Just when I thought I'd sussed it...

[Ajohn]

I should add another comment. Scopes are manufactured some of it automatically and those parts don't usually cause much of a problem. Putting the lot together is something else. People make mistakes and somes time get fed up and don't even bother. Only solution is to have some one else check all of the work - they are likely to have the same problem. Some companies give people a yorki bar for finding a fault - that doesn't work either. Some companies produce interferometry chart with the optics in a jig - not in the scope.

Put it all together and that's why I think dealers should collimate all scopes at the point of sale. They will even find that they should send some of them back.

John

[Astro_Baby]

Having been put therough the mill with collimation myself recently ( I am pretty new to it but learn fast ) can I make a suggestion.

I think the offset issue is a red herring, I also think your collimation is so far out a laser wont help but will actually hinder. My laser is ok for fine tuning but its nigh on impossible to get the collimation into the ball park using the laser.

My suggestion would be this - and it worked on my SW200 that arrived hoplessly out of collimation.

1/ Centre the 2ndry in the back/forward direction. Put a piece of white card down the tube between the primary and the 2ndry to block the secondary. NOw put your eye to the focuser. IS the 2ndry more or less centred to the focuser tube - you should see the 2ndry as a white disc ( reflecting the white card ) againts the black background of the focuser tube and the opposite side of the scope tube. If its not then loosen the tilt screws and wind the 2ndry up or down on its main screw to get it central and more or less face on - ie you should be seeing a circle.

2/ Now use a colimation cap to fine tune the secondary - to get all the usal done, can you see the 3 primary mirror clips etc etc. Dont worry about fancy ultra precision collimation at this stage - you need to get it into the ballpark first. JUst aim to get the 3 mirror clips into view ( you may need to lose the coliimation cap for this and do it just with your eyeball at the focus tube. Go for the three mirror slips and the dot as close as you can manage to centre. It wont centre most likley as the primary is probably out of whack but I always try for best centre I canb manage with all three mirror clips showing.

3/ Primary mirror - adjust to the best collimation pattern you can get - centre dots aligned. The classic bullseye. Its a pain with a colli cap or Cheshire cos its so damn hard to adjust and see what your doing. Little by little does the trick here. You should be able to reach at least one of the primary collimation screws while looking through the cheshire/colli cap though.

4. retune secondary if needed.

Personally I think you have a rotation error on the 2ndry - ie its turned slightly.

By the way I have given up using a laser on my 8" - it ALWAYS produces erroroneous results for no reason I can understand ( I have checked and the lasers collimation is more or less spot on ) BUT in the scope it always produces a strange set up. I just stick to a Cheshire with the 8" now.

In the little 5.1" the laser and cheshire always agree - in the 8" they never do so I have abandoned the laser.

[Andrew*]

What scope is it anyway?

Skywatcher 8" newtonian.

Andrew, everything you need to know is in Tony4563 and KK's last posts. I also suspect that your secondary mirror has swivelled a bit on its axis, if that doesn't resolve things check that the circle/marker in the centre of your primary is actually central. It is rare but occasionally they can be placed a few millimetres off-centre during manufacture.

I will check that. The second

Get the secondary centred in the focus tube by eye, reflecting the whole of the primary, and all this clever stuff happens automagically. Use a laser and you'll not be far out either, probably within 1/10th of a degree of the absolute correct secondary angle if the laser dot is even close to the primary centre.

My suspicion is that your secondary mirror is rotated along the 'scope axis by a tiny amount, so try turning it by a tiny amount and see what happens.

Forget the theory, thats only needed to design the thing, not to collimate it, but it helps if you know what the offset is there for, so you can understand why it looks lopsided from certain angles. If the design is correct, like in the SW 200mm Newt., you don't have to reshape the secondary to get it to look round, just move it until it looks round, end of.

That's a great explanation.

I know for a fact that it's partly an issue of the secondary being rotated. What I do is loosen the secondary central screw and rotate it while looking through the focus tube, until I can see the entire primary, then tighten it again and re-check (the action of tightening rotates the secondary). When I can see the entire primary, the secondary is not round, nor is the reflection of it from the primary. It is elliptical, with the bulges at an angle of perhaps 20° to the angle of the secondary support, as seen from the focus point. When I try to correct this using the allan key nuts, it just starts obscuring the primary again.

When inserting the collimator, the dot is off by an inch. Clearly the secondary is off at an angle. So in the same manner as before, I rotate the secondary back so the dot is on the primary centre. Either that, or I do it by adjusting the angle using the allan key bolts. Looking through my film canister again, the primary is not entirely visible.

I keep on going back and forth. Correcting by sight, then with the collimator, then by sight again. Sometimes I correct the angle, sometimes the rotation, sometimes both, but every time, the same pattern emerges. The dot's off by the same amount in the same direction, or the primary's not visible, also always by the same amount in the same direction.

What I also did in between was bring the secondary closer to the top of the tube, which made it appear more central in the focuser tube. This didn't help. And as I've said, the spider adjustment knobs are out of thread travel.

Can you take a picture of the 2ndry and post it?

Yes, I'll take some pics next time I have the courage to open that tube cap! I'm sure they'll explain the situation better and quicker than words!

Thanks all for your help

Andrew

[Kaptain Klevtsov]

OK, to get the secondary rotation sorted (along the OTA axis, use a small hand mirror, there's one in her bag BTW, to look at the gap where the secondary mirror adjustment happens. It will probably be wedge shaped at the moment, but you need it so that all three adjustment screws are equal. That gets the 45 degree bit close. Next, turn the secondary assembly until you get a close ish alignment and can see the primary mirror when viewing through the focus tube. That will get the rotation close enough to not matter.

Kaptain Klevtsov

[Astro_Baby]

Hmmm - the eliptal issue happened with that dodgy first SW 200P that I had delivered. Some of these problems seem the same especially the elliptical issue - I never got to the bottom of it because in the end what with all the other probs I decided to send the tube back.

Question - is it a new unit or old ? And wahere are you - anywehere near someone who could take a look with you ?

[Ajohn]

When you have adjusted the 2ndry holder for no wedge as per Kaptain you may find that you have to move the 2ndry off center away from the focuser before you can see the mirror even both ways in it with the 2ndry level with the focuser tube. Please let me know. I'm curious as to how many manufacturers have gone that way as per that 10ins F4.3 meade I have. With the meade the 2ndry has to be offset and down too so that it isn't central in the focuser. That's also why a usual laser align messed up the scope. It might also be why a number of people have serious problems with lasers.

Having to sort out offsets away and down will be a bit of a pain so I suggested working out what the offset away from the focuser should be. The scope only needs to be very very roughly aligned to measure where the focal plain is. Given that one offset is right the rest should be easy. I'm 100% sure even thinking about 2 offsets would make my head hurt.

John

[Andrew*]

Sorry for taking time getting these pics.

This is as good as I can get the secondary by sight:

(click to enlarge)

I would say it looks fairly good (disregard the primary!). However, it's not! The laser dot is about an inch off.

Please continue to offer your advice!

Andrew

[M42]

Hi Andrew. You don't say which scope you have. This may sound as though it is not too important but it is. I will explain.

I own a Meade 10inch Schmidt-Newtonian. This has a corrector plate on the front to which the secondary mirror is attached.

I had a hell of a time trying to collimate mine a couple of years back. In the end it turn out to be the secondary was rotated a little bit out of alignment. The cure was simple, just take hold of the secondary mirror cell and turn it back to alignment. Then collimate using a laser and finally doing a star test.

Vega is nicely placed at this time for star testing. If you finf Vega too bright then either use Capella or Arcturus.

I wouldn't rely too much on a laser unless it is a very good/expensive one. There are a lot of cheap ones out there and they are not aligned properly. They can of course be adjusted. This involves a flat surface and a mirror that is known to be square on to the laser, or a line known to be horizontal drawn on a wall.

If you have a long tube newt you may need help with adjusting the primary mirror screws as you look through the eyepiece.

DO NOT forget to loosen the locking screws first! A common mistake made by people who don't know what they are doing.

As for adjusting the focuser this is a little bothersome but this is how you go about it.

Remove the secondary. If you can remove the spider vanes with the secondary in place all the better.

Now you need to mark the inside of the tube directly opposite the focuser. You MUST be spot on here. Now put your eye right in the focuser tube. If the spot is not in the center loosen slightly the mounting screws for the focuser and twist push pull the focuser until the spot is centered. Now tighten the mounting screws. Your focuser is now square to your tube.

Reattach spider vanes and secondary, and collimate as usual.

That should sort you. If you are still have problems get in touch with your local society and see if they can help.

Keith

[M42]

Sorry forgot to mention something. As for the hole reinforcer that has been placed on the mirror to mark the center. IGNORE IT! They are very often NOT in the center of the mirror. Remember these are made on a production line in China, where someone is working for a pittance. They are not going to be too worried if they get it off by a little bit. I am sure the one on my scope is off by about 5mm. In fact my laser says the center of the mirror is right in the middle of the side of the reinforcer. But all the mirrors are visually aligned and a star test confirms my collimation is just about perfect.

Keith

[M42]

Hi Andrew have a look at this website. It is a pdf file. Scroll down to page 44.

http://www.meade.com/manuals/TelescopeManuals/LXDseries/LXD-75C.pdf

Keith

[Ajohn]

Looking at the picture something looks wrong with the 2ndry position in the tube. Measure the distance from the tube to the 2ndry holder along the vanes starting on the focuser side and working round clockwise.

Also have you squared up the bottom of the 2ndry holder as per kaptains comments?

John

[Andrew*]

Looking at the picture something looks wrong with the 2ndry position in the tube. Measure the distance from the tube to the 2ndry holder along the vanes starting on the focuser side and working round clockwise.

Also have you squared up the bottom of the 2ndry holder as per kaptains comments?

That's a good idea...

Yes, it's even all the way round.

Andrew

[Kaptain Klevtsov]

Coming back to this again and again now. Am I correct in thinking that the mirror clips which are visible in the photo should be equi-distant from the centre circle? The thing is, they don't look to be to me.

Can you post a picture of how it looks when the laser says its good, and also try turning the laser 180 degrees in the focuser to double check its collimated itself.

Kaptain klevtsov

[Ajohn]

Best get that movable part of the 2ndry level with the rest of the holder 1st and post another photo afterwards if it isn't. But if that's ok -

Will the 2ndry go down further than that so that the image of the mirror is central in it? Forget the spot for a while. From the picture it's obvious that the outline of the 2ndry mirror isn't central to the focuser. No point in doing anything until it is. I repeat forget the mirrors image in the 2ndry for a while.

Looks like the tube is flocked - has the scope been taken completely apart?

John