Replies

[Charlie Miller]

Greetings,

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.

Charlie Miller
Home-builder

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.

[Nathan Volle]

Greetings Charlie, I am actually quite new to O C too, and have had your question---as have ALL of us with "starballs"---. looking for that perfect light source. I have used a Starlab for 5 years doing presentations---including teaching week-long workshops in high school.

Greetings,

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.

Charlie Miller
Home-builder

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.

After long searching and experimenting, I found, (for my needs), the perfect compromise bulb for brightness and point source quality. It may have been mentioned by someone, sometime, on OC---but I have never seen it. It may be that others have ruled it out for filament size or for something else---but it is DEFINITELY the best compromise I have ever found for the standard Starlab projector and I think would be a good choice for you even if you found something better later.

I would welcome others comments on whether they have tried it----and on any drawbacks they may have found----for their particular use. My guess is that filament size is a problem to some and/or shadowing. For my use, to have enough brightness is a key requirement.

It is called: MAGCHARGER RECHARGEABLE Halogen Replacement Lamp for 6 Volt Rechargeable Flashlight

It is requires 6 volts and draws 1.65 Amps which is 9.9 watts. I get them at Frys Electronics for $4.95 each.
The store sticker says: MAGLITE LR00001 CHARGER REPLACEMENT LAMP.

I hope that might help you---and welcome to O C! Nathan

Attached File(s)

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[Ken Miller]

Greetings Charlie, I am actually quite new to O C too, and have had your question---as have ALL of us with "starballs"---. looking for that perfect light source. I have used a Starlab for 5 years doing presentations---including teaching week-long workshops in high school.
After long searching and experimenting, I found the perfect compromise bulb for brightness and point source quality. It may have been mentioned by someone, sometime, on OC---but I have never seen it. It may be that others have ruled it out for filament size or for something else---but it is DEFINITELY the best compromise I have ever found for the standard Starlab projector and I think would be a good choice for you even if you found something better later.

I would welcome others comments on whether they have tried it----and on any drawbacks they may have found----for their particular use. My guess is that filament size is a problem to some and/or shadowing. For my use, to have enough brightness is a key requirement.

It is called: MAGCHARGER RECHARGEABLE Halogen Replacement Lamp for 6 Volt Rechargeable Flashlight

It is requires 6 volts and draws 1.65 Amps which is 9.9 watts. I get them at Frys Electronics for $4.95 each.
The store sticker says: MAGLITE LR00001 CHARGER REPLACEMENT LAMP.

I hope that might help you---and welcome to O C! Nathan

I have tried this lamp, and didn't like it for two reasons (I reported on it a couple of years ago). First of all it has a glass envelope that causes a strange refraction and generates secondary star images. Secondly, it is so bright that all the projection defects are really noticable. All of this might be ok if you dim the light down enough that the problems are hard to see. In a room that is not fully darkened, it might work well.

Ken

[Nathan Volle]

That's great feedback Ken. Interestingly with a 6.3 volt transformer and the magnetic dimmer I have been using---I have no complaints using it with the Starlab projector.

I have tried this lamp, and didn't like it for two reasons (I reported on it a couple of years ago). First of all it has a glass envelope that causes a strange refraction and generates secondary star images. Secondly, it is so bright that all the projection defects are really noticable. All of this might be ok if you dim the light down enough that the problems are hard to see. In a room that is not fully darkened, it might work well.

Ken

I do know how aggravating double images can be though, and I suppose with the amount the light is dimmed, combined with the background light level----I must have unknowingly reached some acceptable balance that has outweighed my other subconscious criteria. I'll try the Stinger too and compare. Nathan

[Charlie Miller]

That's great feedback Ken. Interestingly with a 6.3 volt transformer and the magnetic dimmer I have been using---I have no complaints using it with the Starlab projector.



I do know how aggravating double images can be though, and I suppose with the amount the light is dimmed, combined with the background light level----I must have unknowingly reached some acceptable balance that has outweighed my other subconscious criteria. I'll try the Stinger too and compare. Nathan

I've switched over to the "stinger-like" bulb (only mine is marketed by Ray-o-vac) and can confirm that the filament size is somewhat smaller and the star images are brighter than most low-voltage incandescent bulbs that are readily available. There is still that annoying problem of projecting a non-point-like filament. As it describes an arc, the star images across the projected hemisphere vary from near-point-like to rather sub-optimal "straight-on" image of the filament.

So here's a thought of potential minor interest. If your projector has a horizon cut-off "basket" inside the starball or stardrum and that basket is designed to "float" along two orthogonal axes of rotation, then one could arrange the bulb socket such that the best-quality images (i.e., most point-like) appear in a preferred part of the projected sky. For example, the star-rise and star-set areas (east and west) could be given best image-size, or perhaps a region of typical interest- along the ecliptic - could be given top priority. So, if your bulb-holder has some degree of positional adjustment, you could do this.

I wish that my star-drum had a larger diameter (> its 16"); a larger star ball would reduce filament size (for a fixed star-hole diameter). but then, of course, you'll need a brighter filament to maint star brightness.