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Posts Made By: William Chang

May 29, 2003 11:18 PM Forum: Pictures of Me and My Telescope and........

28 Years Ago!

Posted By William Chang

Holy Cow! I'm speechless :-) -- William

June 6, 2003 08:12 PM Forum: Telescope Making

Folded Refractor Light Loss ???

Posted By William Chang

Well, a folded 6" f/15 is potentially a travel scope, and would be stable on a much smaller/cheaper mount, compared to a straight-tube f/12. If I can get my hands on an objective I would do it :-) I would use a reasonably priced high quality 3" first mirror instead of 4". On the other hand, a 6" f/8 with Chromacor is much easier to own. Good luck with your project,

-- William Chang (Saratoga CA)

July 27, 2003 12:45 PM Forum: Equipment Talk

Mars in Six Scopes

Posted By William Chang

Would there have been material difference, if you had included 8" SCT, plus 6", 11" and 14" Newts in the mix? Does Aberrator really care :-)

-- William

July 27, 2003 07:19 PM Forum: Telescope Making

Fiberglass tube construction

Posted By William Chang

Keller makes maple plywood drum shells and cylinders (available from Anderson Internation Trading). The cylinders are sold in lengths of 27", 48" and 96" (exotic veneers available); the drum shells in various sizes. I'm considering a 13D 9L drum shell for the secondary cage of an 11" f/4.5 former StarHopper. Thickness goes up to 10-ply, 0.27" for the 100% maple drum shells, up to 7/16" for cylinders.

Has anyone any experience with these plywood tubes, or general comments/suggestions?

-- William

July 29, 2003 02:01 PM Forum: Takahashi

Nexstar 11 GPS vs. Takahashi FS-102

Posted By William Chang

If you get JMI Wheelie Bars and Lymax Cooler for the NS11, you might find it easy to use (e.g. seeing Mars in early morning). I don't have this setup but did research on it (waiting for the right opportunity to own one :-) Folks on SAA recommended this scope when I asked for aperture fever remedy under serious light pollution.

The FS-102 duplicates the TeleVue; you can mount the TV on your GM-8. Or get a Chromacor'ed 5" or 6" refractor, or a 6" Mak-Newt. Wheelie Bars may also come in handy here if the scope is near the upper limit.

-- William

July 29, 2003 04:25 PM Forum: Astro Binoculars

ideas for DIY binoscopes

Posted By William Chang

I haven't built any (see the other thread). I did get a Meade SN6 ($200) to try "over the shoulder" bazooka-style. A star diagonal redirects the light along the tube axis, but I was thinking looking UP rather than down. If this works, a twin-tube alt-az mount (Giro??) -- or a yoke as you suggest -- can be positioned behind the recliner etc. Or it could be a mobile unit :-)

-- William Chang (Saratoga CA)

July 31, 2003 04:12 PM Forum: Telescope Making

Discovery 10" DHQ conversion to truss

Posted By William Chang

Very ingenious! What's the dimension of the "cube"? Please post a report! -- William

August 9, 2003 03:45 AM Forum: Astro Binoculars

Limiting magnitudes

Posted By William Chang

Thanks Mike, this is fun to play with. I can't remember the "formula" (or plot) of the number of stars of a given (sub-)magnitude -- does anyone know?


In principle, one can define a "Richness of Field" function:

RF(visual magnitude limit, aperture, magnification)
= expected number of visible stars in the FOV

possibly normalized for maximum exit pupil (7mm; it could be a function of visual limiting magnitude and observer's age) and eyepiece AFOV (say 60*), and possibly other factors such as elevation, humidity, angle, etc.

Then, for a given visual magnitude limit, one could look for the combination of aperture and magnification that maximizes the RF() function, and call such a telescope a Richest Field Telescope -- RFT. My gut feeling is that the visual limiting magnitude is a major determinant in "scoping out" the RFT aperture/magnification -- i.e. different scopes under different skies.

In the extreme of dark skies, the unaided naked eye (7mm aperture, 1X power, 60* FOV) may come pretty close. But near cities (or under full moon) there are few naked-eye stars, so one needs extra aperture/magnification to darken the FOV background -- in order to see more stars. Using the calculator, I found the L.M. difference between 16X and 38X for a 4" refractor (and 40-year-old-eyes), to be 1.2 for mag-5.5 sky, 1.5 for mag-4 sky, and 1.7 for mag-2.5 sky. Higher magnification helps more when the sky is bright.

Plugging in the star-count as a function of magnitude, one ought to be able to figure out which telescope or binoculars is "RFT" under a given limiting magnitude. Does this make sense to anyone?

-- William Chang (Saratoga CA)

August 15, 2003 03:56 PM Forum: Telescope Making

Cutting PVC 12" tubing >> Salvage effort

Posted By William Chang

There must be a Dremel tool/bit that would work. Or the "drill saw" that cuts with a bit -- have you seen that? Or you can just drill lots and lots of holes...

I designed a three-part interlocking tube that collapses to about 20", and one of these days will order a suitable plywood drumshell and give it a try. I did see, and consider using the plastic tube you've got, but even cut in three it's still heavy -- the wall is just too thick.

How about using it as a backyard pier?

-- William Chang (Saratoga CA)

August 21, 2003 05:30 PM Forum: TeleVue

TV102, Mars, and Gibraltor Mount

Posted By William Chang

Regarding vibration: are you using suppression pads? They make a huge difference. My old Panoramic (delrin sleeve azimuth) is unuseable without them, perfectly steady with them. That said, my new used Gibraltar (fancy) azimuth wiggles when I focus. How do you adjust it so it doesn't?

It can't be that hard to add azimuth slo-mo: have the eyepiece caddies "walk around" the flat round base on rubber rollers, if you think about it... Better yet, I have a design for a barndoor-style equatorial adapter (!) that would go in-between a TelePod head and the tripod. Unfortunately I don't have a TelePod so will probably try it out on a Giro first.

-- William Chang (Saratoga CA)