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DO I need to collimate?


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Apologies for another basic collimation question, but I'm trying to put off the 'evil' (for me) day! But when do I actually need to collimate... What would I notice if my SW 200p needs doing?

I've been using it out of the box for a couple of weeks, gently moving it into the garden and back to view. It SEEMS pretty good: the brighter stars resolve pretty well, though with small spikes/rays, and the smaller ones are quite sharp. The moon also is quite detailed through both the supplied 10 and 25mm eye pieces. I haven't seen any planets yet.

I have a Cheshire but I just know if I start adjusting I'm likely to make things worse :huh:

Thanks in advance, Julian

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Knowledge is power: read about collimation, understand what needs to be done, and you'll not have any fear of it. At the end of the day, you'll need to learn to collimate the telescope so why not use the collimation tool for what it's for? -- measuring the tilt error in the primary and secondary mirrors. Do some reading: http://www.physiol.ox.ac.uk/~raac/collimationLinks.shtml then stick in tool and tell us what you see. Once you're happy with that, try turning one of the primary adjustment knobs clockwise by half a turn. Then look in the tool: what do you see? Turn the knob back to where it was: what do you see in the tool? That's it: you can't mess it up unless you drop a screwdriver down the tube or do something equally silly.

Saying things "seem pretty good" is rather vague. What signatures of miscollimation were you looking for?

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you can't mess it up unless you drop a screwdriver down the tube or do something equally silly.

This is a good point. When you come to it, do the collimation with the OTA horizontal. That way if you do drop something, it's not going anywhere nasty.

James

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it is an interesting little pastime once you get into it. As suggested I did exactly that and played around with it, deliberately put it out, went outside, did the star tests etc. it gives you a feel for your instrument and what tolerances are important to you. While it is easy to get obsessed about it, it depends on your needs also. For visual observing very precise collimating is not the be all and end all. Having an ever so slight incorrect tilt on a secondary, as long as the primary is well collimated with respect to it, in a smaller scope such as mine things like that do not make massive differences to what you see.

Add to that the quality of the equipment. Unless you have an expensive focuser like a Crayford, a simpler but less robust rack and pinion type, or even worse screw in type that I have can easily outweigh the smaller tweaks in collimation, because when used with different eyepieces, the focuser will sit in or out by some distance, this will induce some tilt on the eyepiece ( and therefore an alignment error also).

Don't panic over it, and learn it bit by bit in your own time, it can be fun :smiley: I've got two combination tools, a long a short, a cap, and some homemade improvisations now to do the job on my scope and it has been fun to play around with it.

One tip, as long as you use small tweaks to begin with, see what it does, gain more confidence and put it way out and start again. Easy mistake to make for example is to turn the centre screw half a turn and everything is way out, the secondary is wobbling all over the shop in no time at all.

Good luck :)

Edited by AlexB67
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Don't get paranoid about collimation.

So many Reflector owners seem to regard it with terror. It's not nearly as frightening as you think.

Umadog has offered the best approach to it, by simply advising you to go to school on the subject.

Research it, understand when, and why it is necessary to the efficient working of the optical train.

Plenty of instructions/videos, and advice available online, or here on SGL.

Acquire the grounding first, then you can better understand the questions you may need to ask, and the answers when you get them.

Ron.

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1. don't worry about collimation as it's easy to do once you understand it

2. there's a difference between checking your collimation and adjusting your collimation

3. I would recommend you check your collimation (at least your primary) before each session)

4. if (and that's a key word if) if needs to be adjusted, then adjust in the order of secondary rotation / position up and down the tube (very rare to need adjusting after setting), then secondary tilt (maybe one tiny adjustment per year) then primary adjustment - I need a tiny tweak before each session.

honestly it's easy to do and anything other than mirror damage can be adjusted back again.

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Many thanks for all the advice ... And the links. I've visited the Astro-Baby site a couple of times and, yes, I know it's a super explanation but I'm still not looking forward to my first time. I know I will have to learn very soon and I'm going to look again with renewed confidence thanks to your advice.

Umadog: yes it was a vague statement, but I honestly DON'T know what I should be looking for as I'm a beginner, though I have had a refractor previously. As I said, the stars seem sharp and the moon detailed ... My concern is (was now thanks to the advice) 'if it ain't broke, don't fix it'.

Thanks again all.

Julian

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but I honestly DON'T know what I should be looking for as I'm a beginner, though I have had a refractor previously. As I said, the stars seem sharp and the moon detailed ... My concern is (was now thanks to the advice) 'if it ain't broke, don't fix it'.

Here's how to do it: wait half an hour or so for the telescope to cool down, use an eyepiece that will yield a medium to high power (about 15x per inch should do) and point the telescope at a star. Choose a star that's "medium brightness" and high in the sky to avoid the worse of the turbulence. Focus on it. It should look pretty well point-like. Now defocus SLOWLY. Get the star so it looks about 2 or 3 times the diameter it did when you were in focus. You should notice a the star doesn't look like a disk but is compose of a bunch of concentric rings. It will probably be hard to see through turbulence. It will look small. If you can see the shadow of the secondary and the spider vanes then you've gone too far. The idea is that the rings should be concentric. If they're not, then the primary tilt is wrong. If the primary tilt is REALLY wrong then even in-focus stars will look asymmetrical. If the rings are concentric and stars look symmetrical then your primary tilt is probably good. Once you get experienced you will notice right away when something's wrong by looking at the star shapes. You will notice this stuff at medium to higher powers. When in-focus stars look asymmetrical the medium to high-power planetary views will become very blurry.

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A Newtonian scope is only truly sharp along its optical axis and collimation is simply ensuring that this is in the centre of view. The main off axis aberration is called coma which refractors are generally free of.

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Was just reading above comments and thought to ask: is collimation only for reflector telescopes only and not for refractor telescope like one I own?

Generally speaking you can only adjust collimation on mirror based telescopes (reflectors, SCTs, RCs, etc.) In low-end refractors, the objective elements are fixed at the factory and not user-adjustable. Some high-end models have adjustable objective cells. It is one less thing to worry about.

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Thanks again umadog. I will try that on the next clear night here. I have noticed the out-of-focus effect on stars on my previous observing sessions but I have not attempted it methodically, as you describe above. I've noticed the stars didn't show quite the concentric circles you describe but had a tiny black dot in the centre with bright circles around and tiny dark rays linking said bright circles. Not a very scientific description, so I'll do it properly next time.

Julian

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Just an update ... Using the above advice I decided I did need to collimate. I found the the first part, with the secondary, tricky but it was easier with the primary. Not sure it's that well done but it's sharper than it was. So thanks again to all.

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A Newtonian scope is only truly sharp along its optical axis and collimation is simply ensuring that this is in the centre of view. The main off axis aberration is called coma which refractors are generally free of.

Yes, that's the primary mirror tilt. The secondary mirror tilt ensures that the objective image plane is parallel with the eyepiece image plane (so focus is the same across the field).

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Just an update ... Using the above advice I decided I did need to collimate. I found the the first part, with the secondary, tricky but it was easier with the primary. Not sure it's that well done but it's sharper than it was. So thanks again to all.

Then you did something right! Good stuff. The secondary is tricking because they save money by using setscrews instead of knobs. Do a Google search for Bob's Knobs (it's ok, it's safe to search this!) and you can replace the silly set screws. Secondary adjustments will then become much easier.

Edited by umadog
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