Collimation check

[jason.p]

Is it possible to check the collimation indoors. I was thinking of making up some sort of lightbox or projector with a pinhole and trying it in a darkened room. Would the fact that the focus is going to be short rather than infinity make any difference?

Of course I realise that actually checking on a star is best and that could be done later if stars ever become visible, but in the meantime it would be nice to know if I was somewhere near collimation before I go trying to fix something which ain't broke

As far as I can see it looks OK using the film canister method.

Thanks Jason

[paul mc c]

I usually give mine a quick check in the garage using the light before i take the scope out,it works for me.

[knobby]

I usually give mine a quick check in the garage using the light before i take the scope out,it works for me.

?? How??

[umadog]

You are correct, this is possible. You can make an artificial star and star test it that way. It may need to be quite far away, though, in order for the test to work properly. The artificial star can either be something like a thin fibre-optic or it can a spherical mirror (such as a christmas tree decoration). The diameter of the mirror and the distance from the scope are determined by the aperture of the instrument being tested. Here is some info that will allow you to calculate what you will need:

https://sites.google...ar(pointsource)

http://observatory.m...st/ArtStar.html

http://www.telescope...g_telescope.htm

http://www.cloudynig...uments/star.pdf

These are off-the-shelf units: http://www.hubbleopt...cial-stars.html

My initial advice is that you purchase a Cheshire/sight-tube combination tool. The film canister technique ("collimation cap") is not good for adjusting secondary tilt (see here: http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5052617). That's what the sight-tube cross-hairs are for. Your scope is f/8 and so pretty tollerant of misalignment so even without the combo tool you may be able to get within the tolerance envelope using just a good collimation cap. The combo tool is definitely preferable, though.

[jason.p]

Thanks Umadog. It looks like the Cheshire combo tool may be the way to go after I've sorted the adapter.( I imagine the Cheshire will be the standard 1.25in whereas I've got 32mm at the moment). I'm considering taking out the primary mirror to "centre spot" it, so I'll be starting from scratch, which for me is a bit scary!

The proper tool should help me get things right when I put the mirror back in (hopefully!)

The artificial star is obviously a bit more complicated than just a light in a box, but is certainly do-able. I may well have a go. The LED in a plastic pipe may be the best. I got a bit lost with the maths (need to read up about airy discs), but it seems the tolerances are quite wide, so I should be able to get close enough.

(I just love this "making things out of plastic pipes" etc). Time is my biggest problem. I just don't get enough of it.

If I manage it I'll report back.

Thanks again Jason

[umadog]

Ah, if you don't have a centre spot then you definitely need one. You can't adjust the primary mirror tilt without one and this is the most important adjustment. The best way I know of centre spotting is to use the Catseye stuff: http://www.catseyecollimation.com/ The website is confusing at first, this is the stuff you need: http://www.catseyecollimation.com/template.html Not expensive. The template they supply is easy to use and I think more accurate than other approaches. Astrobaby has a guide on installing a spot with the template: http://www.astro-baby.com/TAL%20Telescope%20Rebuild/Telescope%20Mirror%20Cleaning.htm I encourage you to also use the Catseye spots, since they're reflective and easy to work with in the dark if needed. Based on what he says elsewhere in the site, you'd want the "small" triangle.

What is 32 mm in your set up? 32 mm is 1.26" so perhaps what you're referring to really is 1.25".

[jason.p]

Thanks for info on centre spotting. It seems it is well worth doing and Catseye seems the way to do it. I'm growing quite a shopping list!

My eyepieces and tube are the original TALs that came with the scope and are 32mm (1.26ins), so I imagine a 1.25in collimator may be too "sloppy" to be accurate. It wouldn't be so bad with a standard eyepiece if I wrapped some tape round it, but I wouldn't think it would be good enough for collimation?

I'm getting an adapter so that I can use a collimator and all the eyepieces I'll probably end up buying!

Jason

[umadog]

You're getting a 1.26" to 1.25" adapter? There is such a thing?

I think for collimating your scope you would be fine wrapping a tape around a 1.25" collimating tool. It's actually not very important for it to be centered (although it should be close) but it is important it isn't tilted.

The Catseye Cheshire (http://www.catseyecollimation.com/cheshinfo.html#SMALLPRODS) is nice and works well with the spots they sell. The reason it's good is because it's precisely machined and because it's very easy to see misalignments. Not too pricey at 20 quid or so but you'd still need a sight-tube. The combination tools all the dealers sell would work for that purpose.

[jason.p]

I was hoping there was such an adaptor!

If not I'll have to rely on tape, or drill and tap my tube and fit thumbscrews to hold things in place, which would be a lot cheaper! If it's not that important for the collimator maybe it's a better option. I think what I'll do is centre-spot the mirror and get the Cheshire combo and see how it fits. What shape/colour spot would you recommend? Could you point me to a suitable combination tool/sight tube. Like I've said before, I'm on a steep learning curve!

Thanks Jason

[umadog]

Thumb screw will do you for sure. You're at f/8 and tolerances are nice and easy there. This will do you just fine: http://www.firstlightoptics.com/collimation/cheshire-collimating-eyepiece.html There's also a nicer version: http://www.firstlightoptics.com/collimation/premium-cheshire-collimating-eyepiece.html

I would contact Jim Fly of Catseye and tell him you want to use one of the tools above and ask if you're better off with a standard white ring-binder spot or with one of his spots. The white ringbinder forms a ring around the black central spot of those combo tools (the ones FLO sell) and this makes it easy to use. If you have a filled triangle instead it may be less easy.

[jason.p]

Thanks again