How To Collimate A Refractor 

As a chronic refractor ATM'r, before any optical testing, I collimate the scope and check the collimation of any scope sent to me for testing. There is no reason to optically test a scope that is mis-collimated. For me, there are two physical collimations. The very first one is the focuser by using a quality laser in the focuser, hopefully, shining through a hole precisely centered in the middle of a paper mask placed over the objective. If the laser misses the hole by more than a mm or two, if I can, I adjust the focuser as best I can to make the beam pass through the hole. If I cannot make the adjustment, the scope goes back to the source. If it does "pass", I use a quality Cheshire to then, like you, makes sure the reflection dots merge to form a single dot. If not, and If possible, I then adjust the lens cell axial collimation screw sets to get them to line up. If there is no adjustment capability, and they are far enough off, the scope goes back to the source. Only after the scope "passes" those two tests, does the scope get optically tested (DPAC and star). If I see astigmatism in the star test, I will try to adjust it out with the axial collimation screw sets, even though it may then "fail" a subsequent recheck with the cheshire. After all, the goal is to have no astigmatism or coma at high power, at least for visual use. Best Regards Jeff

This is a similar concept as the Takahashi collimating scope, which uses a small scope behind a Cheshire type diagonal flat, a magnifying eyepiece, and a sliding tube to change the near focus-- this is a novel function to let you shift the focus through the optical train. With a coaxial flat mirror in front (autocollimation), this adjustable near focus and magnification makes it quite easy to have very high precision using the human eye due to the additional reflections to use as reference. I have modified mine by using a cross hair magnification EP from a microscope, and a camera like you did. Even without a mirror, the magnification makes it easier to make more precise adjustment. Btw, many new Takahashi have no Collimation screws, just precise machining. Even fsq85 has no collimatable element for the front doublet.

Hi ! Finally a great turorial on refractors collimation! Love your channel, very technical !

Two TS refractors (80 and 102mm) I purchased had that iron cross artefact. Teleskop services refused to guide me along to get rid of the artefact on the 102mm. They said it would ruin the collimation. They however accepted that I returned the scope. For the second refractor (80mm), I decided to loosen those sets of 3 screws to the point where they started to feel loose. That fixed the iron cross issue. I tested it down to -10°C.

There are two ways to collimate a refractor lens. One, to adjust the tilt of the whole lens assembly relative to the focuser/cameras, and two to adjust the centring and sometimes even the tilt of the lens elements relative to each other. The impact in this case looks to have been partially mitigated by tilting the entire lens assembly, however it is clear from the star images that the lens elements themselves need to be adjusted relative to each other. This is usually achieved by means of a set of screws around the periphery of the lens cell which push into the lens elements. In this case they seem a bit tight and need slackening off maybe a 1/16th turn to see if that gets rid of the distortion. Then a star test is needed and the image is checked just before the airy disc changes to a point. There will be concentric rings which if broken on one side, will show coma or other effects.

Nice analysis and process. Thanks for adding to the hobby.

Here's a plausible explanation of the centering screws in the lens cell: They indeed do serve to center align the lenses, and this process should be done *prior* tightening the retaining ring, otherwise and accident could happen. Once the retaining ring holds the mirrors in the lens cell, the centering screws have served their purpose and, to eliminate any pinched optics, they should be unscrewed a little bit.

Wow ! You are quite the inventor !

Thanks for this. It's very informative.

Perfectly simple and easy!

Great video! I have an Esprit 80ED and have pinched optics. Have been looking for a detailed video like this to help me try to resolve the issue.

Great content as always

Very informative video, the way you have explained it really makes it easy to understand, thanks for the research & explanation 👍

👍👍👍👍 Excellent job. Thank you!

It's a big objective and a collimatible lens cell seems like a luxury. My armored binoculars with their smaller lenses not so much.

Excellent video, like!

Merci pour ce tuyau...encore ;)

Very interesting video! Subbed

Hello,didn't knew that refractors need collimation too,but i guess it applies for apochromatic refractors,until now i knew that only newtonians need collimation

I have an AstroTech 80mm ED doublet grab and go scope I use with a Twilight One Mount. Carry it in my king cab truck wherever I go. So far so good. I'm a retired engineer and have shims to .001 of an inch with .1mm being .004". for those who can only afford a reflector with a poor focuser I add the following. I bought an inexpensive 102mm tabletop reflector with a poor focuser I shimmed until it was centered and drilled and tapped holes in the plastic housing and put nylon screws to hold the focuser tube in place within .002 inch or .05mm instead of that wiggle it did before. My grandsons use it and use a laser collimator to keep the secondary in place as Zhumell's primary on the 102 cannot move. Really pathetic focuser. You can buy shims, taps and drill bits at an auto parts store if you have a bad focuser on a reflector.

Awesome video. Thanks a lot. Out of curiosity, are you using a 20 micrometers artificial star (as opposed to 50 microns from Hubble or 100 microns from astrozap) because of the shorter minimal distance? Or are there other considerations?

Nice video! out of cuirosity is that 3rd printer in your affiliate link something that you use for yourself too as a 3d printer or you use another 3d printer? :) Thinking of buying one.

At 5'20" you mentioned a 10mm lens. Where in the assembly is that placed?

Hello Julien, where is the link to the German website describing the Gryzbowski-Style Collimator. Thanks!

Wow! thank you for posting this. I was wondering if you had the DIY Chesire adapter save in another format (like .stl). I tried to open/download the file but it was 101 files in a format that my pc couldn't recognize/open. Thanks!

Great video, I already have a Cheshire eye piece, but I don't have a 3d printer, is there anyway you could make me one of those projection adapters for me to purchase ? or would that open too much of a Pandora's box ?