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

[kevin@aquilagroup.com (Kevin Ferguson)]

At 02:22 PM 8/21/96 +0100, Klaus Peter Schroder wrote:
>The longest focal lengths found with barlows, however,
>are -6cm, giving you 12 cm from the barlow, over the diagonal into
>the focusser. 

My speculation on why that is:
For a normal (accessory type, not built in) Barlows, longer FL would need to
have bigger diameter to avoide vignetting, as it would need to be positioned
farther into light cone. This would require more in focuser travel in all
scopes, and would intrude on aperture in Newtonians. Besides, nobody wants
an ungodly long tube sticking out thier focuser, so astronomical Barlow
lenses are designed to be rather fast (short FL). 

Edmund lists Negative acromats from -25 to -400 mm  focal length.  Thier
"RKE Barlow" is listed as having a -109 mm FL (it appears to extend ~75mm
into focuser, beyond shoulder) ...So there exist off the shelf solutions.
Most lenses sold as telescope Barlows are >= 1" diameter, and the idea with
a small scope is to be at about half that...so "standard" barlow lenses are
not the ideal anyway.

With a longer FL barlow, it may be feasable to go beyond "normal" Barlow
limit of x3, to say x5 or so, and still have acceptable color...assuming
that aberations are better controlled with increasing f# as in positive
lenses. (seems like they aught to be, but not willing to ray trace and find
out)  Increasing the power gives more throw, so a bigger primary could be
accomidated.

At any rate, this "trick" is most needed with rather small scopes.  A
standard configuration results in a rather hefty obstruction, and  increase
in EFL allows planitary magnifications with nice mid-focal length eyepieces.
Planets are good targets for small scopes, so it is sensable to optimize for
that end.   Doing this with a large aperture scope is likely to result in
unusably long EFL, and would make the scope less useful for the deep sky
work aperture lovers lean toward.

In addition to designs in Sam Brown, S&T had an all spherical job based on
this idea. 6" f/8 (effective) with ~ 3 foot total tube length.  June '94 I
think (?? Sepia toned picture of Tanguska devistation on front cover, for
sure)  The homade Barlow lens was designed to correct primary's spherical
aberation, as well as making the scope physically shorter.

Finally, if you have copy of Sam Brown, you will find some designs using
transfer lenses, which would allow the primary image to be placed very near
the diagonal...on the diagonal if you don't mind looking at the dust on one
edge. 

-KF- 
with appologies for mixed units.

 
"Is that tubing 4 inches diameter?" 
"No, gov, it's 100mm...How many feet would you like?"

-Reported by english author writing for home shop machinist.