Refractor collimation?

[yeti monster]

Chatting with my Aussie pal from Thailand about 'scope etc. I was explaining collimation of reflectors and he asked whether it is possible to check/adjust the optics in his refractor.

So, does anybody have any words of wisdom on this subject?

[Merlin66]

Yes, you can use a Cheshire eyepiece to reflect light into the refractor and look at the reflected beams from the rear and front element ( assuming an air spaced doublet) There should be a larger central circle and subsequent smaller circles ( from the various surfaces) all concentric.

Suiter has a detailed section on this in his book " Star testing of astronomical telescopes", page 121 (1st Ed)

Ken

[yeti monster]

He did mention that his scope is an air gapped doublet. I'll pass on the information, heck, I'll even pass on the forum details too..............

[brianb]

Collimating a refractor is not an easy exercise, even if adjustment is provided it's probably only for squaring on the objective cell. Within the cell the components also have to be accurately centred with respect to each other. If you have an air gapped objective, best advice is not to disassemble it - for any reason!

As with all other scopes, the just out of focus diffraction pattern should be symmetrical - and, when observed with an insanely high power in excellent seeing, one or two faint but complete & symmetrical diffraction rings should be seen around the central Airy disk when observing a brightish star which is central in the field. (2nd mag is about right for a 3-4" scope.) If the pattern is brighter on one side than the other, and the star is in the middle of the field, the collimation isn't quite right.

[yeti monster]

I did think thatit would be a difficult and perhaps risky task to go messing with lenses etc. I think the question was more rhetorical than to do with any serious misalignment that he was experiancing. Still, these things are interesting to know.

[Macavity]

Indeed so, Merlin. I did have a go with a typical Cheshire eyepiece on my Startravel ST102. My example seemed to show quite a lot of "focusser droop" i.e. the eyepieces were pointing quite a WAY off from the centre of the objective! I tightened the "grub screws", then ALSO used some plastic Dymo tape to pad out the focusser to better fit within the OTA.

I'm not sure it made a VISIBLE difference, to the performance of the scope (there are other limits to the capabilities of these generally FINE scopes!) but it was intellectually(?) satisfying. And I suspect I learned quite a bit during the process.

[John]

I've collimated my 5" F/9.4 refractor which, conveniently, has a collimatable lens cell. I used this method:

Refractor Collimation

The interesting thing is that I've checked the collimation on my 6" F/8 refractor which does not have a collimatable lens cell) and it looks as if it's a little off but the performance under the stars is just as good as the one I've adjusted !. I'm starting to conclude that medium focal ratio refractors are quite tolerant of some mis-collimation .....

[F15Rules]

Hi John

I see you now have the 6" frac you talked about:D...is the Konus an early Synta/Celestron CR6/Meade AR6 etc type? How do you rate it versus your 5 incher?

[ollypenrice]

The problem with refractor collimation is not finding the defect so much as doing something about it! There are various ways in which the cells and lenses are held in.

However, a trick I heard, and which is supposed to work for the cheap (but good) Synta variants being talked about above, is this; take the tube and hold it vertically, then settle down and tap the sides with a finger for a few minutes. (Obviously if it was star testing well to start with, don't!) Then test it again. The writer suggested that a bit of loosening and resettling often helped matters. I think I came across that on Ed Ting's site.

Olly

[Macavity]

Heheh - Yeah, I tried the SYNTA "objective tapping" thing too? And, since curiousity got the better of this cat, I even removed the objective from the cell. I sense that the latest air-spaced doublets are now separated by (fortunately) "sticky" pads and fairly intractable? At which point, I left it alone. I did spot a graphite line along the side of the elements for alignment(?). Again, I noticed no change in star diffration pattern before and after. I sense relieving some of the "ten ton truck" pressure on the rubber lens retaining gasket was no bad thing though...

If I EVER get a decent refractor, I've determined to leave well alone.

[John]

Hi John

I see you now have the 6" frac you talked about:D...is the Konus an early Synta/Celestron CR6/Meade AR6 etc type? How do you rate it versus your 5 incher?

It's an early synta, ie: with the non-collimatable lens cell. It's an ex-astro society scope that has hardly been used - the objective is completely dust free and unmarked !.

I've only had one proper session with it and it star tested well and showed nice detail on Mars when the scope was fully cooled. I had 15 mins or so on the moon last night (before clouds stopped play ) and, again, very nice views. The CA seemed well controlled at focus but makes itself known either side of focus of course - luckily I try to do most of my viewing in focus

I'll need to put the 2 scopes side by side I think but from what I've seen so far the 6" is impressive, especially so when I think what it cost me

Of course I may then have the quandary of whether I can keep both ...... I know my wifes opinion on that one already !.

[F15Rules]

John,

Think I saw that one...a fetching shade of yellow??

Of course you need to keep them both...in case your wife wants to view at the same time as you!