Why is exact aligning of the Corrector Plate and the Mirror cell necessary?

[Bill Christie]

When I broke my corrector plate, Meade would not supply another.  I had to buy another telescope.  The reason they gave was that the corrector plate and the mirror have to be perfectly aligned in the factory. 

My question is what is the exact technical reason for this?

[DSOMAN]

The reality is that the telescope is assembled in China and the corrector plate is attached there with minimum alignment. There is no technical reason that a replacement corrector plate can't be installed by the owner. The corrector is symmetric and rotational alignment is not needed. Same for the mirror.

The corrector plate does need to be centered and flat relative to the mirror however. I don't even know if they do this well in China as I have seen several SN scopes where the corrector was off center a little.

The good news is that the corrector alignment is somewhat forgiving.

Brent

[Mark Sibole]

Actually all corrector plates have an alignment mark on them.
It is used to the best alignment to the primary mirror
This reduced coma and other things
So yes generally each scope has to be aligned with the marks to make sure everything is correctly seated.
Most manufactures will not sell you a corrector plate and require you to send it back to them for repairs.

[Bill Christie]

Thanks.  So what I'm hearing from you folks is:

a) that the corrector plate is symmetrical about the optical axis
b) this means it does not need to be rotated about the optical axis to any special angle
c) It is important that the corrector and the mirror are mechanically aligned (i.e. in alignment like the two ends of a cylindrical tube)

If this is so, then surely Meade should've just been able to sell me a corrector plate, so that I could just remove and replace my broken one.  I would've been happy with that.  Instead I had to either ship the scope all the way to Meade for repair or (as I did) buy a whole new replacement telescope.

I get the feeling I had to spend money unnecessarily.

[Mark Sibole]

From my understanding the corrector plate is matched to the prinary for best preformance
Like matched optics.
All I can go by on this is what Ive read and have been told in the past.

Regards

Mark

[DSOMAN]

Bill,

Don't feel too bad. I don't think any manufacturer would sell you a corrector or mirror. They probably feel that most people would not be able to install it correctly and they are probably right.

The SN's have really great optics and is a great scope for astrophotography once you replace the stock focuser. There are also some mods you can do to the mirror cell to improve performance.

Brent

[Bill Christie]

Well, in the end, I did have to go and buy a whole new SN-8 telescope.  Never mind that all I needed was a new corrector plate. Just because us dumb users are not technical enough to unscrew screws and insert a new one.

As with all Meade SN-8's the wobbly rack and pinion focuser assembly is just so bad that it had to be replaced.  Few people who are serious about astrophotography or deep sky viewing do not replace that terrible focuser.  I now have two of these duds sitting on my bookshelf.  It seems that everyone just accepts that the end-users have just got to learn how to replace focusers, which coincidentally involves removing and replacing the corrector plate!

 My question is, given that a half-decent focuser is so critical to a scope's performance (not only for photography) and given also that Crayford focusers abound in even cheap scopes, why oh, why doesn't Meade just put a simple Crayford in a Meade SN-8 and be done with it. 

It would save hundreds of SN users having to do it themselves!  Think of the great PR when Meade made the announcement.  The market interest in Meade SN-8's, especially for photography, would greatly increase as a result, I'm very sure.

Regards,
Bill Christie
zodiaclight.com

[MistrBadgr]

It will depend somethat on the focal length of the primary mirror as to how critical positioning is.  I thought I was pretty good at such things until I started working with a ten inch LX-200 Classic.  I learned a lot over the 30 or so actual hours put in on just getting the corrector plate in the right spot.  The big thing I ran into was getting the secondary mirror centered correctly.  I was finally able to do so well enough when I think we had it within a quarter of a millimeter of dead center.  I know with a 1 mm deviation, there was no way the scope could be collimated.  We had excessive coma no mater where we moved the center spot.  We could just get so close to removing coma with a star collimation and then the coma would simply pop to the other side with the next tiniest move.

Hope this helps,

Bill Steen