Immediate snag in Collimating

[Bathurst]

Hi guys, Ive followed tutorials and read countless threads on here but my first time Collimating has hit a snag already at point 1 of the operation:

- Round and centre the secondary in the focuser

I've loosened the Allen screws slightly and can turn the Secondary with the Philips head up and down until it's centered, trouble is when I centre the Secondary it's nowhere near a "Circle", it has two straight edges top and bottom..dunno the proper word!

When I tried adjusting the Allen keys and got it as close to a "Circle" as I could the Secondary was miles off the Primary when looking through the Collimation Cap after removing my pieces of card....and I mean miles off, when I got it back to roughly inline with the Primary the "Circle" had gone back to square one.

The below picture as reference the "white" area is what I'm on about with the "circle", to me as above it has two straight edges top and bottom of the White area, what's going wrong?

[knobby]

Not sure what lines ... Looks pretty ok to me .

It's the silvered surface of the mirror that needs to look round , if it's off or egg shaped it's no good.

Yours looks pretty ok to me and seems to be pretty well centred in the focuser tube .

[Gomtuu]

Looks okay to me too!

[F1Bird]

I am certainly not the person to talk to about collimation issues but I think the straight lines you refer to are just the secondary mirror holder. As long as the silver / white bit is circular as it is in the picture you're ok.

[brantuk]

The mirror actually looks quite round and it seems to be concentric with the outer light brown circle which I assume is the "card" you refer to.

The straight edges you describe are the mirror edge (or blackened semi circle) to the left of the reflective surface of your secondary mirror. When you look at the primary through the focuser tube - can you see the three primary mirror clips around it's circumference? If so then you're in business.

Which collimation tool are you using? If it's a laser - all you now need to do is turn the three secondary colimation screws till the beam hits the primary dead center. Then use the primary collimation screws to reflect the beam straight back to the secondary and out the focuser tube.

A dot will appear on the laser target - just turn the primary screws till the beam go's down the black hole in the center of the target and that's it - collimation done. Now pop out and do a star test to check. Hth

[Bathurst]

sorry guys thats a reference picture to show what I mean, that's NOT my scope!

[Charic]

Looks good from here too. The 3 Alan bolts have to be loosened, then the centre bolt is adjusted to move the secondary mirror too or away from the primary mirror, but centred under the focuser tube. all good so far. When you  adjust the centre bolt, you can also rotate the secondary mirror along its axis, so when centred and just tweaking things up a bit, if the mirror is rotated any, the secondary mirror will look elliptical in shape. You picture looks good to me. Once the mirror is centred, next you need to tilt the mirror (3 Alan bolts/locks) till you can see the 3 primary clips from the  main mirror. That's about it for collimation. If you then use a laser, you would finally adjust the primary mirror adjusters until the laser beam goes back into its own hole, or a Barlowed laser (the Barlow diffuses the laser beam) effectively showing a shadow of the primary mirror centre dot . this is then centred around the laser bulls eye. job done

Just seen your update. if there's any flat spot, its possibly due to axial rotation of the secondary, showing an elliptical shape. if In doubt, just start again, following your guide. keep the scope horizontal, so you don't drop any tools onto the primary mirror.

Also in the pic you provided, the flat top from White centre to left and the flat from white bottom to left is just part of the elliptical shape of the secondary mirror, quite normal.

[Bathurst]

OK I carefully took the Secondary mirror out as I was just curious as to its shape and I believe I found out why I cant get a perfect circle (as per the picture in the first post which is a reference not mine!)

With the red line & arrows I can clearly see two straight edges of the mirror, they look like darkened blurs near the red bits. Should they be like that? As before when I get the Secondary to as near a circle as possible it's simply not pointing at the Primary in any shape or form.

Thanks for the responses so far by the way, getting a little stressed at my first time

[Charic]

Don't get stressed with it, have fun, you wont break anything as long as your cautious. not many folk would take the mirror out. Its a brilliant learning curve. You`ll soon be showing others what to do.

Try and re-centre everything and then send us a pic.SGL will put you right, but as in your last pic, it sure looks like a flattened edge to the mirror?

Just realised the plectrum in the corner is your camera/mobile? its all deceiving!

Just hold the mirror in hand at 45°  do you still see flat edges?

[Bathurst]

Just seen your update. if there's any flat spot, its possibly due to axial rotation of the secondary, showing an elliptical shape. if In doubt, just start again, following your guide. keep the scope horizontal, so you don't drop any tools onto the primary mirror.

Also in the pic you provided, the flat top from White centre to left and the flat from white bottom to left is just part of the elliptical shape of the secondary mirror, quite normal.

yeah those flat bits are fine and I see them no problem, but see my post directly below yours.

[Charic]

Is the telescope new, and do the flat points look like they have been physically ground flat? If its new, then I would suggest its meant to be like that. The blur you mention could be absorbed by darkening the whole of the edge of the glass edge. I've read that blackboard paint works well. I still don't think that the 'flats' your seeing would prevent the telescope from functioning correctly. get the secondary mirror back in and try to re-align again. I've just checked on my Skyliner and there are no flat spots.

[cantab]

Yup, since the flats are in the mirror itself, don't worry about them when collimating. Just make the rest of it look circular, not elliptical, and you'll be fine.

And while you have the secondary out, if you were finding the secondary adjustment felt erratic, that can be due to the back of the secondary holder being scratched where the adjustment screws press against it. A washer cut from a milk bottle gives a nice smooth surface for the screws to press on, it made the world of difference on my 4 1/2".

[Bathurst]

Yup, since the flats are in the mirror itself, don't worry about them when collimating. Just make the rest of it look circular, not elliptical, and you'll be fine.

And while you have the secondary out, if you were finding the secondary adjustment felt erratic, that can be due to the back of the secondary holder being scratched where the adjustment screws press against it. A washer cut from a milk bottle gives a nice smooth surface for the screws to press on, it made the world of difference on my 4 1/2".

Just bolded and underlined your bit, that's exactly what I'm talking about, I simply cannot get it circular, it stays elleptical and the closest I get to circular it's not anywhere in line with the Primary, I can visually see it miles out when I look down the tube as well as the Collimation Cap.

Thanks for the help guys I really appreciate it, but I think it's a bit hard to portray just exactly what I'm seeing via text, really need someone to help me in person I think. Spent the best part of 4 hours on this now and no matter what adjusments I make I cannot get that Circle

[wookie1965]

Dont worry ive been there took me 2 days the first time. Take your time and follow all the steps in this tutorial its the one i used but when you do remember that when you first present the secondary circle it may go off slightly but you can get that back with the adjustment screws, another thing make two washers out of a milk carton to put in between the stem of your secondary holder and spider that will help with the collimation.

http://www.schlatter.org/Dad/Astronomy/collimate.htm

[Charic]

....., I simply cannot get it circular?

Looks like you wont by your explanation. If this is a new scope, have the secondary exchanged. If its not new, then as circular as possible with a flat top / bottom should still work fine. Once the secondary is as central as possible, just nip up the Alan adjusters in turn, until you can see the 3 primary mirror clips. remember the secondary adjustment is in two parts. getting it central as possible, then tilting to see the primary clips. Once that's all done, the primary mirror can be tweaked to its final position. You`ve not done anything wrong or broken anything. this will all come as natural as breathing  to you eventually.

[knobby]

If I look at your photo of the secondary ...and turn my ipad about 45 degrees it looks pretty circular.

Don't get hung up with the perfect circle bit , just get it in the middle of the focuser and aim it so you can see the primary mirror.

Get it close and then start tweaking !

[Bathurst]

....., I simply cannot get it circular?

Looks like you wont by your explanation. If this is a new scope, have the secondary exchanged. If its not new, then as circular as possible with a flat top / bottom should still work fine. Once the secondary is as central as possible, just nip up the Alan adjusters in turn, until you can see the 3 primary mirror clips. remember the secondary adjustment is in two parts. getting it central as possible, then tilting to see the primary clips. Once that's all done, the primary mirror can be tweaked to its final position. You`ve not done anything wrong or broken anything. this will all come as natural as breathing  to you eventually.

Exactly my point, hard to get my point across I bought the scope last year so not technically "new", thought i'd give Collimating a go before the skies cleared (sometime in the Summer!!).

I'll start a fresh next time I have a lot of spare time, maybe a second go will enlighten things a little bit but right now with the stress levels hitting maximum I'll best leave it well alone!

[Charic]

iI'm trying to locate a 130P image on Google to see if its a regular issue or a one off. Maybe someone here will look at their 130P to check for any flattening to the edges of the mirror.  I hope you can get some rest. take care

[brantuk]

I've had a think about this and (in general terms) it doesn't actually matter what the shape of the mirror is - it can be octagonal or square or diamond shaped. What does matter is that what you are trying to do is deflect light at 90 degrees so it comes out of the focuser tube dead central. You can do that with any shape mirror so long as the light hits a 45 degree plane and comes off it at 45 degrees (making a 90 degrees angle to the primary).

In telescope terms - the secondary is made elliptical so that when it's tilted at 45 degrees it becomes a circle when you look at it from the 90 degree focuser tube (ie 45 degrees to the plane of the secondary). Also - when you look centrally down the tube - it makes a small circle to minimise the size of the central obstruction and increase light grasp of the aperture.

The fact you have a small flat either side of it is inconsequential. Unless there's anything else wrong with the mirror, I think you just have to position it correctly and you'll be fine. Make it appear as circular and concentric as possible when viewed from the focuser - then refine the tilt using the collimation screws to aim the laser at the center of the primary (assuming you're using a laser collimator). Hope that helps

[Moonshane]

I agree with Kim. In all honesty even if your secondary is slightly miscollimated, you'd be hard pressed to see any difference at the eyepiece if you can get all the primary reflected and that is collimated accurately. a slightly misaligned secondary has hardly any effect in sharpness or contrast, just a tiny effect on brightness. on the other hand a tiny misalignment of the primary makes a big difference to sharpness and contrast.

once set the secondary is unlikely to need a regular adjustment but the primary will need regular adjustment taking seconds once used to it. just get it about right, enjoy the session and then gradually work on your skills to fine tune collimation, concentrating on the primary.

[Charic]

I've sent you a PM to a link that may help a little. Hope it helps.

[Bathurst]

I've had a think about this and (in general terms) it doesn't actually matter what the shape of the mirror is - it can be octagonal or square or diamond shaped. What does matter is that what you are trying to do is deflect light at 90 degrees so it comes out of the focuser tube dead central. You can do that with any shape mirror so long as the light hits a 45 degree plane and comes off it at 45 degrees (making a 90 degrees angle to the primary).

In telescope terms - the secondary is made elliptical so that when it's tilted at 45 degrees it becomes a circle when you look at it from the 90 degree focuser tube (ie 45 degrees to the plane of the secondary). Also - when you look centrally down the tube - it makes a small circle to minimise the size of the central obstruction and increase light grasp of the aperture.

The fact you have a small flat either side of it is inconsequential. Unless there's anything else wrong with the mirror, I think you just have to position it correctly and you'll be fine. Make it appear as circular and concentric as possible when viewed from the focuser - then refine the tilt using the collimation screws to aim the laser at the center of the primary (assuming you're using a laser collimator). Hope that helps

I see what you're saying, think that was my problem last night was the fact it never became a perfect circle (as per every tutorial I've read/watched). Maybe I have got a dodgy Secondary mirror and if anyone has my scope (see sig) I'd be interested to know if you too have the flattened edges as per mine in the post above...just to make sure I'm not going mad!

Nope no laser, just a Cap & a Cheshire.

[brantuk]

Actually - thinking a little more - I just realised that having a flat each side of the secondary has a positive advantage when positioning the mirror. If either of the flats disappears and the other comes across the view - then the mirror isn't square on to the focuser and won't appear circular. Meaning the mirror is slightly rotated away from the focuser tube.

You can use this property to get a good alignment by simply ensuring you can see both flats evenly on either side of the mirror. It'll be perfectly lined up then - and all that's left is the tilt and collimation.

I'm afraid I have no experience with Cheshire collimation tools - I think Shane does - so perhaps he'll come back and comment - or someone else will.

[Astronomical]

I've been struggling with pretty much the same issues. I used the 35mm film canister option which seemed fine...then I bought Cheshire collimator. First problem I couldn't see the primary lugs so it was difficult to align the secondary to the primary. Second I adjusted the secondary so far out that I had to start again! Eventually, using the film canister to align the secondary and then using the Cheshire I got the primary about right. We'll at least I think it's about right.....

[Bathurst]

Think I've got a few hours spare today so will attempt it again later.