Collimation cap/Cheshire Collimating Eyepieces

[Jiggy 67]

The world of collimation has always been a bewildering one to me. I have tried it in the past with mixed results. I have only ever tried it using a Cheshire. I have read a number of different instructions that have always confused me because they refer to the Cheshire and a collimation cap separately. My question is...does a Cheshire have a collimation cap built in, there appears to be one at one end or do I need a separate cap for the primary, I was always under the impression that the Cheshire did everything??

I am considering investing in a Hotec Laser Collimater but do I also need a cap or will my current Cheshire suffice??

Sorry of this sounds a stupid question but confusion reigns!!

Cheers

Sent from my iPhone using Tapatalk

[Astro Imp]

You might find reading and re-reading Astro Baby's guide helpful:- http://www.astro-bab...ation guide.htm

[AlexB67]

That may alleviate some of the confusion. I had the same issue, but you are not to blame for that when a lot of sellers define Cheshire not so precisely. A combination tool is a sight tube and Cheshire in one. You can use such a tool to both accurately get the secondary mirror and primary collimated. I use a simple cap for the primary though, but found since I acquired a combination tool recently it is much easier to do the secondary precisely.

[Jiggy 67]

Thanks Alex, your earlier thread has helped, particurlarly the reference to not being able to see the mirror clips, I have had the same problem in the past, probably due to using a long tube Cheshire. Might just get a collimation cap for this reason and continue with the Cheshire for now

[AlexB67]

Thanks Alex, your earlier thread has helped, particurlarly the reference to not being able to see the mirror clips, I have had the same problem in the past, probably due to using a long tube Cheshire. Might just get a collimation cap for this reason and continue with the Cheshire for now

You can also get a short version of the combination tool. Not seeing the clips was not really am issue of the cap in my case, but something else. The problem I have with a long combination tool however is that I have to raise it up, otherwise I cannot see the secondary in full, and when I do that it induces some slop, largely due to not so good focuser in my scope, it is not the tool I blame but it does make it less accurate to use for that reason. I am going to get one of these,

http://www.ebay.co.uk/itm/Skys-the-Limit-Cheshire-Collimator-for-Newtonian-Telescopes-short-/380641200932?refid=store

It should work better in my scope I feel, not having the slop issue so much, and I should be able to see the secondary properly with the short sight tube, and with the focuser turned in fully.

[Moonshane]

hi mate

I use a Cheshire (actually a combination tool (Cheshire/sight tube) before I get jumped on :0) ) for everything. if you use an extension tube it's generally possible to see the outline of the secondary down the small hole in the Cheshire (actually the sight tube bit). if you ever want to come over to Stockport, just give me a shout and I'll happily run through my process, you'll have no issues ever again after about 10 minutes.

[Jiggy 67]

Hey Shane,

come on you blues!!!..........

if you use an extension tube it's generally possible to see the outline of the secondary down the small hole in the Cheshire (actually the sight tube bit).

You see thats my problem!?!?!...different people describe the same item in different ways :huh: what do you mean by extention tube?? I think my problem has always been using a Cheshire to align the primary, I think its too long, hence the need for a collimation cap. I take it you'll be at PSP 2013?....I'll expect a full demo

[Moonshane]

ha ha - the Premiership is already in the bag mate! (we've paid for it after all so it's only fair!). sorry yes, I was a bit misleading. this thread explains what I mean the extension I relate to is a standard 50mm one that you put in the focuser.

for Cheshire/sight tube/collimation cap read 'cheshire'. you'll get it before then I reckon but if not then we can while away the days collimating to your heart's content :0))

[AlexB67]

hi mate

I use a Cheshire (actually a combination tool (Cheshire/sight tube) before I get jumped on :0) ) for everything. if you use an extension tube it's generally possible to see the outline of the secondary down the small hole in the Cheshire (actually the sight tube bit). if you ever want to come over to Stockport, just give me a shout and I'll happily run through my process, you'll have no issues ever again after about 10 minutes.

If I understand you rightly, I could also achieve this by removing the 1.5x lens piece from my barlow, so it has no lenses in it at all, so it effectively just becomes an extension tube. Not sure if that is a recommended way, but that way I can slip the sight tube into the barlow and put that in the focuser, this allowed me to see the whole secondary. I am still not that keen on how steady the whole thing sits though, since back to back runs, removing it, putting it back, still show differences that made me not trust it entirely. The cap itself I always find reproducible because it sits accurately in the same place every time. The Heritage 130P. if there is one criticism of it I do have, I would say it is focuser.

Another thing I also did as a double check, I put the coli cap in the top of the barlow, that way I can see the outline of the bottom part of the barlow ( once again without any lenses in it of course ), the Barlow acts as reference circle to see if the secondary is centered in the circle of the Barlow. With the cap alone, when I put my eye right up against the peep hole, the outline of the hole appears fuzzy, but this method gives me a nice sharp round outline. I used this to double check the centering of the secondary because that also sits steady in the focuser.

I guess I tried to be inventive , Despite my troubles, star tests looked really good though, so whatever the case, I don't think I am far out. I still ordered the shorty anyway because I suspect the secondary is not as good as it should be, though for visual I doubt the difference will be that noticeable. Collimation experiments can be fun after all I find and I think the short sightube will be better for my type of scope and focuser system

In any case Alan ( at sky's the limit ) said I can return it if I don't like it, or don't get out of it what I want.

[AlexB67]

If I understand you rightly, I could also achieve this by removing the 1.5x lens piece from my barlow, so it has no lenses in it at all, so it effectively just becomes an extension tube. Not sure if that is a recommended way, but that way I can slip the sight tube into the barlow and put that in the focuser, this allowed me to see the whole secondary. I am still not that keen on how steady the whole thing sits though, since back to back runs, removing it, putting it back, still show differences that made me not trust it entirely. The cap itself I always find reproducible because it sits accurately in the same place every time. The Heritage 130P. if there is one criticism of it I do have, I would say it is focuser.

I forgot to ask though Moonshane, if you got any other suggestion how to circumvent the above slop I'll gladly hear it I can put the sight tube in the focuser all the way down for a nicer snug fit by itself, ( even though I still feel when I do that it has a little tilt on it to not make me not trust it. I can confirm that by referencing what the coli cap is telling me by itself, they don't agree by a small amount. In any case the combination tool is not so useful in that case since I am back to square one, not seeing enough of the secondary.

[Moonshane]

is your focuser a helical one? maybe try wrapping the threads with ptfe tape? the sort you use for plumbing.

[AlexB67]

See in this thread, it is a fixed type cheapo design,

I did consider tape, but even if I do screw the focuser all the way in, the slight tilt on the sightube is there anyway, because of how it sits in the eyepiece holder, it is not a very snug fit when I tighten the eyepiece holder screws (as you would for a normal eyepiece ).

The combi tool is big and heavy and seems to me (IMHO ) to be better used on heavy duty focusers, not so much the heritage , the tube is almost long enough to be not far away from touching the secondary So it is already a bit sloppy before I started adding extensions, unscrewing the focuser, it just gets worse. I suppose I could tape the sight tube itself to tighten it up a bit.

A normal 1.25 inch eyepiece fits in snugly and firmly, of course that does not have the long tube dangling downwards coming out of the bottom of the focuser ( pardon the pun ) and all that additional weight straining it.

Anyway I would not try to wreck your brain over it, if no further ideas, because the shorty is coming from Alan on Monday anyway, I think that will fit well enough in the focuser all the way in, and will be a better fit in the eyepiece holder, because it sits more like a normal eyepiece without the extension. Besides when I get it I can cross reference it against the cap barlow method I mentioned above. I discussed it with Alan on the phone somewhat and we were on the same wavelength as a way forward. I'll see where I go on Monday with it.

No doubt if I still run into trouble I'll be nagging this useful forum to help me out, besides, despite all of the above, I don't feel the collimation on my scope is bad by any means anyway, but like to fiddle with it and know I am getting the best out if it From what I read in some detailed articles, the secondary is not as critical as is the alignment of the primary anyway.

http://web.telia.com/~u41105032/myths/myths.htm

The star test, well it looks as good as I have ever seen it, and I pretty much had to redo the secondary from scratch, as it got moved out of position badly at one point.

[umadog]

Your "Cheshire collimating eyepiece" is actually composed of two tools

1. The long tube with the cross-hairs is called a "sight tube" and is used to measure errors in the tilt of the secondary mirror. The long tube also does double-duty in helping you to centre and round the secondary in the focuser.

2. The angled shiny plate is known as a "cheshire" and is used to measure errors in the tilt of the primary mirror.

The collimating cap does exactly the same thing as (2), but the shiny face isn't tilted. The first pass of the laser does the same thing as (1). The second pass, or better yet Barlowed, laser does the same thing as (2). Note the laser won't help you round and centre the secondary. It only helps in the tilt of the primary and secondary.

[AlexB67]

Your "Cheshire collimating eyepiece" is actually composed of two tools

1. The long tube with the cross-hairs is called a "sight tube" and is used to measure errors in the tilt of the secondary mirror. The long tube also does double-duty in helping you to centre and round the secondary in the focuser.

2. The angled shiny plate is known as a "cheshire" and is used to measure errors in the tilt of the primary mirror.

The collimating cap does exactly the same thing as (2), but the shiny face isn't tilted. The first pass of the laser does the same thing as (1). The second pass, or better yet Barlowed, laser does the same thing as (2). Note the laser won't help you round and centre the secondary. It only helps in the tilt of the primary and secondary.

where, who, what I never want to use a laser or mentioned one, or is that in reference to the first post ?. I think the combination tool is adequate for my needs and appreciate it does both parts, but I am an old fashioned person and lasers are for modern people , which is kinda strange I know, seeing I did my research many moons ago using a lot of infrared lasers and infrared spectrometers . Now they were big toys like this one. forgot how big it was exactly, but it took up a large part of a laboratory floor by the time you attach all the glassware to it to do the experiments.

[AlexB67]

The shorty arrived this morning, a nice snug fit indeed BUT I can't focus on the cross hairs very well, and as with the cap the outline of the circle when I look through it to see if the secondary is concentric is also fuzzy looking, so while it solves the problem I got it for, it introduced some new ones, I think it will be going back for that reason. On the positive side I can trade it in again and buy another eyepiece . In the mean time I modded the long combination tool with a bit of tape, and it seems to be holding well/or better. If only I could get a good snug fit with the long conbination tool I'd love to use it, because it is so easy to look through and everything is so clear.

Moonshane I'd still love to hear of your extension tube. Note that I have a 1.25 inch focuser though. My barlow trick and removing the lens serving as extension tube for the combination tool is almost useless, it's too sloppy that way and the tool sits very high up, so any alternatives ?

What I need ideally is an extension tube that the combination tool can fit into, so that the tool still goes all the way through it, if that makes sense, so it is not sitting too high up, but about a third of the way up from where it would sit when placed in the focuser normally by itself.

[Moonshane]

I think the basic problem is your focuser. unfortunately, if there's a lot of slop in a focuser, you can only do your best with collimation and try to replicate observing positions and conditions. your idea should work though to see the full secondary down the Cheshire.

[AlexB67]

I did a bit of modding with some tape, one thin layer around the sight tube and it has helped, so it can sit about a third of the way up while the focuser is screwed all the way in, I put some tape on the focuser thread also , I feel more convinced now because both the coli cap and the long sightube are giving me consitently the same results, so I know I can trust it. When I look at the cross hairs, the result looks text book now as should for a shorter focal length scope, as in the attached picture.

In any case I am feeling comfortable now the scope is collimated to be operating optimally, considering that when you focus an eyepiece in the end of the day, it has sufficient slop also, so that will introduce errors, therefore I feel my collimation is well within the error bounds cause by the focuser.

So the short combination tool is going back the sky's the limit. I'll stick with my long sight tube and tape job.

Now I feel comfortable with it and saw the state of the secondary in bright day light with the sun coming in this though the window this morning it showed the state of it. I am going to remove it completely, Secondary is at the "needs a wash stage" I think. I also managed to get finger print on it on one corner at some point, so once done I can have the pleasure of doing the collimation all over again