Collimation help please.

[wyx]

Hi when im using a chesire tool to collimate the primary am i supposed to line up the 'reflection' of the cross hair with the circle on the mirror or align the actual crosshair of the tool? I can align the reflected crosshair ok but not both. Am i doing something wrong? I've read astrobabies guide and still not clear on this. I have a heritage 130p and there is some play with the tool in the focuser which may be a factor. Would be grateful for any advice! Ta

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[Albenn]

I too have one of these scopes and did my first collimation soon after I got it using the Astro baby guide and a Cheshire tool. I found that when I'd finished, the circle on the primary mirror was aligned with both the wire cross hairs (fuzzy ones) and the bright cross hairs. There was a very, very slight difference between the alignment of the two sets of cross hairs, but this could have been down to the fact that the wire cross hairs do appear quite fuzzy and it would be nigh on impossible to get them perfectly aligned because of this. However, I believe that this slight misalignment is perfectly adequate for viewing only.

This level of accuracy being adequate was confirmed later that night when I managed to get my first light with the scope and got the classic 'polo mint' shape when I defocused on a star.

Did you fully insert the Cheshire up to the hilt?

Before you adjusted the primary, did you check that the secondary was centred in the view through the Cheshire by looking at the reflection of the primary mirror clips in the secondary to see if they could all be seen equally? If you can't initially see the clips then back out the focuser until you can.

I found that I had to bring the secondary slightly closer to the front of the OTA to get all the clips looking equal and once I had done that, the 'fuzzy' crosshairs were also centred to the secondary as viewed through the tube.

I then adjusted the primary until the circle was centred in the reflected cross hairs.

I do not now profess to be an authority on collimation but just thought I'd pass on my experience on collimating my Heritage to see if it could be of any help to you.

Follow the guide, be very methodical, take your time and don't adjust anything unless you are sure it needs it.

Believe me, If I can do it - anyone can!

[wyx]

Before you adjusted the primary, did you check that the secondary was centred in the view through the Cheshire by looking at the reflection of the primary mirror clips in the secondary to see if they could all be seen equally? If you can't initially see the clips then back out the focuser until you can.

Thank you!! This is the bit i missed, i followed what you said and now all my crosshairs align!

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[Albenn]

Glad my comments were of assistance.

[pat]

If I remember correctly Astro Baby uses the crosshairs in the sight tube. The cheshire doesn't have xhairs. All articles that I've read use the secondary adjustment screws, not the primary adjustment screws. You use the cheshire to center the primary spot inside the annulas ring of the cheshire. By useing the primary screws. I can't use the fussy xhairs and they interfer with seeing the primary spot. I manually adjust the secondary by Astro Babys instructions. Then I use my Tectron cheshire without the xhairs. I've read where people cut theirs out, but I'm sure there is opinions on doing this. Pat

[umadog]

Pat is correct. A sight tube has cross hairs and a cheshire does not. Usually, however, one buys a combination tool comprised of a cheshire and a sight tube. In this case you see the alignment indicators for the primary and the secondary simultaneously.

The order of steps goes:

1. Adjust secondary position by rotating it on its axis or translating it until it is round and centered in the focuser.

2. Adjust secondary tilt until the cross hairs of the sight-tube point at the primary spot. Go back to 1 and adjust if needed then back to 2, until both steps are satisfied.

3. Adjust the primary tilt so that the primary spot and mark on the cheshire are concentric. Go back to 2 and re-adjusted if needed. Then back to 3. Iterate back and forth until the steps are satisfied.

I find this page useful: John Reed Home Page Collimation - Cheshire

[wyx]

Thanks for your replys, im beginning to learn something. This Collimation is like a dark art!

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[Tinker1947]

My Cheshire has built in cross hairs.....this is it...

First Light Optics - Cheshire Collimating Eyepiece

[wyx]

Pat is correct. A sight tube has cross hairs and a cheshire does not. Usually, however, one buys a combination tool comprised of a cheshire and a sight tube. In this case you see the alignment indicators for the primary and the secondary simultaneously.

The order of steps goes:

1. Adjust secondary position by rotating it on its axis or translating it until it is round and centered in the focuser.

2. Adjust secondary tilt until the cross hairs of the sight-tube point at the primary spot. Go back to 1 and adjust if needed then back to 2, until both steps are satisfied.

3. Adjust the primary tilt so that the primary spot and mark on the cheshire are concentric. Go back to 2 and re-adjusted if needed. Then back to 3. Iterate back and forth until the steps are satisfied.

I find this page useful: John Reed Home Page Collimation - Cheshire

Ok i must have a combination tool. So in reference to step 2. If im using the focuser to centre the crosshair on the primary spot rather than tilting the secondary is that 'cheating' and thus not correctly collimated?

EDIT: that link you sent is the best one I've seen, thankyou. I think i know what im doing now. I adjust the focuser so i can see the whole of the primary, in doing this it just so happens that my crosshair is centered, this obviously means my secondary already is collimated!

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[umadog]

I like that link too.

Don't touch the focuser. You adjust the secondary tilt and position in order to see the primary. You're not cheating, this is just the easiest way of doing things. It's actually not a dark art It just seems like it if you're not clear what it is that you're trying to achieve. Let me explain it this way:

The objective (mirror in this case, but it could be a lens) focuses light onto a plane, known as the focal plane. In a camera, it is at this plane that the CCD/film is located (at the focal length of the lens). If you point your telescope at the moon and put a piece of paper at the focal plane, you will see an image of the moon projected onto the paper. If that was CCD, you'd be taking an image. The instrument isn't focusing light to a point, it's focusing it down to a flat plane. So what? If you move the tissue paper back and forth you will enter and leave the plane of focus (blurring and focusing the image). So much you know. Now imagine that the tissue paper is tilted. Now some regions will be in focus and others out of focus. That's bad and that's what happens when a telescope isn't collimated: the image plane is tilted and/or not centered in the focuser. This is the case because the primary and/or secondary mirrors are misaligned. Your goal is to adjust them so that the focal plane of the instrument perpendicular with the focuser axis. So think of your focuser as fixed and that you're adjusting everything else around it. Obviously you are observing with an eyepiece and an eye, but the idea is the same as the tissue paper. Now take a look at this thread: Telescope Reviews: Concise thread about autocollimators+improvements

Concentrate on the first three posts (the rest talk about the autocollimator, which a different tool). These posts show diagrammatically what I just described.

Once you realise what it is you're trying to achieve, collimation ceases to be a dark art and becomes an obvious series of steps. If something doesn't look right you will be able to figure out why and what to do about it.

[Moonshane]

All good advice above. The other thing about collimation is that for visual use anyway, you should always get it as accurate as you can but don't worry too much if it looks correct. Also, if you get good sharp (visual) images, especially at higher magnifications then the collimation must be OK by default.

[umadog]

Indeed, Moonshane is correct and brings up the other point: how do you know when your alignment is good enough?

First of all, the adjustment of the primary tilt is the most critical thing. So spend more energy on that. "Being collimated" isn't black or white. There is a point beyond which more accurate alignment won't be evident. As Moonshane says, the more you magnify the better your alignment needs to be. Generally, however, it will be obvious when something's wrong. It's instructive to purposely miss-align the primary and see what effect different degrees of misalignment have. Look for asymmetric and blurry stars even in the center of the field of view. The effect should appear at high powers for small degrees of misalignment and then at progressively lower powers as you misalign more. You will also see loss of detail on the moon and planets. Do a Google search for "star test" and learn about that.

[wyx]

Clear skies last night so i did the star test and i was pretty much spot on! Also got to see Saturn, Good times!

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