jmrp

Lee - 

it can be done - if you are intent on cutting the F10 tube you can get 
an F5 telescope using the following information and a commercially 
available 0.5x focal reducer on the focuser tube end:

D80F10 to D80F5    this is just one example - cut 100mm off tube    
if this does not work for you - you can use a 100mm extension tube
at the focuser (no focal reducer) to restore original focal length                                                                                                                                                                                                                                                                                                                                    

Focal Length Objective             800mm    
Focal Length Reducer(FR)         200mm    
FR dist behind objective lens   600mm    
Back Focal Length                    100mm
Effective Focal Length              400mm
Total Length                             700mm

Bob

A manual GoTo was described using the values for RA/Dec  

https://www.dpreview.com/forums/thread/4363903

Alternatively I have done a similar GoTo using the Hour Angle (HA) Circle settings and a graduated setting circle on the Dec bracket - I then use the HA Stellarium value for any DSO