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Hi Steve, I live in Newcastle which is probably miles from you.

I'll work my way through this thing, I must be doing something basic wrong. The thing that is nagging me is (I put a piece of coloured paper behind the secondary and a piece of white card in front of the primary), and I see a perfect circle but it is dead centre on the yellow background with no obvious offset. This is what makes me think that the secondary is mis placed in the tube.

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Zog, Zog, Zog............

Reading your last post-

""I see a perfect circle but it is dead centre on the yellow background with no obvious offset"":hello2:

THAT'S EXACTLY how it should look!!!!!

The "off-sets" are not obvious to the eye ( unless they are ABSOLUTELY screwed up). The manufacturer of the SW scopes has stated that the secondary mirror is glued to the support in a position to achieve this off-set.....

When you look at the secondary, the reflection of the main mirror and edge clips should also appear circular and concentric with the secondary.

If your laser is itself collimated properly then, in this position, the laser been will hit a point ( don't worry WHERE that point is at the moment) on the secondary and reflect down to the main mirror.

Some slight adjustment of the secondary, should bring the spot into the centre of the main mirror.

Adjust the main mirror to return the beam BACK to the secondary; this return beam should hit EXACTLY the same point on the secondary ie there's only one laser "spot" visible on the secondary.

Forget everything else, for the moment....

This setting will get you very, very close to collimation. Certainly enough to get the telescope out and under the stars to do a final check. Any subsequent improvement should be with the main mirror adjusting screws. Don't touch the secondary any more.

I think you are very close; you just dosn't realise it yet!!!

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First off, the star test says the scope's OK so really that's all that counts, anything else is faffing around for the fun of it.

I have a flextube 300 like you. Next time I collimate it (i.e. next time I take it out to observe) I'll try and note where the laser hits the secondary. But I don't expect it to be in the centre - Merlin66 has explained this. As I've said in another thread, as long as you centre the secondary circle in the focusser tube you'll get your off-set automatically.

The one little point I wanted to make is about getting the primary centred, i.e. getting the 3 clips in view. When I first collimated my flextube I found this difficult and for a moment I wondered if the secondary was too small (a reported problem on some other SkyWatcher models). Instead I realised that the primary had been screwed very far up the tube. So I loosened all the primary bolts and got the mirror sitting further down the tube, then adjusted it to get the clips in view. This time it was easy.

Another thing I noted is that my Cheshire/sight-tube is a bit long for such a fast scope. Ideally you want the dimensions of the Cheshire to match the f-ratio, otherwise when you peer through the Cheshire you may not be able to see to the edge of the primary. But I find I manage.

Once you've got your secondary centred under the focusser, pop in your laser and adjust the three secondary screws until you're hitting the centre of the primary. This adjustment can be awkward on the flextube and similar scopes because of the inadequate screws - try inserting a washer made from a bit of milk carton above the secondary holder and beneath the tips of the screws - the difference is amazing, and the laser spot won't jump around on the primary.

Then you adjust the primary, looking through the cheshire and tweaking until it all lines up. I do it every time I take the flextube out and the whole job takes a couple of minutes.

If the cross-hairs don't line up it may be because the secondary is rotated slightly with respect to the central axis of the focusser (i.e. like you've reached into the top of the tube and given the secondary a twist clockwise or anticlockwise). This will result in a little light loss at the eyepiece, that's all. It means you're not perfect but no one's going to notice. If it's cloudy then fiddle a bit more with the secondary to try and straighten things up. If it's clear then get on with the business and enjoy.

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DONE IT :(:):D

I still couldnt get it right, those cheshire cross hairs just wouldn't go central to the secondary reflection so I went back to basics, tightened the primary screws right down and backed them off by 2 turns, used a small torch to light up the cheshire and it was still wrong. Then a thought hit me, it couldn't be.......could it ?

I took my brand new Orion precision centering adaptor out of the focuser and dropped in the ordinary 2" to 1.25" adaptor. Added the colli cap and had a peep. WOW, the secondary, which had seemed perfectly circular and central to the focuser was set way too far down the tube and too low as well. A couple of turns on the centre screw and a quick twist of the secondary holder and all looked well. Removed the white card from the tube and yep, all 3 mirror clips on view, one slight adjustment of the secondary and they were all evenly spaced and equal sized.

Acid test time, in with the cheshire, a quick gulp, and then a peep. Oh my God, the cheshire cross hairs intersected the secondary shadow perfectly and the centre donut reflection was directly below and touching the shadow. A quarter turn on a primary screw and it was all lined up. Dropped the laser into the focuser and the dot hit the donut dead center. Added the barlow and using a mirror looked up the focuser tube, the donut shadow was perfectly aligned on the target.

I have a collimated scope and I've learned not to trust a piece of equipment just because I believed the advertising blurb.

Thanks to everyone who has offered advice, and especially to Astro baby for her excellent tutorial :)

Bill

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Now muck it all up and try again. Practise makes perfect as they say

Agreed.

When I first got my scope, I once spent all night screwing up the collimation - twisting secondaries, adjusting the primary -the whole shooting match. In this way, I learnt quick ways to get it back to true and gained masses of confidence along the way until it doesn't matter what I see through a collicap / peephole or whatever, collimation never seems far away.

One tip I got from a user group, was to modifiy the 2ndary adjustments to ensure that in no way could the secondary move / twist when I adjusted.

Drill a "dimple" in the places where the screw bear upon

Remove screws and with a sanding attchment on a drill, sand to a fine(ish) point

Re-assemble

Picture below

ScopesinMay2009036.jpg

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Drilling dimples strikes me as drastic. The problem with secondaries on most scopes is that the screws bite in through overtightening, creating dimples, which makes collimation more difficult, because the secondary always wants to jump back to the same position. Inserting a washer (e.g. piece of milk carton) is one popular solution to this, another is to lubricate the tips of the screws with graphite. This makes secondary adjustment smoother and easier.

Drilling dimples makes secondary adjustment impossible: your secondary is fixed. On a solid-tube dob of aperture up to about 8-inches this may or may not be OK - I never really needed to adjust my secondary on my solid-tube 8-inch. But if for any reason you need to remove the secondary (e.g. cleaning, flocking) you've made life a lot harder for yourself when you reassemble and recollimate.

The beauty of a Newtonian is the ease of adjustment, enabling the user to get tip-top condition. You wouldn't lock the tuning pegs on a guitar.

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Drilling dimples makes secondary adjustment impossible: your secondary is fixed.

...err... not exactly following this reasoning. What are your assumptions ? All I know is that I can now adjust my secondary to much better precision, with total repeatability.

As one screw moves down, the other(s) moves up - we rely on the elastic properties of all materials concerned (confession - I am going to design my own secindary adjuster out of titanium in the near future but right now, this works perfect)

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When I used to adjust the secondary mirror I found and also when Steve tried was that the screws being flat bottomed would move around causing collimation difficult to obtain.

After this little dimple mod and sharpening of the screws the collimation is very precise.

But then Acey's little mod of a plastic washer would work very good as well, especially with pointed screws. The screws will dig in very slightly thus very similar to Steve's design.

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What are your assumptions ?

I'm assuming that when you shine a laser off the secondary onto the primary and see that the spot is off centre, you want to be able to tweak the secondary screws so that the spot moves in a smooth and predictable way until it lands right on centre, rather than leaping around because the secondary has a tendency to pop into some preferred position. This is presumably the opposite of what you want, and if that works for you, then fine. It wouldn't work for me.

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  • 2 weeks later...

Bobs Knobs and milk carton washers now fitted, and what a difference. The knobs make everything so much more convenient and the washers damp down the movement of the secondary nicely.

Setting the secondary is now a breeze. Many thanks for all the excellent advise.

Bill

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I had many a nightmare about getting collimation spot on.

The best method I now use to actually use my scope at night and look at objects and enjoy my hobby is as follows.

Take three letters,

O

C

D

The above letters will take your enjoyment away from your hobby. So never use them.

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