Collimating problem.

[Bolton rob]

Hello all.

Im collimating my skywatcher 200p explorer. I have both Cheshire and laser collimator. I have managed to get the secondary mirror into the right position. When I put a laser into the the scope to adjust the primary mirror I'm have a problem. When I'm adjusting it I look at the laser to make sure it goes into the centre on the target thing on the laser collimator. Yet when I go look into the scope from the top the laser is still at least an inch above the centre of the primary mirror. Please can anyone help?

[umadog]

Let's go through this step by step. You have two do three different things:

1. Round and centre the secondary in the focuser

2. adjust errors in secondary tilt

3. adjust errors in primary tilt

You need tools to see those three sorts of error and minimise them. You have and a laser and, I suspect, a Cheshire/sight-tube combination tool (the one with the cross-hairs), which provide a visual read-out of those errors. The combo tool will measure all three errors. A regular laser will measure (2) and (3), which also happen to be the most important two.

The laser collimator is just a device for positioning a laser beam in the centre of the focuser, aligned with the focuser's axis. The first pass of the beam (focuser to primary) measures errors in the tilt of the secondary mirror. It does the same job as the cross-hairs of the sight-tube in your combination tool. In both cases, you adjust the secondary screws until the marker (laser or cross-hairs) points at the centre spot. The second pass of the beam (or better yet use the barlowed method, but let's leave that out for simplicity) goes from the primary back to the focuser and so to the laser collimator. This second pass, like the cheshire part of your combination tool, measures primary tilt errors. You adjust the primary until the laser return beam points back at the laser source. You will need to iterate back forth between the secondary and primary adjustments to get them both correct. You will also need to check that your laser is collimated (rotate in focuser draw-tube, beam should move minimally).

The laser and cheshire should give similar readings. If they do not, it could be because the laser is not collimated or because one tool is sitting differently (is tilted, perhaps) in the focuser. Lack of centrality in the focuser isn't a major contributor to collimation errors.

Here are links: http://www.physiol.ox.ac.uk/~raac/collimationLinks.shtml

[Jason D]

When I put a laser into the the scope to adjust the primary mirror I'm have a problem. When I'm adjusting it I look at the laser to make sure it goes into the centre on the target thing on the laser collimator. Yet when I go look into the scope from the top the laser is still at least an inch above the centre of the primary mirror. Please can anyone help?

Adjusting the primary mirror should not move the laser beam on the primary mirror surface. You are tilting the primary mirror when adjusting it. You are not shifting it sideways.

To move the laser beam to the primary mirror center spot, you need to adjust the secondary mirror.

Jason

[Bolton rob]

Looking through the Cheshire everything seems to be centre. I aligned the finder scope with the the main reflect scope and it seems to be working. Yet the laser don't seem to be hitting middle of the primary mirror. I know it's its bouncing back from the primary mirror to the secondry mirror coz I can see the laser on the target of the collimator.

[Moonshane]

if your laser does not match the cheshire and / or hit the primary centre then either your laser is not well aligned, your focuser has some slop or your secondary is not correctly aligned. it's vital to ensure your secondary is correct before adjusting the primary (which takes seconds). I am in Stockport and would gladly help you if you can come to me. personally I don't bother with lasers, a cheshire is more than adequate.

[Bolton rob]

I would to take you up on the offer.. Sad part is I don't drive. I have managed to get it all sorted I think. Well aligned my finder with main scope everything was coming through crystal clear. So I presume that my laser is wrong. Bad part is I don't know what you use to take the sillicone cover off. Which i just posted a topic about.

[Bolton rob]

Well I'm highly frustrated now. I have had my skywatcher 200p 5months. At first I thought it was just me. It really needs collimating . After months of being unhappy with it and learning how to collimate I finally get a Cheshire and and a laser. All the screws and stuff I need to adjust as stupidly tight. I feel as if I'm going break it.

Anyhow like thank everyone what took time to help me on here

[bingevader]

If you are worried about the laser, then leave it be and just use the Cheshire for the time being.

You don't need to adjust the finder 'scope during the collimation process, just the primary and secondary mirrors.

Have you tried Astrobaby's guide?

I only use a Cheshire and a old 35mm film canister, but I'm very lucky in that my 'scope keeps its collimation very well indeed, I tend to un-collimate it and re-collimate it each time rather than actually adjusting any fault that may have occurred.

It can be daunting at first, but like most things in astronomy, once you get your head around it, it's not too difficult.

Cheers

[Bolton rob]

I know what I need do. When I turn the Allen keys on secondry mirror it's just goes back to where it started.

[Bolton rob]

I put some paper in the tube to help see. I have the primary mirror centre bang in centre of secondary mirror. The problem is the seem be more paper at the top than the bottom.

thanks again

[bingevader]

If you are using a piece of card or paper, it is only for adjusting the 2^o mirror and ensuring it is central to the focuser, you need to take it out to adjust the 1^o to the 2^o.

When adjusting the 2^o, its only the central bright circle that needs to be centred, don't worry about being able to see the edge of the mirror.

Like this:

You need to hold on tight to the 2^o when loosening the screws so you don't bend the spider vanes. The screws will only loosen in the one direction to start with, but once loose, you'll be able to move them individually and see what effect each has on the mirror. This bit is after you've removed the paper though!

Cheers

[astronymonkey]

Hi Rob,

collimating for the first time can be daunting and as Moonshane says the primary mirror adjustment is the easy bit...... first you need to set the secondary.

You say that everything is really tight so rather than trying to adjust the screws in the secondary by tightening them you can also get the same affect by slackening off the opposite ones. Often when factory set everything is done up very tight so there is no room for the secondary to tilt when being adjusted, slackening the opposite side a bit at a time give the space it needs to tilt as you tighten the bolt.

cheers