New scope collimation - Cheshire or Laser?

[Kevin C]

Hi all, I've been lurking around for some time now and have just signed up

I've been using binoculars for about 9 months now and wanted to get a telescope to get better views of things, mainly wide-field 'deep(ish) sky' and nebula etc. but also planets.

My good lady wife (I have to say that now!) has just bought me a Celestron Omni XLT150 reflector on a CG-4 GE mount and I will probably need to collimate it sometime.

Would you advise a cheshire or a laser to do this job?

I've never had to do this before, so I'm hoping it won't be too difficult or make me wish I had a refractor!

Thanks for any assistance in this matter

Kevin

[jeremy1]

Hi. It seems some swear by Cheshire and some swear by Laser. Personally I have a Laser and find it quick and simple to use. I think what ever does the job and you are happy using is the answer.

[Islander]

A cheshire is a fixed device and pretty foolproof whereas a laser collimater will require its own collimation checking and, if necessary, adjusting. It's not that hard to do but it does mean an extra check in the overall process.

[Stephen]

You can't do a full collimation with a laser, as such I'd recommend both

[Quatermass]

I have allways used an old film canister with a hole in it works fine but a nice cheshire would be a good choice.

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[umadog]

There are several stages in the collimation process and not all the tools will do all the stages. Before going into them, you need to know that there are two different passive tools in common use: the cheshire and the sight tube. These are usually combined into the cheshire/sight-tube combination tool that you see on sale in most places. This tool has two different components which do different things (to be explained below).

1. Centering and rounding the secondary in the focuser. The sight-tube is the best tool for this, as it nicely frames the secondary in the focuser. You can get away with just a cheshire if pushed, though. The laser will not help you do perform this step.

2. Adjusting the tilt of the secondary. This is achieved using the first pass of the laser beam or the sight-tube. You can't do this step accurately using only a cheshire or film canister with a hole in it. This is discussion on why not (with a demonstration): Telescope Reviews: My collimation is that bad!

3. Adjusting the tilt of the primary. This is done with the return beam of the laser (which is the error-prone way of doing it, barlowing the laser will work better) or a cheshire.

If possible, buy 2" tools. If you don't want to spend that much then consider buying a nicer 2" to 1.25" adapter. The cheaper ones can cause the collimation tool to tilt and so add errors to the process.

[Kevin C]

Thanks for all the info

Looks like I might have to get both!

[Digz]

Top info by umadog.

What I would add to this, is that although lasers are good, you get what you pay for and a critical thing with lasers - and this will sound daft at first - is they also need to be collimated!

By this, I mean the laser must be dead centre and when the laser collimator is put on a surface and rotated the dot must not move i.e. it must be stationary and not scribe an arc or circle on the wall.

Most lasers are colimatable (if thats a word) and the high end ones will be spot on. Despite that your still going to need a cheshire or sight tube to complete the job.

Might also be worth searching 'barlowed laser collimation' here on the forum.

[umadog]

Definitely worth learning about the barlowed laser technique. This is the only way you should consider using a laser to adjust the primary.

[RikM]

I use a Cheshire/sight tube combination tool (often sold as simply a Cheshire eyepiece in the UK) Using the sight tube barrel and the cross wires to centre and align the secondary and then the reflective Cheshire and primary mirror centre spot to align the primary I find quick, easy and accurate.

I did use a barlowed laser for aligning the primary but it was no more accurate than the Cheshire. The batteries ran out months (years?) ago and I have had no inclination to replace them.

[eekeek]

Hello umadog,

To be clear, a cheshire/sight tube combination tool will do a good job of all 3 steps?

Many thanks

[Islander]

Hello umadog,

To be clear, a cheshire/sight tube combination tool will do a good job of all 3 steps?

Many thanks

Yes, people were collimating newtonians with sight tube type devices long before laser collimators were available

[umadog]

Islander has, of course, answered your question. I'd just mention that the critical step is the adjustment of the primary and for that, a good Cheshire is as accurate as a good laser. The reason is that both approaches rely on two passes of the focal length (the light goes from the focuser to the primary and then back to the focuser) and the read-out is pretty similar (especially if you use a barlowed laser). For most applications, this level of accuracy is perfectly good enough.

[michael.h.f.wilkinson]

I got myself a Cheshire/sight-tube combination, for the little 4.5" mini-Dob of my kids. Seems to work well.

[Kevin C]

I'll probably get a Cheshire and see how I get on with it first. Thanks for all the replies, this is a great place to learn!

[rory]

hijacking the thread here, but is a cheshire a cheshire ,end of ? or is it like most things, you get what you pay for ? seen as low as £15 and others alot higher.

im curious... thanks.

[umadog]

The more expensive Cheshires tend be 2" tools, so you don't need an adapter. Other than that, they are just more accurately made. i.g. They are machined very precisely so they fit into the focuser without wobbling. The reflective area is exactly in the right place, etc. The Catseye ones come with their own center markers and this makes small collimation errors more obvious. If you have a combo tool with a sight tube then the cross-hairs will more accurately positioned on a more expensive tool.

No all of the pricier tools will be made equally. I only have experience with Catseye, but the general consensus is that they're the best out there. If you're paying GBP then they're not insanely priced by any means and they will come with all the tech support you could want.

[RikM]

What you will normally see in the UK is a combination Cheshire and sight tube tool. The cheapo £25 SkyWatcher ones work perfectly fine. The more expensive ones are just a little smarter fit and finish. I don't believe they are any more accurate.

If you are talking about the Cats Eye sets that include a Cheshire then these are far more accurate than anything else I am familiar with. But here the Cheshire is just one part of the tool-set, so you really can't compare performance or price.

[rory]

thanks guys.

[Kevin C]

I went for the cheshire (From FLO) and checked my new 8" dob and it's pretty good 'out-of-the-box'. I'm not going to mess with it unless I have to.

(I did get the 'premium' one though, as someone else pointed out here, It's a nice colour!)