Primary mirror movement affecting collimation on SW 200P

[TwoPi]

Hi,

Fitted my new MoonLite to my SW200P yesterday and after some initial confusion about which holes in the Newtonian installation kit to use (and a brief period when the kit was upside down so the focuser was half the secondary mirror's diameter out of alignment! ) it's now all in and I'm very chuffed with it.

I then went through a full collimation exercise and after getting it bang on with the OTA close to horizontal, I then moved the scope towards the zenith and heard a slight clunk. After looking through my cheshire I realised the collimation dot was now outside the doughnut marked on the primary. It occurred to me that I have heard this clunk when moving the scope before, but never really taken any notice of it.

Anyway, after a bit of playing around I realised the primary mirror is shifting slightly when moving the scope between 0 and 90deg altitude - I can feel it when I hear the clunk.

Just done a search on SGL and read the various lengths people have gone to to solve this problem. In the end I've loosened the collimation locking allen nuts so they're no longer engaged, tightened the collimation screws all the way, backed them off about half a turn and then re-collimated the primary mirror with the scope at about 45deg altitude.

As a result, there's no longer a clunk or huge shift in collimation; instead I would say the centre spot moves from one side of the doughnut to the other when moving the scope from 90deg to 0deg altitude, with it being slap bang in the centre at 45deg.

I have two questions:

1. Is this amount of movement significant / am I likely to notice it when observing (not had chance to try out a star test yet)?

2. Is there anything further I can do without taking the mirror out of the OTA, something I'm not particularly keen to try to be honest...

Thanks.

[NGC 1502]

Hi Gav. If you are sure the issue was the primary mirror shifting (and from what you have tried that did help, it sounds like what was happening) then I'd guess there's not a lot further you can do without some dismantling.

I'd try it at night, do an out of focus star collimation check, see how it goes and if the views are affected by any minor shifting of the primary.

You may be fine, no further action needed

If more work is needed, you could consider the following -

You could fit stiffer collimation springs. If the springs can be compressed between your fingers, they are not stiff enough. You may have to hunt around to find suitable ones.

It's possible there is minor radial movement of the primary within its cell. Some thin packing pieces (tape etc) could be fitted between the edge of the primary and the cell. You need to leave a very tiny amount of slack to ensure the mirror is not being pinched. Careful you don't clamp the mirror in place with the mirror clips for the same reason.

Hope you sort it, Ed.

[Moonshane]

that's a fair amount of movement. I'd try to resolve it if possible. it may also be your secondary vanes or even your central bolt/secondary adjusters. are they all reasonably tight? if not the secondary can sag and move about causing the same effect. as Ed says a few lateral packers should short you out but you'd need to take the mirror and cell out - not as scary as it seems, just take your time.

[NGC 1502]

Hi Gav, further thoughts. As Shane has said in post #3, check secondary vanes etc.

I'd also check for focuser drawtube slack, or if your cheshire is a good fit in the focuser, this could very easily cause what you are seeing with collimation shift.

Regards, Ed.

[Tinker1947]

I did have movement with my Primary mirror, solved the problem by screwing the primary mirror screws right in then collimating from there, my mirror movement was due to the slack in the adjusters as there were right near the end on the thread....

[laser_jock99]

I found x-y mirror shift to be a problem with my 12" F4 Newt. The link below shows how I sorted it:

I can slew this big scope around the sky now and the collimation barely shifts a whisker. Usually a 1/8th turn of one adjuster screw will bring it back on. With a sensitive F2.9 set up I routinely need to check collimation before each imaging run anyway. The fact it now holds so well is a real time saver.

[TwoPi]

Thanks for all your replies, very informative & encouraging as usual.

The secondary mirror & vanes seem secure (and of course the cheshire fits like a glove in my new MoonLite ) so I tried tightening the collimation screws still further but the problem persisted.

Finally I bit the bullet and removed the cell from the OTA. I discovered that one of the three clips was looser than the others and I could push the mirror against it. Tightening it up, re-fitting the cell and then re-collimating... hey presto, solid as a rock now - the cheshire dot stays dead centre!

Wasn't sure how to tell if I was pinching the mirror though, so bit worried I might have tightened the errant clip *too* much. I tried to just keep tightening slightly and seeing if I could still move the mirror. Guess a star test will be the only way to find out...

Thanks again for your help.

[laser_jock99]

Yep- check a slightly out of focus star and make sure it's round- anything else could indicate a problem. Also make sure the scope is cooled down first. Tube currents give strange effects too!

[NGC 1502]

Thanks for all your replies, very informative & encouraging as usual.

The secondary mirror & vanes seem secure (and of course the cheshire fits like a glove in my new MoonLite ) so I tried tightening the collimation screws still further but the problem persisted.

Finally I bit the bullet and removed the cell from the OTA. I discovered that one of the three clips was looser than the others and I could push the mirror against it. Tightening it up, re-fitting the cell and then re-collimating... hey presto, solid as a rock now - the cheshire dot stays dead centre!

Wasn't sure how to tell if I was pinching the mirror though, so bit worried I might have tightened the errant clip *too* much. I tried to just keep tightening slightly and seeing if I could still move the mirror. Guess a star test will be the only way to find out...

Thanks again for your help.

Hi Gav, glad you seem to have sorted it. I'd run the check in post #8.

It's right to worry about clamping the mirror, but too much slack is an issue as well, as you have found.

Often mentioned is a 'credit card' gap between mirror and clip, but I think that's too much, just a 'smidge' is good.

Regards, Ed.

[TwoPi]

Thanks Ed, last night I tried the test laser_jock99 suggested and the central circles all seem nice and concentric with no obvious pinch points (though seeing wasn't great so outer circles were quite turbulent).

Does make me wonder if the telescope was ever collimated before it left the shop, though I guess screws can become loose in transport.

Anyway, happy now!

[Geordie mc]

Knowing some courier firms its a miracle it's in one piece never mind collimated and securely set up.I had to collect mine from the Fed-Ex depot. As I stood waiting some double doors were kicked open followed by my mount box being kicked along the ground while the ota box was thrown through the air to land at my feet. Unbelievable!