Focussing problemd - focuser bore & 2" to 1.25" adaptor

[haitch]

Originally added to my thread in swap shop - thought it's more appropriate here now...

Good news & bad.

The good... The 2" to 1.25" adaptor arrived today thanks Gaz.

The bad... It seems skywatcher have made the bore of their focusers a fraction of a mm smaller than the standard and consequently it won't fit Anybody else had this experience?

Now got to think what is the best thing to do.

Do I...

A) Enlarge the bore of the focuser?

Reduce the OD of the adaptor?

C) Keep hunting for an adaptor that fits and is short enough?

D) Move the mirror further up the tube to allow it to come to focus?

I think the latter is probably the simplest and quickest as I just need to drill 3 new holes in the plastic tube for the cell to attach to. I've also not got any told for turning/milling the metal parts. My only worry is will I be losing some light as the mirror & secondary are presumably a certain distance apart and if they are closer together the light being focussed will effectively be a wider disc and may not all hit the secondary? Or is it that a cm or so won't make much difference. Presumably to get 1cm more in-focus I move the mirror 1 cm? The secondary seems pretty large anyhow - is there some calculation of secondary size at a given distance for known size primary and focal length?

Any advice gratefully received!

[philsail1]

"haitch"

Depending on how much metal you have left on the "shoulder" of the adaptor, you could try sawing a single straight cut down the barrel of the adaptor. Clean all the "swarf" off and put a touch of grease or vaseline on the barrel, and try pushing it into the focuser. If the metal on the adaptor is not too thick, it should sqeeze up by a fraction of a millimetre and allow the adaptor to be pushed home.

You could also try (if the fit is too tight by just a "fraction" of a millimetre) wrapping some medium grade "emery" cloth around the barrel of the adaptor, and gripping it lightly in your hand, rotating the barrel of the adaptor, thus (after much rotating!) taking off the amount of metal you need to get the adaptor to fit in the barrel of the focuser. (You could finish off the "sanding" by using a finer grade of emery, then paint the adaptor black.

That's about all I can think of.

Regards,

philsail1

[GazOC]

Strange Paul, I plonked it in a 8" Skywatcher before I posted it. Maybe 5 minutes with a metal file will do the trick?

[haitch]

I think the problem is the focuser not your adaptor Gaz. The focuser and old adaptor are a bit of a weird shape actually. It's difficult to show on the focuser but you can see the original adaptor is a bit odd on this photo.

(click to enlarge)

I've managed to file it down (took about 1/2mm off I guess) by taping it to a drill chuck and turning it with a file held against it.

Trouble now is when I tighten the retaining screws it pushes it to one side because of the hollow to take the tapered part of the old adaptor. Wondering if I'd have this problem with a 2" EP.

I'm going to try and modify it further but I think I'll end up having to go with moving the mirror if I can't get it to sit straight otherwise EPs won't be aligned with the light path of the scope.

[GazOC]

I've got a adapter like that but I've always been able to use it interchangeably with the one I sent you.

[Astro_Baby]

And vice versa - its my old focuser and it worked fine witha swap of bits from other stuff. Muchly perplexed here.

[haitch]

Now I understand. Just had a brainwave and delved into the bottom of the box that Astrobaby sent the focuser in. Where I'm trying to put the adaptor isn't where a 2" EP goes. It takes an adaptor for 1.25" or another for 2". Here, see, there's the other one I've just found in the bottom of the box. Astrobaby packaged it really well and I didn't find it before in the mass of polystyrene chunks (not wanting to empty them on the floor).

(click to enlarge)

Looks like I'm going to have to move the mirror then otherwise I wouldn't get a 2" to come to focus (when I manage to get my hands on one) as the adaptor for that raises it to the same height as the original 1.25" adaptor.

I feel so stooopid now.

[haitch]

Thanks everyone - I'll remember to look through packaging throughly next time I buy something! :oops: