Adjusting the focuser?

[Spec-Chum]

I think my focuser is off square. Not by much, but in order for me to get the secondary reasonably central in the focuser tube I had to reduce a spider vane to its lowest amount, and even then the secondary is slightly high in the focuser tube. Again, I stress it's not much, and everything seems to work OK, but I shouldn't have to take a spider vane to its extreme to achieve this, surely?

Can you advise what I need to do to "raise" the top end of the focuser to effectively lower the secondary in the tube. I can see 3 small grub screws next to 3 larger hex bolt, but I don't know what they do, and I don't want to fiddle without knowing what I'm doing.

It's not a big issue, and I think my collimation is fine, really, but it's a niggle I want to eradicate lol

[Spec-Chum]

Behold my artistic skills to show the issue:

It's somewhat exaggerated but should get the idea across.

[NGC 1502]

When I had a SW 200 Dob, the focuser had three pairs of screws to square the focuser to achieve the adjustment you desire. If it doesn't have those adjusters, you can get the same result by shimming the focuser with washers or similar spacers. It's all quite doable with basic DIY. Please come back if you need any clarification.

HTH, Ed.

[JamesF]

If you're talking about the screws I think you're talking about, you should be able to use them to re-align the focuser. There's a ring inside the focuser mount that the dome-headed screws screw into, "pulling" the focuser body towards it, and the grub screws rest on, "pushing" the focuser body away. By appropriate slackening and tightening of those screws you should be able to re-align the drawtube.

James

[knobby]

Hi Spec ...

Not an expert but the 3 grub screws will allow you to centre the secondary under the focuser tube, as you slightly tweak each screw you'll see the movement.

To accurately align the focuser tube, the spider needs to be removed and a ' target ' made on the opposite of the tube ... deep stuff.

I think all this sort of stuff is explained better if you look on you tube, it makes more sense when you see it in action...

Good luck

And as James said, if you do decide to adjust the focuser, its the 4 grub screws ... take it easy, small steps and keep re checking

[Spec-Chum]

Cheers all, it looks like I'm on the right lines then.

Knobby, I'm not particularly concerned with having it dead on, so I'll leave the mirror in situ, I just want it a little better

I'll have a play after work tonight...

[NGC 1502]

I'm not particularly concerned with having it dead on, so I'll leave the mirror in situ, I just want it a little better

What I would do is to centre the secondary within the tube, using the adjustments on the ends of the spider, then adjust the focuser to get the secondary centered under it.

Keep at it, you won't find this hard if you take your time.

Regards, Ed.

[Spec-Chum]

What I would do is to centre the secondary within the tube, using the adjustments on the ends of the spider, then adjust the focuser to get the secondary centered under it.

Keep at it, you won't find this hard if you take your time.

Regards, Ed.

Yeah, plan. I'll readjust the spider as I've got 1 extra long vane and 1 extra short at the mo lol.

It doesn't seem too difficult but I didn't want to adjust something I wasn't meant to.

Which is unusual for me, as I'd be the first to press a button that says "DO NOT PRESS" just to see why...

[knobby]

How about you measure from the centre of the Philips screw at the back of the secondary to the edge of the tube and adjust it til its equal spaced!

Then align the focuser with the secondary ... oh it goes on and on but what else are you going to do in the rain and clouds!

Edit ... well done Ed, I must learn to type quicker

[umadog]

This sounds like a problem I had for a while. Perhaps this helps: The problem I had was that the secondary was centred in the tube and I had virtually no adjustment room on the spider. With the scope collimated, not all the primary mirror was visible in the secondary. In the end I collimated the scope. Rotated the secondary until all the primary was visible. Then, with the sight-tube in the focuser, altered the focuser tilt until the sight-tube cross-hairs pointed at the primary spot. In other words, I altered focuser tilt in the situation where one would normally alter secondary tilt. Things are better now.

So what I didn't do was paint the target opposite the focuser, remove the secondary, and square the focuser. It doesn't really matter if the focuser is square WRT to the UTA. All that matters is whether you can centre the primary in the secondary with the optics axially aligned. Tilting the focuser to achieve this seemed like the way to go. So that's what Ed said, I realised.

In your case you might want to start by re-centring the secondary in the tube. But see how it goes.

[Spec-Chum]

Yeah, I know my secondary is not central, but that was done on purpose to get it near central in the focuser.

In hindsight I should have adjusted the focuser to match the secondary (as this was central to start with), not vice versa, but I'll know for next time!

[Saleratus]

It's more important to adjust the focuser to be square with the tube than with the secondary. Then adjust the secondary to center it in the focuser. A lot of secondaries can be taken off, and you can then put one or two flat washers in between the secondary adjustment screws and the top of the secondary itself. This will push the secondary down a little further for you.

Good luck!

[knobby]

Minefield alert !

[Spec-Chum]

I'll stick with making sure the vanes are equal and adjusting the focuser to that lol

[Spec-Chum]

Is it safe to temporarily remove the focus locking bolt? It's getting in the way of the bottom dome screw thing, can't get me Allen key in lol

Just didn't want any bits falling off haha

[JamesF]

I'm sure that will be fine.

James