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Collimation crisis!!


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OK, so it may be overstating it a bit BUT I just got a second hand 8" Newtonian (skywatcher 200p) and have been checking it over for alignment etc and while the image isn't bad based on observation of Jupiter at 166x magnification, I've been trying to get the little paper circle into alignment with the reflection of my eye and it just won't go! What am i missing? Could the spider be twisted? Could the main mirror need adjusting somehow (I thought the main mirror just sat tight in it's carrier and adjusted with the thumbscrews on the end...). I've checked that the spider is centered in the tube and have adjusted the secondary up and down with the main bolt and adjusters and can't get that damn circle into the middle of the viewfinder...all advice and thoughts welcome including 'stop worrying about it...' if that's what I should do.

Thanks all.

BTW, this is my first Newtonian so I may be doing something really dumb - just tell me what it could be please.

Bob

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Adjust the secondary first. Make sure it looks perfectly circular and centered in the focuser tube. Also look down the center of the open end of the scope and ensure it is central and circular. It's actual oval but when placed correctly it should appear  circular. Now look in the focuser tube again using a tube cap with a small central hole in it. You should be able to see all three primary mirror clips equally spaced around the circumference. You may need to rack the tube in or out to see this.

Then you should be able to place a laser in the tube and adjust the secondary collimation screws until the beam hits the center spot on the primary. Now you can adjust the primary collimation screws to direct the beam straight back at the same spot where it leaves the secondary. It should now be hitting the target on the laser collimator.

Fine adjust the primary till the beam is on the center spot of the target. Now do a star test with an eyepiece on a bright star. Find the focus point and it should be a pinpoint of light. Rack inwards a little and you'll see the bright airy circles that should be concentric. Now do the same racking slightly outwards and you should see the same concentric circles. Then you're ready to start observing. :)

+1 for Astrobabys guide as well.

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AHA!! Thanks all for this so far, will get this guide and read/implement it later tonight, but big thanks to brantuk, I have a laser collimator but never thought to use it to centralise the secondary with the primary too. That's job number one tonight...

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Couldn't resist a quick browse of Astrobaby's guide and noticed it mentions overtightening the retaining brackets of the primary, just out of curiosity really - how tight should they be?

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When you say: "That's job number one tonight..."

Don't do it outside do it inside in the light.

How tight, well have read that you should just be able to slip a sheet of paper between mirror and the adjuster. That however will depend on the paper used.

What the problem is is that people tend to tighten them, as in "That'll never move again!!!"

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I mean tonight, when I get home from work :) appreciate the thought though, and the input on the brackets - a bit of googling suggests a business card is the thickness to go for, but that sounds quite loose to me, I'd be worried about the mirror moving as it tracks across the sky.

Thanks.

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The mirror is supported on the edge, top, and bottom, by the mirror clips. The trick is to make sure it's held in place without any pressure anywhere otherwise you could "pinch" the mirror and distort it - typically a pinched mirror results in distorted stars. The edges are held in place by virtue of their position on the cicumference and the position of the screw holes in the bracket.

Gravity holds the mirror to the bottom of the bracket. So you only really need to worry about how you tighten them down on the top. As mentioned above - a sheet of thin paper should be able to slide between the clip and the mirror surface so there's no pressure on the mirror at all - but it has no room to move when tipped by altitude adjustments.

Best done by hand - turn the screws only a quarter turn each time and test slide the paper. Once the paper starts to stick - back off a quarter turn and the job's done. :)

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Update - went really well last night so thanks to you all for the plethora of info that combined to get me collimated last night. Also, checked the mirror brackets and they were cranked up tight!! Now reduced to a gentle hold so thanks again brantuk and ronin.

Here' sto clear skies :)

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