Re: ATM Focal Length of first mirror

[akelly@ghgcorp.com (Albert F. Kelly)]

>I would plan the mirror cell so you could have considerable adjustment 
>there if you need it.  As I remember earlier discussion here (maybe six 
>months or so ago) suggesting that the focal point of a newtonian should 
>be about 0.5 inch beyond the end of the focusing tube at its lowest 
>position.  Perhaps another member of the group can confirm or correct 
>this if it is in error.
                
I would not depend on this approximation for a variety of reasons.  First,
it leaves insufficient wiggle room for reaching the focal plane of all
possible eyepieces. Second, it does not consider the needs of other imaging
systems than the eye.  The Cookbook 245 CCD camera, for example, requires at
least 0.9" beyond the end of the focusing tube since the chip is located
0.9" inside the front face of the housing.  Of course, if the CB245
photographer wants to use a turret and/or filter wheel to make object
acquisition simpler or to do tricolor work, then additional spacing will be
required, probably at least another 1" or so.  

My rule of thumb would be to set the focal plane two inches outside of the
end of the focusing tube at its lowest position.  Of course, if one is
certain that CCD or film photography is not a goal and has determined the
full suite of eyepieces that will be used, this guideline may be invalid.

Al Kelly