Refractors

I'll admit it.  Since getting into Dobs in 2001 I have considered refractors to be the instruments for the effete or optical purists.  (OK, I'm tongue in cheek here... Takahashi and AP owners need not send hate mail.  <g>)   My adherence to the Dob philosophy (large aperture, low cost) transferred to my relationship with refractors, and the only one I owned was a Short Tube 80 (ST) f/5 I purchased for use as a finder -- for a Dob, naturally-- for the close-out price of $99 including tripod and eyepieces.  Other than the special case of solar observing, this situation persisted until I picked up a Televue Ranger in mid-2006.  It was a fun scope to use, and opened my eyes to the possibilities of a rich field/wide angle scope that could be used to compliment-- rather than replace-- a Dob under dark sky conditions.

Let's introduce the three refractors I now own:

Apogee ST80 (80mm f/5 achromat) "liberated" from use as Frankenscope's spotting scope when I decommissioned the old Dob.  This particular unit had the lens cell loosened and remounted (to make sure that it was not pinched) and flat black paint added to the inside of the tube to reduce glare, so it counts as 'semi-customized'.

Televue "Ranger" 70mm f/6.7 (sometimes called a "semi-apo" scope because of the reputed presence of one ED glass element in the doublet).  See Televue Ranger for more details

Orion ST120 (120mm f/5 achromat)  See Orion ST120 for details.  This scope counts as customized-- not much else you can call it when a fanatic takes a hacksaw and a yard of flocking paper to a scope!

All three now have a place in my observing kit.  This sounds self-indulgent, eh?  Fortunately the three cost me a total of $500, so I can afford some redundancy! 

"Small Refractor shootout"

Round 1:

Taking the Ranger and the stock ST120 to dark skies in November revealed that:

Round 2:

Taking all three scopes back to the same dark sky site (G.W. Pad rest of 2006) in December and setting them up side by side facilitated some interesting testing on the following deep sky objects:

Pleiades

Here the Ranger showed that it rendered the sharpest and most pinpoint star images of the three scopes-- probably due to its longer focal ratio (f/6.7 vs. f/5) and higher optical quality.  It showed more nebulosity around the stars than the ST80 did.  This was due in part to the higher quality diagonal on the Ranger (a 99% reflectivity Astrotech model, vs. a cheap and visibly distorted generic one on the ST80) and in part to the eyepieces involved (Panoptic 22 vs. Meade 26 SP).  I was able to swap the two diagonals--whereupon the ST80 showed somewhat more nebulosity--  but could not simultaneously swap the eyepieces since the ST80 lacks the requisite in-travel.  The ST80, for its part, showed fainter stars than the Ranger-- size clearly matters!  The ST80 rendered perfectly acceptable views-- surprisingly so, for a bargain basement scope of this price.  The ST120, as one would expect, showed both fainter stars than either of the other scopes and more nebulosity.  Optically, the modified ST120 was the scope of choice, especially since it combined this performance with a field of view a third wider than either of the others.

M31 Andromeda Galaxy

Here the ST80 again rendered good views, which were noticeably brighter than those through the Ranger.  However, the Ranger again showed more structure and more dust lane detail than the ST80.  Here is where the performance of the modified ST120 pulled markedly ahead of either of the other scopes, though.  M31 has arms that stretch ~5 degrees when seen under dark skies, and the 4.25 degree FOV of this scope was able to do them justice.  It also had enough light grasp and sharpness to render an image that crossed that intangible threshold between a small scope and a 'big scope' viewing experience.  Perhaps the most fitting accolade I can bestow on this scope is that it gave me a view that looked like that of images of M31, rather than something you would expect to see with the naked eye. 

Having beaten its rivals handily, the ST120 took its victory lap on a cruise through the Milky Way near Cygnus.  Here again, the experience was unusual and impressive-- something akin to having the light grasp of a small Dob coupled with a field of view 2-4 times as large as that of any reflector I've ever used.  This scope is going to become a regular companion for dark sky observing-- though otherwise the Ranger will be my ready use 'suburban small refractor' because of ease of use and suburban light pollution that doesn't play to the ST120's strengths

        The 20 x 80 binos on a Bogen 3265 tripod head

  Special shootout-- modified ST120 refractor vs. 20 x 80 binoculars

Once I retrieved the 20x80 Oberwerk binoculars I lent to a fellow NOVAC member, I decided to test the binos head-to-head against the modified ST120 The two contenders seemed pretty evenly matched in terms of:

I tested the two contenders in terms of:

The ST120 won by a knock-out when equipped with the Nagler 31 (which is how I'll be using it).  It trounced the binos in terms of pinpoint star resolution both on and off-axis.  The binos looked "OK" in sharpness, until compared to the scope, which had star images that focused down to a crisp pinpoint appreciably smaller than the same stars as seen through the binos.  Field of view was impressive for both instruments, but the ~1/3rd wider field on the ST120 gave it that "port hole" feeling that is a Nagler trademark.  The only 'horse race' was in light grasp.  While I had expected the binos to pull ahead here, the ST120 again triumphed.  Stars that were faint direct vision objects for the ST120 were tougher to see in the binos. 

If "Clothes make the man" then "eyepieces make the scope!"

The superlative performance of the ST120 in these various tests is due in large part to 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).   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-- but if you've already got the big Naglers, this scope is an attractive option if you want cheap wide field thrills.