Gear or motor speed selection for Larscan camera.

[Sofjan@xxxxxxx]

Hi Guys

    Today during my short lunch hours I was trying to come up with a 
calculation so that I don't have to build the camera first before knowing the 
motor rpm and the resulting shutter speed. 

  Here goes off my table napkin. Please review it and tell me if I got it 
wrong. 

 Given 
Foc. Length 15 mm 
Formula from Dusariev book for calculating the resulting shutter speed. 
 
Length of negative for 360deg = 15*2 * 3.1416 = 94.248  = 94.25 mm 
Slit width = 1.5mm 
I mark the 220 negative for the length of 94.25 mm  and count how many 
revolution it took for the spool to wind the complete length.  I come up with 
 2.5 revolution when the spool is empty and 1.5 rev when the spool is almost 
full  so I used 2 rev as the number.
Motor rpm = 24 RPM 

  
So: 
   For 1:1 gear  ratio between motor  and take up spool  
   Time needed for the camera to wind tru 360deg worth of neg. is   
    Tn  =  2 Rev  /  24 Rev/min 
           =  1/12 Min  = 60 sec / 12 = 5 sec  
    
    equivalent shutter speed =  Tn divided by  Number of slit width in 360 
deg neg. 
                                            =    5sec /  ( length of 360 deg 
neg / slit width) 
                                            =    5 sec /  ( 94.25 mm /1.5 mm)
                                  =     1  /  12.56  Sec 
                              =    Or  just round to 1/15 sec 

    For 2:1 gear  ratio between motor  and take up spool  
   Time needed for the camera to wind tru 360deg worth of neg. is   
    Tn  =  2 Rev  /  48 Rev/min 
           =  1/24 Min  = 60 sec / 24 = 2.5 sec  
    
    equivalent shutter speed =  Tn divided by  Number of slit width in 360 
deg neg. 
                                            =    2.5sec /  ( length of 360 
deg neg / slit width ) 
                                            =    2.5 sec /  ( 94.25mm /1.5 mm 
)
                                  =     1  /  25 Sec 
                              =    Or  just round to 1/30 sec 


If we can get a 120 rpm motor  then 

   Tn = 2rev / 120 rev/min
         = 1/60 min = 1 sec 
    equivalent speed =  1  sec /( 94.25/1.5 ) 
                                 =   1/62 sec  = 1/60 sec  


   How about my calculation  guys?  Please tell me if i'm right on track . 


Sofjan Mustopoh