[AZ-Observing] Re: Morning meeting with Mars...., first light on Mars (again!)
- From: Joe Larkin <joeclarkin@xxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx
- Date: Fri, 4 Jul 2003 12:37:09 -0700 (PDT)
Some of the best planetary images I've seen were with a very thin
(maybe 1") 17" mirror. So I'm not really afraid of a thin mirror. I
was surprised to see this mirror, though. It looks like a potato chip
next to a full thickness mirror. The weight difference is significant
and does make the tube noticably lighter.
I'm mostly concerned with deformation. I was expecting a 1.25 inch
thick mirror. My research indicates a 9 point floatation cell should
be okay, and my observations seem to say that images are acceptable
at least. To me, acceptable is a mirror that can take ~25x per inch
without much in focus degradation. I would prefer better, and this
miror may be better, but I need more time to find that out. Seeing
held me back more than anything else. But I think I need another
eyepiece or barlow so my next magnification step is a little smaller
than 230x to 400x.
I do know the theory of star testing, but the differences always seem
subtle to me, even on a poor mirror. Seeing wasn't up to it last
night. There were no stars visible this morning, it was too bright. I
tracked Mars past sunrise and it still looked decent.
I think the collimation was decent. I need to add a center spot to
the mirror, but even without it, things were concentric in a chesire
sight. Further tweaking at high power helped a bit.
I think I am going to keep this mirror. It is quite an improvement
over my previous mirror. It was a good price and was delivered pretty
quickly. I wanted a decent mirror for Mars, and I think I've got
that. My next telescope plans may be to send my original mirror out
for refiguring. But if this mirror is on the better side of good, I
may just keep using it and then my next upgrade would be a decent 15+
inch dob.
Joe Larkin
--- Jack Jones <spicastar@xxxxxxxxxxxxxxx> wrote:
> Some people swear by a thinner mirror and regard it a plus. The
> weight
> savings alone is an advantage, not to mention faster cool-down. If
> you
> can pick off details on Mars I think you have a good one there. As
> concentricity in the rings is what to look for when collimating, so
> is
> symmetrical ring images when going inward and outward from the
> focal
> plane. If you can't tell the slightly inward focus from the
> slightly
> outside focus image, it's a great mirror. If inside is different
> then
> there could be a little spherical abberation or over/under
> correction,
> but it's a tough test anyway and should be forgotten in the
> interest of
> peace of mind as long as you see good detail on planets etc.
>
> Jack Jones
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