Calling all Cats Eye collimation people

[Catanonia]

Do you find that one you have collimated with the cats eye, take out the auto colimator and insert it again the image is different. Ie out of collimation ?

I find it so, also as I rack the focuser in and out it changes too !!

My method is as follows.

1. Rough collimate

2. Insert the auto collimator and tighen very loosely with one screw as suggested in the instructions. I have to do this overwise the collimator will fall out in the obs. I rry to keep it in the same orientation using the 2 sight holes.

3. Focus until in the offset the relections are about the same size.

4. Collimate

Am I doing this right ?

I check with the cheshire and the sight tube and it looks ok. But if I rack in our out it changes and this gets me worried as it is dependent upon how good I am at focusing by a small amount.

On the plus side, it looks like the primary doesn't shift when the scope is swung around unlike a lot of GSO 8 inchers out there. So I will hold off the silicon mod on the primary for now.

[Jason D]

The autocollimator is a sensitive tool. Make sure to place the autocollimator in the same orientation each time. For example, always point the 2nd pupil towards the drawtube set screw. You should always get the exact images if you remove the autocollimator then reinsert it. If you don’t then we need to discuss further.

If you rotate the autocollimator and the images “jitter”, that is OK. If the movement is more than a “jitter”, then describe what you see to discuss further.

If you rack the focuser in and out, make sure reflections P+2 have about the same size. If you can discern that reflection 2 size is smaller or larger than reflection P then rack the focuser until both reflections have the same size. If reflection 2 is larger then rack out the focuser and vice versa. You want to collimate with both reflections have the same size.

The autocollimator is a highly sensitive tool. That is why you can see the errors you have mentioned with the autocollimator but not with the sight-tube.

Jason

[Catanonia]

Thanks Jason.

I done some more testing and collimations. The position of the focuser or your focal length has a massive impact on the collimation with the cats eye. Something I am concerned about. If I get the donuts the same size and get good collimation in both primary and offset holes then all is good.

If I come back to it and don't get the exact same focal length with the cats eye then it can be off by a bit. This is concerning as there is a large degree of error here depending upon whether you get the donuts exactly the same size. That can be an art in itself and virtually impossible. The problem is that final collimation results depend upon this being bang on.

[Jason D]

The autocollimator is a very sensitive tool. It will pick up and magnify focuser imperfections. If the center spot reflections appear about the same size then the autocollimator mirror is very close to the focal plane. That is where the tool performs best.

When you remove the autocollimator then reinsert it in the exact orientation without racking the focuser, do reflections shift? They should not. Let me know if you do. If you see small shift when you rotate the autocollimator or when you move the focuser slightly, that is OK.

Jason