Re: ATM Curious about Central Obstruction Determinations

[BodyByJay@aol.com]

In a message dated 98-07-03 12:52:42 EDT, you write:

<< >In general conversation, if asked what the central obstruction is on the
 >scope, which should I mention, the obstruction of area ? 9%,   Or the
 >obstruction of diameter ? 30%. >>

IMHO the best way to accurately state the percentage of obstruction (the area
of the primary obstructed by the secondary and holder) would be to measure the
diameter of the holder and the thickness and lenght of the portion of the
spider vanes that are within the incoming light cone and compute the area of
all of these things.  Then compute what percentage of the primary is shadowed
by the obstructions.  Seems the shadow would actually obstruct more of the
primary.  I don't know how to compute the actual shadow though... maybe
someone else does?

I understand some commercial spider vanes have thicknesses of amost 0.10".  It
would also be interesting to see what the actual thicknesses are of some of
these commercial spider vanes for comparison purposes.  I never see this
listed in the devices spec or drawings.    Jay D. Anderson