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Telescope Focuser Comparison Chart

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JMI Focuser
Comparison Chart

Focuser Model

EV- EV-XT RCF-
Type
Part Number 1n 1c 1r 2n 2c 2r 3n 3c 3r B XTn XTc 1 MIN1
Telescope-Specific Design Newt
and
Gen
Cass Refr Newt
and
Gen
Cass Refr Newt
and
Gen
Cass Refr All–
(Build
Your
Own)
Newt
and
Gen
Cass Newtonian
and
Generic
Inside Diameter of Drawtube 2" 2" 2" 2" 2" 2" 2" 2" 2" 2" 3" 3" 2" 1.25"
Picture Link (may not match your options as ordered)  
Body
Modular Design Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Semi Semi
Black Anodized Aluminum Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Std Std Yes Yes
Gold Anodized Aluminum                     Opt Opt    
Base
Base is Interchangeable Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
Refractor Base     Yes     Yes     Yes Opt        
Cassegrain Base   Yes     Yes     Yes   Opt   Yes    
Combination Base (for surfaces from the specified radius curve to flat) 3.625"     3.625"     3.625"     Opt 10"      
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 Yes             Opt Yes Yes    
Single Speed — 0.58" of Travel per Revolution of Knob       Yes Yes Yes Yes Yes Yes Opt     Yes Yes
Rubber-Grip Gold Knobs Yes Yes Yes Yes Yes Yes       Opt Yes Yes    
Knurled Aluminum Knobs             Yes Yes Yes Opt     Yes Yes
Vertical Load Capacity 8 lbs 8 lbs 8 lbs 8 lbs 8 lbs 8 lbs 4 lbs 4 lbs 4 lbs 8 lbs 8+ lbs 8+ lbs 2+ lbs 2+ lbs
Motor — Integral Miniature Opt Opt Opt Opt Opt Opt Opt Opt Opt Opt Opt Opt    
Quick-Release Manual Override included with Optional Motor Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
Locking/Drag Brake on Motorized Version Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes    
Locking/Drag Brake on Manual Version Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
Drawtube Bearings — Sealed Stainless Steel Ball Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
Drive Shaft Bearings — Sealed Stainless Steel Ball Yes Yes Yes Yes Yes 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 Yes Yes       Opt Yes Yes    
Drawtubes
2.0" Drawtube (L)   Yes     Yes     Yes   Opt        
2.5" Drawtube (L)                   Opt       Opt
3.0" Drawtube (L) Yes     Yes     Yes     Opt 3.4" 3.4" Yes Yes
4.6" Drawtube (L)                   Opt       Opt
5.75" Drawtube (L)     Yes     Yes     Yes Opt        
Compression Ring in Drawtube Yes Yes Yes Yes Yes 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" 9.0" 4.0" 3.0" 9.0" 4.0" 3.0" 9.0" Var 4.9" 5.1" X+D X+D*
Minimum Height (H) 2.3" 2.4" 4.5" 2.3" 2.4" 4.5" 2.3" 2.4" 4.5" Var 3.7" 4.0" 1.8+D 1.9+D*
Drawtube Travel  1.7" 0.6" 4.5"  1.7" 0.6" 4.5"  1.7" 0.6" 4.5" Var 1.2" 1.2" 1.48 1.62*
Maximum Penetration 0.7" 0.0" 1.2" 0.7" 0.0" 1.2" 0.7" 0.0" 1.2" Var 0.0" 0.0" L-1.8 1.1*
Weight
Total Weight of Standard** Focuser without Motor (oz.) 22 21 43 21 17 39 18 17 39 Var 46 45 12 8
Total Weight of Standard** Focuser with Motor (oz.) 24 23 45 23 19 41 20 19 41 Var 47 47    
Mounting
Spacing of Mounting Holes (Square Configuration) 2.75     2.75     2.75     Var 3   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)     Yes     Yes     Yes Opt        
SCT Large Format Visual Back Adapter — 3" (ADPT3XT)                     (Opt) Std    
SCT Large Format Visual Back Adapter — 4" (ADPT4XT)                     (Opt) Opt    
Other Telescope-Specific Mounting available (see Build-Your-Own EV Focuser) Opt Opt Opt  Opt Opt Opt Opt Opt Opt          
Output Adapters
1.25" Eyepiece Adapter (ADPT2-1.25) Std Opt Opt Std Opt Opt Std Opt Opt Opt     Opt  
1.25" Eyepiece and T -Thread Adapter (ADPT2-1.25T) Opt Opt Std Opt Opt Std Opt Opt Std Opt     Opt  
2" Eyepiece Adapter (ADPT3-2)                     Yes Yes    
SCT Threaded Output Adapter — 2" (ADPT2THREAD) Opt Std Opt Opt Std Opt Opt Std Opt Opt     Opt  
SCT Large Format Threaded Output Adapter — 3" (ADPT3THREAD)                     Opt Opt    
Upgrades
EV-1 Upgrade (EVUPG1)       Opt Opt 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 Opt Opt Opt Opt Opt    
Type
Telescope-Specific Design Newt
and
Gen
Cass Refr Newt
and
Gen
Cass Refr Newt
and
Gen
Cass Refr All–
(Build
Your
Own)
Newt
and
Gen
Cass Newtonian
and
Generic
Part Number 1n 1c 1r 2n 2c 2r 3n 3c 3r B XTn XTc 1 MIN1
JMI Focuser
Comparison Chart
EV- EV-XT 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.
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