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Posts Made By: William Chang

February 5, 2006 07:31 PM Forum: Equipment Talk

Fujinon 16x70 vs 4" refractor

Posted By William Chang

Re: binoculars

Although I haven't used an Apogee RA88, based on the concept and people's reviews, that's the one I would recommend. It's quite heavy, but mounts like an alt-az telescope.

The SkyWindow concept has potential issues such as mirror flatness (they do sell a more premium version, which raises the question of how premium), dust on the front-surface hard-to-clean mirror, and the fact that you have to do mental geometry to figure out where in the sky you are pointing! (Remember that the image is upside-down and the angle of reflection is not constant, IIRC.)

If my skies were darker, I think I would invest in a nice large pair of binocs. But like I said, 35-40X makes a huge difference in darkening the sky background.

Another thing in the RA88's favor is the possibility of using light-pollution filters.

-- William

p.s. And as you know, I'm building a Borg 100 Achro binoscope :-)





February 7, 2006 10:21 PM Forum: Telescope Making

Off axis Newtonian Binoscope

Posted By William Chang

Well, a 4.5" f/8 is probably very close to spherical (it might even BE spherical despite claims otherwise), but even then you still cannot tilt them -- you would get the spherical abberation of a 9" f/4 (or worse). However, you can try the following.

Mask off half of each mirror (instead of obstructing the center). Maybe the absence of spider diffraction and large central obstruction will end up helping. You might also use smaller secondary mirrors and bring the scopes closer together, masking off a narrow edge of each mirror where the head obstructs. The trade-off might end up advantageous. (The standard "binewt" construction does not favor small primary mirrors. Bringing the foci together and avoiding head-blocking would require extra large secondaries, about 1.5" in your case.)

Hope this helps,

-- William (Saratoga, CA)

February 8, 2006 02:37 PM Forum: Equipment Talk

Another new idea

Posted By William Chang

Interesting idea, Floyd. I want one for my f/5 12.5" ELT but with a twist smile

I would like the four holes to be "keyholes" catching the balls so the entire UTA with poles can be lifted off and the poles don't fly apart. I.e. like the SM Hybrid. Not sure if the geometry allows it (I think so, with the round holes on the exact geometric diagonals, and the slots over the off-diagonal socket holes), or if the plastic material is stiff enough. Was planning to make one using thin plywood (but didn't have any on hand).

What do you think?

Best, -- William (Saratoga, CA)

March 1, 2006 11:40 PM Forum: Beginning Astronomy?

Are 8" Dobs hard to track with?

Posted By William Chang

Belated reply... I ran into several difficulties with a Dob.

It is almost impossible to share views with family & friends. Too many times I got the kids to come out to the cold, only to have them wait around while I re-locate the object (DSC would help). Also can't expect them, untrained, to track so no high mags for planets or globular clusters.

Another difficulty can also be alleviated with DSC. The best view I've had of Mars was at dawn. I even got the kids out of bed but alas, the sky had brightened to the point even Mars was invisible naked-eye, and unfindable.

In general, too much time was spent re-acquiring an object -- after consulting a map or swapping in a high power narrow view eyepiece, for example. A powerful finderscope would help (80mm with light-pollution filter, if needed).

My 2 cents, -- William

March 2, 2006 08:16 PM Forum: Telescope Making

How to repair Edmund red tube & Unitron mounting bar?

Posted By William Chang

Thanks for the suggestion. In fact, Harald had such a spare -- but used it up when he encountered the same problem himself! -- William

March 2, 2006 08:50 PM Forum: Refractors

Using 2 meade ar6 refractors to make a binocular

Posted By William Chang

Johann, I don't get the part about using a Celestron German Equatorial -- your neck will have to twist to match the angle of the binoscope unless it rotates...

I'm working on an alt-az mounted pair of Borg 100 Achro. My design is a variation of Matsumoto's EMS (see David's earlier response), from first principles (below). Two mirrors per side, right angle, correct image, true stereo. If I can't get that to work, I have a simple three-mirror-per-side design that is left-right reversed, but still true stereo not pseudo-stereo (in which convex objects look concave and vice versa). Certainly not for the faint of heart!

Imagine a large RACI (right angle correct image) prism diagonal, that works by having two orthogonal reflecting surfaces tilted back 45 degrees (there's an invisible centerline). Now use only half of the available clear aperture, say the right half, shifted left and centered over the optical axis. The final image will emerge, at a right angle, shifted to the left of the main tube axis. Do the reverse with the other tube. This way, RACI and decreased IPD are simultaneously achieved.

My implementation will use oversized elliptical mirrors instead of prisms. The underlying geometry is fascinating but not well published AFAIK. Probably originated in a military optics manual somewhere.

-- William Chang (Saratoga, CA)

March 3, 2006 11:44 AM Forum: Eyepieces

Mid priced EP shootout.... your opinion as best

Posted By William Chang

You probably don't need eyeglasses since you like the Ultima. Even then, the eye-relief get real short real fast. I couldn't use a 7.5 Ultima even without wearing my eyeglasses. I verified that a 12.5 Ultima has much less eye-relief than a nice 12.5 "conehead" Ortho. So while the Ultima are excellent eyepieces, I ended up only keeping one 30mm. Also, the 35mm had too much eye-relief resulting in blackout; and many samples of the 30mm barrel were oversized -- this is frequently mentioned and I had several at one time.

I actually like the Burgess 17 and 12 Wide Angle (never tried their SWA). My favorite eyepiece is the 12mm Brandon; the 2" 30mm Vernonscope/Brandon WF is also very nice and much lighter in weight than my TeleVue 32 Wide Field. If you can find them, a used TeleVue 24WF is a bargain at $150.

-- William

March 28, 2006 08:19 PM Forum: Telescope Making

Obsidian Optics mirror arrived

Posted By William Chang

Albert Highe's super-elegant designs: http://www.ultralightscopes.com/

His mirror cell design is also shockingly simple -- pivoting bars on the outside of a triangular frame. (You might also consider a variant where the pivoting bars are instead on the inside of a larger triangular frame, that attaches via collimation bolts right at the tube/box wall.) For travel, I would suggest you first build a removeable mirror cell/box then build a scope around it.

-- William (Saratoga, CA)

p.s. I have a "carry-on" 11" f/4.5 project using four anti-parallel poles and a drumshell UTA -- but you probably don't need something quite that compact.

March 29, 2006 12:28 AM Forum: Meade

Mead MTS SN8

Posted By William Chang

The 6" version of the MTS is fairly common, and usually priced around $250-300 IIRC. So the 8" would be somewhat more, if a Collector is around to pick it up in person. Shipping a scope like this is probably not a good idea due to high cost (heavy, bulky mount) and risk of damaging the corrector plate (which should not be removed, unlike the primary mirror). The modern version of the SN8 is priced around $300-350 (used) for the OTA with UHTC coating.

-- William

March 29, 2006 07:01 PM Forum: Telescope Making

Finder ideas for a reverse bi-Newt?

Posted By William Chang

Here's something I posted on CN and Yahoo Binewt group:

Date: Thu Feb 16, 2006 4:35 pm
Subject: Correct Image Periscope Finder for Binewt

I just had a brainstorm while trying to design an over-the-shoulder
finder for my binewt project. I bought a 700mm FL Mayflower refractor
and was thinking, if I cut the tube a couple places and added two
elliptical secondaries to bend the light backwards... then it dawned
on me there's an elegant solution.

Simply mount a right-angle finder pointing UP at right angle to the
main scope optical axis -- like a periscope. Then fix an elliptical
secondary inside the dew shield (make one if needed), and cut a big
hole in the dew shield in line with the main scope axis. The finder
will provide a correct image, pointing over the viewer's head. And
two of these side-by-side will make a bino finder!

;-)

Someone must have done this before?

-- William Chang (Saratoga, CA)

p.s. earlier I had thought a RACI finder was needed, but that is not so