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Telescope Focuser Comparison Chart
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JMI Focuser
Comparison Chart

Focuser Model
EV- RCF-
Type
Part Number 1n 1c   2n 2c   3n 3c   B 1 MIN1
Telescope-Specific Design Newt
and
Gen		 Cass   Newt
and
Gen		 Cass   Newt
and
Gen		 Cass   All–
(Build
Your
Own)		 Newtonian
and
Generic
Inside Diameter of Drawtube 2" 2"   2" 2"   2" 2"   2" 2" 1.25"
Picture Link (may not match your options as ordered)        
Body
Modular Design Yes Yes   Yes Yes   Yes Yes   Yes Semi Semi
Black Anodized Aluminum Yes Yes   Yes Yes   Yes Yes   Yes Yes Yes
Base
Base is Interchangeable Yes Yes   Yes Yes   Yes Yes   Yes Yes Yes
Refractor Base                   Opt    
Cassegrain Base   Yes     Yes     Yes   Opt    
Combination Base (for surfaces from the specified radius curve to flat) 3.625"     3.625"     3.625"     Opt    
Curved Base with the specified radius (standard/optional)                     10"/
5"		 3.75"/
6"
Flat Base                     Opt Opt
Black-Wrinkle Painted Metal Base                      Yes  Yes
Drive
Dual Speed with Dial Indicator — 0.064" and 0.58" of Travel per Revolution of Knob Yes Yes               Opt    
Single Speed — 0.58" of Travel per Revolution of Knob       Yes Yes   Yes Yes   Opt Yes Yes
Rubber-Grip Gold Knobs Yes Yes   Yes Yes         Opt    
Knurled Aluminum Black Knobs             Yes Yes   Opt Yes Yes
Vertical Load Capacity 8 lbs 8 lbs   8 lbs 8 lbs   4 lbs 4 lbs   8 lbs 2+ lbs 2+ lbs
Motor — Integral Miniature Opt Opt   Opt Opt   Opt Opt   Opt    
Quick-Release Manual Override included with Optional Motor Yes Yes   Yes Yes   Yes Yes   Yes Yes Yes
Locking/Drag Brake on Motorized Version Yes Yes   Yes Yes   Yes Yes   Yes    
Locking/Drag Brake on Manual Version Yes Yes   Yes Yes   Yes Yes   Yes Yes Yes
Drawtube Bearings — Sealed Stainless Steel Ball Yes Yes   Yes Yes   Yes Yes   Yes Yes Yes
Drive Shaft Bearings — Sealed Stainless Steel Ball Yes Yes   Yes Yes   Yes Yes   Yes    
Drive Shaft Bearings — Nylon                     Yes Yes
Drive Shaft Pusher Bearing — Sealed Stainless Steel Ball Yes Yes   Yes Yes         Opt    
Drawtubes
2.0" Drawtube (L)   Yes     Yes     Yes   Opt    
2.5" Drawtube (L)                   Opt   Opt
3.0" Drawtube (L) Yes     Yes     Yes     Opt Yes Yes
4.6" Drawtube (L)                   Opt   Opt
5.75" Drawtube (L)                   Opt    
Compression Ring in Drawtube Yes Yes   Yes Yes   Yes Yes   Yes    
Formula Variable (D)                     L-3 L-3
Formula Constant (X)                     3.25 3.5
Maximum Extension (E) 4.0" 3.0"   4.0" 3.0"   4.0" 3.0"   Var X+D X+D*
Minimum Height (H) 2.3" 2.4"   2.3" 2.4"   2.3" 2.4"   Var 1.8+D 1.9+D*
Drawtube Travel  1.7" 0.6"    1.7" 0.6"    1.7" 0.6"   Var 1.48 1.62*
Maximum Penetration 0.7" 0.0"   0.7" 0.0"   0.7" 0.0"   Var L-1.8 1.1*
Weight
Total Weight of Standard** Focuser without Motor (oz.) 22 21   21 17   18 17   Var 12 8
Total Weight of Standard** Focuser with Motor (oz.) 24 23   23 19   20 19   Var    
Mounting
Spacing of Mounting Holes (Square Configuration) 2.75     2.75     2.75     Var 2.75 2.625
Cassegrain Bases — 2" (EVBASC)   Yes     Yes     Yes   Opt    
SCT Large Format Visual Back Bases — 3" (EVBASL)   Opt     Opt     Opt   Opt    
SCT Large Format Visual Back Bases — 4" (EVBASX)   Opt     Opt     Opt   Opt    
Refractor Base (EVBASR)                   Opt    
Other Telescope-Specific Mounting available (see Build-Your-Own EV Focuser) Opt Opt   Opt Opt   Opt Opt        
Output Adapters
1.25" Eyepiece Adapter (ADPT2-1.25) Std Opt   Std Opt   Std Opt   Opt Opt  
1.25" Eyepiece and T -Thread Adapter (ADPT2-1.25T) Opt Opt   Opt Opt   Opt Opt   Opt Opt  
SCT Threaded Output Adapter — 2" (ADPT2THREAD) Opt Std   Opt Std   Opt Std   Opt Opt  
SCT Large Format Threaded Output Adapter — 3" (ADPT3THREAD)                        
Upgrades
EV-1 Upgrade (EVUPG1)       Opt Opt   Opt Opt   Opt    
EV-75 Upgrade (EVUPG75) Opt     Opt     Opt     Opt    
Accessories
Digital Read Out (DRO)** Opt Opt   Opt Opt   Opt Opt   Opt    
Type
Telescope-Specific Design Newt
and
Gen		 Cass   Newt
and
Gen		 Cass   Newt
and
Gen		 Cass   All–
(Build
Your
Own)		 Newtonian
and
Generic
Part Number 1n 1c   2n 2c   3n 3c   B 1 MIN1
JMI Focuser
Comparison Chart		 EV- RCF-

Focuser Model
Definitions

Std = Standard Feature
Opt = Optional Feature
Var = Varies Depending on Options Chosen


Cass = Cassegrain Telescopes (including SCTs)
Newt = Newtonian Telescopes
Refr = Refractor Telescopes
Gen = Generic Application


Input is the end in which the light enters ( >[] ) and Output is the end from which the light exits ( []> ).
 
Some values in the chart  are shown as formulas, including variables or constants (see example 1 below).  You can find each variable or constant (shown in parentheses) along with its description in the left column (see example 2 below).  The value of the variable will be listed in the description in the left column (see example 3 below) or will be listed within the chart itself as a value or formula (see example 4 below).

1.
2.
3.
4.

 E-H
Maximum Extension (E)
3" Drawtube Length (L)
L+X		 To calculate this value, subtract the value of the variable H from the value of the variable E.
The variable E represents the maximum extension of the drawtube.
L=3
Drawtube length L plus formula constant X (which differs depending on the particular focuser).

See picture (below) showing dimensions, including some of the formula variables.  All lengths are in inches.

If the formulas in this table are more frustrating than helpful, please contact us for the information you need.
Miscellaneous Information

*  For an RCF-mini focuser with a 2.5" drawtube, Maximum Extension (E) is 3.0, Minimum Height (H) is 1.9, Drawtube Travel is 1.1 and Maximum Penetration is 0.6.
**  Please note that additional options, such as DRO and DFC, will increase the weight and decrease the number of knobs on the focuser.
Output adapters and extension tubes are not included in weight and height values but mounting adapters are included.
We recommend using software like NEWT for optimizing the aftermarket placement of our focusers on your telescope.   NEWT is a Newtonian telescope design program that lets you quickly try various design parameters and see the results by casting rays through a telescope model on the screen.
For best results printing this chart, select landscape mode under print options.  You may wish to disable any extra header and footer information placed on the page by your browser.