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Post by Ron Walker on Sept 6, 2022 12:19:24 GMT -7
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Post by Ron Walker on Sept 6, 2022 12:19:47 GMT -7
Posted by: mrgare5050 Jul 12 2007, 08:35 AM
wait i take 2 days off and TIM arrives! dudes, actually ive got another newbie emailing me, i'll try to get him over here
i made constellation templates out of tin, actually roof flashing, then transferred them to vinly for needling. im the worlds only 54 yearold man who recently purchased a pin cushion (its an apple!) g
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Post by Ron Walker on Sept 6, 2022 12:20:12 GMT -7
Posted by: Ken Miller Jul 23 2007, 12:55 PM This stuff on pinhole sizes and star magnitudes is great stuff. I was just now trying to find my way back to this discussion, and it took me a while. It would be nice if this kind of detail could be put together somehow in a way that we could easily reference it. I'm thinking along the lines of basic design principles for Do-It-Yourself planetariums. This would include pinhole principles, lenses, light sources, mounting, and motor drives.
The pinhole discussion helped me understand why the strictly pinhole projectors cannot do as nice a job of showing the sky as the projectors outfitted with lenses for the brighter stars. The sky projected by the pinhole-only projectors looks nice, but it's harder to pick out the constellations. Now I understand more about why that can happen. Without lenses you have to make compromises between the size of the projected star disk and the range of brightness that can be achieved. Star disks that are too large do not look realistic, and without the full range of brightness it is harder to pick out features in the sky.
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Post by Ron Walker on Sept 6, 2022 12:20:38 GMT -7
Posted by: charles jones Jul 24 2007, 12:20 AM Hi Ken -
Actually, optical planetarium projectors have the same problem reproducing the sky. Each lens, afterall, projects a section of the sky depicting brighter stars by reproducing them with larger diameters than fainter stars.
In the later Zeiss models, quite a number of the brighter stars ( 40 + I think) had separate projectors to compensate for this. The custom built Korkosz (not sure of the spelling), all optical projector, in Boston had, in effect, three separate star globes with different lamps, each depicting different magitudes stars. It had two hemispheres for one criteria of magnitude stars, then four quarter spheres for another magnitudes, and another set of quarter spheres for another set of magitude stars. All this to make the sky more realistic.
Actually with pinpoint projection, I have found that the star images seem to be about the same diameter when projected on the dome . Each pinhole crudely focusses the image of the lamp's filament or arc on the dome. The larger pin holes are brighter. But there is a point when the pinhole is large enough and does not act as a lens any longer and reacts more like a "shadow box" effect. That's where the pin hole appears more as a round disk on the dome.
In some ways, the early Spitz models, with lenses to create a brighter image for the bright stars , was a great innovation--something the optical projectors didn't do.
All in all, it is easier to depict a more realistic sky when only showing stars to the 3rd or 4th magnitude. Who needs to show 5th and 6th magnitude stars that the average audience will never see?
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Post by Ron Walker on Sept 6, 2022 12:21:14 GMT -7
Posted by: Tim Mitchell Jul 24 2007, 01:02 AM Well Hi Guys, I have been offline for a few days and just got back on a few hours ago. I have been working on a star chart with gores, it isn't perfect, but it should work for the purpose of putting holes in a globe. I used the mercator projection chart that Ron pointed me to for the star positions, and it worked pretty good, but there were a few errors which I had to correct, I think that I have got them all, but if you find any more extra stars let me know. wink.gif I have sized the stars according to the drill size that should be used, all except the small ones of course. I also put smaller dots in the centers of the larger circles to aid in drilling. This chart is already reversed so is perfect for the making of a projector, it includes stars up to the sixth magnitude, if you need a chart with only one magnitude of stars then I can make one for you and e-mail it across. These are the colors, Yellow/red-0 mag, orange/red-1st and second-mag, green-3rd mag, purple-4th mag, orange - 5th mag, blue - 6th mag The dots are sized according to their respective hole diameters within their range of magnitude. I calculated the dots with seventh magnitude in mind as a possible addition, on a 19" sphere (the chart does not have 7th mag on it yet). The scale can be slid up and down if needed. The 19" globe chart is better for precision when drilling, however the file is a bit larger than the 10" sphere one. Click on the links to download the JPEGs. www.buffaloriverforge.com/snake.web/gore%20star%20map%2010%22%20globe.jpgwww.buffaloriverforge.com/snake.web/gore%20star%20map%2019%22globe.jpgMagnitudes with respective drill sizes. 7th mag. #80 6th mag. #75 5th mag.#66 4th mag. #55 3rd mag. #44 2nd mag. #29 1st mag. #3 0 mag. 21/64th of an inch. -1 mag. 33/64th of an inch. Just size it for your globe and then print it out and cut the blue background out. I hope someone finds it of use, Cheers, Tim.
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Post by Ron Walker on Sept 6, 2022 12:21:51 GMT -7
Posted by: Ken Miller Jul 24 2007, 07:40 AM 33/64 inch sounds way too big. I've been concerned about the "cartoonish" or unrealistic look of stars that are projected with holes as large as 3/16 inch or slightly smaller. A lot does depend on the size of the dome, the diameter of the star globe and even the size of the lamp filament, and the thickness of the material that the pinhole is drilled through.
We need a good technical presentation on the details of pinhole projection. I don't feel qualified myself.
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Post by Ron Walker on Sept 6, 2022 14:07:41 GMT -7
Posted by: Tim Mitchell Jul 24 2007, 08:42 AM Hey Ken,
The size was calculated from the area of the hole and therefore the amount of light that is allowed through, I forgot to mention that the larger holes have lenses on them to focus the light, like the spitz.
Tim.
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Post by Ron Walker on Sept 6, 2022 14:08:00 GMT -7
Posted by: Ken Miller Jul 24 2007, 08:47 AM QUOTE(Tim Mitchell @ Jul 24 2007, 08:42 AM) * Hey Ken,
The size was calculated from the area of the hole and therefore the amount of light that is allowed through, I forgot to mention that the larger holes have lenses on them to focus the light, like the spitz.
Tim. OK Tim. That explains it. What is your strategy for selecting and obtaining lenses?
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Post by Ron Walker on Sept 6, 2022 14:08:30 GMT -7
Posted by: Ken Miller Jul 24 2007, 03:42 PM QUOTE(charles jones @ Jul 24 2007, 12:20 AM) * Hi Ken -
Actually, optical planetarium projectors have the same problem reproducing the sky. Each lens, afterall, projects a section of the sky depicting brighter stars by reproducing them with larger diameters than fainter stars.
But there is a point when the pinhole is large enough and does not act as a lens any longer and reacts more like a "shadow box" effect. That's where the pin hole appears more as a round disk on the dome. I think Ron Walker pointed out a relationship between the size of the lamp filament and the size of the pinhole. Is this the transition point between lens and shadowbox? Did Ron say that the pinhole needed to be smaller than the size of the lamp filament? This is why I would like to see all these details pulled together in one place.
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Post by Ron Walker on Sept 6, 2022 14:08:56 GMT -7
Posted by: Ken Miller Jul 24 2007, 03:47 PM By the way, every time I see that Ron hasn't posted anything for several days, and he doesn't jump into a discussion like this, I start worrying about him. I hate to be a busybody, but where are you Ron?
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Post by Ron Walker on Sept 6, 2022 14:09:35 GMT -7
Posted by: Ron Walker Jul 24 2007, 05:28 PM QUOTE(Ken Miller @ Jul 24 2007, 03:47 PM) * By the way, every time I see that Ron hasn't posted anything for several days, and he doesn't jump into a discussion like this, I start worrying about him. I hate to be a busybody, but where are you Ron?
Alive and well, thanks! laugh.gif
Just trying to get my info straight in my head before I post.
Also want to actually measure some of the star holes on the A3P star globe and see how they correspond with Tim's calculations.
Still am back at getting a picture of a Greenly punch. Also had a chance to see one of those strange laser planetariums that Edmond Scientific was selling. More later.
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Post by Ron Walker on Sept 6, 2022 14:10:05 GMT -7
Posted by: charles jones Jul 24 2007, 10:32 PM Hey Ken -
A pinhole can act as a lens. We know there are pinhole cameras, even eye glasses fitted with pin holes instead of a lens. Can't say if they were any good!!
From my experimentation, I found that the small pinholes crudely focussed the image of the lamp's filament on the dome. That's why I noticed "crescent" shaped stars at certain angles in my star globe using the GE 605 bulb (the bulb the early Spitz projectors used). And, as I recall, the closer the screen is to the pinhole (the relative dome size), the more distinct the image of the filament is.
The fact that we dim the stars down to faint images, erases the picture of the filament. What I remember discovering is that the larger size of the pinhole would let more light through, but the focussed image (the filament) on the dome remained the same size as the fainter stars. This must be when pinpoint projection works to it's advantage for planetariums. The image size is the same, but the brightness varies. Perhaps, someone else can verify this.
Also, the fact that a pinhole doesn't have the clarity of a true lens, the Spitz planetariums used this to their advantage. A projected lamp filament looked so fuzzy, it was mostly hard to distinguish it from a fuzzy dot of light.
An arc lamp eliminates any image of a filament, so the effect is a dot (the arc), is projected on the dome. The actual diameter of the stars projected should remain the same, but the brightness varies depending on the pinhole size.
As you say, you reach some transposition point when the light projected is no longer through a "pinhole" sized opening. Then you'll notice the stars appearing as round images on the dome (even if you don't have a lens system like the Spitz projectors). I guess we need a physicist to explain just when the pinhole doesn't act as a lens anymore and why.
Oddly, this is not true in optical projectors--the brighter stars are really depicted as larger circles on the dome. And the stars projected through the lens systems of the Spitz units are circles as well.
Either way, we show the difference in magnitude by the size of the opening the light is projected through including the digital planetariums.
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Post by Ron Walker on Sept 6, 2022 14:10:25 GMT -7
Posted by: mrgare5050 Jul 25 2007, 05:00 AM
another name can be born here, i've experienced it often, 'the weakest link' or the 'dullest star'.. as you add sharpness to your sky, the next level of fuzzy stars begins to bug you. say you add lenses to the 1st mag stars (slog through 'the quest for cheap lenses' board for some of our attempts to find something REALLY cheap, which is whats holding me back personally), then the 2nd maggers begin to bug you. its like the wallpaper hanger who alone probably notices the one crooked piece above the door, but if our skies dont please us we will drift somewhat .
gare with a philsophical sidebar
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Post by Ron Walker on Sept 6, 2022 14:11:10 GMT -7
Posted by: Ken Miller Jul 25 2007, 08:16 AM Having an assortment of projectors, I've been able to do comparisons of pinhole projections of various sizes along with lensed projections. My limitation is not having a dome to get the full effect (I would like to see Owen tell us more about the effectiveness of using a large tent for his shows -- I do have an inflatable dome, but no place right now to set it up). The Spitz projectors with lenses for the brightest stars do a significantly better job of representing the sky and making the brightest stars and constellations "pop out".
As I have pointed out before, the pinholes and lenses both tend to project images of the lamp filament. There is a distance at which the pinholes do focus a sharp image, and that depends on the size of the starball and possibly the size of the pinhole. At longer distances the image defocuses and looks more star-like, so the size of the dome is an important factor.
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Post by Ron Walker on Sept 6, 2022 14:13:50 GMT -7
Posted by: Ron Walker Jul 25 2007, 02:21 PM If I remember my physics correctly a hole smaller then the light source size will project the image of the light source more clearly then when the hole becomes larger then the light source. Once the hole becomes larger the the light source the projection becomes softer. Thus for star projection you would always want the light source to be smaller then the holes your projecting through.
Also the need for lenses is dependent on the faintest star that one wishes to reproduce. There comes a time when further reducing the size of a pinhole will actually produce a larger projected star because of refraction. Because of this a number 80 drill is usually the smallest size used. If we only reproduce a sky of the 4th magnitude and assign the #80 drill for them, then you can probably get away without any lenses even on the first magnitude stars as their holes will not be terribly large. However, if you want to reproduce to the 6th magnitude and assign the #80 drill to them, then the relative size of the first and second magnitude stars becomes so large that they no longer look star like on projection. The only way to get the full brightness of the brightest stars without them looking unnaturally big is to use a lens to "collimate" the light into a beam that doesn't do the normal expanding but rather concentrates all the light from a larger hole to a smaller dot on the screen.
Also, the larger the star globe, the smaller the star projected at any given distance. There is no way to get away from optical refraction however so your kind of limited to that #80 drill.
Most small projectors with star globes up to about 12 inches are usually limited to about 750 stars or 4th magnitude and will project in domes up to about 20 feet in diameter.
Medium size projectors with star globes of about 18 inches will project well in domes as large as 40 feet in diameter and will reproduce up to 3500 stars to about 6th magnitude.
Large size projectors with star globes of 36 inches will project well in domes as large as 60 feet in diameter and will project as many as 9000 stars to about 6.5 magnitude.
In all these cases the smallest magnitude projected is a hole produced with a #80 drill. They project smaller because of their distance from the light source.
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