RE: Picking a Mark IV Lens

["Griffing, Dan" <DAN.GRIFFING@kla-tencor.com>]

Tom,

Aren't you mixing two separate issues.  Star magnitude
affects the stars you can see in the whole field of view,
even in the center where its clearest.  Your 70% estimate
of good area on the Mak affects the width of the useable field
which will affect the number of units you will need to scan the
same area of sky, but not the end result.

I also wish to caution you about the big factor of risk of
committing yourself to a course of action involving a
large unknown, i.e. the development of a "home-built"
system in which you will have to be successful in
so many areas for the assumptions of your analysis
to be true.  With the Mak, nearly everything is 
known about it.  For the Mak unknowns, you can 
build a single prototype without being committed to 
using the Mak for the whole project.

On the other hand, the home-built must be designed
and constructed first which is a significant percent of
the project's cost which must be spent even to test
your hypothesis.

My bottom-line advice is that it is usually more
financially prudent to buy something "off-the-shelf"
than to design it yourself, unless you have an
overwhelming justification to do otherwise.
On the other hand, just wanting to design and build 
something yourself is part of what amateur astronomy 
is all about and it is such  a justification, especially if 
its your own money.  But the home-built option is a 
more risky and less financially conservative path. 

I do applaud your financial analysis, however, in evaluating
the end goal and how much each incremental unit of quality
will cost.  It is quite proper to evaluate ends versus means
especially with respect to those who will have to be putting
up the means.

Thanks,

Dan Griffing
Engineer