This short 2” mirror-type star diagonal is designed to swing through the fork arms of most catadioptric telescopes when used with the #7785SC short SCT adapter listed as a recommended option below. This is helpful for keeping eyepieces and diagonals from colliding with the base of computerized telescopes when observing at the zenith in the altazimuth mode.
The 2” tubulation at the front of the diagonal is 1” long, compared with the 1-7/16” length of the standard TeleVue 2” diagonal. When the diagonal and short SCT adapter are assembled, they have a combined depth of 4.5” from the bottom of the adapter to the furthest edge of the diagonal. You will need at least this much distance between the start of the SCT rear cell threads (not simply the back plate of the scope) and the base of the scope for this diagonal to clear the scope base.
This short diagonal is designed specifically for use with Schmidt-Cassegrains using adapter #7785SC, but can also be used with refractors and other telescopes, although the longer tubulation standard #7780PC diagonal is recommended with these scopes for increased locking stability. When this short diagonal is used in with 1.25” eyepieces, it is recommended that the #2715FC flat top 2” to 1.25” adapter or #2715CON eyepiece converters be used in place of a standard 2” to 1.25” high hat adapter.
The diagonal has Everbrite dielectric mirror coatings for 99% reflectivity. In addition to higher reflectivity than a conventional aluminum or silver coated diagonal, the Everbrite coatings also provide higher contrast and resolution during critical planetary, lunar, and binary star observations. A safety stop at the bottom of the 2” eyepiece holder tube prevents 2” eyepieces or 2” Barlows from hitting the surface of the mirror. However, if you use the #2715FC 1.25” adapter, it is possible that an exceptionally long 1.25” Barlow may project far enough into the diagonal to contact the mirror surface. Any damage caused in this way is not covered by warranty.
The mirror housing and its 2" focuser and eyepiece holder barrels are precision-machined from a single block of solid aluminum, rather than being simply a pair of less-rigid tubes screwed into a flimsy plastic mirror housing or sheet metal stamping, as is the case with lesser diagonals. This single unit construction assures permanent collimation of the diagonal’s parts. The 2” diameter barrel that fits into the focuser or visual back is internally antireflection threaded for higher contrast. It is also threaded to accept standard 2” (48mm) filters. A groove cut into the outside of the barrel acts as a safety stop to keep the diagonal from falling should the scope’s visual back or eyepiece holder thumbscrew loosen slightly from cold or use.
The 2” eyepiece barrel uses a non-marring soft brass compression ring to hold eyepieces in place, rather than a thumbscrew that contacts the eyepiece barrel directly. The thumbscrew that tightens the clamp ring is tethered internally in the diagonal so that the thumbscrew can’t be lost in the field .
This 2” diagonal can be used with f/6.3 focal reducers on an f/10 SCT. Because of the extra path length of the diagonal, the f/6.3 focal reducer will actually produce an f/5.5 focal ratio. Eyepieces longer than a 27mm Panoptic may vignette severely in such a system, and some eyepieces (such as 2” Naglers) may not reach focus.
This is an absolutely premium 2” diagonal that will give you all the visual performance your scope is capable of delivering. It's a worthy upgrade for virtually any Schmidt-Cassegrain or Maksutov-Cassegrain that uses a 2” star diagonal.