This 2.5” drawtube travel Feather Touch focuser from Starlight Instruments is a precision-machined Crayford-type 2” focuser for reflectors. The friction-drive low profile focuser’s zero image shift/no backlash design and non-marring compression ring eyepiece holder make visual observing a joy. The 2” drawtube travel allows it to be used both visually and photographically. The 2.5” drawtube travel and internal brake system allow it to be used both visually and photographically, generally without the need for extension tubes or modifications of the light path length to allow long physical length CCD camera/filter wheel combinations to focus properly.
Optional mounting base adapters are available for installing the focuser on a reflector optical tube or a truss-tube Dobsonian diagonal mirror cage. (A base adapter is required in all cases for installing the focuser on a telescope.) You have your choice of four adapters. The #FT-SFB standard flat base adapter is for installing the focuser on the flat focuser mounting board of a truss-tube Dob’s diagonal mirror cage if you are sure the mounting board is precisely at right angles to the light path. The #FT-SFBL standard flat base leveling adapter is similar, but has four leveling screws for fine-tuning the collimation of the focuser if needed. The #FT-CCB curved base adapter is custom-machined to install the focuser flush against a conventional reflector’s curved optical tube. The #FT-CCBL custom curved base leveling adapter is similar, but has four leveling screws for fine-tuning the collimation of the focuser if needed.
The focuser body is 1.6” tall, exclusive of mounting base. With drawtube fully extended, the top of the drawtube is 3.4” above the focuser base.
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 the 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!”