Are you collimating your scope correctly?

This is where you can read up the answers to some FAQs on astronomy and star-gazing in Singapore. Members' contributions are welcomed.
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VinSnr
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Post by VinSnr »

Canopus Lim wrote:I have read one way of squaring the secondary mirror in Newt. I find it an excellent way. First, it is necessary to centre dot the elliptical secondary mirror. I do this by putting a small dot of paint onto the centre. That small dot will not affect the image as it is like dust. Having centre dot it, using a laser or cheshire, adjust the spider such that the laser beam or the crosshair (in cheshire) strikes that dot or is aligned with it. The secondary mirror will be in the exact centre when viewed from the focuser draw tube. There is no need to put a piece of white paper behind the secondary mirror or use estimation from the cheshire to determine if the secondary mirror is centred. (the cheshire gives the crosshair but it is still necessary to estimate if the secondary mirror is in the centre)
actually the right way is to offset the secondary mirror....and not place the secondary exactly center below the focuser. But most people find offseting the secondary a little difficult to do, so most would just square it.
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Canopus Lim
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Post by Canopus Lim »

For those interested on offsetting secondary mirror can refer to this.

http://skyandtelescope.com/howto/scopes ... _812_1.asp

I am using (C) and VinSnr probably does (B). Both will give good collimation. I am fine with (C) and it is simpler to do.
AstroDuck
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VinSnr
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Post by VinSnr »

weixing wrote:Hi,
The focuser position is same for both... What I mean is that I can collimate to either 1) The smaller brightest white "donut" in the center, but the two bigger "donut" (one grey colour and one green colour) is slightly offset or 2) The two bigger "donut" merge to one, but the smaller brightest white "donut" is off center. All this done without touching the focuser, just adjusting the lens cell comillation screws.

Currently, I adjust the collimation to 2 which I think is correct, but just wonder why the brightest smaller "donut" is not in the center... is it because the hole on the reflective surface in cheshire eyepiece is not round or not in the center??? Or should I adjust to collimation 2??

Hope those refractor expert can give some advice.

Thanks and have a nice day.
think a few possibility

1) The cheshire face is not exactly at 45 degrees
2) The hole may not be exactly at the centre.
3) Focuser need shimming.

I think the only way to tell is to star test it.
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