Just another Collimation question

[RonVen]

I have a Celestron Astromaster 114eq. Ive been finding soom detail issues when viewing planets and stars. I made a collimation cap from a 35mm film canaster. Looking in the focusing tube - the intersecting lines and the primary clips seem perfectly centered. I do not have a donut on my primary lens, so should I "donut" the center of my primary mirror? If I do this, would I then look into my focuser with this same collimation cap to see that the "donut" is in the center of the intersecting lines to see that my primary mirror is collimated? Also, I'm confused as to the benefits of a cheshire collimator as opposed to the collimation cap. Does the cheshire have any added benefits that the collimation cap doesn't have. Also, would leaving the correcting lens in the focuser impede the cheshire collimator? By the way I have left the self focuser in the focusing tube. There seems to be advocates of leaving it in and advocates of taking it out during collimination. Sorry for so many questions. Thanks for your help.

Ron

[umadog]

Ok, one reason you're confused is because there are some persistent nomenclature issues with these tools. It's explained at the bottom of this page: http://www.physiol.o...tionLinks.shtml But I'll repeat it here briefly. A "collimation cap" with a silvered or white inside surface is a Cheshire. A Cheshire is tool used for adjusting the primary mirror. The tube thingy with the angled plate and the cross hairs is not "a Cheshire." Not. Despite what the shops call it. It is a combination tool composed of 1) A Cheshire (the angled plate) and 2) sight-tube (the cross-hairs). So it's a Cheshire/sight-tube combination tool. Or "combo tool".

The sight-tube is used to set the tilt of the secondary mirror and the Cheshire is used for the primary tilt. The tube does double-duty to help round the secondary as viewed from the focuser. The 45 degree angled plate is just to help illuminate the silvered surface. It doesn't have any other alignment-related purpose. Some Cheshires, such as those from Farpoint or Catseye, don't have an angled plate at all. Not necessary.

Aligning the telescope requires you to set the tilts of both the mirrors. So your home-made Cheshire will only do the primary tilt. Whilst this is the more important adjustment, you would also want to set the secondary tilt. If by "intersecting lines" you mean the reflections of the secondary mirror supports, then ignore those. They don't provide any useful information.

No tools, including a Cheshire and a sight-tube, will work without a centre spot accurately placed in the middle of your primary mirror. They all work using the spot as a reference. The best you can do without the spot is to round the secondary in the focuser and centre the primary reflection in the secondary. I don't know the protocol for removing the barlow lens in your scope. I would guess it would have to come out, though.

Edit: you can, although it's not as easy, adjust the primary tilt using a star test. I won't go into that here. Use Polaris because it doesn't move. Use high power. The star test won't help you adjust the secondary, but it'll get things good enough. I would do that and see how things go. Otherwise, what issues are you having with the views? Perhaps it's just bad seeing?

[knobby]

Slight off topic moment but Umadog... Your replies are always well thought out, understandable and generally very accurate! Just had to say that

Thankyou

[RonVen]

Thanks Umadog. Your post really gave me a lot of clarity. It seems the more I read, the more confused I got. Thanks again.

Ron

[RonVen]

Ok, one reason you're confused is because there are some persistent nomenclature issues with these tools. It's explained at the bottom of this page: http://www.physiol.o...tionLinks.shtml But I'll repeat it here briefly. A "collimation cap" with a silvered or white inside surface is a Cheshire. A Cheshire is tool used for adjusting the primary mirror. The tube thingy with the angled plate and the cross hairs is not "a Cheshire." Not. Despite what the shops call it. It is a combination tool composed of 1) A Cheshire (the angled plate) and 2) sight-tube (the cross-hairs). So it's a Cheshire/sight-tube combination tool. Or "combo tool".

The sight-tube is used to set the tilt of the secondary mirror and the Cheshire is used for the primary tilt. The tube does double-duty to help round the secondary as viewed from the focuser. The 45 degree angled plate is just to help illuminate the silvered surface. It doesn't have any other alignment-related purpose. Some Cheshires, such as those from Farpoint or Catseye, don't have an angled plate at all. Not necessary.

Aligning the telescope requires you to set the tilts of both the mirrors. So your home-made Cheshire will only do the primary tilt. Whilst this is the more important adjustment, you would also want to set the secondary tilt. If by "intersecting lines" you mean the reflections of the secondary mirror supports, then ignore those. They don't provide any useful information.

No tools, including a Cheshire and a sight-tube, will work without a centre spot accurately placed in the middle of your primary mirror. They all work using the spot as a reference. The best you can do without the spot is to round the secondary in the focuser and centre the primary reflection in the secondary. I don't know the protocol for removing the barlow lens in your scope. I would guess it would have to come out, though.

Edit: you can, although it's not as easy, adjust the primary tilt using a star test. I won't go into that here. Use Polaris because it doesn't move. Use high power. The star test won't help you adjust the secondary, but it'll get things good enough. I would do that and see how things go. Otherwise, what issues are you having with the views? Perhaps it's just bad seeing?

The stars seem to flare off and Jupiter and Saturn don't seem that sharp. When I focus out, I seem to get the reflection of the secondary mirror supports and eventually the star/planet. Since I'm new at this (about 1 month), I don't know if it's the telescope's shortfall or mine. The telescope's GEM isn't the most stable and even touching the slow motion cables seems to wobble the view. Like I said I'm new at this and maybe more experience will resolve some of these issues. Thanks again.

Ron

[umadog]

Very glad it was useful

You mean the stars flare off to one side? That sounds like collimation-related issue. Probably the primary mirror. You will get bad planetary views (well, all views, actually!) if it looks like that. You could try the star-test to help you adjust the primary: go to Polaris, defocus a bit, and turn the primary screws until it looks better. Do a Google image search for star test and see how little you need to defocus. You want to see the diffraction rings, not the shadow of the secondary mirror. You also need high power for this. Do some Googling on it. If you're having a tough time with it (the wobbling won't help you) then maybe you should centre spot the mirror and use your home-made Cheshire/colli-cap or even buy a combo-tool.

Seeing the shadow of the secondary is normal if you defocus to a great degree. Don't worry about that.

[RonVen]

Thanks again umadog. I'm waiting for my cheshire/coll-cap I ordered. I'll centre spot my primary and give it a go (collimation). Thanks for your help.

Very glad it was useful

You mean the stars flare off to one side? That sounds like collimation-related issue. Probably the primary mirror. You will get bad planetary views (well, all views, actually!) if it looks like that. You could try the star-test to help you adjust the primary: go to Polaris, defocus a bit, and turn the primary screws until it looks better. Do a Google image search for star test and see how little you need to defocus. You want to see the diffraction rings, not the shadow of the secondary mirror. You also need high power for this. Do some Googling on it. If you're having a tough time with it (the wobbling won't help you) then maybe you should centre spot the mirror and use your home-made Cheshire/colli-cap or even buy a combo-tool.

Seeing the shadow of the secondary is normal if you defocus to a great degree. Don't worry about that.

Thanks for your help. I did order a Cheshire/colli-cap. I'll centre mark the primary and give it a go (collimation). Your posts have been very helpful in getting me on the right track.

Ron

[RonVen]

Thanks again umadog. I'm waiting for my cheshire/coll-cap I ordered. I'll centre spot my primary and give it a go (collimation). Thanks for your help.

Thanks for your help. I did order a Cheshire/colli-cap. I'll centre mark the primary and give it a go (collimation). Your posts have been very helpful in getting me on the right track.

Ron

One last question. Will the corrector lens in my focuser impede the sight tube from going in all the way? Or do I just put the sight tube down as far as it will go (up to the corrector lens). Thanks for your advice, I'm still new at all this.

Ron

[umadog]

Yes, it may well do. I also don't know how easy it would be to conduct the collimation with the lens in place (e.g. you may not see the spot clearly). I think you might have to remove the barlow lens (which is what it is). Just give it a go and see how you do. Let us know how you get on!

[RonVen]

Yes, it may well do. I also don't know how easy it would be to conduct the collimation with the lens in place (e.g. you may not see the spot clearly). I think you might have to remove the barlow lens (which is what it is). Just give it a go and see how you do. Let us know how you get on!

My cheshire/site tube is on its way (from Optics Planet ). As soon as I get it, I'll give it a go and update you. Thanks again for all your help.

Ron

[RonVen]

Yes, it may well do. I also don't know how easy it would be to conduct the collimation with the lens in place (e.g. you may not see the spot clearly). I think you might have to remove the barlow lens (which is what it is). Just give it a go and see how you do. Let us know how you get on!

I got the Cheshire and the problem was the Cheshire flopped in the focuser and eventually stops at the corrector lens. The focuser tube circular dimension is actually more than 1.25", but the black top ring is 1.25" (where the screws hold the eyepieces in) and I've read that the ring holding the corrector lens is also 1.25". So, when you take out the corrector you have 2 spots that are 1.25", which stabilizes the collimating tool (cheshire or laser). So, unless you take the corrector out, the cheshire or any other 1.25" collimator won't work.

What I did was to first "donut" the primary mirror and then I collimated only using a collimation cap. I followed garyseronik.com guide entitled "A beginner's Guide to Collimation". The secondary mirror seemed to be spot on, but I did have to adjust the primary mirror. I took the scope out yesterday and it seemed sharper. In any case, I think I'll work with it a while and if I need to collimate it again, I'll take the corrector out. Thanks for your help through this process.

Ron