Re: BOUNCE atm@best.com: Admin request (fwd)

David Beach (D.Beach@irl.cri.nz)
Wed, 22 Mar 1995 10:16:54 +1200

>Basically, I plan to replace the fork mount with a modified german
>equatorial.
>
>A new diagonal mirror will be placed on the scope's optical
>axis to reflect the light from the secondary on the corrector plate out the
>side of the scope and through the declination axis tube. No matter what the
>declination orientation, the image will be sent out the declination axis
>instead of down the original baffle tube.
>
>The declination axis tube will be fairly short (but long enough so that the
>observer will not be in the light path when the scope is pointing toward the
>celestial north), and will be supported by external bearings. A second
>diagonal mirror will be fixed to the support platform directly over the
>polar axis and will reflect the image toward the north celestial pole into
>the eyepiece. This second diagonal and the eyepiece will rotate with the
>entire platform around the polar axis but are not connected to the dec.
>tube.
>
>With this arrangement, the observer sits in comfort, looking down the polar
>axis into the eyepiece no matter what dec. and RA the scope is pointing at.
>(Of course, as the dec and RA are changed the image will undergo some
>interesting rotations from the observers perspective).
>
>I realize that dimensions will be critical, and counter-balancing etc. will
>have to be done.
>
>My questions relate mostly to the optical issues, including:
>
>Has anyone done this before? (Does this kind of a mount have a name?)

Yes, it is called a Springfield mounting, after Springfield, Vermont, where Russell Porter first introduced the concept. Many such have been built, including one of my own. The best reference is the (out of print) Amateur Telescope Making, Book 2, (known as ATM2 to afficionados) pages 333 to 360. However, this original version involves considerable pattern making and casting of aluminum, and modern material technology gets around this.

>
>I'm assuming the additional diagonal mirror needs to be only the diameter of
>the current current tube...is this correct?

Not necessarily. You must make the extra diagonals big enough to accept the full diameter of the light beam at that point *plus* a bit more for the extremes of the field that is to be unvignetted. The first diagonal may have to be somewhat closer to the secondary than you suspect, in order to give sufficient distance to the focus to accommodate the dec and polar light paths. This position would have to be *very* carefully chosen so as not to obstruct the converging primary beam. Were you expecting to use the existing dec axis on the tube? I can't see how you would have enough focal distance without using a Barlow lens.

>
>I'm assuming the dec. tube will have to be baffled to reduce light scatter;
>is there some other source of light pollution problem inherent in the
>design?

No. Baffling follows the normal rule of avoiding grazing incidence surfaces, etc.

>
>Other that the need two use two diagonal mirrors does this design present
>any specific optical degradations

Generally a larger central obstruction because of the first diagonal, with a consequent redistribution of light from the Airy disc into the diffraction rings.

>
>Are there any other mods I could make to improve the design?

Draw it actual size before starting!

>
>Am I crazy?

We're all crazy, Terry!

>
>TIA,
>
>Terry Chrisman

---------------------------------------------------------------------- A. David Beach MNZIP MRNZS................Industrial Research Limited Scientist....(d.beach@irl.cri.nz).........24 Balfour Road Machine Vision Team.(http://161.29.40.7/).Parnell Production, Automation and Control........PO Box 2225, Auckland Ph +64-9-303-4116....Fax +64-9-302-8106...New Zealand ----------------------------------------------------------------------