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How good is good enough?



 
 
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  #11  
Old February 14th 04, 10:58 PM
Thad Floryan
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Default How good is good enough?

@hpl.hp.com () wrote in message ...
I recently got an Orion 127 mm Mak-Cas telescope, and although the images
seem to be pretty good, I was wondering are there any tests for determining
if a given telescope is performing as well as it should? How sharp a point
should a star focus to? It seems to depend on the brightness of the star
too. At high (150X) magnification, I do see little rings around some stars:
are these the Airy circles? I'm new at this so any pointers would be
appreciated.


Don't laugh, but daytime testing can reveal glaring deficiencies in a scope.

One of my "tricks" after reassembling a scope is to attempt to read the label
and serial number of a microwave dish about 1/4 mile away from my back yard.
You could do the same for gear mounted on distant power or telephone poles.

I recently did this with my Orion Apex 90 after I flocked its baffle tube and
did a rough indoors collimation and, by golly, the digits in the serial number
were crystal clear and I only needed to perform a minor additional tweak later
that day (actually night :-) with a star test.

Another test is examining leaves on a distant tree and see if you can detect
the veins in the leaves and, hopefully, the absence of any blue/purple color
fringing around the leaf's edges; this is also a good test for refractors.

FWIW, the Apex/StarMax 127 (I have one, too) is overall a very good performer
and there are some exquisite astrophotos taken using them that people have
posted to the "OSAO" Yahoo group.
  #12  
Old February 15th 04, 07:11 PM
Harry Leopold
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Default How good is good enough?

On Sat, 14 Feb 2004 9:26:54 -0600, Rod Mollise wrote
(in message ):

snip

Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_


BTW, Rod, I just ordered your book. Diane gave me a gift certificate at
Amazon and told me to order a good book on telescopes or astronomy.

It is amazing how some people react to getting a big bunch of roses for Feb.
14. ;-)

Especially if they don't have to remind you.

--
Harry F. Leopold
aa #2076
AA/Vet #4
The Prints of Darkness

The Internet is full, we can not accept any more posts until further
notice. Thank you. - Kalinka Djnepropetrovska

  #13  
Old February 15th 04, 07:11 PM
Harry Leopold
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Posts: n/a
Default How good is good enough?

On Sat, 14 Feb 2004 9:26:54 -0600, Rod Mollise wrote
(in message ):

snip

Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_


BTW, Rod, I just ordered your book. Diane gave me a gift certificate at
Amazon and told me to order a good book on telescopes or astronomy.

It is amazing how some people react to getting a big bunch of roses for Feb.
14. ;-)

Especially if they don't have to remind you.

--
Harry F. Leopold
aa #2076
AA/Vet #4
The Prints of Darkness

The Internet is full, we can not accept any more posts until further
notice. Thank you. - Kalinka Djnepropetrovska

  #14  
Old February 16th 04, 05:44 PM
Richard
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Posts: n/a
Default How good is good enough?

@hpl.hp.com () wrote in message ...
I recently got an Orion 127 mm Mak-Cas telescope, and although the images
seem to be pretty good, I was wondering are there any tests for determining
if a given telescope is performing as well as it should? How sharp a point
should a star focus to? It seems to depend on the brightness of the star
too. At high (150X) magnification, I do see little rings around some stars:
are these the Airy circles? I'm new at this so any pointers would be
appreciated.

Albert


Your scope is apparently in good collimation and is producing correct
airy discs. You next task would be to examine Jupiter (for instance)
or Saturn to see how contrast is. If at around 150x the planet is
bright with black sky around it, chances are the scope is decent, the
optics
smooth. If the planet is surrounded by a bright glow, then some of
the
light is being diffused, reducing contrast. Three things can make
this happen;
The central obstruction contributes a tiny amount, but this won't
radically
effect images. If the optical surfaces are rough, this will
definitely cause
a loss of contrast, also, if the optics are spherically aberrated
(curves are
imprecise) this will diffuse and blur a planetary image because the
rays hitting
different parts of the optics are coming to a different focus position
from
one another. In order not to misdiagnose any problems, it's important
to let
the scope reach the same temperture as the outside air and to have
good, steady
seeing conditions (when stars appear not to twinkle).
When the scope is properly cooled
down and the seeing is good, (star images when defocused are stable,
without any waves going through them or motion) you should be able to
detect clearly
Cassini's division in Saturn's rings, and festoons or cloud whorls in
the belts of Jupiter, as examples.
-Rich
  #15  
Old February 16th 04, 05:44 PM
Richard
external usenet poster
 
Posts: n/a
Default How good is good enough?

@hpl.hp.com () wrote in message ...
I recently got an Orion 127 mm Mak-Cas telescope, and although the images
seem to be pretty good, I was wondering are there any tests for determining
if a given telescope is performing as well as it should? How sharp a point
should a star focus to? It seems to depend on the brightness of the star
too. At high (150X) magnification, I do see little rings around some stars:
are these the Airy circles? I'm new at this so any pointers would be
appreciated.

Albert


Your scope is apparently in good collimation and is producing correct
airy discs. You next task would be to examine Jupiter (for instance)
or Saturn to see how contrast is. If at around 150x the planet is
bright with black sky around it, chances are the scope is decent, the
optics
smooth. If the planet is surrounded by a bright glow, then some of
the
light is being diffused, reducing contrast. Three things can make
this happen;
The central obstruction contributes a tiny amount, but this won't
radically
effect images. If the optical surfaces are rough, this will
definitely cause
a loss of contrast, also, if the optics are spherically aberrated
(curves are
imprecise) this will diffuse and blur a planetary image because the
rays hitting
different parts of the optics are coming to a different focus position
from
one another. In order not to misdiagnose any problems, it's important
to let
the scope reach the same temperture as the outside air and to have
good, steady
seeing conditions (when stars appear not to twinkle).
When the scope is properly cooled
down and the seeing is good, (star images when defocused are stable,
without any waves going through them or motion) you should be able to
detect clearly
Cassini's division in Saturn's rings, and festoons or cloud whorls in
the belts of Jupiter, as examples.
-Rich
 




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