ATM atm: secondary size with use of barlow attached to it

[Klaus Peter Schroder <schroder@ast.cam.ac.uk>]

Hi all, 
have just gone through the digests from the time of my absence.
Just a note to the barlow/secondary topic:

This is a very good idea which I came across many years ago.
It is fantastic for all high resolution work. In practice, however,
there is a limitation that has not yet been noted here:

To maintain a 2x factor on the focal length, any barlow must be
placed just one-times its (negative) focal lenght in front of the
original focus, which in turn moves out by another f-barlow. This
gives you twice the barlow lens's focal length from its place to
the final focus. The longest focal lengths found with barlows, however,
are -6cm, giving you 12 cm from the barlow, over the diagonal into
the focusser. This is just alright for a 6" newtonian, manageable for 
an 8" with sled-focusser but does not work for any larger primaries.
With 10" (and more), you inevitably get a 3x (and more) of a factor. 
That may still be fine if you specialize in planets, but should be kept
in mind to dampen enthusiasm of those, who are looking for a general
purpose scope. - Of course, if you could taylor-make your barlow by
grinding and cementing your own negative achromat...... I know why I
envy all glass-pushers (being an ashamed optics-buyer, myself)!

Cheers, Klaus-Peter