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Is this collimated.

insomnia

By insomnia
in The Astro Lounge

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insomnia

insomnia

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My first collimation attempt. Took me over an hour to hopefully get the mirrors aligned. But I need a second opinion as reading up on tutorials, they all say about the primary mirror clips being visible. To me everything looks aligned, but I can only see 1 clip, is this ok ?

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coulthamst

coulthamst

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To see the primary clips, try racking the focuser out.

As to the out-of-centre secondary reflection, if I've got this right ( says he, stepping where angels fear to tread), if your scope is faster than f6 there should be an appropriate amount of offset applied to the secondary for perfect results. The displacement seen appears to be about right, and in the right direction with regard to the vanes for this to be the case, and out of centre secondary offset is to be expected. Everything else seems to be very well collectively centred.

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insomnia

insomnia

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I have given the secondary a good going over. Not getting any different result from my first attempt.

I have fully wound out the focuser

It's a 250 flextube Dob, so the off-set primary looks about right.

It's raining and thick cloud at mo, so a star test will have to wait.

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Moonshane

Moonshane

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it's not far off and you may find that at f6 it would be fine visually.

check that the secondary surface shows round down the empty focuser (step back a bit maybe 6 inches) and is central in the focuser tube (use a collimation cap).

as others have said, the donut is not quite central to the cross hairs and could do with being down and right a bit in the pic.

don't worry about the clips, the first two steps above will sort that. my 6" does not have any clips, it's siliconed to the cell.

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themos

themos

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this part of the edge of the brighter part of the image is where I think something's wrong. It doesn't have the same curvature as the opposite side. I think you are actually seeing one edge of the primary (the one near the red arc) but not the opposite one. That's why I think you need to rotate the secondary.

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Mr Q

Mr Q

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To see the primary clips, try racking the focuser out.

As to the out-of-centre secondary reflection, if I've got this right ( says he, stepping where angels fear to tread), if your scope is faster than f6 there should be an appropriate amount of offset applied to the secondary for perfect results. The displacement seen appears to be about right, and in the right direction with regard to the vanes for this to be the case, and out of centre secondary offset is to be expected. Everything else seems to be very well collectively centred.

I agree. The operating manual for my 10", f4.5 newt says just that (for photographing purposes). This slight offset will not make any difference in visual observations.

Indeed, do a out-of-focus star test on a bright star near the zenith. Racking the focus in or out to form a circle of light with concentric light and dark rings on medium power with the shadow of the secondary at its center will show that you are, for all practical purposes, well colimnated. As long as the rings are concentric and the secondary shadow appears at the center, you are as close as you can get for all practical purposes.

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Gloris

Gloris

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Don't you have to rack the focuser IN to see the whole mirror better? :).

i'm a bit confused on this too...i'm sure i remember reading in guides that you need to rack out as far as you can when collimating...but i can only see the primary clips at all if i rack all the way in. if i rack out i can't see any of the clips :s

is that normal? or does it mean my secondary is quite badly out? (maybe it's too far up/down the tube?)

side note....i also find that i have to give my secondary a little more tilt/twist in order to see the primary....or else it's reflection is too high on the secondary, so i only see half a circle (but i've checked the spiders and they all measure exactly the same length :s).

collimation i've found to be sometimes very easy, and other times a complete nightmare....i've only done it a few times on my main scope, and twice on my brothers lil scope (stored at mine now because my nephew kept knocking into it ha ha ha)

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umadog

umadog

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i'm a bit confused on this too...i'm sure i remember reading in guides that you need to rack out as far as you can when collimating...but i can only see the primary clips at all if i rack all the way in. if i rack out i can't see any of the clips :s

is that normal? or does it mean my secondary is quite badly out? (maybe it's too far up/down the tube?)

You want to achieve three things when you're playing with the secondary:

1. You want the cross hairs pointing at the primary centre spot.

2. You want the primary mirror centred in the secondary and for it all to be visible.

3. You want the secondary mirror centred in the focuser and round.

If you achieve (1) and (3) then (2) should happen automatically. To help you achieve (3) you want the sight-tube positioned such that its inner edge tightly frames the secondary. This will make it easy for you to judge if the secondary is centered in the focuser and round. Play with the position until it looks good. You play with the secondary position only after you have got it pointing correctly at the primary. Read this: Telescope Reviews: Useful info about secondary mirror alignment

Once (1) and (3) are right then you put your Cheshire pinhole to roughly where the image plane is. You can find out where that is by placing tissue paper over an empty focuser and imaging a star on it or the moon. Rack in and out until you see a crisp image on the paper. You now pop in the Cheshire and the whole primary should be visible and centered. If not, you play with it until it's right. On some scopes the secondary is too small and you never can see the whole of the primary. Watch out for that: don't try to achieve the impossible! If this is the case, it's not a big deal. Perhaps you're throwing out 10% of the light or so.

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JimStan

JimStan

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I go along with the pinhole in the film cap, but they are almost impossible to find stateside.

However, you really need to look down the center of your focusing tube, as others have suggested. The camera shot looking down the barrel of the scope allows for too much leeway. You will come close, but have a difficult time getting it really accurate.

Stars are the best targets, but you can get away with lights a long ways away on the horizon, or even reflections of a light source off a shiny, curved surface, such as a chrome bumper on a car.

Good luck ! Jim S.

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insomnia

insomnia

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I gave my 250 dob another crack at fine tuning the collimation tonight. A few things still puzzle me. No matter what I do with the secondary mirror, I can not get any clips in view without the primary being seriously off- centre. The primary is clearly bigger than my FOV through the cheshire.

The scope has a dual speed Crayford, fitted by the previous owner.

Would a non factory fitted focuser cause collimation problems.

I'm still waiting for the rain to stop so I can do a star test.

At least with all the cloud and rain, I am getting plenty of collimation practice.

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RikM

RikM

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I've been going through the same on my 250 flex and no matter what I've tried I can't get the clips in shot.

You won't ever be able to, the secondary mirror is too small. This doesn't matter.

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