[Second Time Around]

Ah, OK. I've learnt something there - thanks!

[CraigT82]

4 hours ago, Acrab67 said:

Well yes, the Glatter Parallizer partly solves the problem from the point of view of the placement of the Cheshire, but there is still the deviation caused by tightening the adapter in the focuser. By the way, I don't understand why a Cheshire hasn't been made directly in 2"

The whole point of the HG parallizer is to ensure that the accessory is parallel with the focuser drawtube. 

[Acrab67]

 

1 hour ago, CraigT82 said:

El objetivo principal del paralizador HG es garantizar que el accesorio esté paralelo con el tubo del enfocador. 

Yes, but it cannot avoid the deflection caused by the tightening of the HG adapter by the focuser itself

[CraigT82]

30 minutes ago, Acrab67 said:

 

Yes, but it cannot avoid the deflection caused by the tightening of the HG adapter by the focuser itself

It does, the OD of the parallizer has a specific shape (not round) that ensures parallelism between accessory and focuser drawtube.

IIRC the instructions say that the thumbscrew of the focuser should be located opposite to the thumbscrew of the paralliser for it to work optimally.

[Acrab67]

3 hours ago, CraigT82 said:

It does, the OD of the parallizer has a specific shape (not round) that ensures parallelism between accessory and focuser drawtube.

IIRC the instructions say that the thumbscrew of the focuser should be located opposite to the thumbscrew of the paralliser for it to work optimally.

Thanks for the info, that's something I didn't know.