JMI Reverse Crayford Focuser

JMI Reverse Crayford Focuser


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JMI Reverse Crayford Focuser is a light-weight, low-profile 2″ focuser with a new revolutionary design – JMI’s low-cost, high-quality answer to plastic focusers.

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JMI Reverse Crayford Focuser

JMI Reverse Crayford Focuser is a light-weight, low-profile 2″ focuser with a new revolutionary design – JMI’s low-cost, high-quality answer to plastic focusers.  In the reverse Crayford design, the drive-shaft pressure is applied to a bar which “pulls” the drawtube against its supporting bearings as apposed to directly “pushing” the drawtube as seen in the normal Crayford design.  This design gives zero image shift and zero backlash as with our EV focusers.

The highly acclaimed Crayford design uses high pressure on a metal drive shaft pressing against a 3″ metal drawtube which is held by bearings on the opposing side.  This direct surface-to-surface drive mechanism eliminates backlash and shifting of the drawtube as seen in other designs such as the rack-and-pinion focuser, and allows for very fine adjustments.

A flat or curved base choice must be specified up on purchase.  For curved bases, you need to specify the radius.  If you don’t, you will get the standard size radius.  The Reverse Crayford Focuser has a standard radius of 10 inches (5″ diameter).

Please place a note in the shopping cart comments area specifying “Flat Base” or “Curved Base” and the appropriate radius for the curved base (Diameter = 2 x Radius).

Note:  When using a laser collimator, keep in mind that there will be slight deviations in the laser beam when moving the drawtube through the full length of travel.  This deviation is not noticeable visually, is only a small fraction of the amount present with a normal rack-and-pinion focuser and is completely eliminated with the higher quality of the EV focusers.  Due to the design of this focuser, the knobs are at a 90-degree orientation from most, placing the drive shaft perpendicular to the optical tube when using a curved base.


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Package Includes:

  • RCF Focuser

JMI Focuser Comparison Chart

Q.What are the differences between the NGF and RCF focusers?
A.The RCF focusers are based on a new revolutionary reverse Crayford design (U.S. Patent No. 6,297,917).  This new design allows us to produce light weight metal focusers that are price competitive with plastic versions.  Another difference is the ability to change out the base of the focuser by removing two screws which makes it easy to change between a flat base and a curved base.   The bases are made from formed aluminum rather than machined aluminum.   One more difference is the orientation of the knobs.  On NGF focusers with a curved base, the drive shaft is normally parallel with the optical tube.  The design of the RCF focuser places the drive shaft perpendicular to the optical tube when using a curved base.
Q.Can I add a Motor to my RCF focuser at a later date?
A.Yes, the Motor attaches very easily.  A gear slides onto the drive shaft and is secured with a setscrew and the motor unit attaches to the drive block with two screws.  
Q.Will an RCF focuser work on my Meade Starfinder?

“I only wish I had bought two [RCFs].”  (P. G.)

The RCF-mini1 focuser is great.  There was very little trouble fitting it onto my small scope.  Works great!  …  Where is the deviation when collimating?  I have none, but that’s good.”  (G. G.)

“Just a quick thank you for the prompt and excellent service in delivering my latest JMI purchase, an RCF focuser kit.  JMI is proving now, more than ever, to be a real leader in amateur astronomy.  Thanks again.  (R. M.)

“Many thanks to you.  The order arrived in very good time, is assembled (and wowed a number of friends with the high quality parts and design) and is to be installed in my dob…  Congratulations on an excellently made product.”  (F. H.)

“I just wanted to thank you people for producing an excellent low-cost focuser.  I purchased a Monorail focuser a few weeks ago and I am impressed with the quality of the design.  I am a quality control manager of a machine shop in Texas and have seen a lot of mechanical products and know that this focuser is an excellent quality focuser.  I recommend that anyone who needs a high quality focuser at an affordable price should purchase one of yours.”  (R. N.)

“Received my replacement part and the new invoice today.  Thanks.  As always, it’s a pleasure dealing with some of the best ‘good guys’ in the astro business!”  (R. M.)

“After a long and exasperating search, I have not been able to find a better deal for this type of focuser.  It appears that JMI puts great emphasis in the quality, function and reasonable price of their instruments.  I truly look forward to receiving my order and seeing for myself what the rave is all about.”  (M. D.)

“The whole amateur community owes companies like yours a very great deal; uncompromising quality at the cutting edge is what has driven this hobby forward so amazingly in the last twenty years and where our entire future lies.”  (J. K.)