Re: [ATM] Help with star test interpretation - TUE,SA or something else?

["David Harbour" <scarab2@cox.net>]

Jay,

A harder, more intense outside perimeter to your inside focus extrafocal
diffraction disc is is an indication of overcorrection; also, I am
suspicious that the "crest" of your "high" zone is not at the .7071r
location (radius wise on your mirror). Overcorrection is childishly easy to
repair; you should truly use Foucault to guide you. Please read this
article:

http://www.atmsite.org/contrib/Harbour/Foucault.html

If, after a survey with Foucault, your mirror does show "flatter" (i.e.,
"irregular") outer regions, they are childishly easy to fix, too. Look at my
article(s) on area formatted ring laps, here:

http://www.gordtulloch.com/harbour/index.html

The two articles you should look at (about using Zulu laps) are: "Figuring
With the Area Formatted Ring Lap" and: "Some Basics".

You should be able to bring that mirror to book in a single afternoon, pal!

Davey

----- Original Message ----- 
From: "J Killea" <jkillea@yahoo.com>
To: <atm@atmlist.net>
Sent: Saturday, June 25, 2005 12:17 PM
Subject: [ATM] Help with star test interpretation - TUE,SA or something
else?


>
> It's been pretty gratifying trying to star test my uncoated 8" f/6.4  ( f
= 1300mm).  Mirror seems passable but on my first star test I instantly
realized that I was going to need help interpreting these patterns.  Any
thoughts are welcomed, so here goes, with no photos...
>
> First off, I have had the most success testing on actual stars - my
artificial star setup is a royal pain to get working.  Virtually impossible
to find the darned glinty thing using my 5mm EP, and when I do, stabilizing
the scope at horizontal is hard.
>
> Stars:
> Inside focus I get a bright outer ring.  Inner rings hard to distinguish,
but maybe I'm just not experienced enough, maybe I've got slight dew on the
mirror (don't know),  plus it's hard to keep up with the star at this
magnification, so image is always in motion.  Polaris is OK, but hampered by
Bay Area light pollution.
>
> Outside focus, the bright ring has moved in toward center just a little
and there is a bit of haze around the outside of it.  Haze portion alone is
just slightly wider than bright ring itself.  Inner rings are just a bit
easier to see.
>
> Jupiter:
> 4 moons easily visible, 2 dark planetary bands.  No red spot visible, but
I think it's been absent during my observing times.  But here's what I'm
concerned about:  just beyond the planet's edge ("limb"?) is nice black,
dark space, but then there is some glow from the panet's brightness outside
of that region.  (Thinking back, that dark gap might be seen with the 32mm
EP, and no dark gap with the 5mm EP.)
>
> My questionable diagnosis:
> Star test indicates either, TUE or general SA from undercorrection.  Could
a shallow, low zone also cause these effects ( at about 75% radius)?
>
> Planetary haze bugs me - seems like the classically described effect of
TDE.  Not affected by turbulence, but could atmospheric moisture contribute
much to this effect?  Or just humidity from the lawn I am standing on? Dew??
>
> Last thing:  a fine-control mechanism for this Dob would be very useful -
any thoughts?  (The bearings are smooth, but still feel like I need better
control especially at 260x  !)
>
> Thanks for the help,
> Jay
>
>
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