Focuser advice, too much play

[Adz]

Hey all,

From my last thread you might have gather that not only do I understand collimation, I can completely do it now! Thank you to all the people and their patience for newer members like me.

I would like to ask about my focuser though. It is the standard one that came with my 200mm Newt and when I zoom in and out with it there is a hideous amount of play up and down (if you were looking through an eyepiece).

Now I thought Ad come on don't just jump on the forum every time you can't figure out something. I found the plastic screw on the top of the focuser that seems to control the easy in which it moved in and out. Unfortunately when using my cheshire, if I adjust this screw to try and stop the play it brings the secondary out of alignment again! How do I know that with the screw tightened up, the cheshire is now in the right place and there is also some wobbling that is not making me feel confident about it all.

I found the two alan key adusters above and below the plastic screw, what do they do? I tightened then so they were both finger tight and I think it has made some difference along the line.

I guess what I am getting at is, I have found the importance of a cheshire over a laser and I take Astrobabys words with great respect of her experience. But using the cheshire instead of a laser seems to be altered by the negative effects that put me off a laser (moving around in the 1/25" adapter, moving around focusing in and out).

How should I combat this, or is it something we all live with?

Also could anyone mention whether aligning a focuser is important and how?

Many thanks,

Adam

[Freff]

Hi Adam

I feel your pain mate. I went through this until I used a Hotech SCA collimator. I know people say it is expensive, it is. But it's the only way I have been able to eliminate 99% of the slop from my setup.

The collimator centres itself in the SCA and the SCA centres itself in the focuser. No thumb screws to tighten and offset the alignment except the locking one.

I have also bought the Hotech T-adapter for the same reason.

[Adz]

Thanks Tony, how does this get over my problem with the actual play in my focuser tube? I would say it is about 3mm which is loads!

Would I have to use the Hotech racked all the way in? But then perhaps collimation would go slightly out when I come to zoom out to gain focus with an eyepiece.

[Freff]

Hi Adam, I don't pull the focuser tube out when collimating. The tube screw and rack anti-slip screw are tight.

When I'm viewing then I use the Orion SCA and take it it's the best I can get it. When imaging I'll use the Hotech camera SCA T-adapter.

I think after you eliminate the worst of the slop and finally do a star check, you have done all you can.

[Astro_Baby]

All but the very best focusers contain slop in the mechnaism. Its just one of those things you either live with or upgrade to a more serious focuser like a Moonlite or Feathertouch.

You can tune some focusers to be a bit better but most low cost focuser are - ummm low cost - and that means slop.

[Glider]

If it's the basic Skywatcher Rack and Pinion you have then you can do a lot to reduce the slop using thin, single sided adhesive PTFE tape.

Search SGL for my old thread on slop and sag in a SW R&P focuser.

If this appeals then I've got yards of the stuff and can cut you some.

[Adz]

Thanks for a good response everyone! Glider I shall give things a go and get back to you if I don't have any, thanks for the offer.

Tony what is the tube screw and rack anti-slip screw that you mentioned, which and what do they do?

Cheers

[Peter Drew]

It's very important to adjust out "rocking" or "slop in" rack & pinion focusers if possible and to check whether the drawtube is square to the axis, otherwise no matter how well your self adjusting laser fits it can still give incorrect indications of collimation. There is not, or shouldn't be much clearance between the drawtube or its housing, overdoing the adjustments can cause the drawtube to foul the housing makining it stiff or even scoring the surface. Adjustment is a balancing act between the mesh of the rack and the pinion and tension of the two small grub screws. When properly adjusted, the drawtube should be suspended centrally in its housing with no rocking or backlash. The large tensioning screw is to lock the drawtube in its focused position, not to avoid the problems described. .

[Freff]

Tony what is the tube screw and rack anti-slip screw that you mentioned, which and what do they do?

Cheers

It locks the drawtube to prevent slippage ie with weighty cameras etc.