I'm a new member with an old projector - a home-made 16" diameter version of the Steven B. Smiith 20" drum style. My 16" drum's strength is its dense star field (equivalent to the norton star alas 6.4 magnitude field), but because of its relatively small diameter, a strong design demand is the light source.
I'm writing to ask about experiences (and my observations) about light sources. I've migrated to the "bi-pin" still of bulbs used in Maglight (they have high intensity ad small filaments -- two very good things), but I'm still looking for the optimal. One weakness of the maglite style bulbs (or at least the Rayovac "High intensity" 3.6V, 0.3A bi-pin model I'm using) is that it's small glass envelope precludes distributions of light evenly throughout a solid angular region of 340 degrees (the angular coverage of my device). Old research I did on the Spitz A3 indicated that they used to use a bulb that had two important design parameters: a large glass envelope (thus increasing the distance from the filament to the base -- and thus the angular coverage of the light field) and a filament that seemed to be carefully placed within the center of the spherical shape of the bulb. This latter design aspect prevents secondary images developing as a result of filament light reflected off the inside surface of the glass envelope.
Perhaps I'm mentioning things that are well known; I apologize if this is the case. But I'm still searching for that bulb that has:
(1) the smallest (point-like) filament
(2) appropriate brightness (of course, dependent upon dome size, etc.)
(3) whitish color temperature (this is a toughy for incandescent bulbs, with there yellow-weighted spectra)
Note: The proper combination of items (2) and (3) are important, as if I'm compelled to "dim" the bulb because
it is simply too intense, then I get further away from white color temperature.
(4) large coverage of the star sphere / cylinder (i.e. globe of stars)
(this seems to be a drawback of these small "maglite" type bulbs, darn it! I'm loath to thinking of going to a two-bulb
configuration, due to the demand for precise location and masking, etc.)
(5) filament centered within an ideally spherically shaped glass envelop, to prevent the formation of
"secondary" or ghost images due to the lack of such a geometry.
Anyway, I'd appreciate hearing from any who have looked at this issue. I do find that the mag-lite bubs are pretty good, but
I'm striving for the best -- because it is THE limiting factor for my 16 star cylinder.
Thank you for any/all help out there.
P.S. I have emailed with Gare and hope to send him a photo or two of my star cylinder and perhaps a shot of a star field (i'm currently working on re-doing all the condenser lense mounts and all that, however).
P.P.S. Thank you for this oppotunity and venue.
Iowa City, IA
Iowa Space Science Center project
Other Projects: Science education, Spitz A4, Home-built planetarium
(6.35 limiting mag); single fisheye digital projection system. Spitz 373,
and the Spitz Jr.!