Star testing for Cats?

cloud_cover · Post by **cloud_cover** » Mon Mar 21, 2011 5:03 pm

I'm curious whether my VMC is poorly collimated or something
When I defocus on a bright star, I see an almost symmetrical circle with just a bit more oblongish on one side but that's about as much as I can get.
Occasionally I get a bit of shield-shape diffraction rings, sort of pointed in one direction.
Of late I've managed to sort that out and the diffraction pattern is about as circular as I can get it. Not perfect, but almost
However, I notice when using a Nagler 9mm EP (power 217x), stars tend not to come to sharp focus: it seems like they are splayed out a little with spikes coming out in this or that direction.
Wondering is that a problem with my scope's optics, collimation, eyepiece or just poor seeing?
In contrast stars were pinpoint sharp in my Onyx but of course its focal length was just 500mm so the highest power I've ever went to was 139x
Having said that, I did manage to clearly split Rigel Kentarus and Alpha Crux although I'm guessing that's not difficult (was star testing - noticed 2 diffraction rings and then 2 stars when in focus. Nice.

Gary · Post by **Gary** » Mon Mar 21, 2011 5:45 pm

Maybe can try star testing your VMC together with Shirox's side-by-side. Perhaps this can reduce the effects of seeing from influencing your analysis. I think Shirox may also be interested how well his VMC perform in a star test.

So Lorong Halus Wetland ob session soon? Please invite me along cause I am also curious to find out.

cloud_cover · Post by **cloud_cover** » Mon Mar 21, 2011 5:52 pm

Sounds good. sometime this week?

But you'll need transport to get to the wetlands and its quite brightly lit up by outdoor lamps - the white light kind

Gary · Post by **Gary** » Mon Mar 21, 2011 5:55 pm

Sounds good! Congrats to Shirox. He just sold his AZ mount. His spanking new toy will be here in about 2 weeks.

I can bring my PORTA mount along for his VMC in the meatime.

So what say you shirox?

orly_andico · Post by **orly_andico** » Mon Mar 21, 2011 7:50 pm

kevin

the spikes are due to the spider vanes!

that's why the refractor doesn't have them.

cloud_cover · Post by **cloud_cover** » Mon Mar 21, 2011 7:56 pm

Erm, those are not cross-spikes. I see them only on bright stars. They're more like coma (except the VMC is not supposed to have them, at least not in the center of FOV at f/9.75) and stars don't quite focus into pinpoints. They're more like smudges.