This 1.5” drawtube travel Feather Touch low profile 2" focuser from Starlight Instruments is a precision-machined Crayford-type focuser for reflectors and catadioptric scopes alike. In addition to making visual observing a precision joy, this premium friction-drive focuser’s zero image shift/no backlash design is a must for critical CCD imaging and astrophotography. It totally eliminates the image-shift inherent in any telescope that moves the primary mirror to focus.
Optional mounting base adapters are available for installing the focuser on catadioptric rear cells as well as on reflector optical tubes and truss-tube Dobsonian secondary mirror cages. (A base adapter is required in all cases for installing the focuser on a telescope.) Non-vignetting short adapter #FT-MSLG for Meade 10” and larger scopes will generally let the focuser swing through the fork arms of these particular SCTs. Mounting base adapters #FT-SCT for 7" through 9.25" Celestrons and Meades, #FT-CLG for 11" and larger Celestrons, and #FT-MLG for 10" and larger Meades have enough internal length to allow the long drawtube travel of this focuser to be used. However, they typically will not allow the focuser to swing through the fork arms of the smaller aperture scopes, depending on the length of the accessory stack attached to the focuser. German mount SCT scopes do not have this potential swing-through problem and the focuser can readily be used with any base adapter.
The focuser body is 1.6” tall, exclusive of mounting base. With a standard SCT mounting base, the overall focuser body height is 3.7”. With a short SCT base it is 2.7” tall. The fully extended focuser drawtube adds 1.3” to either height.
The focuser’s 2” eyepiece holder uses a soft brass compression ring to hold 2” eyepieces and the optional low profile Easy-Grip 1.25” eyepiece adapter (#FT-ADEP). This non-marring design will not scratch the chrome barrels of your eyepieces, as a conventional thumbscrew eyepiece holder can. The compression ring also has improved holding power over conventional thumbscrews, even nylon-tipped thumbscrews. There are two thumbscrews on the draw tube. One is used for 2” eyepieces; the other for 1.25” eyepieces when using the #FT-ADEP 1.25” adapter.
All running parts are polished stainless steel for durability and wear resistance. The focuser’s floating hardened stainless steel raceways are matched to stainless steel ball bearings for improved smoothness over conventional Crayford focusers. The result is a drawtube that moves with a buttery smoothness and zero backlash. This steel-on-steel running gear design will virtually never wear out or degrade with age or use. The hardened stainless steel drawtube friction plate likewise will not wear out as will the softer aluminum plates used in lesser focusers.
There are coarse and fine focus knobs with a 10:1 reduction ratio for unrivaled accuracy and repeatability. Each revolution of the fine focus knob results in a mere 0.070 inches of drawtube travel, making critical focus effortless. Fine focus repeatability is on the order of <0.0005 inches. Moderate finger-pressure tightening of the drawtube tension screw allows the lifting capacity to be adjusted up to a nominal maximum of five pounds, although greater payloads are possible by further tightening the tension screw.
Because a Feather Touch Crayford focuser is so smooth in operation and moves so freely, it can only hold a limited amount of weight (generally about 1 lb.) without drifting out of focus when the focuser drawtube is not horizontal and has a vertical component. The focuser therefore contains an adjustable tension internal brake system to allow astrophotography and visual use with heavy eyepieces without the possibility of the focus shift.
The brake consists of a lever arrangement internal to the pinion block. Adjusting the thumbscrew on the pinion block causes the lever to press a friction pad onto the pinion. This increases the torque needed to turn the focus knobs. If the thumbscrew is turned all the way in (only 1 to 1½ turns), the pinion shaft and the drawtube are locked in position for long exposure astrophotography. Visually, partial tightening of the internal brake offers a virtually infinite range of braking force to accommodate various eyepiece weights.
There are somewhat less expensive versions of this focuser available on special order. One has the compression ring, but does not have the variable force internal brake. Instead, it uses a simple thumbscrew to lock the drawtube in place as needed. A second has the internal brake, but does not have the compression ring eyepiece holder. Instead, it uses a nylon-tipped thumbscrew to hold the eyepiece in place. The third has neither the compression ring eyepiece holder nor the internal brake. While these are all somewhat less expensive than this version with both compression ring and internal brake, we feel that the small savings you will see are not worth what you give up in performance. Accordingly, we cannot recommend these lesser versions.
A letter to Starlight Instruments from Jack Newton, well-known astrophotographer, said,
“I have just installed your zero backlash Feather Touch Focuser on my Meade 16-inch telescope. This focuser is the finest I have ever used! It is built with the precision of a Swiss watch. Your superb workmanship has produced a focuser in a class by itself.
“I attached a dial micrometer to measure the focus differential required for demanding CCD tri-color imaging and cannot get over the smooth performance of this incredible instrument. My Pictor 1616XT and a large filter wheel have a combined weight of over 7 pounds. Even bearing this heavy load, the focuser moves with micrometer precision.
“Thanks, Starlight Instruments, for this wonderful contribution to my CCD focusing!”