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Collimation?


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Hey again guys,

Last question for the day, this time I mean it :)

I think my telescope was out of allignment, so I tried to collimate. Basically, The secondary mirror supports (not sure what theyre called!) were uneven, and I measured the aperature of the scope which was 22", and made sure the centre of the secondary mirror unit was centered at 11". When that was sorted, I used a bottle cap with a hole in (primitive, I know, but on a budget at the moment!) to check alignment. Basically, The circles were way off, so I took the tiny allen key and loosened the secondary mirror, adjusted it and retightened until when looking through the cap, it was more or less centered. It now looks like a perfect circle to my untrained eye, whereas before it was only a half circle! I realise I won't have collimated it entirely correctly with laser precision, but is what I've done to some extent collimation? I haven't touched the primary mirror...but it seems to be okay, I managed to focus (well, a bit blurred due to close range) on a small bird and it seemed central..

Also, why is it that you cant see the X shape of the secondary mirror supports when looking into the scope? I don't understand why they aren't reflected!

Sorry again for the long post, I've studied some guides but just wanted some reassurance!

Thank you :)

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You have been adjusting secondary positioning which is the 1st step in collimation. The 2nd step are the adjustments to the primary - this might be why the secondary plus it's supports are not visible when you look into the eyepiece drawtube (I assume this is what you mean when you say "looking into the scope"). The tilt of the primary mirror may well need adjusting now.

I'm a bit surprised that you could focus on a bird - many newtonains have a closest focus that is a long, long way off !.

Star testing is really the only way to test the scope so wait for dark, cool the scope and have a look at Polaris inside and outside focus.

Edit: Just noticed the aperture of your scope - it may need the secondary to be offset and therefore not prscisely centred in the tube.

Edited by John
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Hey all,

I have read a few articles already. What I mean is, I can see it when I look into the scope without a lens in, when collimating, but with a lens it it can't be seen!

If I've got a good circle and things are focusing pretty well, is there a need to mess with the primary mirror? Is there not much chance the primary will be fine right out of the box? The secondary definitely wasn't!

Con

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You have been adjusting secondary positioning which is the 1st step in collimation. The 2nd step are the adjustments to the primary - this might be why the secondary plus it's supports are not visible when you look into the eyepiece drawtube (I assume this is what you mean when you say "looking into the scope"). The tilt of the primary mirror may well need adjusting now.

I'm a bit surprised that you could focus on a bird - many newtonains have a closest focus that is a long, long way off !.

Star testing is really the only way to test the scope so wait for dark, cool the scope and have a look at Polaris inside and outside focus.

Edit: Just noticed the aperture of your scope - it may need the secondary to be offset and therefore not prscisely centred in the tube.

Alright, will do! I understand that if I have symmetrical rings around a blurry polaris, all is well?

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Hey all,

I have read a few articles already. What I mean is, I can see it when I look into the scope without a lens in, when collimating, but with a lens it it can't be seen!

That's because all of the light bundle for one object point in the field collapses to one single point on the focal plane, including the shadow of the vanes, and the eyepiece converts it back to a cylindrical light bundle (with vane and central obstruction shadows again) and your eye focuses it again on the retina (on one single point).

Same thing as looking through a fine grid with your eyes focused to something far away: you can't see the grid. Focus on the grid instead and it becomes visible again. Just eyeballing things isn't going to work.

Look at a star and go well *out* of focus and you'll see the spider vanes again.

If I've got a good circle and things are focusing pretty well, is there a need to mess with the primary mirror? Is there not much chance the primary will be fine right out of the box?

No, there isn't, certainly not if you messed with the secondary placement and tilt.

But the first thing to get is knowledge.

Edited by sixela
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Thanks for the reply. I've watched several videos and read a few guides now..I find it odd that the primary mirror needs adjusting, especially as it seems to lay quite flat!

I will take it out in a bit and see if it appears alright, what I am seeing now seems to coincide with the guides, so I think I'll just have to wait and see.

Con

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The primary may appear to sit flat but the tiniest tilt will throw the collimation way off. If you imagine a fixed knitting needle standing perpendicular in the center of the mirror and the same length as your scope - when you tilt the mirror just a tiny bit you get a huge movement at the other end of the needle.

The tip of the needle (or light ray) has to be precisely positioned on the secondary to get the light reflected at 90 degs off the secondary into the focuser tube for good collimation.

Do you really have a 22" diameter mirror there? Awesome size :)

Edited by brantuk
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The primary may appear to sit flat but the tiniest tilt will throw the collimation way off. If you imagine a fixed knitting needle standing perpendicular in the center of the mirror and the same length as your scope - when you tilt the mirror just a tiny bit you get a huge movement at the other end of the needle.

The tip of the needle (or light ray) has to be precisely positioned on the secondary to get the light reflected at 90 degs off the secondary into the focuser tube for good collimation.

Do you really have a 22" diameter mirror there? Awesome size :)

Definitely made a huge error, I meant 22CM! Oops...I wanted to get it out tonight to mess about with the colimation, I think now the secondary seems adjusted the primary will be pretty simple..Damn fog..

Con

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