GSO 8" Newtonian mods

[multiweb (Marc)]

Originally Posted by RobF

Great work Marc. Sounds like you're stretching every bit of performance out of her!

Yeah I think it'll do nicely and give me some very nice pictures. It required some stiffening mechanically but optically it's fine. I've just sorted the OAG last night so the next test should be on the money

[Lee]

Looks like a good idea... will have to try this....
This week after my abortion of an attempt with CCD Inspector, I collimated with the laser, then tightened up the locking screws, only to get this rubbish....

Everything looked better after loosening off the locking screws....

Are RC's this much trouble??

[multiweb (Marc)]

Originally Posted by Lee

Looks like a good idea... will have to try this....
This week after my abortion of an attempt with CCD Inspector, I collimated with the laser, then tightened up the locking screws, only to get this rubbish....

Everything looked better after loosening off the locking screws....

Are RC's this much trouble??

Is that a corner at 1:1? It doesn't look that bad. Are you sure it's not field rotation? What was the sub length. GSO RCs use a corrector as well I believe so maybe spacing needs tweaking?

[Lee]

1:1 near centre frame.. sub length 10s... I think the tightening pinched the optics somehow??

[multiweb (Marc)]

Originally Posted by Lee

1:1 near centre frame.. sub length 10s... I think the tightening pinched the optics somehow??

ah... yeah if it's center frame then you have an issue. What does the out of focus pattern look like, both in and out of focus? Is it oblong?

[Lee]

I haven't checked into out of focus patterns.... Although during focusing at the start of the night, the donuts are round (fairly)...

[Lee]

Well... thanks to this helpful post I've installed some stiffer springs, some C-700's from CSC spring company (Bunnings have the range). They really hold the mirror cell a lot stronger than the standard ones, it really takes an effort to compress them, the standard ones compress under the mirrors weight....
I've left out the locking bolts, and am getting round stars across fields from one side of the sky to the other so far....
Thanks for the post....
I still think I need a better spider....

[cometcatcher (Kevin)]

Interesting thread.

Way back when I was making newtonians and mirror mounts, I would use valve springs from the car wreckers. This worked for all scopes from 6" to 16".

I am thinking of getting a GSO 8" F5 for imaging. They make these things much cheaper than I could build one.

[Wavytone]

My first 8" Newtonian used valve springs in the primary mirror cell, but like most things with springs it never remained collimated for long as the wing nuts on the back would shift. Not a great idea IMHO.

In the end I removed the springs and used lock nuts on the inside of each threaded rod, opposing the wing nuts. A bit more fiddly to adjust during collimation, but once done it DID remain collimated - well enough that I could remove the entire cell from the tube to clean the mirror, put it back and no adjustment was needed.

[Shiraz (Ray)]

just to say thanks. following earlier discussion on this thread, decided to re-check my secondary. The darn thing has developed astigmatism and there is now nothing I can do to get it to be flat - guess it must have been made out of poorly annealed material and recently decided that it would de-stress itself. Anyway, your thread has been very helpful regards ray

[multiweb (Marc)]

Originally Posted by Shiraz

just to say thanks. following earlier discussion on this thread, decided to re-check my secondary. The darn thing has developed astigmatism and there is now nothing I can do to get it to be flat - guess it must have been made out of poorly annealed material and recently decided that it would de-stress itself. Anyway, your thread has been very helpful regards ray

That's really unusual Ray. Never heard of plastic deformation with glass. I could be wrong but I've had stressed glass always going back to its original shape in the past. Have you tried to stick it in the freezer for a couple of days?

[Shiraz (Ray)]

Beats me?? I used this secondary in a planetary scope for a couple of years and would undoubtedly have noticed if it had any problems - it certainly does now though.

I am not intending to stick it in the freezer - will use the flip top receptacle next to it - only $50 for a new one

EDIT: Found this on a mirror blank site http://castle-emerald.com/ This is important because when glass is manufactured, it invariably includes internal stresses (which are impossible to avoid) and as these stressed regions exert internal pressures within the glass, they act to deform the mirror over time. During the re-annealing cycle that our blanks are subjected to, these strains are allowed to ease leaving a blank that will not deform over time.

Maybe it can happen - hope the primary is made of sterner stuff