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)

 6V_Nicad_Bulb.JPG ( 33.31k ) Number of downloads: 4

 

[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.

[Nathan Volle]

Thanks Charlie---If you had details on your Ray-o-vac bulb I would be interested----meaning voltage and current and part number.

From the beginning of my quest to find the right bulb I have been in the continual process of relaxing my expectations. The 6 volt Nicad is still the best I've found for my purposes.

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.

I still want to try anything that might help though, and appreciate your useful thought on adjusting the bulb filament orientation. Also I would be interested in what type lenses you used on your cylinder and how you mounted them. Do the lensed stars project small and round even with overly large filament bulbs? Did you ever project more than one star with one lens? The picture below, (taken without a tripod), is of the Starlab starfield showing cassiopeia etc---Deneb is bright and way above the star cylinder near the top. This gives some idea of the quality, (or lack of), that I have come to tolerate. Any thoughts or advice is welcome! Nathan

Attached File(s)

 Starfield_Cassiopeia_upload.JPG ( 21.57k ) Number of downloads: 4

 

[Charlie Miller]

[attachment=2931:41JgdZKF...S500__1_.jpg]

Thanks Charlie---If you had details on your Ray-o-vac bulb I would be interested----meaning voltage and current and part number.

From the beginning of my quest to find the right bulb I have been in the continual process of relaxing my expectations. The 6 volt Nicad is still the best I've found for my purposes.



I still want to try anything that might help though, and appreciate your useful thought on adjusting the bulb filament orientation. Also I would be interested in what type lenses you used on your cylinder and how you mounted them. Do the lensed stars project small and round even with overly large filament bulbs? Did you ever project more than one star with one lens? The picture below is of the Starlab starfield showing cassiopeia etc---Deneb is bright above the star cylinder. This gives some idea of the quality, (or lack of), that I have come to tolerate. Any thoughts or advice is welcome! Nathan

Dear Nathan,

Sorry for neglecting your bulb question. The Ray-O-Vac Krypton bulbs (with bare-lead "bi-pin" connections) that I've had luck with are sometimes listed as T3AAA-2 or T2-AAA bulbs, with the number suggesting the number of 1.5V batteries being used. I buy mine at Menards, a midwest-based home-DYI store. I've attached a picture: they come packaged two per card and cost about 3-4 buck for two. Ace Hardware also sells these (on-line at least). The 2T and 3T differ in current, voltage, and brightness, as expected. The 3T is rated at 3.6V, 0.3A, and about 1.08 A, with 2900 candlepower (beats me what the lumens value is) and a 45 Hr average life. For my 16" star drum, the 3T is too bright for my 14x14' room... for relatively white color temp, you don't want to run these bulbs at too-low of a current.

I've thought about building either a relay or diode-controlled automatic "bulb changer" to prevent star black out due to bulb burning out. Of course, another way is to time how long you've burned a bulb and replace it prior to burn-out.


I [attachment=2931:41JgdZKF...S500__1_.jpg]

Attached File(s)

 256792_300_1_.jpg ( 71.17k ) Number of downloads: 4

 

[Ken Miller]

[attachment=2931:41JgdZKF...S500__1_.jpg]
Dear Nathan,

Sorry for neglecting your bulb question. The Ray-O-Vac Krypton bulbs (with bare-lead "bi-pin" connections) that I've had luck with are sometimes listed as T3AAA-2 or T2-AAA bulbs, with the number suggesting the number of 1.5V batteries being used. I buy mine at Menards, a midwest-based home-DYI store. I've attached a picture: they come packaged two per card and cost about 3-4 buck for two. Ace Hardware also sells these (on-line at least). The 2T and 3T differ in current, voltage, and brightness, as expected. The 3T is rated at 3.6V, 0.3A, and about 1.08 A, with 2900 candlepower (beats me what the lumens value is) and a 45 Hr average life. For my 16" star drum, the 3T is too bright for my 14x14' room... for relatively white color temp, you don't want to run these bulbs at too-low of a current.

I'll have to add the 3AAA bulbs to my list. I know that I have considered them in the past, and ended up not using them, but I don't remember why.

From what I read, candlepower is one of those measurements that has often been abused. You can't always believe it. Lumens is considered to be more scientific. Also, a lot depends on how directional the beam of light is. Lumens measures the total light output in all directions. Sometimes a factor of 4 pi has been used as a conversion factor, but it isn't necessarily accurate. In this case you would end up with more than 200 lumens, which doesn't seem likely. If you consider that the power input is much less than with the Stinger (xenon) bulb, and the bulb technology is similar (actually, xenon technology is supposed to be brighter than krypton), the Stinger should actually be the brighter bulb.

Ken

[Owen Phairis]

I'll have to add the 3AAA bulbs to my list. I know that I have considered them in the past, and ended up not using them, but I don't remember why.

From what I read, candlepower is one of those measurements that has often been abused. You can't always believe it. Lumens is considered to be more scientific. Also, a lot depends on how directional the beam of light is. Lumens measures the total light output in all directions. Sometimes a factor of 4 pi has been used as a conversion factor, but it isn't necessarily accurate. In this case you would end up with more than 200 lumens, which doesn't seem likely. If you consider that the power input is much less than with the Stinger (xenon) bulb, and the bulb technology is similar (actually, xenon technology is supposed to be brighter than krypton), the Stinger should actually be the brighter bulb.

Ken

Hi Ken,

As I recall from my physics 101 days that lumen is definitely related to and based on candle power or now the candella. 1 cp at 1 foot distant on 1 foot square is one lumen. Also works for 1 meter distant and one square meter. Lumen being the measure of light flux per unit area, where cp is a measure of luminous intensity and of course all the inverse square radiation equations hold. I think the 4 pi refers to an imaginary sphere with an imaginary perfect point source at the center. I do not think we want to even get into eye spectral curves and all the other units of measure of light. It all gets very confusing.....I guess that why we have light meters...

Owen

[Ron Walker]

Hi Ken,

As I recall from my physics 101 days that lumen is definitely related to and based on candle power or now the candella. 1 cp at 1 foot distant on 1 foot square is one lumen. Also works for 1 meter distant and one square meter. Lumen being the measure of light flux per unit area, where cp is a measure of luminous intensity and of course all the inverse square radiation equations hold. I think the 4 pi refers to an imaginary sphere with an imaginary perfect point source at the center. I do not think we want to even get into eye spectral curves and all the other units of measure of light. It all gets very confusing.....I guess that why we have light meters...

Owen

Thank God for light meters.

[Owen Phairis]

Thank God for light meters.

I was thinking about some of my light meters. When one does photography one has a propensity towards collecting things for the camera, studio, and darkroom. Not pictured below are the laser power meters or my favorite light meter of all time, the Gossen Luna-Pro. I think it is tucked away with one of the 4X5 cameras.

A couple of my meters measure candle power in foot candles like the large one in the center with the remote head. I believe in my shed is a radiant integrating sphere and a standard source as well.

Owen

 LightMeter_003E.jpg ( 76.37k ) Number of downloads: 0