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The old chestnut of collimating....


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Hi all, could someone please tell me if my scope is collimated?

In the picture, the cheshires crosshair is slightly higher than the reflection of the spidervane. Is this normal? Do I need to adjust the vanes?

There is also a very slight crescent shaped reflection of the tube in the primary mirror reflection (look just above the primary mirror reflection). How do I straighten this out?

The secondary mirror, although it looks circular, has a flat edge at the top. Is this normal?

Had a nightmare collimating this for the first time even with AstroBaby's guide. I think I moved the secondary when I shouldn't have....

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It's impossible to tell whether your scope is collimated unless you take a picture through a centred pupil.

what is apparent is a tilt/rotate error: the secondary's major axis is not towards the focuser and that's compensated with tilt. But that's an error you can usually safely ignore (all it does is make the secondary, and thus the fully illuminated field, slightly elliptical as seen from the focuser, but as long as you can see the entire secondary that's really not that important).

Apart from that, it's either really badly miscollimated or the camera's not in the middle of the focuser, with the latter (fortunately) much more likely. From a tool with a centred pupil, the cross hairs and the reflection of the cross hairs should overlap. They don't here, so either the camera is way too low of there's massive miscollimation.

By the way, there's no centre spot that I can see on the primary. Since you have a Cheshire, I'd certainly consider putting one on the mirror if there isn't one.

Ignore the spider vanes - due to a perspective effect, the optical axis doesn't usually hit the geometrical centre of the primary and depending on how that offset is implemented (secondary holder centred or offset, secondary glued offset or centred) the spider vanes can get offset with respect to the rest.

The silhouette of the secondary's reflection is *always* offset away from where you see the focuser intrude and towards the back of the scope. In your case it's offset a bit more anticlockwise, which is what hints there's a tilt/rotate error (the secondary's far side needs to move down a bit).

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Yes you have moved the secondary. Lossen screw gently and only move a small amount. Recheck.

Thanks, but for which problem with the collimation? I think theres a few things wrong with the secondary but whatever I try, I cant seem to rectify it. Any chance someone could list everything wrong on the picture with a brief method of how to fix it? Cheers!

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The first part of collimating a newtonian is to sort the secondary, ignoring,

for the time being, the reflections of the spider.

In the pic, only one primary clip (at the top) is visible. You need to get all of

them, as best you can, visible and equidistant within the secondary reflection.

Use only the secondary adjustments to achieve this. You may have to try the

focuser fully in to do this, but it looks like it is in your pic, protruding into the

the light path on the left.

Having done this, then using only the primary adjustments, sort out the spider

reflection you can see.

I usually finish off by collimating on a slightly out of focus star, using just the

primary adjustments for this.

Take it slowly, you will get there.

Good luck, Ed.

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Any chance someone could list everything wrong on the picture with a brief method of how to fix it? Cheers!

No, because we don't know if the camera was centred, and if it wasn't, then the effects of that parallax dominates all the "errors" we see. And we also don't know what collimation tools you have, you haven't said whether there is a center spot on the primary (despite my question), etc. so we can't tell you how to fix it either.

As I said, the one error we can see (a rotational error of the secondary) is the least relevant one, so I'd rather not make you chase red herrings if there are big fat tunas around.

If you want to know what it has to look like, look up "Jason D"'s avatar.

Or this thread:

http://stargazerslounge.com/equipment-help/121989-centring-secondary.html

Edited by sixela
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In the picture, the cheshires crosshair is slightly higher than the reflection of the spidervane. Is this normal? Do I need to adjust the vanes?

You need to adjust the 3 set screws until the cross-hairs are aligned with the reflection of the primary mirror center spot reflection. Ignore the spider vanes reflections.

There is also a very slight crescent shaped reflection of the tube in the primary mirror reflection (look just above the primary mirror reflection). How do I straighten this out?

The crescent is the reflection of the area around the primary mirror. Align the cross-hairs as I explained above and see what happens.

The secondary mirror, although it looks circular, has a flat edge at the top. Is this normal?

I do not see it. I only see the flat edge to the left which is the reflection of your protruded focuser.

Jason

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OK. So I 've lined up the primarys centre spot in the crosshair, which has fixed the crescent shape issue, but no matter what I try, I cannot get all 3 clips in view. Am I still doing this wrong? At one point I could even see the edge of the secondary through the cheshire. Shall I post another pic?

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Hi Astrobaby! Yes you're guide was my step-by-step, a very good one too! I've got the Skywatcher 130p. Ah, so a collicap is actually something completely different from a collimation tool? This could be where I'm going wrong. Basically I think everything looks right now apart from the clips issue. The crosshair and its reflection pretty much line up and the primarys centre spot sits in the centre. Am I far off from sucess? This is one of the hardest things I've had to do, all the help from you guys is really really appreciated. Can't imagine what this must be like on a massive scope!!!

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I had a similar problem in seeing the three clips and centering the secondary with the cheshire fully inserted into the focuser. I got over this by pulling it out until I could see all of the secondary mirror with a clear gap around it, but that meant the cheshire could now move around as it was slighly loose in the focuser. So I used plumbers tape to take up the slack and get a good fit. I could the set up the secondary and after that fully insert the cheshire for the final primary mirror collimation.

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Yes, I find a collimation cap (collicap) good for initial lining up of

the secondary. You can make one from a 35mm film can. If you

don't have one, try asking at a High St processors, last time I

did that they gave me a bag full !

Link here for making a collicap -

CosmicOne - Ultra Cheap Collimation Eyepiece

All the best, Ed.

I did this but I've discovered that the Astro Engineering metal dust caps have a hole dead centre, just pick off the AE logo sticker and there it is, a colli cap.

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Right, I think I'm there (maybe)! I still don't have a collicap, but this is my collimation without the use of one. Since I dont have a cap, I tried just using my naked eye. the centre dot was directly over my pupil and (what looked like) an equal distance from the clips. I suppose the only surefire is a startest tonight, but from these images, am I far off?

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Hi

I recently collimated for the first time a Skywatcher 200P and then an Astromaster 130 (after fitting a centre ring on the primary). I used AstoBabe's guide (and very good it is too). For both scopes I had to use a collimation cap to set up the secondary. Focuser tube fully out for centralising alignments and fully in to align with the primary clips. I could not see all three clips of the primary with the cheshire in any position. This is more or less what AstroBabe says in her tutorial. Using different coloured cards one opposite the focuser tube and pne in front of the secondary helps to figure out what is what.

Datman

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Hi Astrobaby! Yes you're guide was my step-by-step, a very good one too! I've got the Skywatcher 130p. Ah, so a collicap is actually something completely different from a collimation tool?

Not really, but it doesn't have a tube, so its aspect ratio is never too narrow. If a collimation tool is too narrow, then it's possible that even if you rack the focuser in completely you can't see the whole of the primary reflection (and if you rack the focuser out, the secondary may clip the reflection of the primary before you can actually see all of it).

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