cat's eye infinity and collimation

[troypiggo]

This is driving me nuts and I'm not sure if I'm being too pedantic or if it is warranted.

I have the Cat's Eye collimation kit with the TeleCat, Infinity, and BlackCat. No real dramas with the TeleCat and initial collimation of the secondary and primary, but when using the Infinity for precise secondary collimation I just can't get the 2 "hot dot" reflections to line up perfectly. I can get them pretty close so they're overlapping, but it's certainly not as perfect as the photos in the manual show.

I just have a 10" Black Diamond and maybe it's the quality of the secondary and it's screws? Or it could be my technique? I try very slightly loosening one screw, then tightening another, but it seems after a few goes the whole secondary moves out of collimation a lot, like a jump, and I have to start the whole process again.

Should I just stop and be happy with the hot dots overlapping mostly and don't worry about them being "perfect"? Or is the difference between perfect and almost very noticable?

[Moon]

Sounds like it. To give yourself a reality check, get it all lined up, then give the focuser or secondary a little push and you'll see how sensitive it is.
I figure if it's that sensitive, gravity and other issues mean the eyepiece or camera is never going to be exactly 100% where the collimation tools were, so don't go overboard. Get it as good as you can, then move on.
James

[jeff65]

I just received my Catseye tools and am learning how to use them. A couple of suggestions:

Have you tried slightly rotating the secondary itself while viewing to see if this brings the reflections closer together? You may have to back off the secondary screws to allow this. I inserted a large diameter M5 washer sandwiched between two milk jug washers atop the secondary holder to make this easier to do without loosening the secondary collimation screws.

Have you checked that the focuser is square to the tube axis? I haven't figured out the best way to do this. Jim's explanation in the instructions using is a little hard to follow without any diagrams.

[Logieberra]

Troy

Using the same scope as you, and same collimation gear.

You need to upgrade the secondary screws. Throw the grubs away, rubbish, and add bolts. Add wing nut heads to these bolts. Makes things MUCH easier.

Send me a PM to remind me, and I'll post a pic of the mod in this thread.

Logie.

[troypiggo]

Thanks mate.



I've gently twisted the secondary vanes to see the effect. The hot dots do come into align sometimes (depending on where they were). But if I back off the screws to manually rotate the secondary, it jumps too far and I have to start again.

I checked the focuser is square first of all. It's a Moonlite, so can collimate it as well. Pretty sure that's fine.



Thanks mate, that's what I'm thinking. Will look into that down the track. I know Peter (peter_4059) swapped the springs around as well so that the screws are pulling rather than pushing or something, and he reckons it's much better/easier as well. Think that was either a Bird or Quark trick.

[jeff65]

Yes, that's why I used the washers. They allow the secondary holder to rotate even when the alignment screws are fairly tight. Also, I've heard that the marks worn into the back of the secondary holder by the screws can prevent you from getting the right degree of rotation.

[erick]

Absolutely! Very common on the GSO secondary mirror holders. I had some left over teflon bearing pads from earlier dob days. I made up three small pads, drilled a little indentation to take the screw heads and put those in place. Now I have smooth movement back and forwards in the correct direction for each of the three secondary adjustment screws. (They have only fallen out once - when the scope fell over and hit the ground!)

[tlgerdes]

Hi Troy,

I had a chat with Jim Fry last year about some of these sorts of issues, one that affected me was focal plane distance.

When you insert an eyepiece or camera, your focal plane is between 2-6cm above the focuser. Whereas when you collimate you probably arent collimating at your focal plane as the catseye tools are quite low profile.

This problem is easily visible as one of the two overlapping images is larger than the other. It just means racking out the focuser to "expected" focal plane distance (this could also mean adding an extension tube).

[troypiggo]

Thanks again, guys. So maybe the grub screws have been done up too tight causing the indentations in the back of the sec holder. I had thought of that and tried sanding it back a bit but they're too deep.

If I was to glue some plastic there, might that help? Should they only be done up loosely?

[multiweb]

Keep it square, no thumb screws, just flat on its face and as close as possible to the focus plane.

[troypiggo]

?? I don't follow Marc.

[multiweb]

Oops. I should have read the whole thread :P I thought you had problems with the telecat and infinity set.

[jeff65]

I use a 30mm OD stainless steel M5 flat washer in between the collimation screws and the back of the secondary holder. The main screw attaching the secondary to the spider is M5 so it's a perfect fit. The collimation screws press against this instead of the secondary holder.

To make the secondary holder easier to rotate, I use another washer cut from a plastic milk jug in between the washer and the secondary holder.

And finally, to reduce scarring on the stainless steel washer, I use another plastic milk jug washer between it and the collimation screws.

This is all just slipped on over the secondary attachment bolt after the spring goes on. No need to glue anything. By not gluing you can always rotate the washers if they get too marred by the screws in one spot.

[troypiggo]

Gotchya. That makes perfect sense, thanks Jeff.

[Jason D]

The fundamental advantage of the milk jug washer is its elasticity. It allows fine tuning the secondary mirror by tightening/loosening one set screw without the need to touch the other two. Use one or two jug washers. Too many milk jug washers will give too much elasticity – not a good thing.

The scope in Catseye video http://www.youtube.com/watch?v=Vj12cx3tnsM is mine with the milk jug washers.

No need to use steel fender washers. The milk jug washers alone are enough.

Jason

[troypiggo]

Thanks mate. Last night I gave it a go. I found 2 washers weren't enough, the secondary still jumped around a bit, nor smooth. 3 washers worked nicely. Will do some more testing over the next couple of nights.