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Collimation help needed


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I have a SW 130M, and would like to collimate it. On all the tutorial videos I've seen, there is a small O on the primary mirror. Mine doesn't have it; does it appear once the collimator is installed into the eyepiece?

If not, how do I centralise it accurately?

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Hi, there is a thread here which discusses adding a centre spot and collimating without one.

Hopefully that will help you out (personally I'd be looking at the less scary collimating without option!) :)

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See Astro Baby's guide to collimation for an in depth tutorial. Do a search on the forum and it will come up.

Other than that, a simple and cheap way to get good results is to get a plastic milk bottle top and drill a small hole in the centre. Put this over the eyepiece. The hole ensures that your eye is looking down the centre of the focuser.

You should be able to see the whole of the primary (big) mirror, you should see the three mirror clips at the edges holding the primary. if you can't see all three mirror clips, then you can't see the whole of the primary. To sort this, adjust one of the three screws at the front of your scope to adjust the angle of the secondary until you can see the whole of the primary. Remember as you tighten one screw to loosen the others so that the mirror can tilt. When happy, do them all up, but not too tight.

Provided that you can see the whole of the primary, the view down the focuser tube now needs to show the secondary and primary as being concentric, so that the view of the secondary is in the middle of the primary. If it isn't, adjust the screws at the back of the telescope (these move the primary) until the view is nicely concentric.

That's it.

You can also buy a laser collimator, there is an excellent tutorial on you-tube showing how to use one (sorry but I forget the link).

I'm not sure about the central spot on your model, but someone else who owns one might be able to chip in?

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I agree with 0113mountainman. Astro Babys guide is excellent. I have a pretty new 150mm reflector and tried to have a go collimating with a cheshire collimator. My secondary was out, I thought, and it took a while fidgeting with screws etc to centre it. Once that was it, getting the secondary to capture all three primary mirror clasps took a little time too. Adjusting the primary was a piece of cake. i'm hoping that once the secondary is sorted, it wont have to be done that often. I wouldn't mind the primary, it's easy once the secondary is allright. I think the best thing to do is just have a go and get up close and personal with your equipment, it will be an education in itself. Nothing to be afraid of though, just have a go.

Bart

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Hi, there is a thread here which discusses adding a centre spot and collimating without one.

Hopefully that will help you out (personally I'd be looking at the less scary collimating without option!) :)

It discusses it, but doesn't explain how.. I'm guessing a measuring tape and an indelible ink marker, or maybe a small round sticker?

Also mentioned is collimating without the O, and a 'star test', but again doesn't explain how..

Any suggestions would be much appreciated

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hello i collimated my scope for the first time today and im pretty sure that you can see the circle(centre of mirror) once the cheshire was is place.just had a quick check and yes i could see it with the cheshire in place but i would wait to hear from someone with more experiance in cheshires. to confim this before you remove the primary mirror as im still new and could be doing it rong

star

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star_chaser, if there isn't a centre spot on the primary, you will NOT see it with a cheshire. What you will see in the cheshire is a reflection of the dark-disk-within-bright-disk that is on the cheshire's sloping shiny side.

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Also mentioned is collimating without the O, and a 'star test', but again doesn't explain how..

A star test isn't that hard to do. You look at a bright star with a high power eyepiece, above x100 is good.

Defocus, racking the focuser both inwards and out on both sides of focus alternately. You will see circular airy disks and the central obstruction shadow of the secondary. If your scope is collimated, the secondary shadow will be exactly in the centre of the defocused image on both sides of focus and the airy disks will be concentric. If they off to one side, then you adjust the primary to get the central shadow centralised and the disks concentric. Now slowly focus on the star, the circular image should shrink equally on all sides to a nice sharp pinpoint - a perfectly focused image of the star. Job done :(

Please don't be frightened to adjust the scope, the F5 focal ratio design needs to be adjusted to get the best image and is designed to be. You can't hurt it and if you find the image not doing what you want, just go back the other way with the same screw. The trick is remembering which way you turned that last screw and by how much!

Be aware that if you travel anywhere, you will probably have to collimate when you arrive at your observing site due to the disgraceful state of our road network. I went over a pothole yesterday that could have been mistaken for Copernicus!

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The "O" is just a page renforcer for a ring binder, placed in the exact centre of your primary mirror.

If I were to stick one on my primary, how would I guarantee exact centre?

Knowing my luck I'd drop it and it would stick to the primary somewhere unintended

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If you go to SkyandTelescope.com website there are a number of help topics down the left. The How To section has a subsection called Do it Yourself and a section in there is called Collimating your Newtonion or something similar. It gives you instructions on how to place the circular sticker in the centre.

Have a look and see if it helps.

Bart

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star_chaser, if there isn't a centre spot on the primary, you will NOT see it with a cheshire. What you will see in the cheshire is a reflection of the dark-disk-within-bright-disk that is on the cheshire's sloping shiny side.

so i need to put a sticker on my primary :):(:D. thanks for the info tho im going to wait for a clear night and do a star test to see how close i am (maybe i got lucky)

star

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A star test isn't that hard to do. You look at a bright star with a high power eyepiece, above x100 is good.

Defocus, racking the focuser both inwards and out on both sides of focus alternately. You will see circular airy disks and the central obstruction shadow of the secondary. If your scope is collimated, the secondary shadow will be exactly in the centre of the defocused image on both sides of focus and the airy disks will be concentric.

Except it's slightly wrong. Far from focus, the secondary silhouette is actually slightly offset. That's why you need an exit pupil no larger than 1mm (i.e. 130x on that scope) and need to stay very close to focus (where the diffraction pattern is really one and you see three to five concentric rings) to accurately star test.

[As a side note, the star needs to be centred in the field of view, another reason for using as high a magnification as possible.]

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sorry to the op for butting in but is this placement of the circle a 1 shot deal no room for error. is the primary salvageable if a mistake happens ?

does the circle stay on the mirror ?. has everyone done this ??

and i take it my scope is now most likely worse than before

sorry again for the slight hijack and the not to correct information :(.i though it all went a little to smoothly

star

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The sticker is often put on by manufacturers and the ones they use arent reinforcing rings - they are slightly smaller.

The reinforcing ring idea is simple enough and workes well with an indelible marker and its how my TAL-1 is done and also my sisters weird old newtonian as well. Whatever you mark the ring with to make it black make damn sure it wont run - I made a few test ones up and socked them while attached to a plate to check the ink wouldnt run because runny ink on a mirror is never going to be a good thing and the mirror will get wet with dew from time to time.

Best not to make mistakes to be honest and if you follow the guide I put a link to and take your time and have a dry run before hand on something (like the lid off a tin) to get the process right it should go smooth.

If it doesnt then so long as the reinforcer is close to centre you could either soak it off or remove it by prising it away VERY CAREFULLY with a thin object. The area around the centre of the mirror is not used by the scope as its in the shadow of the secondary mirror.

If on the other hand you are cack handed and drop the sticky on the mirrors main part then your best bet is to soak it off pronto and bear in mind you dont want to be picking at it or trying to scarpe it off with a paint scraper :(

Have a read of the guide I posted up the page, practice on something like an old tin lid or bit of card and all should be well.

By the way long focal length scope often dont need the centre spot because their sweet spot on the mirror is so large. Thats the case with my cranky old TAL 1 but because I am OCD I put a spot on anyway just for my own sanity.

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sorry to the op for butting in but is this placement of the circle a 1 shot deal no room for error.

Sure there is. Q-tips and acetone...

is the primary salvageable if a mistake happens ?

I've respotted at least twenty mirrors.

does the circle stay on the mirror ?

Yes.

has everyone done this ??

I've run out of a stack of twenty CatsEye centre spots spotting mirrors for others...

By the way, you can collimate a scope less precisely without a centre spot, with a sight tube with cross-hairs.

1) Make sure the secondary itself is concentric with the sight tube's inner edge where the primary reflection is almost as large as the secondary itself (insert some bright paper opposite the focuser, to see where the secondary ends) and that it appears round (if it's misrotated it will appear as a skewed ellipse).

2) Make sure to tilt the secondary so that the primary reflection is concentric wit hthe secondary.

3) Tilt the primary so that the crosshairs and reflected crosshairs overlap. That's extremely hard to do with any precision because you can't focus on both at the same time, which is why people use a centre spot and a Cheshire.

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