Re: ATM Advice w/ specs on 12.5" Newt

[mark cowan <cowanm@open.org>]

BodyByJay@aol.com wrote:

> I just purchased a JMI 2" NGF-DX2 focuser for the 12.5" Newt I am building.
> Apparently the racked in height is 1.85" (including .25" base and 0.10"
> plate).  When fully racked in about 1.1" of draw tube extends into the OTA.
> The fully racked out height is about 3.0".
>
> The draw tube, when fully racked in protrudes into the light cone .25".  I
> determined this by taking the ID of the OTA of 15.4 and subtracting the front
> aperature diameter of 13.7" to get a space of 1.7".  Half of 1.7" is .85" and
> so that is the space between the OTA inside wall and the light cone.  The
> focuser protrudes to about 1.1" so the draw tube can come into the light cone
> when fully racked-in by .25".   Is this bad?



> Optical diameter of primary:                            12.3 (12.5")
> Optical diameter of diagonal minor axis:        2.12 (2.14")
> Focal ratio:                                                                    f/5.309
> Focal lenght:                                                                   65.3"

By my calculation:  arcsine(((15.4/2 - 1.1)-(12.3/2))/65.3) = 0.4 degrees is the edge of 
the
intrusion into the FOV.

IE, if you weren't looking at a 0.8 degree FOV or larger, there'd be no obstruction at 
all.  And
the obstruction will only be for 2" along the extreme edge of the widest field.  I don't 
think
it'd matter; the benefits of using the low profile focuser far outweigh it...  8^)


Mark Cowan
Salem, Oregon