0
Orion 120ST Short Tube Refractor
Nov 22 2005 02:28 AM |
John Zimmerman
in 120mm - 130mm Refractors
Discuss this article in our forums
Orion 120ST Short Tube Refractor
By John Zimmerman July 2005
The Orion 120ST (Short Tube)
is a 120mm achromatic, f/5 refractor. As the Orion ads state, this is designed
for wide field observation of nebulae, star clusters, and galaxies - the implication
being that it is not as well suited for planetary or lunar observation. Orion
sells this as a package that includes an equatorial mount. It may also be purchased
with just the OTA (Optical Tube Assembly). I ended up purchasing the OTA and
Orion's AZ-3 alt-azimuth mount to create the package you see in the photos.
The cost was greater than if I had purchased the equatorial package, but I had
my reasons for doing what I did.
Until the purchase of the 120ST, all of my scopes were GOTO. I wanted an alternative to the high tech, sometimes complex (and frustrating) world of computerized observing. The alt-azimuth mounted ST was the answer. It is simple, uncomplicated, quick and easy to set up, and it delivers nice views of most everything.
As indicated above, I bought the OTA and alt-azimuth tripod. A set of tube rings was also purchased to facilitate mounting the scope on the tripod. I already had a right angle finder scope and a 2" diagonal to complete the package. As I have come to expect from Orion products, everything was well packaged and showed good quality in construction and workmanship. I really like the appearance of the OTA ? the dark grey finish of the tube and the black wrinkle finish of the tube rings and mount make this look like a very professional instrument.
With everything assembled, the total weight is only 18lbs - that's a pretty light weight package that includes a 5" aperture scope, mount, and tripod. And while not as physically compact as an 80mm short tube refractor, the 26" length of the OTA does not take up a lot of room - you can easily lay the entire scope in the back seat of a car. One very nice feature is the tripod shelf - it simply sits in place with three pins that go through holes in the spreader - you don't have to spend any time tightening nuts - just set the shelf in place.
Since the 120ST arrived in the daytime, initial use was for terrestrial viewing. Wow! What a difference 120mm of aperture makes in terms of image brightness. Previously I had been using a 3.5" Questar for terrestrial viewing, and the difference in brightness was remarkable - the shorter focal ratio of the 120ST also contributed to the brighter image. What was really impressive was the relative absence of chromatic aberration (CA) on terrestrial objects.
With nightfall came the test of the 120ST as an astronomical instrument. Setup was just as I had hoped. It is easy to carry outside - just set it down, remove the dust covers, insert an eyepiece, and you're good to go. No leveling, aligning, or other distractions. The phrase "Zero to Observing in 60 seconds" came to mind!
So how good is this package? The first thing I found striking were the extremely wide fields of view. A 40mm Plossl with a 46 degree apparent field of view yielded 15x and a 3 degree field of view. This, combined with the ease of use an alt-azimuth mount, made slowly sweeping the night skies a rewarding experience. All of my other scopes are f/10 or greater, so views such as this were new experience for me. I also quickly discovered that my optical finder, with its 7 degree field of view, was somewhat redundant. I later replaced it with a red dot finder.
For deep sky objects, the 120ST performs very well. Because it is a refractor, stars appear as sharp, pinpoints of light. This makes open and globular clusters appear very nice. Performance on nebulae and galaxies is good with nice contrast and sharply defined detail. Double star performance is also good. Epsilon Lyra was nicely split at 80x.
Planetary and Lunar performance isn't bad, though probably not as good as with a longer focal ratio achromat. Jupiter showed considerable CA - a V fringe filter from Orion helped minimize this, but it was still evident. Some smearing of planetary detail was evident, an effect I haven't seen in my other scopes. This seemed to be related to seeing conditions combined with CA. In moments of good seeing, a number of belts could be seen, and hints of detail within the belts was evident. Lunar features looked good, though again, not as crisp and well defined as you would get from a longer focal ratio instrument.
The 120ST star tests well. It shows concentric rings both inside and outside of focus. The objective cell cannot be collimated, so it is fortunate that it comes well collimated from the factory.
Here are some other things to note. The focuser is smooth, and accepts 2" accessories. The alt-azimuth mount is a good fit for this OTA. Damping time is less than 2 seconds at the highest power (150x) I could muster using a 2x Barlow. Tracking objects with the mount isn't too bad - it has slow motion controls on each axis that facilitate this. There is some slop in altitude movement, but for a scope designed for wide field views, this isn't much of a problem.
Overall, the 120ST on the AZ-3 mount is a good combination for terrestrial and astronomical use. It is portable, very easy to use, and for astronomical use, delivers good images for deep sky work. As a lunar and planetary scope, it is not as competent as more traditional, longer focal ratio instruments, but serviceable nonetheless. For the amateur who already owns a good all purpose scope, the 120ST makes a nice addition.
By John Zimmerman July 2005
The Orion 120ST (Short Tube)
is a 120mm achromatic, f/5 refractor. As the Orion ads state, this is designed
for wide field observation of nebulae, star clusters, and galaxies - the implication
being that it is not as well suited for planetary or lunar observation. Orion
sells this as a package that includes an equatorial mount. It may also be purchased
with just the OTA (Optical Tube Assembly). I ended up purchasing the OTA and
Orion's AZ-3 alt-azimuth mount to create the package you see in the photos.
The cost was greater than if I had purchased the equatorial package, but I had
my reasons for doing what I did.Until the purchase of the 120ST, all of my scopes were GOTO. I wanted an alternative to the high tech, sometimes complex (and frustrating) world of computerized observing. The alt-azimuth mounted ST was the answer. It is simple, uncomplicated, quick and easy to set up, and it delivers nice views of most everything.
As indicated above, I bought the OTA and alt-azimuth tripod. A set of tube rings was also purchased to facilitate mounting the scope on the tripod. I already had a right angle finder scope and a 2" diagonal to complete the package. As I have come to expect from Orion products, everything was well packaged and showed good quality in construction and workmanship. I really like the appearance of the OTA ? the dark grey finish of the tube and the black wrinkle finish of the tube rings and mount make this look like a very professional instrument.
With everything assembled, the total weight is only 18lbs - that's a pretty light weight package that includes a 5" aperture scope, mount, and tripod. And while not as physically compact as an 80mm short tube refractor, the 26" length of the OTA does not take up a lot of room - you can easily lay the entire scope in the back seat of a car. One very nice feature is the tripod shelf - it simply sits in place with three pins that go through holes in the spreader - you don't have to spend any time tightening nuts - just set the shelf in place.
Since the 120ST arrived in the daytime, initial use was for terrestrial viewing. Wow! What a difference 120mm of aperture makes in terms of image brightness. Previously I had been using a 3.5" Questar for terrestrial viewing, and the difference in brightness was remarkable - the shorter focal ratio of the 120ST also contributed to the brighter image. What was really impressive was the relative absence of chromatic aberration (CA) on terrestrial objects.
With nightfall came the test of the 120ST as an astronomical instrument. Setup was just as I had hoped. It is easy to carry outside - just set it down, remove the dust covers, insert an eyepiece, and you're good to go. No leveling, aligning, or other distractions. The phrase "Zero to Observing in 60 seconds" came to mind!
So how good is this package? The first thing I found striking were the extremely wide fields of view. A 40mm Plossl with a 46 degree apparent field of view yielded 15x and a 3 degree field of view. This, combined with the ease of use an alt-azimuth mount, made slowly sweeping the night skies a rewarding experience. All of my other scopes are f/10 or greater, so views such as this were new experience for me. I also quickly discovered that my optical finder, with its 7 degree field of view, was somewhat redundant. I later replaced it with a red dot finder.
For deep sky objects, the 120ST performs very well. Because it is a refractor, stars appear as sharp, pinpoints of light. This makes open and globular clusters appear very nice. Performance on nebulae and galaxies is good with nice contrast and sharply defined detail. Double star performance is also good. Epsilon Lyra was nicely split at 80x.
Planetary and Lunar performance isn't bad, though probably not as good as with a longer focal ratio achromat. Jupiter showed considerable CA - a V fringe filter from Orion helped minimize this, but it was still evident. Some smearing of planetary detail was evident, an effect I haven't seen in my other scopes. This seemed to be related to seeing conditions combined with CA. In moments of good seeing, a number of belts could be seen, and hints of detail within the belts was evident. Lunar features looked good, though again, not as crisp and well defined as you would get from a longer focal ratio instrument.
The 120ST star tests well. It shows concentric rings both inside and outside of focus. The objective cell cannot be collimated, so it is fortunate that it comes well collimated from the factory.
Here are some other things to note. The focuser is smooth, and accepts 2" accessories. The alt-azimuth mount is a good fit for this OTA. Damping time is less than 2 seconds at the highest power (150x) I could muster using a 2x Barlow. Tracking objects with the mount isn't too bad - it has slow motion controls on each axis that facilitate this. There is some slop in altitude movement, but for a scope designed for wide field views, this isn't much of a problem.
Overall, the 120ST on the AZ-3 mount is a good combination for terrestrial and astronomical use. It is portable, very easy to use, and for astronomical use, delivers good images for deep sky work. As a lunar and planetary scope, it is not as competent as more traditional, longer focal ratio instruments, but serviceable nonetheless. For the amateur who already owns a good all purpose scope, the 120ST makes a nice addition.
0 Comments