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.

[charles jones]

On the subject of lamps:

Ron, Owen, Ken -

Do you think there is a danger of the Stinger bulb ceasing to be manufactured in the near future? Should we all be hoarding these while they are available? I see a number of conversion kits for Streamlight and others, from incandescent to LED lamps. And the trend to LED for flashlights is obvious.


And it is interesting to find out that some projectors are using super bright LED's -- not for pinhole star globes, but I assume for planet and auxiliary optical projectors. Any idea what they are?

Of course, LED's would not be dimmable, should one want to lower the intensity.

Charles

[Owen Phairis]

On the subject of lamps:

Ron, Owen, Ken -

Do you think there is a danger of the Stinger bulb ceasing to be manufactured in the near future? Should we all be hoarding these while they are available? I see a number of conversion kits for Streamlight and others, from incandescent to LED lamps. And the trend to LED for flashlights is obvious.


And it is interesting to find out that some projectors are using super bright LED's -- not for pinhole star globes, but I assume for planet and auxiliary optical projectors. Any idea what they are?

Of course, LED's would not be dimmable, should one want to lower the intensity.

Charles

Charles,

There is one interesting thing that I have observed with all of my planetarium projectors that seems to be universal. When ever I dim the stars to try an better make out the constellations I am always disappointed. With all the projectors I have it seems to work out so much better and is much more effective by leaving the star intensity at maximum and binging up the dome floods a little. Has anyone else noticed this? Bottom line - I never dim the fixed stars, not a concern to me about dimming LEDS.

Owen

[Ken Miller]

Charles,

There is one interesting thing that I have observed with all of my planetarium projectors that seems to be universal. When ever I dim the stars to try an better make out the constellations I am always disappointed. With all the projectors I have it seems to work out so much better and is much more effective by leaving the star intensity at maximum and binging up the dome floods a little. Has anyone else noticed this? Bottom line - I never dim the fixed stars, not a concern to me about dimming LEDS.

Owen

Owen

I guess I have noticed that same effect. Raising the background light level does make the constellations stand out better. Does the following make sense? When you dim out all the stars, the eyes can accomodate to a certain extent so that everything looks relatively the same. When you bring up the background light, you increase the minimum threshold so that the dimmer stars are washed out, and the eyes cannot accomodate for that.

Dimming has been useful for me, because as eyes accomodate to dark, you can reduce the light level to reduce all the irregularities that become more visible, and at the same time save bulb life (not so much of a problem with LED's).

As I said before LED's are completely dimmable. I do it in my LED light pointers, to set the optimum illumination, and I do it in LED red dot finders that I have built. I am an electronic engineer, and I have worked with LED's a lot. I have often changed the LED light level by adjusting the drive level for a particular application.

Ken

[Owen Phairis]

Owen

I guess I have noticed that same effect. Raising the background light level does make the constellations stand out better. Does the following make sense? When you dim out all the stars, the eyes can accomodate to a certain extent so that everything looks relatively the same. When you bring up the background light, you increase the minimum threshold so that the dimmer stars are washed out, and the eyes cannot accomodate for that.

Dimming has been useful for me, because as eyes accomodate to dark, you can reduce the light level to reduce all the irregularities that become more visible, and at the same time save bulb life (not so much of a problem with LED's).

As I said before LED's are completely dimmable. I do it in my LED light pointers, to set the optimum illumination, and I do it in LED red dot finders that I have built. I am an electronic engineer, and I have worked with LED's a lot. I have often changed the LED light level by adjusting the drive level for a particular application.

Ken

Ken,

Yes, that makes perfect sense. It seems so much better for me to bring up the dome lights. You should try it, and see the difference.

It has never bothered me about burning out light sources, or bulb life.

Yes, on dimming of LEDS. I have several mineral rotating light stands that use dimming leds to change the lighting on the Quartz sample. I sure would like to order and play with some of those new really bright ones! Yesterday, while at the hardware store I bought a neat little flashlight. It has about 6 white leds and a red laser in the center. The red laser is really BRIGHT! I had hoped to use it in the planetarium but the switch is the type that sequences through off, leds, laser. No way to go from off directly to laser. The good news is that it takes 3 AAA batteries, I hate those laser pointers that take hard to find batteries....

Owen

[Ron Walker]

Ken,

Yes, that makes perfect sense. It seems so much better for me to bring up the dome lights. You should try it, and see the difference.

It has never bothered me about burning out light sources, or bulb life.

Yes, on dimming of LEDS. I have several mineral rotating light stands that use dimming leds to change the lighting on the Quartz sample. I sure would like to order and play with some of those new really bright ones! Yesterday, while at the hardware store I bought a neat little flashlight. It has about 6 white leds and a red laser in the center. The red laser is really BRIGHT! I had hoped to use it in the planetarium but the switch is the type that sequences through off, leds, laser. No way to go from off directly to laser. The good news is that it takes 3 AAA batteries, I hate those laser pointers that take hard to find batteries....

Owen

Yeah, what is this thing about AAA batteries. I can understand the thinking for not using D cells but there is no reason whatsoever for not using AA cells. All the AAA's do is fade faster, but that's probably the reason they use them, so you have to buy more.