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Paulus17

Collimation-bag of nerves!!!

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Well i have put the Chesire collimation eye piece into my newly acquired Dob and it looks like things are out of alignment :sad:

I have just read Astro babys very good tutorial on how to adjust it all,but to be honest i feel very very nervous at having a go at it,and confused at what to do.There are no instructions with the Chesire.

I will have another read,then probably another and another, of ABs link and look at it again.

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Hi , I no exactly how you feel , I was here many years ago , but there is really no reason to be worried . True .

When you are ready . start with any one of the three main mirror collumination knobs ( bolts ) and pick one , does not matter which and give it a small 1/4 turn either way if possible looking into the cheshire ( its easier with 2 people , 1 looking and instructing and 1 turning ) and the dot will move , one way or the other using the 3 knobs the dot will slowely become centralised as the knobs are turned in or out , depending on which way the dot moves , do it in small increments .

Have fun and learn , as its not hard .

Brian.

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You should have got collimation instructions in the box with your 200p - I did at least. They were in one of the 2 booklets. Astro babys guide is great but just look at one step at a time as the whole thing can seem a bit overwhelming. You may well not need to do everything!

Don't just go straight for the primary though. Out the box my secondary was a fair bit out so I sorted that (the bit about seeing all 3 mirror clips) then I found the primary was actually spot on. I've tweaked the primary a couple of times since then due to carrying it about and it's all honestly very easy - much easier to do than to describe. Take your time, small movements each time then check in the cheshire (and reverse what you did as you will almost certainly have turned the bolt the wrong way - sods law!), and when you've done it there will only be quick little tweaks after that.

Edited by Foggymike

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I was in the same boat as you a year ago. I was pointed towards this colli guide http://www.propermotion.com/jwreed/ATM/Collimate/Chesire.htm which shows using its diagrams what is happening to the light path as you adjust the mirrors which I found really helped get my head around it. Using this guide along with astro baby's really cleared things up for me. It is a daunting process when just starting out but a year down the line you will look back and wonder what all the fuss was about :)

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Thanks for the encouragement,but i still feel unsure.

I have attachhed a piccy of what i am seeing through the Chesire.

Yes i did get the instructions ith the scope,but again i find em confusing,at the moment.

post-23363-0-27308100-1339755809_thumb.j

Edited by Paulus17

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Just run a check from the beginning, is the Secondary mirror in line with the Focus Tube, thats the center screw behind the secondary, Check the Mirror clips that's the 3 outer screws behind the secondary, use a focus tube dust cap with a tiny hole in the center, then work your way through the tutorial, the likely hood of the secondary needing attentions is very small, most of the adjustments will be with the Primary Mirror screws.

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One thing that can go wrong that's easy to avoid, is dropping a tool onto the primary mirror, whilst adjusting the secondary.

Drill a hole through the handle of the screwdriver, and make a loop with string to go around your wrist, or tape the string to your allen key or whatever tool in use.

Thankfully, I didn't learn this the hard way........

And don't worry, after a while collimation gets a lot easier with practice, just like any other thing you learn.

Regards, Ed.

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As Tinker says. You just need to work through the tutorial from beginning to end. Astro baby mentions about putting card into your tube to block the reflection of your primary, I found this incredibly handy as it removed the confusing relfection and just allowed me to concentrate on centering the secondary under the focusing tube. You just need to jump in and start playing. You will soon start to see what turning the adjustment screws is doing. With your scope it may be easier to enlist someone to help you due to its size.

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I moved to a 12" truss tube Dob from an SCT so regular collimation has become part of my observing prep. At first it was a bit of a faff but after the first few efforts everything seems to fall into place and now what I considered a nuisance is really no trouble whatsoever. As has been said stick at it and, with time, it gets much easier.

Dave ;)

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Right thank you all again.

Good tips there Ed which i will do.

Can anyone tell from the image i put up what needs doing?

Or is it just best to have a go at it all and start learning while we have this lovely weather,for ducks. :Envy:

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Paulus17, just follow astrobaby's guide step by step. it really is easier than it sounds. next week you'll be wondering why you were so nervous. If I can do it believe me anyone can.

The bit that says to put a piece of card in to block the primary mirrors reflection when you're adjusting the secondary really does make things a lot clearer. if you want a look at a video before you start look at astronomysheds youtube clip about collimation. Above all DON'T WORRY. as long as you don't drop any tools onto the mirrors you can't really break anything.

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once you have done it and appreciate the relationship between the things you adjust, it's dead easy. follow the process step by step and you'll eventually wonder what all the fuss was about. if you can, just read about what you should see and then adjust things in the right order to get them. constantly reading the instructions gets confusing in my experience. as above, just adjust something and see what the effect is; you'll soon understand.

even with my 16" f4 dob on my own it takes very little time (maybe 5 minutes) to do a full collimation

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Orion Telescopes have a great Newtonian collimation vid in YouTube. I can't get access to YouTube at work to give you a link, so try a search. I used it and it was great.

Alexxx

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If you keep the tube horizontal you eliminate the chance of dropping something onto the primary.

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To be honest, if you have not had practice with collimation, a laser collimator is easier to use than a cheshire - although not quite so accurate. Some may disagree, but I would certainly advise you to invest in one - they are not too expensive and you will find it useful to have alongside the cheshire.

One snag is that the primary mirror has to be centre-spotted - or to be more accurate, it has to have a small paper ring (like those used for reinforcing pages in a ring binder) stuck to the exact centre. If your 'scope does not already have this, you will have to remove the mirror cell from the tube and do it yourself - it is not difficult but I can understand how some people may be nervous about this - I was! But you only have to do this once. Note that the ring won't affect your viewing in any way.

The second snag is that the laser itself has to be collimated before you can use it. There are various guides as to how to do this.

The point about using a laser is that you can 'see what you are doing' whilst making the adjustment - especially when collimating the secondary (which you should do before the primary). You can twiddle the screws and watch the spot moving about on the mirror as you adjust: this makes it very easy to home in on the centre. Then, when you go to adjust the primary, if you have twisted the laser so that the cutaway portion faces towards you, you can similarly 'watch what you are doing'.

But if you are a perfectionist, your Cheshire is better for that final fine tuning.

Do the collimation with the 'scope exactly horizontal and with the focusser pointing upwards. Then you eliminate errors due to the flexing of the focusser itself.

Edited by 661-pete

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The mirror has to be center-spotted no matter which tool you use. The laser will allow you to adjust the tilt of the primary and the tilt of the secondary. It won't allow you to round the secondary in the focuser. For that you will still want a sight-tube. The laser should be as accurate as a good Cheshire. I suspect it will be more accurate than a sight-tube for adjusting the secondary tilt but only if the laser is well collimated. For adjusting the primary tilt you should use the barlowed laser technique. If your laser is good then there's no reason to resort to a Cheshire for fine-tuning.

I wouldn't align the scope with it pointing upwards. Firstly there's no reason you'd see less focuser flex with the tube in this orientation. It's far more important not drop stuff onto the primary (as mentioned above) and this is more likely with the tube in this orientation. Secondly, focuser flex occurs to different degrees depending on the eyepiece you're using so it's pretty hard to account for it. It's possibly better to collimating with the flexed focuser in order to take this into account.

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The laser will allow you to adjust the tilt of the primary and the tilt of the secondary. It won't allow you to round the secondary in the focuser. For that you will still want a sight-tube.
True, but centring the secondary in the focuser is less critical than getting its tilt right.

I meant collimate with the focuser pointing up, not the OTA itself! Unless you're very clumsy you're not likely to drop anything through the focuser tube onto the secondary!

The question of focuser flex is certainly a good point, but difficult to allow and compensate for unless you always use the 'scope in the same orientation, with the same eyepiece. Probably best to collimate with the focuser unflexed.

And the one thing that's really difficult (I've never tried, myself), is checking whether the focuser is square-on to the tube. I believe a Cheshire will show up that sort of error, but it's often difficult to correct - depends on the mounting of the focuser.

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OK,well i had a go at it,got through half a bottle of whiskey doing it to calm me nreves :grin: and have got it all looking in line now with the bulls eye look,but do i have to tighten the adjustment screw in the middle on the secondary mirror??

My head is spinning at the moment,as i am still not sure what i am doing,i like to be shown how to do things rather than read instructions :shocked:

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Well done, not so bad eh?! The middle screw moves it in/out and the 3 small ones adjust the tilt. If you tighten the middle one after finishing with the smaller ones it should just snug everything up without changing anything - nothing needs to be really tight at the top or bottom, just comfortably snug so it doesn't come undone or move too easily.

You just need a clear night for a star test now - hope you kept some whisky back as it could be a while coming!

Edited by Foggymike

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The spinning head might have something to do with the whisky.

Thanks for posting and thanks everyone else for sharing your thoughts on this. I will be tackling a decent collimation for the first time this weekend and I am not confident. However, the discussion and tips have been very useful and are giving me more confidence.

Paulus - keep us updated about how it looks now collimated!

Martin

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Great thread. I've been considering scopes for a few weeks now. The 200P Dob seems like great starter scope but I'll admit that the thought of collimating it was starting to put me off. Reading this thread has made it seem a little less daunting! Just got to make the final, FINAL decision now...

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once you have done it and appreciate the relationship between the things you adjust, it's dead easy. follow the process step by step and you'll eventually wonder what all the fuss was about. if you can, just read about what you should see and then adjust things in the right order to get them. constantly reading the instructions gets confusing in my experience. as above, just adjust something and see what the effect is; you'll soon understand.

even with my 16" f4 dob on my own it takes very little time (maybe 5 minutes) to do a full collimation

Agree entirely with this. As long as you don't do anything to damage the optics - which basically means dropping the secondary or dropping anything on to the primary - it can be fixed if you get things wrong. I actually used to deliberately put my Newt way out of collimation just to make the adjustments obvious and to practice. Take your time and relax.

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the purpose of the central bolt is 1) to allow the position of the secondary to be adjusted up or down the tube - this is unlikely to be wrong with a new scope. 2) to hold the secondary in position so it does not drop down the tube.

it should not be necessary to tighten the central bolt as this pulls the secondary up the tube while the secondary adjusters push it down the tube to maintain 'pressure'. once you have got the tilt of the secondary correct, work around the three adjusters gradually to ensure there's no movement of the position and so they are all tight. this will naturally tighten the centre bolt.

it all sounds so much more difficult in words which was my point above.

if you can put a wanted ad for someone to assist and show you it will all seem much easier. sounds (and looks) like you are pretty close, just the final adjustments needed.

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Congratulations on the first collimation, although I'm sure everyone will be interested to hear if everything still looks just as well aligned when the whiskey's worn off!

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I actually used to deliberately put my Newt way out of collimation just to make the adjustments obvious and to practice. Take your time and relax.

funnily enough this is exactly how I teach people to collimate. at a star party I was at last October, several people asked for help with collimation. their scopes were actually pretty close so like a devil, I adjusted the secondary for them, showing them what to do then put the primary quite a bit out and 'insisted' they then do this stage which is actually more vital yet more easily adjusted.

using the basic theory and for the first time collimating, they all got it first time no problems.

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