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Orion ST120 Redux

Posted by norman sullivan   07/13/2009 12:00AM

Earlier this year I submitted a review of this telescope based on first impressions. Judging from the response, several observers have had a similar favorable impression of this versatile and economical telescope.
This update will discuss peripherals.

In my opinion, the mount selected makes a huge difference in observing. One reason for the popularity of Dobsonian mounts is certainly ease of use, but they can also track reasonably well. Few will argue that an f5 refractor is best seen as an RFT instrument, and my experience has verified that it is weakest when pushed to its maximum theoreyical power of 240X, at a 1/2 mm exit pupil - but still usable during times of excellent seeing. At 25X to 50X it really shines, and this is where the choice of mount becomes important. I would personally not buy Orion's total package, which includes a manual Eq mount with one or two axis tracking optional. Even after you have set it for your Latitude and carefully polar aligned it, odds are the finder will always be positioned at an awkward angle. This is especially true if you use a Rigel "Quickfinder" or Telrad. Resetting the tube in the rings is difficult and unadvisable - you may have to re-set a dozen times in one observing session. An alt-az mount offers a much better potential solution, but there are a number from which to choose and some work much better than others. The standard
"HD" alt-az served marginally well, but to hold the tube rigid at any particular angle requires the altitude bearing nut to be tightened down considerably, which means easy panning in the altitude mode cannot be achieved - movements are unacceptably jerky. Also, the slow motion travel is limited to a few degrees. Another disadvantage of this type is the near impossibility of viewing anything at the zenith. Both Orion and Vixen make offset alt-az mounts that get around this problem; my choice was the Vixen Porta II. The slow motion altitude control is excellent, and the tube can easily be moved in either axis by a light touch, yet remains where it is set. Viewing at the zenith presents no difficulty. It seemed a bit strange at first that the tube rings mounted "sideways", but takes only a short time to accustom to this orientation. The weight ratig is "5 Kg" - I believe this to be a very conservative rating. My ST 120 with a 50mm right angle finder and Orion "Stratus" eyepiece tips the scale at about 11 lbs., but the mount has no difficulty holding at any angle.
While on the subject of eyepieces, I now believe it is better to have a few good ones than an extensive collection of OEMs and star-party specials. The 21mm "Stratus" does very well and gives a nice big picture window effect. The Astro-Tech "paradigms" also do well, and are comparatively inexpensive. I especially like the 12 and 15mm sizes in this line.

Finally, for a short focus refractor, a right angle finder scope takes some of the strain out of finding DSO's. No rule says you can't use a light reflex sight in combination with it.

Admittedly, a 120mm instrrument is not likely to find too many 11th magnitude galaxies, but if you still get a thrill from finding, viewing, and showing to others, the Ring nebula, or the Veil, or admiring the Double Cluster for its aesthetic beauty, I highly recommend this combination, for optical excellence and ease of use.