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Newbie Collimating Advice


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I am going to be buying a 10" Skywatcher Dobsonian in the very near future. However, I'm aware that a telescope of this aperture and focal ratio will need frequent collimating (especially if I buy a collapsible one).

I intend to use the Chessire Collimating Eyepiece (I hope this is adequate; I want to keep the costs down) from FLO. However, before I buy the telescope I want to make sure that collimating is something I will be able to to do. I did look at Skywathcer manual but found the collimating section a bit confusing (in fact it put me off buying the telescope). I know there are a lot of threads about collimating in this section of the forums but I would appreciate it if someone can point me to the best link (with effective, easy to understand steps) for a beginner like me.

Many thanks,

Ralf

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ps - my guide has changed its address - its now at Astro Babys Guide to Collimation

Both fantastic tutorials but I used the Andy Shotglass one to learn.

Once you have done it a few times you will wonder what all the fuss was about.

It reads alot more complicated then it actually is.

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I agree with Doc - I'm no expert, but I've learnt a lot by having a go multiple times until it just "clicks" - there is not much to worry about and plenty of time to practise !

Good choice of 'scope - you should have lots of fun

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Its not as hard as it seems BUT its fair to say the first time can be scary and unpleasant.

The best advice I can give is NEVER collimate unless you have to and leave lots of time. DONT try collimating as a beginner if you plan observing soon after.

Best is to try it during daylight and with LOTS of time to spare.

Most collimation is primary adjustments and they arent actually that hard. Secondary mirror adjustments can be a nightmare at times so try and leave the secondary alone as much as is possible.

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Thank you everyone.

I am going to be transporting the telescope to Cyprus (unfortunately nowhere in Cyprus sells the telescope I want). Although I'm not yet sure what's the best way to do this I am pretty sure its going to need a fair bit of collimating by the time it gets there. That's why I wanted to find out about how to do it before I buy the telescope.

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  • 2 months later...

Here's a good no nonsense you tube video using a LASER for alignment:

How can a laser go out of collimation? Its a perfectly straight line....?

Unless of course it gets dropped & knocked out of line:eek:

To drill a hole DEAD CENTRE in a cap or film canister requires great degree of skill, even done in a centre lathe it could be 2-3 thousandths of an inch out.

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Here's a good no nonsense you tube video using a LASER for alignment:

How can a laser go out of collimation? Its a perfectly straight line....?

A) that YT video applies only to the SCA laser collimator which is self centering

:) most commonly available/affordable laser collimators will likely as not be out of collimation.

Yes the beam is sent in a straight line but needs to be centred first !! No good if it's in a straight but going slightly off :)

Personally this is why I gave up and sold my laser, caused far more problems than it solved. A good "cheshire" and a quality colly cap will be perfectly adequate. PS the laser can only be effective once the secondary is aligned !

PPS I should know I'm STILL in the process of trying to resolve my collimation issues (long story)

Ralf, if you're likely to buy a Newt ,YOU WILL at some point have to go thru the collimation process. Scary at first but hey you get used to it ! And there'll always be someone to help.

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  • 1 year later...

If you are getting a collapsible scope and a big one, can I point you toward the tutorial on this site regarding barlowed laser collimation? Sorry can't post the link in this thread as I'm on my phone.

A cheshire is nice on a small scope where you can reach the screws at the same time as looking through the eyepiece. The barlowed method can be done from the rear and takes care of the lasers inefficiencies (centralisation).

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Thank you everyone.

I am going to be transporting the telescope to Cyprus (unfortunately nowhere in Cyprus sells the telescope I want). Although I'm not yet sure what's the best way to do this I am pretty sure its going to need a fair bit of collimating by the time it gets there. That's why I wanted to find out about how to do it before I buy the telescope.

Have you tried Planatarium Vega in Nicosia. My scope cost about 100 Euro more than the UK price but I would have had to at least double that to have it shipped myself. He collimated it while I was in the shop so I got a good lesson in collimating before I took it away.

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Ok thanks for the update Astro Baby. Anywhere else you suggest is a good guide? Ours is half done and the skys look quite clear so far!!! We've completed all the secondary mirror alignment, now about to start on the primary mirror - wish us luck lol :D

Edited by Dubbs75
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  • 3 months later...

Hi,

I've been struggling for a while now to get my collimation nailed. I've followed Mel's fantastic guide and from what I can see it should be spot on. I'm pretty sure the secondary is well centred under the focuser and the beam from my Skywatcher Laser Collimator (which is collimated) is smack bang in the middle of the doughnut, the returning beam is also bang in the middle of the collimator window.

After adjusting the collimation last-night when I'd clamped the OTA into the rings, I performed a star test. Before I got to checking the de-focused image of Polaris I noticed that I couldn't get the star to focus to a tight single point of light. The focused image I see looks quite like

Collimation.jpg

which I tried to correct by adjusting the primary but those adjustments only seemed to move the image around the EP and didn't actually sharpen it. I read that collimation after the OTA has been clamped on is best as the rings themselves can affect collimation.

So,

a) Can the laser indicate that collimation is bang on even if the secondary is not well centred?

:) Can the laser be way way off and be giving me a bum steer that I'm collimated when in fact as the focused image suggests, I'm not even close?

c) Could it be something else like cooling etc, I did leave the scope outside (covers off & dew shield on) for at least an hour before getting started but did notice that the primary looked foggy?

Any suggestions as to how to diagnose further would be appreciated.

Thanks

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the returning beam is also bang in the middle of the collimator window.

But it's an incorrect reference if the forward beam is only 1-2mm off. Do cross check with a collimation cap close to the focal plane, as it's much more precise.

More importantly, though, centre spots from Synta and GSO are always suspect, sometimes up to 3-5mm off!

Collimation.jpg

More often than not, pinched optics can give similar views if you're not familiar with the exact differences. Make sure the clips of the primary don't touch the primary (or at the most rest on it without pressure).
which I tried to correct by adjusting the primary but those adjustments only seemed to move the image around the EP
Well, that's what it's supposed to do. Recentre the object and then see if it has more or less coma.

a) Can the laser indicate that collimation is bang on even if the secondary is not well centred?

Centring the secondary only affects edge of field illumination in very long focal length eyepieces. That's not your main problem, so don't chase that just yet.

:) Can the laser be way way off and be giving me a bum steer that I'm collimated when in fact as the focused image suggests, I'm not even close?

Yes. If the forward beam is really spot-on (but with cheap laser collimators that's extremely hard to ascertain) and the return beam also is, *and* it's all consistent with a collimation cap reading (collimation cap pupil reflection seen exactly centred under the centre spot in the entire travel range of the focuser), then the centre spot location is suspect.

c) Could it be something else like cooling etc,

Possibly, but that doesn't give you this kind of aberration, certainly not if it's also stable. Edited by sixela
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Hi Ivan,

Three things:

1. Can you check with a Cheshire?

2. If you defocus in the other direction, does your Airy pattern look the same or does the "centre" shift 180 degrees?

3. I'm not an expert - just trying to help! :)

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Hi Ivan,

Three things:

1. Can you check with a Cheshire?

2. If you defocus in the other direction, does your Airy pattern look the same or does the "centre" shift 180 degrees?

3. I'm not an expert - just trying to help! :)

1. Unfortunately I don't have a Cheshire though I'm considering getting one.

2. I didn't actually spend much time looking at the de-focused image as the focused star was so "spikey" that I didn't think there'd be any value in it. I'll check next time.

3. Appreciate the help :)

I started again last night, did the following:

1) Removed the primary and checked that the 3 clips were well & uniformly seated, noted that it was a little dusty with some larger hairs and dust particles (not brave enough to clean it yet, it's only 6 months old). After replacing the primary (tape mark indicated the position before I removed it), I tightened the primary adjustment screws to the point where the 3 lock screws (fully screwed in) came into contact with the primary casing. My thinking being that the primary would be fairly well perpendicular to the OTA casing.

2) With the scope out of its rings, I followed Mel's collimation notes again and observed:

a) The secondary concentric under the focus tube

:) The 3 clips uniformly spaced

c) After a & b I connected the collimator and the laser was almost dead centre in the primary doughnut before any adjustment (adjusted it a little to dead-centre)

d) Return beam from primary was way out so adjusted it to centre.

3) Connected the scope to the rings (permanently attached to my pier mounted EQ5). Didn't overly tighten the rings.

4) Checked collimation again and noted that the laser from secondary to primary had moved about 1cm from centre but the primary to secondary was still dead centre. I didn't adjust collimation at this point.

5) I tried tight focus on Polaris again, same results, star looks spiky both sides of focus. I tried several eye pieces and noted the same effect on them all.

6) I de-focused and noted that the airy disc was well formed, concentric rings, dark spot looked slightly off centre though, but to me it looked pretty much like it's supposed to.

Maybe I'm expecting too much. I wear glasses (astigmatism) but when looking through the EP tend not to wear them (don't want to scuff the coating on either the glasses or the EP). I think what I'm going to do next is to take a photo using my DSLR (focused & de-focused) and post it on here to see if I'm trying to fix something that isn't broken. I'm mainly interested in AP and some of the shots I'm getting look ok for focus but it's hard to tell if collimation is out.

M103-1.jpg

Thanks

Edited by IvanT
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If your eyes are astigmatic, chances are that stars look better when you magnify more (and the exit pupil shrinks).

From the looks of it, I also suspect there might be pinched optics. When you reinstalled the mirror, did you make sure that the clips do not touch the primary, but hover just over it?

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