Orion 120mm f5 Short Tube refractor (ST120)

The premise

I had been in search of a Rich Field Telescope, and had looked at various options such as the old Celestron "CometCatcher" and its Meade counterpart the "Comet Tracker".  Quality seemed to be variable on these old astrographs, and the prices were equally variable (some units were collectables... others virtually trashed.)  I opted instead to go with a short focal length refractor, and the one I decided on was the Orion Short Tube 120, an an f/5 120mm achromat.  I planned to install my Nagler 31 eyepiece, which would yield a 4.24 degree field of view at 19X --which was low enough magnification to avoid both the coma and spherical aberration such a fast design and cheaply built quality is prone to, and is also close to the magic 3.5X/inch threshold for a Richest Field Telescope (see http://www.cdcc.sc.usp.br/cda/telescopios/hp-mbartels/scopes/rft.html for a detailed discussion of this.) Using the highly corrected Type 5 Nagler 31 eyepiece further helps overcome the optical limitations of this scope.  (Al Nagler designs and tests these eyepieces down to f/4, so this scope should be well within the performance envelope.)  This would not be my instrument of choice for high powered planetary work... but it yields a FOV 1/3 wider and reaches over 1 magnitude deeper than my Televue Ranger --the recent acquisition that got me thinking about the role a small refractor could play in complementing a big Dob under dark skies.  Besides the Nagler 31 eyepiece, I am using the AstroPhysics MaxBright 2" diagonal that has sat idle in my kit for nearly four years since I got into Dobs and away from using my LX200.  If anyone sees the incongruity of hanging $900 worth of eyepiece and diagonal off the back end of a $300 scope, well... I agree.  <g>  I certainly wouldn't recommend buying such high end gear to hang off of such a pedestrian scope, but since I've got the stuff I might as well use it-- and it ensures that I am getting the most possible out of the scope!

I didn't want to buy an ST120 for list price from Orion.  I noticed that many went for ~$225 on Astromart-- a typical depreciation from list price.  I ran across one for $275, but which included the mounting rings, dovetail, and a 1.25" anti-fringing/chroma correcting eyepiece -- about ~$125 worth of accessories.  I was second in line to purchase this scope, and when the initial buyer pulled out, it became mine!

First impressions: This is a big scope-- much bigger than the Ranger, and much heavier than I had expected.  (I guess I was subliminally associating quality-- or the lack thereof-- with weight.) 

Unimpressive start

Mounted on my EQ2 tripod in place of the Ranger, this scope was too large, too ungainly, and too imbalanced.  As detailed elsewhere on this side (Refractors) taking it to dark skies revealed that it had decent light grasp, but was beaten on detail in M31/Andromeda Galaxy by the much smaller Televue Ranger.

ST120 gets its second wind

I soon realized that, rather than trying to counterweight my light EQ-2 to work with this ~14 lb load, it was better to bite the bullet and use my EQ-5 tripod.  The downside of this is that it requires removing my solar rig "Uncle Beasley" (Budget Solar Trio) from its mount. I had also read enough on-line to realize that this scope vignettes fairly badly with a 2" diagonal in place.  Rather than its native 120mm of light grasp, in my configuration it may have been performing as a ~100mm scope-- which was throwing away nearly half the available light!!!

The other part of the rehabilitation of this scope came from my recent reading of Walter Scott Houston's "Deep Sky Wonders" (http://www.astroleague.org/al/bookserv/obsgd/rev00051.html) drawn by Stephen O'Meara from the 50 years of Houston's monthly column in Sky and Telescope.  Although he had a veritable arsenal of scopes at his disposal, Scotty Houston seemed to do a fair amount of work with an Apogee 5" refractor yielding 20X.  (Here is a link to a dedicated observing list for "rich field scopes" http://www.saguaroastro.org/content/Great-Objects-for-Rich-Field-Telescopes.htm)  This is almost identical to the size and magnification of my ST120 with the Nagler 31 on board.

A final factor was finding a "Richest Field possible" calculator by Mel Bartels which displayed various ways to hit the "sweet spot" in terms of number of stars per FOV starting with the human eye and going up to large Dobs (http://www.cdcc.sc.usp.br/cda/telescopios/hp-mbartels/scopes/rft.html).  Again, a short focal length scope of 4-6" diameter was one of the winning combinations.   

"Mediocre refractor make-over" (aka "pimp my scope")

Once I had decided the scope was worth keeping, I bit the bullet and decided to do irreversible modifications to reduce the vignetting, improve image contrast (by flocking), and improve mechanical operation (re-greasing).

Combating Vignetting:

Mechanical

Contrast enhancement

Ease of Use

These tweaks worked, morphing the scope from an undistinguished "loafer" into an almost a sublime performer in its view of Deep Sky Objects. (see Refractors) The biggest, factor, though is the Nagler 31 type 5 eyepiece, as I verified by testing the scope with other eyepieces.  The Astrobuffet clone of the Widescan III 30mm eyepiece was dreadful in the ST120.  Perhaps the inner 25-30 percent of the Nagler-sized FOV was usable, with the out half of the field looking as distorted as if viewed through water or a film of vaseline.   A Meade series 4000 26mm Super Plossl rendered a FOV and clarity that were a virtual tie to those of the binos, i.e., reasonably sharp stars, with progressively increasing astigmatism the farther out in the FOV you look.  The 22mm Panoptic was worse, failing to render what I considered acceptably sharp images even on-axis.  (This mirrors the performance of these eyepieces in my fast reflectors, too.).  The Nagler 3-6mm zoom was also a winner, rendering crisp images of Saturn at up to 150x (4mm setting), and slightly soft ones at 200x (3mm).   On another night, I verified that there is a significant amount of chromatic aberration when the scope is pointed towards the limbs of the moon -- no surprise there... and looking at the edge of the moon is more along the lines of an optical 'stress test' than an observing interest of mine.  Using the Nagler zoom, the surface of the moon looked very nice @ 200x, and with some effort I was also able to discern the E and F stars in the heart of M42's Trapezium at 200x.  If you ever wanted to see whether premium eyepieces make a performance difference, this is the scope to prove it, since nothing that didn't have "Nagler" on the barrel was a stand-out performer.   Of course, I don't think spending this kind of money on eyepieces to outfit a scope is warranted.  Buy a slower/longer focal length scope that sacrifices some field of view in order to work well with cheaper eyepieces.  HOWEVER,  if you've already got the big Naglers, this scope is an attractive option if you want cheap wide field thrills.

What lies ahead for this scope?  It will NOT replace the 70mm Televue Ranger in my garage as my 'grab and go' lunar instrument  or 'blitz' (five minute observing session) scope, since it is too cumbersome and needs to use displace my solar rig from the CG-5 tripod.  Where this scope will see its use is in dark sky sites when combining nearly 5" of aperture with a 4+ degree FOV gives it a 'niche' not readily filled by a Dob, binoculars or a smaller refractor such as the Traveler. 

The ST120 on the EQ5 tripod

The ST120 compared to the Ranger

Cutting down the focuser draw tube (you can see the removed section at the top of the image), removing its baffles, and increasing contrast via flocking paper

The flocked optical tube under a flash

The flocked tube under more realistic 'worst case' conditions (pointing towards sunlight).       It's pretty dark!!!!