Secondary mirror

[teztwister]

Just a quick question???

Was just wondering if the secondary mirror had to be exactly centered,would a millimeter or a couple of mil make much difference if of center to one side or not.

I mean on a SCT!!

Tez

[barkis]

Tez. The main mirror will have a lateral movement of more than a couple of millimetres due to mirror flop. This flop can be noticed when using the scopes own focuser. You will notice a significant image shift when a change of focus direction occurs. This is normal of course, but when a mirror lock is in place, and either an electric or Crayford focuser is added, no shift happens.

The scopes collimation when tweaked to perfection, should normally stay that way. However. Collimation checks should be done on a regular basis, ensuring maximum performance at all times.

A star test at a fairly high power will show coma if the collimation is not spot on

Ron.

[teztwister]

This is the kind of thing that i mean,if the mirror was just offset would it make a whole lot of difference??

tez

[GazOC]

Ideally it needs correcting but TBH thats the sort of thing I'd try to fix on a cloudy night but I wouldn't let it get in the way of actually observing....

[barkis]

If you looseen all the corrector plate screws, just loosen, not remove, you could perhaps have enough movement to centralise the secondary cell.?

On a cloudy night, as Gaz suggested.

Ron.

[GazOC]

There is no retaining screw on some SCTs so be VERY careful or the secondary will literally fall off if you undo all 3 screws too far.

[barkis]

Woah! I was not saying loosen the Secondary screws, just the Correctors periphery screws. Loosen, not remove. I am not certain there will be enough movement of the plate to allow centralising the secondary cell

[GazOC]

Ah, gotcha Ron!!

[MikeP]

Dunno how LX90's are put together, but I'll describe my C11 which should be somewhat similar.

The corrector has a ring of metal round the periphery with about 8 screws holding it in place. If I remove that ring, the corrector is free to be lifted out and would fall out unless the OTA is upright.

The corrector sits in a kind of rebate, so there is a gap all the way round ie the diameter of the rebate is greater than the diameter of the corrector. My C11 has 4 tiny pieces of what looks like cork spaced evenly around to stop it sliding around in the rebate - it is possible yours needs something like that to centralise it.

Now two cautions ....

Do not rotate the corrector in the rebate - mine has an index mark on the tube and on the glass. I believe this is to ensure the secondary remains in the correct orientation relative to the primary.

Do not overtighten the screws that hold the corrector retaining ring in place - they should not be loose but overtightening can cause distortion of the corrector.

I have no idea whether your secondary being off centre is a problem ie collimating may fix any problems you have with such a small offset, but if you do decide to try to move the corrector over by 1mm, be very careful. A mistake could be quite expensive.

Mike

[barkis]

Good un. Mike, I should have stressed the do not rotate the plate Also.

Ron.