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Cheshire crosshairs


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you can do this of course but it's not necessary as there is enough adjustment in the system to accommodate a few degrees of focuser misalignment. the focuser can only go up and down in one plane assuming it has no slop. if the secondary faces the same plane (i.e. is correctly adjusted) then it is not an issue.

Hi Shane,

I had to adjust my focusser slightly using its collimation screws. It needed tilting slightly to the side, ie not up/down the tube, so I could see the whole of the secondary with the cheshire in place. Perhaps I needed to move my secondary to the side by adjusting the spider vanes but adjusting the focusser seemed the easier option. I'd measured the vanes and they all seemed the same length, so it was odd that the view wasn't quite right down the focusser.

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If its any help , there is a meeting at Rolliston astronomy Club on Friday ( google it) (just near Burton ,I will be there and I am more than pleased to help. Sometimes five minutes being shown is all that's needed.

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be a little careful playing with the colli screws on the focuser as they are normally pretty close from the factory, and inducing one error to correct another doesn't help in the long run. To set the focuser correctly I remove the secondary totally.

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I didn`t know the focusser had collimation screws this might help. Whereabouts on the focusser are they.

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I don't know if they're on all focussers. There's 4 on mine, in the image look at the silver Allen screw bottom left. Just next to it there's what looks like a little grub screw. To raise the focusser loosen the silver screw and tighten the little grub screw. You have to make sure you don't loosen it too much so the nut on the other side doesn't come off.

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What problems can arise from incorrectly setting the screws. I seen them on my focusser but wasn`t sure what they were for.

Personally I don't think there's anything to worry about. You can always put them back. The focuser will probably be fixed to the. OTA with all the corners tightened flush. All the collimation screws do is gradually raise a corner to tilt the focuser in that direction.

This is what I did.

I blocked out the primary with a piece of card. Then with the draw tube fully extended I looked through the cheshire to see if I could see all the secondary. Mine was correctly centred up and down the tube but it was slightly to one side. So I could have fiddled with the spider vanes to move the mirror, which is the correct thing to do if its a long way out. I could tell that if I tilted the focuser slightly that would also correct it. A bit like putting a small wedge under one side of the focuser. So whilst looking down the cheshire I just gradually loosened a nut and then tightened a collimation screw and watched the mirror move into line with the focuser.

As mentioned you can always put them back. There isn't a lot of room the other side of the silver Allen screw so as mentioned make sure you don't loosen it so the nut falls off.

If there's something wrong with doing this please someone say as I don't want to mislead anyone, but I just thought that was what the screws are for, a final fine tuning adjustment.

Once you can view the whole of the secondary in the cheshire then just proceed with collimation in the normal way.

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Thanks for the useful information, I think I will give it a try. I thought the spider vanes needed to be equal distances as altering them would change this. Sorry for all the questions just trying to gain as much information as I can.

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Thanks for the useful information, I think I will give it a try. I thought the spider vanes needed to be equal distances as altering them would change this. Sorry for all the questions just trying to gain as much information as I can.

Yes you're right in theory, if they are equal distance and the focuser sits correctly on a perfect tube then everything should line up. My spider veins all seem the correct length but my focuser wasn't quite right, which I think is odd.

Anyway there's no harm in looking, it's very obvious if its slightly out when you look down the cheshire because you'll see more of an edge around the mirror on one side.

Before you alter one corner, loosen all of them slightly so you don't put an unnecessary strain on another as you change the angle the focuser sits on its mounting plate . Then tighten them all when the adjustments finished.

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As Shane pointed out not all OTA`s are round some are oval so sometimes all the vanes will not be the same length, the two opposite each other should be. So if you need to adjust them you would loosen one side off and tighten the other side up and the same in the other direction.

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Having opposite spider vanes be parallel is more important than having equal spider vanes in lengths. Parallel opposite spider vanes will reduce diffraction spikes. Precise centering of the spider vanes is unnecessary. Reasonable centering is good enough.

Jason

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As Moonshane has said check your secondary so it looks straight not twisted like these

When the focuser is reasonably squared and the spider vanes are reasonably centered then the secondary mirror stalk will most likely end up straight. In case it is little tilted, it is OK. Actually, sometimes you have to tilt the stalk a little to compensate for any little focuser/secondary mirror misalignment.

The reason I am mentioning this point is because we tend to overwhelm beginners with a long list of alignments (I am as guilty). That sometimes drive them crazy. It is important to point out which alignments are important, which ones are forgiving, and which ones have little/no impact. That way, beginners can concentrate on the important alignments in case they can't get all alignments correctly.

Jason

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When the focuser is reasonably squared and the spider vanes are reasonably centered then the secondary mirror stalk will most likely end up straight. In case it is little tilted, it is OK. Actually, sometimes you have to tilt the stalk a little to compensate for any little focuser/secondary mirror misalignment.

The reason I am mentioning this point is because we tend to overwhelm beginners with a long list of alignments (I am as guilty). That sometimes drive them crazy. It is important to point out which alignments are important, which ones are forgiving, and which ones have little/no impact. That way, beginners can concentrate on the important alignments in case they can't get all alignments correctly.

Jason

I know Jason and i appreciate what your saying but what i and Shane where alluding to is the fact his mirror is low in the focuser vertically there must be a reason for this and although he may be a beginner no matter how much we try to help him he is not going to be able to collimate the scope until that mirror is square under the focuser, why the mirror is too low i do not know we need a picture to determine that the usual suspects have been eliminated i.e spider vanes and collimation screws.

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why the mirror is too low i do not know we need a picture to determine that the usual suspects have been eliminated i.e spider vanes and collimation screws.

I agree

Without a photo we can only speculate

Jason

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I do agree with the comments abve about making collimation seem over complicated. as long as the focuser, spider vanes and secondary position are approximately correct and the primary acurrately adjusted you will have a scope which has accurate enough collimation to ensure good images which for visual purposes that are no different to a 'perfectly' collimated scope.

if you understand what the different steps do then you'll also get a better idea about why they matter and how to do them. it's all in the notes.

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Thanks for the useful information, I think I will give it a try. I thought the spider vanes needed to be equal distances as altering them would change this. Sorry for all the questions just trying to gain as much information as I can.

I also agree with the previous comments about keeping everything simple without introducing unnecessary adjustments, so it got me thinking about why I needed to use the focuser collimation screws when my mirror stalk was perfectly positioned in the centre of my spider vanes.

With my focuser flush to the tube without adjusting its collimation screws the secondary mirror appeared slightly to one side when I looked through the cheshire. ie I could see a slight gap to one side, and the cheshire crosshairs were not in the centre. I assumed this meant my mirror wasn't in exactly the correct place in the spider vanes. Adjusting the focuser collimation screws did overcome the problem.

However, I've now put the focuser back so its flush with the tube and found that a very slight rotation of the secondary mirror has repositioned it centrally in the focuser. Ie I loosened the three adjustment screws and rotated it very slightly around the central bolt.

So I'm now back to square 1, ie I can see the whole of the secondary through the cheshire, which means its crosshairs are also central, but my focuser is now also flush to the tube.

So we'll all wait to see your picture but I think you should be able to correct whatever error you have by only adjusting the mirrors without touching either the spider vanes or focuser collimation screws and I apologise if I have unnecessarily confused the issue.

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Sorry for the delay work etc. Taken 2 pictures one through the Cheshire and the other through the 2" adapter. Hopes this shows my problem clearly enough. It shows better through the Chesire with the 3 clips showing but the mirror to low. Ignore the bolts in the primary reflection from the wall, thanks.

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post-15799-0-45569600-1382729067_thumb.j

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I dont know how it looks straight in the 2 inch adapter you can see it is central and yet in the Cheshire it looks low is there any play at all in any part of your focuser other than that i have no idea i have asked Astra baby to have a look for you.

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I am not the best at analysing photos as I am not sure sometimes what I am seeing. based on th first more accurate pic, it looks like your secondary needs moving down a bit toward the primary and you then need to rotate it a little. this should get you there or thereabouts.

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