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re: that dreaded C word


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good evening all, i thought i would just check my collimation on the scope and yes found it was off slightly so i have adjusted it and it's looking dead central, but now i find that i have to have the focuser fully extended to get a clear sharp image.The star test was perfect with concentric rings, is it normal for the focuser

to be fully extended. cheers stu.:)

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Which eyepiece are you using? A very low power one will require you to rack out the focuser more than for a higher power. Try a high power EP, and see if you can reach focus with it.

If your collimation included tightening the mirror collimation screws, and moving your mirror to the rear of the cell, then any tolerance you might have had, will have been cancelled out, resulting in your focusing problem.

Ron.

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Sounds like you've done the collimation just fine, but like Ron says, just moved the mirror too far up/down the tube in doing so (the collimation is independent of how far up/down the tube the mirror is).

By "fully extended", I guess you mean you have focuser at it's maximum height?? with the eyepiece "far away" from the tube?). In which case, the primary mirror is too far up the tube (too close to the secondary). Put the same number of turns on all the collimation screws*** on the primary mirror to bring it back down the tube a bit to correct that without changing its angle. You'll have to tweak the collimation a little bit again once you've done that, but it should stay pretty close.

*** I don't know the details of your telescope, so you'll have to interpret that into things that are sensible for your specific telescope

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hmm, summat doesn't sound right here,

collimation, a process that involves moving things millimetres or fractions of millimetres has caused a focuser, that has a range of 10s of millimetres, to run out of range?

Doesnt sound credible.

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evening all, well the saga continues i have tried most of today to try and sort this out,but i have made it even worse,i can't get things to line up nothing seems to be going right so i am asking for someone who lives close to me if they would show me what i have done wrong or i would bring the scope to you,thats how bad it is. pm me if you can help. thanks stuart:confused::)

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Don't worry Stu, it's not broken or beyond recovery.

So take a break from it. Calm down and relax.

Then come back to it tomorrow with fresh eyes.

I'm afraid I don't have a reflector, so beyond that I can only point you in the direction of Astro Baby's Collimation Guide. That's if you haven't already come across it :)

HTH

Edited by david o
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Whats wrong with the secondary Stu?

The central screw adjusts moving the mirror up or down the tube. The 3 screws adjust the angle so the image hits the center. The seconday needs to appear as a circle in your collimating cap you mentioned, the guide should help.

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as others have said, don't panic. it's all just adjustment so can be adjusted back.

assuming your secondary is pretty much OK then try the following (I think this is how the SW adjusters work):

loosen the small 'locking screws' all the way out (I am personally not a fan of locking screws but that's another story)

turn all the main primary adjusters until they feel like they have fully compressed the springs (I think clockwise like a normal bolt)

then turn these same bolts two full turns the other way - this gives you plenty of slack to collimate but keeps the tension high enough to retain collimation and effectively moves the mirror down toward the end of the tube.

then collimate your primary.

hopefully this will help.

if not then just follow a full collimation from the start which should sort things out.

unfortunately I live nowhere near you.

Edited by Moonshane
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it's getting to the stage were i will give up and put in the loft, go back to fishing..

Well, many of us have felt like that at some point :)

There's a lot of good advice already given, but if I could just

mention this - when collimating a Newtonian, sort out the

secondary mirror first, ignoring for the time being the reflection

of the spider and secondary.

When, and only when, the secondary is sorted, then it's time

to adjust the primary, this time using the reflections you ignored

earlier.

You will get there, just relax if you can, take it slowly.

Regards, Ed.

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good afternoon,i'm back and in a better frame of mind,have spent most of the morning trying to correct the mess i made,well i think i have done it i have taken my time and the superb advice given and seem to have the secondary and primary lined up just waiting now for tonight for star test.

Thank you all very much for your patients and help,now i will just wait and see.:):eek:

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I had this problem a year or so ago with my Meade LXD75 10 inch Schmidt Newtonian. The answer was very simple, I had adjust the SECONDARY to far up the tube so it wasn't lining up with the focuser. This came about when I had to remove the secondary completely because it was so loose that it (the whole secondary housing not just the mirror) would rotate around on the corrector plate.

Keith

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