ATM Barlow extensions

[Suzanne Sarlette/Gerald Pearson <suegerry@mut1.muscanet.com>]

Nils Olof,

I'm posting my reply to the list, because someone out there might be
able to help me understand the "peculiarly small" magnification gains
which I've "measured" with extensions between barlows and oculars.

> Funny you should mention it, I have been trying to tout this idea of
> extension tubes but have very little following AFAIK. This is a wheel
> anybody could invent, I have and maybe you, if not I would like to hear
> where you got the idea (if not from me) and what your experiences are,

"Advanced Telescope Making Techniques", Allan Mackintosh ed., "Volume 2
Mechanical" -- published by Willmann-Bell, Inc.  ---  collections of
edited articles from the Maksutov Club Circulars  ---  2 articles:      
"Barlow lenses, Photographic and Visual Considerations in Use",
P.H.Morgan, p.227-231;    "The Barlow Lense:  Wlhat it is and How to Use
it", R.K.Dakin, p.232-233.

In particular, I was surprised and impressed with Dakin's claim that
"Two good Barlow lenses have been tested at magnification ratios from 2x
to 6x with no appreciable difference in performance."


My results so far have been OK (excellent image quality) and
_qualitatively_ understandable, but a given length of extension seems to
give me less magnification increase than I calculate/estimate that I
should obtain.  

A friend and I tried to measure magnification gain by looking at a
vertical stick about 15 meters away with our scopes, and measuring
exactly how much of the length of the stick is inside the fov of the
ocular+barlow, both with & without the extension;  magnification gain
provided by extension  =  stick_length_without_extension /
stick_length_with_extension.    We measured gains in the vicinity of
only about 1.2x for extension lengths of ~40mm, using a Celestron 2x
Ultima barlow & a Klee 2.8x barlow.   Both of these barlows are short
(roughly 75 mm in length), so I would "guesstimate" that the barlow
lenses must be f.l. = -25mm (negative 40 diopters) or stronger;  this
would predict something like _twice_ the magnification gain that we
actually observed.

Now, our measurement method assumes that the apparent fov is the same
with & without extension, which isn't necessarily true.  But to explain
an apparent magnification gain which is too small, the apparent fov
would have to be ~15% smaller _without_ the extension!  ??????  Does
this make sense???   To eliminate the "vignetting" complication, we plan
to repeat our measurments some time using a reticle at the focal plane
of an eyepiece, and measure "stick lengths" between two reticle marks
(instead of between the field stops).


Now, I would _like_ to be able to calculate the length of extension
which is required to produce whatever magnification gain is desired --
for 2 reasons:    1.  I want to be able to make extension tubes for
myself (& friends & relatives) which are "ideal"  (1.4x & 1.6x).     2. 
I really want to understand what is going on. 


Any thoughts, anyone?

        -- Gerry