Refractor collimation

[John]

I made some adjustments to my 6" F/8 achromat this evening. It's an older design that does not have a collimatable lens cell but my cheshire eyepiece was showing some mis-collimation as described on this website:

Refractor Collimation

Mine looked like the lower of the 2 pictures of the view through the cheshire - the one labelled "what you might see".

What I did tonight was to remove the lens cell with the lens still in it. I then loosened and removed the lens retaining ring, with the lens pointing upwards I then went around the lens cell lightly slapping the cell to "settle" the objective lens elements, then a quick visual check that the 3 foil spacers were where they should be and I replaced the lens retaining ring, only tightening to "light finger tight" as recommended - this avoids the risk of "pinching" the objective lens too tightly in it's cell

After carefully screwing the objective lens / lens cell assembly back onto the tube (being careful not to cross the threads) I then re-checked the collimation with the cheshire eyepiece and I was pleased to see that it now looked spot on - like the upper cheshire view on the web page above.

I'm hoping for some clear skies now to star-test under. I'm keen to get good collimation with this scope because I'm using an Aries Chromacor with it - this is a device that potentially significantly reduces chromatic abberation specifically in the Chinese F/8-ish achro refractors (ie: Synta and Meade) but it's positioning in the optical axis of a well collimated scope is critical to get the best out of it.

I'll post a report on the results of both the collimation and the Chromacor in due course.

[GazOC]

Good news John. Keep us posted mate.

[Doc]

Sounds really interesting John, I look forward to reading this.

[philj]

Thats interesting John, ive come across lack of seating before as well. I didnt initially believe it because how can a cell with no collimation facilities be out but sure enough it was.

The foil spacers are important, if you have 3 make sure they are all the same thickness and exactly 120 degrees apart otherwise there will be uneven spacing between elements and you will get a ghost image as wel as not being able to collimate properly with the cheshire. This last one, ghosting, puzzled me on a current restoration until I stripped the cell and found one foil hand been swapped for a bit of fag packet foil.

Im doing a short write up at the moment on the joys of refractor collimation and things they dont telll you in the books. Ill post it in the equipment discussion section when its done.

Phil

[Merlin66]

Using a collimating laser in the focuser to ensure it's alignment with the centre of the objective is also mandatory! Sometimes the focuser can be mis-aligned to the optical axis.

Ken

[John]

Using a collimating laser in the focuser to ensure it's alignment with the centre of the objective is also mandatory! Sometimes the focuser can be mis-aligned to the optical axis.

Ken

Absolutely ! - and especially with the standard Skywatcher R&P that the scope is still fitted with as it's mechanical tolerances are none to tight. This is doubly critical with the Chromacor as it attaches to the diagonal and has to sit precisely in the centre of the optical axis.

The next step for me with this setup is likely to be a replacement focusser although I don't have the budget for a Moonlight unfortunately so I'll look at the William Optics or Skywatcher crayford upgrades or similar.

[Merlin66]

I found the GSO/TS Crayford to be a good buy.

Ken

[John]

I found the GSO/TS Crayford to be a good buy.

Ken

Thanks ken - I'll look into that one

[Terrynak]

I made some adjustments to my 6" F/8 achromat this evening. It's an older design that does not have a collimatable lens cell but my cheshire eyepiece was showing some mis-collimation as described on this website:

Refractor Collimation

Mine looked like the lower of the 2 pictures of the view through the cheshire - the one labelled "what you might see".

What I did tonight was to remove the lens cell with the lens still in it. I then loosened and removed the lens retaining ring, with the lens pointing upwards I then went around the lens cell lightly slapping the cell to "settle" the objective lens elements, then a quick visual check that the 3 foil spacers were where they should be and I replaced the lens retaining ring, only tightening to "light finger tight" as recommended - this avoids the risk of "pinching" the objective lens too tightly in it's cell

After carefully screwing the objective lens / lens cell assembly back onto the tube (being careful not to cross the threads) I then re-checked the collimation with the cheshire eyepiece and I was pleased to see that it now looked spot on - like the upper cheshire view on the web page above.

I'm hoping for some clear skies now to star-test under. I'm keen to get good collimation with this scope because I'm using an Aries Chromacor with it - this is a device that potentially significantly reduces chromatic abberation specifically in the Chinese F/8-ish achro refractors (ie: Synta and Meade) but it's positioning in the optical axis of a well collimated scope is critical to get the best out of it.

I'll post a report on the results of both the collimation and the Chromacor in due course.

I had problems with an Orion Short Tube 80-A I bought in late December - it would not focus sharply. I got a replacement OTA from Orion, returned the first one, and the replacement scope still wouldn't focus sharply. Getting somewhat desperate, I bought a refractor Cheshire eyepiece, saw that I was seeing two dots through the eyepiece, followed the tweaking steps exactly as you outlined above, and then when I looked through the eyepiece again, BOOM - the Cheshire showed a single dot with perfectly centered outer rings - collimation! When I tested the scope on a slightly defocused bright star, the star test showed a bright dot in the center with concentric rings, although the overall shape was somewhat triangular. I'll try shimming the diagonal or loosening the retaining ring holding the lenses to the cell - hopefully that will get rid of the triangle shape to a perfect circle on a slightly defocused bright star. But at least now everything is centered. Thanks John!

[John]

I'm glad it worked for you

The triangle shape sounds like the retaining ring is still a bit too tight.

[MjrTom]

Very interesting report John, thanks for sharing

[Terrynak]

I'm glad it worked for you

The triangle shape sounds like the retaining ring is still a bit too tight.

Yup, I'm gonna loosen the ring a little bit and then try the scope out again tonight after work - looking forward to getting a perfectly round circle (hopefully). Thanks again!