Cheshire Collimation Eyepiece

[malc-c]

I've been using a Hotech laser collimator for the past year, but since a strip down of the OTA I wasn't quite sure that the optics were perfectly aligned. Yes the centre axis was square, but the results still didn't strike me as being as good as they should.

So I've just ordered and taken delivery of a collimation eyepiece from FLO (quick service I hasten to add) - but it was disappointing to find that there were no instructions on how to use it. OK there are loads of sites that show you what you are trying to achieve, but nothing that said how far the thing should be inserted into the focuser. For example, the the last inch at the end of the tube is slightly thinner than the rest of the barrel (A in the attached image). Do I insert it to this point or should it be inserted right up to the shoulder ( ?

[Tinker1947]

When i use mine same Cheshire same supplier, i insert it in up to point "B" , use a dust cap with a central hole first to align the mirror clips, only done the scope a couple of times hold its collimation really well and a high power EP with a star central and out of focus brings a nice round spiders web....

[Jason D]

You need to insert it all the way-in.

The lower thinner part of the tool has no meaning or benefits.

Jason

[malc-c]

Cheers Jason,

To be honest this was the one of the biggest waste of cash I've every parted with for something connected with this hobby. The cross-hairs at the base of the collimator can not be discernibly focused when viewed through the small hole in the top. The process can't be done easily using room light as the resulting view is very dim, and as the thing relies on the thumb screws to fit the tolerances of the body in the focuser moves the cross-hairs off centre so much it's impossible to class this as the precision instrument needed to accurately collimate the optics.

I've used the Hotech to colimate the optics, and will resort to a star test to check the alignment....

[Tribal-Wolf]

I use my cheshire about 1 in every 10 compared to my laser, gives me a headache when I use it lol.

[umadog]

A good Cheshire actually works very well and has its advantages. I agree, though, that the one in the photo isn't the best. Still, it should be easier to use than you describe. Have you tried shining a torch at the 45 degree opening? That will give you way more light to work with and it can even be used to collimate in the dark. Another alternative is to clip a small LED torch to the spider.

It's definitely true that the fit of the device to the focuser is important. This is why a well-machined 2" Cheshire wins over the 1.25" one you are talking about. However, the 1.25" will work better with a compression fitting (centering) adapter (2" -> 1.25"). This is a worthwhile investment since that shift you notice with the sight-tube may also be taking place with your eyepieces...

[malc-c]

Well tonight was the first time I could try a star test.... I noticed that the traditional diffraction spikes formed by the spider didn't quite focus as sharply as I seemed to remember they did before I stripped the scope for flocking. On defocusing on a bright star using an 8mm Celestron Xcell the rings, whist well defined were not concentric, being compressed on the side away from the primary mirror like the image on the right of this example.

However when focused to my eye the star is quite defined, but the spider defraction spikes seem to be off set to one side slightly and "thicker"...

I had to call it a day as the clouds then rolled in... but it looks like I should of left the scope as it was....as it was performing really well.

The strange thing is that the laser suggest the alignment is fine !!

EDIT:

Still managed to capture a few frames of Venus though...

[umadog]

Nice Venus shot!

The star test is very sensitive so if your laser is slightly misaligned you would see a the star test looking poor but the laser looking good. Also, a scope can drift out of collimation so if you collimate at the start of the night then it might shift out of collimation as it cools. Another thing that it might be is tube currents, which can distort the rings and squish them down on one side so they are no longer round. In this scenario your laser is correct and the rings would have looked good if it wasn't the case that they were distorted. If you weren't looking for it, you might have mistaken non-round rings for non-concentric rings.

A misalignment of the magnitude you show will cause small-scale details to blur together. On a tough target like Mars that's enough to wash out all the detail and turn it into a featureless orange ball (this happened to me last night!). Venus, on the other hand, has no detail to speak of (apart from the phases) so you wouldn't notice the consequences of a small misalignment.

[scuffer]

Just about to buy a collimator , is there a 'better' one to buy ? Is the Premium one at flo worth the extra few quid or should I look for a different one . Didnt want to spend more than £40 if possible but no use buying something that is rubbish

[malc-c]

Nice Venus shot!

The star test is very sensitive so if your laser is slightly misaligned you would see a the star test looking poor but the laser looking good. Also, a scope can drift out of collimation so if you collimate at the start of the night then it might shift out of collimation as it cools. Another thing that it might be is tube currents, which can distort the rings and squish them down on one side so they are no longer round. In this scenario your laser is correct and the rings would have looked good if it wasn't the case that they were distorted. If you weren't looking for it, you might have mistaken non-round rings for non-concentric rings.

A misalignment of the magnitude you show will cause small-scale details to blur together. On a tough target like Mars that's enough to wash out all the detail and turn it into a featureless orange ball (this happened to me last night!). Venus, on the other hand, has no detail to speak of (apart from the phases) so you wouldn't notice the consequences of a small misalignment.

Thanks for the comments and advice. I've ruled out bad thermals as the scope is housed in a ROR observatory, so should be more or less cooled to surrounding temps.

Just about to buy a collimator , is there a 'better' one to buy ? Is the Premium one at flo worth the extra few quid or should I look for a different one . Didnt want to spend more than £40 if possible but no use buying something that is rubbish

Before I stripped the scope down I was getting very acceptable results, maintaining collimation with a Hotech laser colimator. But from reading various posts and websites these are no good for setting up the secondary mirror, hence why a sight cap or cheshire is recommended. Personally I think the price (£28 or more) is high for what is basically a metal tube with a small hole in the top. The cross-hairs on the one I purchased from FLO are naturally out of focus and IMO not worth having... It came with no instructions what so ever which is the biggest oversight, given the disappointment the results give if you get the optics mis-aligned. Now I see why folk drill holes in an old film canister - probably works a lot better !

[Rob L]

I have zero faith in my Hotech, it gives me a different result every time I try (which I believe is down to the poor quality of the standard Skywatcher R&P focuser). For that reason I use a Cheshire.