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#11
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Hey, what about using a green laser for collimation of a big dobsonian?
Roger Persson Brian Tung wrote: .... If you're talking about the concave surface of a telescope mirror, I of course agree that shining a green laser down a telescope is completely irresponsible. But that's not what we were talking about. We were talking about possibly using green lasers to blind pilots. .... |
#12
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"Brian Tung" wrote in message ... Tim Killian wrote: So tell us, what is the safe distance when someone points a green laser in your face? What, you don't know for sure? Mike need not give a hard lower limit for safe divergence to show the logical flaw in Gary's argument. My common experience is that the beams do diverge significantly even after only 100 meters or so, and while I wouldn't want to stick my eye in the beam path, a few kilometers seems more than enough to ensure safety. Yes. To get a beam closer to parallel, requires a longer laser assembly. I used to work with lasers that were certified as 'eye unsafe', at over 10km. They had three features relative to the pointers being discussed. The first was that they were they were over 2 million times the initial power (just over 10KW). The second is that they were in the IR (the eye has a tendency to 'blink' if the light is visible). The third is that the optical assemblies were just on a metre in length. The actual specifications for how parallel the beam is from most laser diode assemblies, are remarkably good for their size, but are still in the order of tens of arc seconds of divergence, rather than than the sub arc second levels needed for the beam to be really tight after a few thousand feet. Remember even one minute of arc, gives a 'spot size' of 1" at 100 yards. But even if there were a laser beam that *could* be kept that tight, one needs to show that the green laser pointers are in fact kept that tight. Experience at tens of meters such as Gary had--as unfortunate as that was--won't extrapolate to thousands of meters. Yes. Distance is definately your 'friend' in this regard. However, I treat any such light, 'with respect', and would avoid any situation involvng looking directly at such a beam, eevn if it was certified as 'safe'. With regards to pilots, the biggest danger, would be in the UK, a pilot flying 'night VFR'. Compared to the normal instrument flight, a pilot in this enviroment is dependant far more on 'seeing', and a light that is completely 'safe', can still have a very significant effect on night vision. When powerful searchlights are aimed up into the sky, notification has to be given to the CAA, of this, not because they will endanger the eyes, but because of the effect on vision. Unfortunately, these laser pointers, are common, and cheap, and are being handled by many people as if they are 'toys', yet in some circumstances can have slight dangers associated with them, and as such need a degree of responsibility in their use. The astronomical club mentioned in an earlier post in this regard, has done the 'right thing', by arranging not to use these when flight is occurring over the area, and this should be a signal to other groups to consider similar care in this regard, to help prevent a beaureaucratic 'overreaction', that old be t th detriment of everybody... Best Wishes |
#13
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On Thu, 30 Dec 2004 22:42:02 -0700, Tim Killian
wrote: So tell us, what is the safe distance when someone points a green laser in your face? What, you don't know for sure? I said nothing about a safe distance. I said only that Gary's experience could not be extrapolated to the distance of an aircraft. Do you disagree with that? Laser beams can be collimated, expanded, even focused (theoretically to a spot 1/4 wavelength in diameter). To make a blanket claim that beam divergence will always protect bystanders shows you have little real understanding of atmospheric propagation. I didn't say that beam divergence will always protect bystanders, nor did I make any claim, blanket or otherwise. I said the events could not be equated, and divergence is a major reason. Other reasons include the windows on the aircraft and atmospheric scattering and/or absorption. Where did this stuff about focused beams come from? Are you suggesting that that's what happened in the incident with the airline pilot? Did someone track the aircraft with a focused laser beam? Do you have any evidence for this? If not, what's the relevance of the ray gun talk? Are you saying that because lasers can be focused Gary's experience was the same as the airline pilot's? Or are you bringing stuff up just to argue? Your tone is consistent with that. In fact, what I objected to might be considered a blanket claim made by Gary -- the apparent assumption that all events would have identical results despite very different circumstances. You've pointed out that variable factors can affect the resulting beam. It seems we agree. As for my understanding of atmospheric propagation, that's pretty much irrelevant, but I do have some knowledge of how our observatory's adaptive optics laser behaves in the atmosphere and how much it diverges at the height of aircraft and beyond. But even without that I'd guess most people know that laser beams diverge. Mike Simmons Mike Simmons wrote: On Fri, 31 Dec 2004 04:17:33 GMT, Gary Honis wrote: I've been watching the news channel reports today on green lasers and the dangers to pilots. I've also been reading the messages here about using green lasers to easily point out objects in the night sky. I don't own a green laser pointer but I have a unique opinion of green lasers because I was flash blinded by one accidentally turned on by Howie Glatter at a star party. When a pilot says he was "flash blinded" by a green laser, I know what he experienced and the disorientation that results. No you don't. The pilot of an aircraft will likely be miles from the laser. The beam will diverge greatly at that distance. Equating that (if that's what it was) with your point-blank experience is ludicrous. Mike Simmons |
#14
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On Fri, 31 Dec 2004 08:04:53 GMT, Stuart Levy wrote:
I have a 5mW green laser, and have tried shining it into my own eye -- briefly (1 second or so) but at short range, so that the beam diameter was smaller than my pupil -- while dark adapted. You're a brave man, Stuart. I'm glad your experiment turned out alright. Mike Simmons |
#15
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"Gary Honis" wrote in message ... I was flash blinded by one accidentally turned on by Howie Glatter at a star party. I would like to hear Howie's response to this message. |
#16
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I would venture to say that automobiles kill more people every day in
the US than willl ever be hurt by green lasers in the course of 10 years. I don't see anyuone calling for banning cars. Instead, we enact laws and rules to govern the use of cars. It's the same with green lasers. Exercise appropriate care in their use and storage to eliminate the possibility of accidental damage. IMO, it's absurd to ban green lasers at star parties. Maybe we do need a 10-minute class on their safe use. Clear skies, Shneor Sherman |
#17
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When a pilot says he was "flash blinded" by a green laser, I know
what he experienced and the disorientation that results. Hi: You do? Well, tell me how some pilot was "flash blinded" by a handheld laser pointer at 5 or 10k feet? How was it guided? How was it pointed into the cockpit? What about the tremendous beam dispersion? I have several friends who are airline pilots, and I respect them tremendously. But, as they will tell you, when you're tired and immersed in what is basically a disorienting enviroment, it's very possible to see things that ain't so, from zooming saucers to laserspots. Unless these incidents were documented with laser radiation detectors, I wouldn't call them "proven." If these incidents were a terrorist attack, yes, it's serious, VERY SERIOUS, but it was NOT done with a 5mw handheld laserpointer. I don't think they (green lasers) should be used in an environment where they can irritate other folks, but they are not useless, especially for educators. I'm very sorry--VERY--that you were injured by a laser flash, but I must also say this is not a common incident. I've been to plenty of star parties over the last 5 years or so since the greenies started becoming popular, and I can't recall somebody creating a problem with one. That doesn't mean that there are not kooks and careless people out there, just that they must be the exception rather than the rule among amateurs. Frankly, I'm afraid I see the beginnings of an anti-green-laser movement not unlike some segments of the anti-gun movement: "We don't care if you're a law-abiding citizen who uses that laser/pistol responsibly. No one should be allowed to have a laser/firearm." Wouldn't ya know it? I FINALLY get a green laser of my own, and now they're gonna be OUTLAWED! ;-) Peace, Rod Mollise Author of _Choosing and Using a Schmidt Cassegrain Telescope_ Like SCTs and MCTs? Check-out sct-user, the mailing list for CAT fanciers! Goto http://members.aol.com/RMOLLISE/index.html |
#18
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Jax wrote: I would like to hear Howie's response to this message. The topic was discussed on the Cherry Springs Star Party list, and Phil De Rosa e-mailed me to ask about it. Phil kindly posted my correspondence with him to the list in an attempt to clarify the details. I'll copy Phil's post, which he e-mailed to me: I know Howie from his attendance at star parties, including ours, and I was very concerned when I heard about the laser incident at CSSP with Thom Bemus. I also banned green lasers from the MDSP, for safety reasons, but I didn't ban Howie from selling them. I also use one at our public starwatches. I was very curious why I did not hear anything from him after people were going back and forth about green lasers after the incident. So I emailed him, and following is the thread of emails: ================================================== ==== What happened, if you care to share it with me? I'm sorry to report that the accident referred to did occur, and although it was an accident, I was responsible for it. It was not a green pointer, but a collimator. It was early twilight, I just finished collimating my scope, and two outside visitors from a group that had been invited to the party by the park rangers came by and asked me general questions about astronomy and telescopes. I believed I had turned the collimator reliably off by unscrewing the battery cap, and I was holding it in my hand. Evidently, in gesturing while talking to the guests, the battery cap jiggled on and the collimator was pointing backwards. I was unaware it had turned on until I heard people calling out. By that time the incident had occurred. A am mortified and incredibly bummed out that it happened. I was most concerned about Gary, but evidently he is O.K. Fortunately the collimator was class IIIa, 5mw , and the contact was momentary, but this shows that one cannot be too careful. I was hoping it would not receive wide circulation, but alas, it is on the internet. I will answer e-mail by concerned parties, but I don't wish to post anything. It would remind me of gawkers at highway crashes. Yours, Howie I would have liked to hear what actually happened sooner, I think it would have been better if you spoke up at the time. I don't know what I would be responding to. Do you have any links to where it was discussed? Did you at least tell Thom Bemus what happened? No, but an accident report was filed with the park police, and I spoke to them then. I'm pretty sure Thom knows. Yours, Howie I think the best thing to do so long after the incident is to let me forward your explanation to the Cherry Springs List server. It's up to you, but I think you should only do so if it will serve a good purpose and not feed the gossip network. If the thread died down, why start it up? If someone recently posted something on it with wrong information or assumptions, I could see a reason. If you decide to, you can post my e-mails to you if you explain that you initially e-mailed me, and include your questions and my unedited responses Yours, Howie ================================================ I am not excusing the incident, but this could have been ANYONE on the field, and someone mentioned the incident again. I think Howie has been in the doghouse long enough. Apparently, he talks with his hands. Must have a bit of Italian in him.....I would have never guessed! Philip J. De Rosa York County Astronomical Society 12/31/04 addendum from Howie: The only positive development to come out this that I know of is that I've belatedly taken up an idea and suggestion that Frank Bov made to me a few years ago to place a thin full diameter O-ring between the collimator battery and the battery cap (Thanks, Frank!). It prevents inadvertent activation from shakeing and vibration. I'm supplying them now, and I plan to send them to past purchasers of my collimators. H. |
#19
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I agree that you can't simply extrapolate the flash blinding by a nearby laser pointer to flash blinding of a pilot at large distances (like thousands of feet). Beam divergence of these inexpensive laser pointers would diverge the beam to the point that the energy is just too low to do anything other than appear to the pilot like a tiny green point in the distance. Assuming an initial beam size of 2 mm and a beam divergence of 5 miliradians, at a distance of 40 feet the beam gets to be 7 mm in diameter. At greater distances the beam continues to expand so that it can no longer be completely accepted by the dark adapted eye. For example, at a distance of 100 feet the beam is 15 mm across and only 23% can enter the dark adapted eye, at 500 feet it is 66 mm and only 1% can get into the eye. At one mile, the beam is 673 mm across and would require the pilot to use a 26 inch diameter telescope objective to collect the whole beam (not just any old concave surface which happened to be in the cockpit. Not a likely chance occurrence. What this tells me is that either someone is playing games with an industrial strength laser or else is using large diameter astronomical optics to focus the pointer laser and reduce the beam divergence. I will not go into detail how this could be done, but if this is in fact what happened, the irresponsible meathead who did it should be prosecuted and have his equipment confiscated. Clif Ashcraft |
#20
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wrote in message oups.com... I'm sorry to report that the accident referred to did occur, and although it was an accident, I was responsible for it. It was not a green pointer, but a collimator. Thanks for your quick response, Howie. I have been accidently hit in my dark adapted eye with a green laser pointer of yours from 20' away with no impact other than it was annoying. Is the collimator more a risk than the pointer? Peace, Jon |
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