Re: ATM More Spheroidalage

["B. Pepin" <atmjedit@conceptsnet.com>]

Nice to read someone speaking strictly on this subject. I have long
suspected the old rule of thumb on spheroidal mirrors up to 6 inches
requiring f/10 for excellent performance was just hokum.

I have an Edmund Scientific mirror, spheroidal, part of one of the kits they
marketed in the 1970s, which I built into an openwork PVC truss tube
Newtonian. They must have had this kind of standard in mind. The
measurements are 83mm x 1203mm i.e. f/14.5, diagonal diameter is 19mm. Based
on star images I suspect the figure is certainly around 1/8 wave. Gives very
fine planetary views and contrast is so good that I am able to make out
details in M31 under a dark sky.

Barlow


----- Original Message -----
From: Dominic-Luc Webb molmed <Dominic.Luc-Webb@molmed.ki.se>
To: Anthony Stillman <atmer@flash.net>
Cc: <atm@shore.net>
Sent: Friday, August 24, 2001 5:26 AM
Subject: Re: ATM More Spheroidalage


>
>
> On Fri, 24 Aug 2001, Anthony Stillman wrote:
>
>    > PS  Below is a table of minimum focal ratios required for spheroidal
>    > mirrors.  Differing wavefront criteria (in waves) and diameters (in
>    > millimeters) are given.  Lambda is 574 nm
>    >
>    >        wavefront    1/10       1/8        1/4        1/2         1
>    >    diameter
>    >
>    >      80 mm          14.0       13.0       10.3        8.2       6.5
>    >     100 mm          15.0       14.0       11.1        8.8       7.0
>    >     150 mm          17.2       16.0       12.7       10.1       8.0
>    >     200 mm          19.0       17.6       14.0       11.1       8.8
>    >     250 mm          20.4       19.0       15.0       11.9       9.5
>    >     300 mm          21.7       20.1       16.0       12.7      10.1
>
>
>
> Well, well... look what the wind blew in... Hi Anthony.
>
> Along these lines, I have been considering the case of the
> 1/10th wave parabolic primary combined with a spherical
> versus hyperbolic secondary in a classical Cassegrain design.
> Don't recall if it was this or another poster asking about
> the folded scopes, but this is of general interest. The above
> values are not new to me and always seemed like pretty radical
> focal ratios for most users. In general, they are not very
> realistic, aside from making some kind of fashion statement.
> Some of these would have tubes over 5 meters long and the
> magnification would be too high for most evenings!
>
> For a Cassegrain, there may be some sanity to this. Perhaps
> a 1/10th wave parabolic primary with a lower focal ratio,
> which we can all make, combined with a properly chosen
> amplification sperhical secondary would turn some Newts
> fairly easily into decent Cassegrains with markerdly higher
> magnification. I like this for moist areas like where I am
> living where we have a lot of mediocre nights and some rare
> nights when much higher magnifications are possible. A simple
> change of the secondary turns a widefield scope into a high
> magnification scope for viewing planets. Take for instance
> the 200 mm mentioned above. A parabolic 8" F/6 should
> behave similar to the F/19 above by use of a 3-amplification
> cass secondary. I would be be tempted to try this with a
> spherical seondary.
>
>
> Cheers,
>
>
> Dominic
>
> North 59 37' 30"
> East  17 48' 10"
>
>