Orion 120ST Refractor
Over the past few years I have used many scopes and setups
trying to find the scope for me.
Basically, I wanted the largest aperture scope I wanted to
deal with size and weight wise.
After trying assorted Dobsonians, 8 inch and smaller Reflectors on an
Equatorial mount, refractors of six inch and smaller on assorted mounts, I have
settled on this setup: Orion's 120ST on Vixen's Porta mount.
I have upgraded the focuser to William Optics double speed
which really helps at powers of 100X or above to keep chromatic aberration to a
minimum especially when used with Orion's V-block filter. With the rack and pinion focuser that
comes with the scope it was very difficult to find that perfect focusing spot
at powers of around 80X or more. Also, the fine speed adjust really helps to
keep the vibration to a minimum during the focusing process.
I have considered a new scope that William Optics has
released recently, the 110mm doublet, but for my viewing conditions (during the
Summer mostly cloudy for instance) I can not justify the price difference: The
Orion is $299 + shipping and the WO 110mm is $1400.
My seeing conditions are light polluted urban skies with
typically high humidity so I'm regulated to the brighter Messier Objects,
double stars, the planets and the Moon.
Primarily, I like to look at colorful double stars like Izar and
Rasalgethi. When Saturn and
Jupiter are up I look at them for a few minutes and move on. Generally, my seeing will not allow
high enough power to make Mars interesting except when it comes close like it
did a few years ago.
Jupiter is the most interesting planet because I can see
quite a bit of surface detail especially when viewed through a V-block filter
at 100 to 120X. I have found that
the Baader Planetarium Semi-APO filter provides too much correction for viewing
Jupiter but does a better job than the V-block on the double star Izar at 150X
(with Vixen 4mm LV eyepiece). It
really reduces the glare so that the separation is easier to see between the
yellow primary star and the bluish secondary star.
The Vixen Porta Mount handles this scope very well as
dampening time is very quick: under 3 seconds. I tried the 120mm f/8.3 version but found the dampening time
greater and bothersome. Plus, I
really like the wide field viewing the f/5 gives.
Even on nights where I can barely detect the mag 4 stars in
Ursa Major the Ring and Dumbell nebulas are easy to see in the 4.7 inch optics
where with a 3.1 inch scope they are barely detected and quite disappointing. I tried many scopes under 120mm and
found, with my skies, a 4 inch scope is passable but even for the brighter
Messier objects the 120mm is noticeably better.
People interested mainly in planetary and/or Moon viewing
would probably be happier with a long focal length achromatic refractor, like
Celestron's f/9.8 102mm, or an APO.
The view of Jupiter in the 120ST is pretty good with the
V-block filter but is not quite as sharp as in the Celestron 102mm without a
filter. Also, I was able to push
the power a little higher in the 102mm than 120ST before details started
softening which often allowed more details to be seen. In the 102mm, 120 to 150X was the
typical maximum power range where for the 120ST 100 to 120X was the typical
maximum power range.
For powers up to around 70 I use either Orion's Sirius
Plossls or Edmunds Optics RKEs but when I get to 80X or higher I use Vixen's LV
eyepieces. The Plossls and RKEs
are 4 element eyepieces and let more light through than the LVs (which have 7
elements) which is good for DSO viewing but when I reach about 80X this is Moon
and planet power as DSOs become too dim to be interesting in the 120ST in my
seeing conditions. At a darker
sight this may not be true. I use
the LVs because they maintain 20mm of eye relief where a 4mm plossl will have
around 4mm of eye relief which is uncomfortable to me.
I really like the Vixen Porta Mount because you can move the
scope around manually to get to a part of the sky quickly and then there are
the slo-mo controls to track an object with, I've tried a number of mounts, like William Optics Eazy
Touch, and just couldn't get the hang of tracking without slo-mo controls at
powers of 100X or higher. But, it
appears, many people do not have this issue and are very happy without slo-mo
controls.
In conclusion, the 120ST and Vixen Porta Mount is portable
and yet shows enough of the sky to keep me interested when certain accessories
are used with it.
Blair Slayton
August 2007
