This precision-machined Feather Touch #FTF3015B-A 3" rack and pinion focuser and 117mm adapter package from Starlight Instruments is designed for non-slip/no backlash focusing during imaging with the Astro-Tech AT10RC and AT12RC Ritchey-Chrétien astrographs. The 109mm threaded adapter of the standard #FT-3015 has been replaced by a 117mm adapter in this package to allow the focuser to be used on an Astro-Tech 10” or 12” R-C. The package price is less that the cost of the focuser and adapter bought separately.
The 117mm adapter threads onto the rear cell of the Astro-Tech 10” and 12” Ritchey-Chrétien astrographs. The focuser body in turn slips into the adapter and is held firmly in place by three large Delrin-tipped screws that also act as the focuser rotation mechanism. A built-in Teflon-lined focuser rotation collar allows the FT-3015 focuser to be smoothly rotated a full 360° in the 117mm adapter for the best photographic composition prior to critical focusing. The three large focuser rotation lock knobs assure a solid non-tilt locking of the focuser at the desired angle.
The fine-pitch brass rack and hardened stainless steel pinion gear of this Feather Touch focuser are factory-matched and burnished to provide a very smooth engagement between the rack and the pinion. The 3” diameter drawtube glides on three stainless steel straps that mate with Teflon bearing surfaces to provide very tight yet extremely smooth motion. This design eliminates the need for grease between the drawtube and housing. A greaseless drawtube prevents the attraction of dirt that might eventually cause focusing smoothness problems. The drawtube has 13 machined internal knife-edge baffles, finished in glare-resistant matte black, to reduce reflections and improve contrast.
A built-in adapter steps down the 3” diameter drawtube to a 2” accessory holder that uses a soft brass compression ring with three thumbscrews to hold 2” accessories firmly in place. The three thumbscrews are spaced equally around the accessory holder to tighten the compression ring evenly and reduce the possibility of tilt in the imaging chain such as you might find with a single thumbscrew accessory holder.
There are two coarse focusing knobs for focusing with either hand. The right knob also has a smaller concentric knob with a 10:1 ratio reduction gear for microfine focusing. 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. Combined with the ultra-low backlash focuser design, this provides exceptionally precise focus control during critical CCD imaging. The metal focusing knobs have ribbed gripping surfaces so they are easy to operate, even while wearing gloves or mittens in cold weather.
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 significant vertical component. The focuser therefore contains an adjustable tension internal brake system to minimize the possibility of focus shift when focusing with heavy imaging systems without.
The brake consists of a lever arrangement internal to the pinion block. Adjusting the thumbscrew on the pinion block under the focuser 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. A large lock knob on top of the focuser body provides an additional positive lock on your photographic focus. Partial tightening of the internal brake offers a virtually infinite range of braking force to accommodate various accessory loads.
The 3" diameter focuser drawtube has two laser-engraved scales (one on either side of the drawtube). These are marked in 1mm increments so you can note individual focuser positions for easy return to the approximate correct focus when switching between different imaging equipment combinations.
To minimize drawtube flexure under heavy loads when imaging, the drawtube travel of the FT-3015 focuser is kept short, at 38mm (1.5”). To make up for the longer all-purpose drawtube travel found in scopes used for both visual work and imaging (typically 80mm to 135mm or more), the Astro-Tech R-Cs come with three thread-on extension rings (two 1” and one 2”) to fine-tune their back focus. These can be installed singly or in combination between the rear cell and the optional #M117x1 adapter ring to place your camera’s imaging chip at the telescope focal plane. Many long imaging equipment trains will require no extensions.
For example, with the 10” AT10RCF, the available back focus with the focuser alone and no extension rings installed is 127mm (5”). This is reduced in 25mm (1”) steps by adding the appropriate extension rings as needed. One 1” extension and the focuser provide 102mm (~4”) of back focus. The 2” extension ring and focuser provide 77mm (~3”) of back focus. The 2” extension ring and one 1” extension ring, plus focuser, provide 52mm (~2”) of back focus. The 2” extension ring, both 1” extension rings, and the focuser provide 27mm (~1”) of back focus.
With the 12” AT10RCF, the available back focus with the focuser alone and no extension rings installed is 180mm (~7”). This is reduced in 25mm (1”) steps by adding the appropriate extension rings as needed. One 1” extension and the focuser provide 155mm (~6.1”) of back focus. The 2” extension ring and focuser provide 130mm (~5.1”) of back focus. The 2” extension ring and one 1” extension ring, plus focuser, provide 105mm (~4.1”) of back focus. The 2” extension ring, both 1” extension rings, and the focuser provide 80mm (~3.1”) of back focus.
With careful choice of extension rings, it can be seen that there will be no situation when the focuser drawtube will need to be extended more than a fraction of its 38mm travel, providing an exceptionally rigid camera mount for whatever equipment load you might want to add. A little experimentation will be called for to determine what combination of focuser and extension ring(s) will be needed for your particular combination(s) of imaging components.