Sometimes, you gotta laugh. Otherwise you go crazy.

[cathalferris]

Ordered Destiny 3-vane curved spider for my 8" newt. It arrived, so I disassembled the temporary spider I had been using (using 6mm threaded bars ) and installed said professional spider. I put in a large plate washer as a bearing surface to improve the design - it needed it.

So I went ahead and siliconed the secondary to the new support, leaving it aside to cure for a day or two. Then I reinstalled and collimated - a dream compared to the old way..

Tonight isn't clear but it's clear enough that it would be worth a look to try out and see if I see anything different. So I haul the stuff out to the observing spot at the end of the estate, unpack, connect, put together and start to align. First view through the eyepiece and Vega showed as a huge astigmatic smear, maybe 50 arcsec long

As I know that the mirrors have performed very well in the past I can only presume that the secondary has deformed under the silicone.

I've redone it and I've to wait another day or two to test it.

It was bad enough that the smear of light that was Jupiter was so bad that I could only make out the main bands about 3 mm away from the centre of confusion.

Poo!

[Mr Q]

This may be a 'stupid' question but did you check the colimation before observing with the new spider instalation? It doesn't take much to mis-align the optical path.

[Merlin66]

When I was using the "big" Dobbie's I used matchsticks as spacers when siliconing the secondaries to the support.....sounds like this may be your problem....

Good news is, when you get the support sorted out it will perform 100%!!!!!

[cathalferris]

I had already done the siliconing with matchsticks to the old secondary, and I've got a good combo tool so the collimation was good. I'll find out in a little while, if there are any breaks in the cloud.

I didn't have the matchsticks to hand last week, but I went out and got some to tohe attachment the most recent time. Been curing now for 2 days so I've got fingers crossed that I'll get my pinpoint star images back.

[Merlin66]

A Ronchi grating (RON-Key) helps to see these types of problems....

[cathalferris]

A second attempt to silicone with 3 small dabs behind the mirror led to astigmatism and what appeared to be about 4 waves of undercorrection again. I was more than a little frustrated - tempted to throw the whole thing at the wall! Common sense prevailed - I've gone to the local B&G and picked up a small roll of double sided no-more-nails foam tape. I've put three bits on the diagonal holder, two thicknesses of tape on each point. Now the diagonal mirror is safely fastened to the spider centre post, with enough flexibility to remove any tendency for strain.

I took the time to rinse the primary as it was in need of a dousing with warm water (no rubbing, just a dustoff and soak of the stuff stuck to the primary). I also flattened any high points on the spider centre post by sanding it on a large flat stone - worked well enough as the centre post was a polyethylene type plastic with saw marks on it. Reassembly and recollimation went without a hitch, and looking at Jupiter I can now focus to a point again. And the curved-arm secondary has proven itself as well - no 6-sided diffraction bars around anything.

Overall - I'm now happy enough with the experience, and I can recommend that if you have three 6mm threaded bars as your spider vanes, it's well worth going to the curved vane focuser if you don't already have a good thin-vane secondary.