Posts Made By: William Chang

Well, on first glance it's a crazy idea, but then after a while the benefits begin to surface.

Both scopes are horizontal so won't have differential balance issues due to altitue etc. Altitude rotation can be either well-supported scope(s) or front mirror(s). The whole structure rotates on an azimuth table. Easier co-collimation mechanism than long, cantilevered OTA(s). To adjust IPD just use the built-in focuser(s). Fine-focus with a helical inserted into the back diagonal(s). Viewing height is constant, but viewing angle is not so good unless you added another diagonal. Availability and cost of large, high quality elliptical front mirror(s) limit aperture to 4" or less, unless you ground your own.

-- William

In general, to figure such things out I do a mental raytrace of a beam at the top (12 O'clock), and one at the side (say 3 O'clock), remembering that the focused image will be reversed. I.e., without any diagonals whatsoever, top stays top so it ends up bottom, right stays right so it ends up left. With a star diagonal, top becomes bottom so it ends up top, right stays right so it ends up left. Hope this helps,

-- William

42, with 13 and 11-year-olds, and indulgent wife (of me).

The great meteor storm a few years back, and somewhat enthusiastic family, renewed my interest after a very long hiatus. Three scopes deserve a lot of credit too: an Astroscan for "being there", a TeleVue Genesis, and an 8" f/6 Dob "Hamler Classic".

-- William

I was going to suggest it might be a Cassegrain primary whose secondary has been lost -- but your mirror is pretty thin for that. Still, it could be an ATM. (I happen to have collected a couple sets with around f/3.6 primary...)

I think it doesn't hurt to give it a try. Could make a very nice RFT!

-- William

It took me a while to derive the angle of incidence and then the rotation of the ellipse (Figure 3).

For the geometric optician: Consider isosceles triangle (-1,-2,0)(-1,-1,0)(0,0,0) formed by the lightpath. It is bisected by (-1,-1,0)(-1/2,-1,-1/2) into two 30-60-90 triangles. The bisector is orthogonal to the prism face, hence angle of incidence = 60*. Project the lightpath (-1,-1,0)(0,0,0) parallel to the bisector onto the prism face (-1,-1,0)(0,0,1)(-1,1,0), yielding (-1,-1,0)(-1/2,0,1/2). This is the orthogonal projection of the bending lightpath with respect to the reflective surface; i.e. the bending lightpath appears straight. Interestingly, the angle formed with the prism edge (which is parallel to the prism roof line) is alpha/2, half the cross-section apex angle of the prism (alpha = 2*arctan 1/sqrt 2).

In Figure 3, which shows the prism faces unfolded like a book opened flat, light travels parallel to the main diagonal line when viewed from above. When the faces are folded back up until they are orthogonal, and then tilted up 45*, the lightpath will bend 120* on each face and 90* in combination, with a fully inverted image and no image rotation. The view through an eyepiece will be Right Angle Correct Image.

To make a mirror version, I will attach Newtonian elliptical secondary mirrors to the inside of an aluminum angle, positioned according to Figure 3. Figure 4 shows "end caps" for the aluminum angle casing (also made from aluminum angle) with eyepiece barrel and helical focuser at the two ends. The helical focuser will be slightly repositionable to allow centering with a hollographic collimator.

The mirrors are respectively 2.6" and 2.3" minor axis, or 1.9" and 1.6" effective entrance/exit clear aperture for the 1:2 aspect-ratio ellipse. A single unit makes a full-aperture 2" RACI diagonal, which is not available on the market except from EMS for around US$1000 (each). A pair would make a binoscope back. Unlike the EMS 60*-elbow-pair design, interocular distance adjustment and co-collimation are left to the mount.

Another design, shown in Figure 5, simply uses collimatable Newtonian secondary fixtures with the mirror holders cut at 30* rather than 45*. Definitely not rocket science once the geometry and measurements are known.

Finally, HomeDepot sells 60* pipe elbows. It is a fun puzzle to give to someone: fit two 60* elbows together so the openings are perpendicular (hint below).

-- William Chang (Saratoga, CA)

(spoiler)

Hint: use the reversability principle -- in/out openings are symmetric -- so counter-rotate them in equal amounts until done.

Do you need eyeglasses?

I do, so I use Pentax XL 21/14 and 8-24 zoom. Other favorites include the classic 18 Ortho (Celestron/Vixen Circle-V) which has no rectilinear distortion. I also have a Borg flip/turret which holds a set of featherweight Vernonscope Brandon plus the 2" 30 WideField in straight-through. These are all classic EPs that are/were sold with spotting scopes (Pentax ED, Celestron SS80, Questar Field, Brandon Birder).

Generally speaking, you need longer focal length EPs for longer focal length scopes. And avoid eyepieces that have a tendency to "black out", since day-time pupil is going to be smaller.

-- William

What I have:
32 Wide Field
and 1.25" pairs:
24 Wide Field
16 T5
10.5 Plossl
7.4 Plossl
3-6 zoom
Having sold the 17T4 (heavy and some lateral color), 13T1, 9T1 (which I miss!), and some Plossls. Is there a Nagler in the sub-12mm range with decent eye-relief? :-)

-- William (Saratoga, CA)

p.s. Pentax-wise, pairs of 21XL and 8-24 zoom (covers 8-16 with good FOV), plus 14 and 5XL singles.

Thank you all for the advice... I took the plunge on Paul Hyndman's MN86 (yes, that one).

Mak-wise I have a nice M615 with beautiful contrast especially binoviewed. But I guess I just had to try an MN at some point, with everyone gushing over them. Given the weight, I will have to Dob-mount it, on top of a Tom O. compact equatorial platform. I hava used my 30-lb SN10 on a Giro/DDR/pier (with a big counterweight) but the MN weighs 40. In the end, I choose the 86 over a local MN76 because even that weighed 30 lb... and the 86 fits snugly in a golf case

-- William (Saratoga, CA)