lutz@ur-tut.UUCP (Dave Lutz) (10/16/86)
A few weeks ago I submitted a question about the safety and
usefulness of welders glass as a telescopic solar filter. I would
like to thank the many people who responded. The following is a
summary of the responses I got.
Most people advised against looking at the sun directly with a
telescope at all no matter what type of filter you are using. The
chance that your filter might be damaged allowing the direct sunlight
to hit your unprotected eye is just too great. This is especially
true when using an in-the-eyepiece filter because the heat generated
by the concentration of sunlight can crack the filter. The only
truly safe way to view the sun is the projection method.
Concerning the usefulness of welders glass as a filter most
people advised against it. Welders glass is not intended for this
use and it is not optically flat. This means that the image could
have severe distortions in it. It is especially poor for first
hand viewing because it does not block the full spectrum of infrared
and ultraviolet light. This means that even though you may not feel
it you have a very good chance of damaging your eyes. One person
stated that he had looked at the sun very briefly through welders
glass and has experienced some loss of vision. He stated that it
is not noticeable during the day but he has problems looking at
faint objects through his telescope.:-(
A few people said that there was an article in Sky and Telescope
concerning the issue of solar filters and viewing the sun. It is
supposed to be a very good article and it even deals with the question
of using welders glass for this purpose. I'm sorry to say that none
of them could remember what issue it was in. If you can find it it
would be a good article to read if you're interested in the sun.
Thanks again for the many responses.
Dave Lutz
allegra!rochester!ur-tut!lutzhogg@utcsri.UUCP (John Hogg) (10/16/86)
The best filter to use for looking at the sun is (surprise) one that has been designed for looking at the sun. These can be obtained for about $36 Canadian at current rates, and you get a wonderful bonus: there's a sextant attached. The model I'm thinking of is the Davis Mk III. It's cheap, it's plastic, but it works. The disadvantage from the astronomer's point of view is that these filters cannot be installed on a telescope. However, they are designed to be used for staring at the full sun for extended periods, and therefore presumably filter out all harmful rays for direct viewing. I've used mine in past for eclipse watching with no noticable effects. (The issue was moot this time around.) -- John Hogg hogg@utcsri.uucp hogg@csri.toronto.cdn
tj@alliant.UUCP (Tom Jaskiewicz) (10/17/86)
In an article, somebody writes: > >>surface, like a screen or flat sheet. Or you may want to use a safe >>filter. An easy, cheap filter is welder's glass -- number 14 or >>darker. Just look in the yellow pages under "welding supplies." Give > > Would this be good for making a homemade solar filter for a telescope? >I would probably make the type that fits over the end of the scope tube. > > Would it be safe??? NO! The spectrum produced by arc-welding is DIFFERENT than the solar spectrum. Welder's glass will not necessarily protect your eyes while viewing the sun. I've heard of one scientist who used his lab equipment to test welder's glass. He found that sometimes it will work, and sometimes it won't. It depends who the manufacturer is. (I heard this in 1983). Similarly, what harms a camera is different than what will harm your eyes. A 5.0 or 6.0 neutral density photographic filter will let you take a picture of the sun, but won't protect your eyes if you look through it. -- #################################################################### # uucp: decvax!linus!alliant!tj ## Bernese are mountains of love. # ####################################################################
cmpbsdb@gitpyr.gatech.EDU (Don Barry) (10/21/86)
I have run tests as well on Welding filters, and have found the
#14 and #15 filters I have tested to be perfectly safe for direct
viewing of the sun. UV is no problem because of glass' inherent
absorption of it, as well as the design of the filter (which is designed
to filter a color temperature hotter than that of the sun's surface!).
Infrared radiation is also taken care of (one reason why these are rather
thick), even though the vitreous humour of the eye is rather opaque to
wavelengths much beyond 1000 nm.
Although with the sun, safety must *always* be the rule, several common
materials have a spectral density suitable for solar viewing. exposed,
black-developed B&W film (say, from a leader, or custom-made) is safe if
the density is sufficient to make the sun's image of reasonable brightness.
****WARNING**** - the same is *not* true for color film, whose dyes exhibit
in some cases absorbances with dangerous valleys near the visible region in
danger zones to the eye. B&W silver emulsions have a uniform absorbance
which renders them rather safe.
If in doubt, don't do it. Projections are always the safest way. And
*Never* use a filter material on the back of a telescope eyepiece, even
those little filters that come with telescopes. I have heard first-hand of
one such filter that *exploded* after several minutes viewing, fortunately
when no one was looking. The heat in this area of the telescope is most intense.
--
Don Barry (Chemistry Dept) CSnet: cmpbsdb%gitpyr.GTNET@gatech.CSNET
Georgia Institute of Technology BITNET: CMPBSDB @ GITVM1
Atlanta, GA 30332 ARPA: cmpbsdb%gitpyr.GTNET%gatech.CSNET@csnet-relay.ARPA
UUCP: ...!{akgua,allegra,amd,hplabs,ihnp4,seismo,ut-ngp}!gatech!gitpyr!cmpbsdbmarkm@star.UUCP (10/21/86)
In article <779@ur-tut.UUCP> lutz@ur-tut.UUCP (Dave Lutz) writes: > A few people said that there was an article in Sky and Telescope > concerning the issue of solar filters and viewing the sun. It is >supposed to be a very good article and it even deals with the question >of using welders glass for this purpose. I'm sorry to say that none >of them could remember what issue it was in. > The article is "Safe Solar Filters" by B. Ralph Chou, School of Optometry, Univ. of Waterloo, Ontario. It was published in the August, 1981 issue of SKY AND TELESCOPE. The author states "...that metal-coated glass or Mylar filters are safe for visual and photographic work. Properly treated black-and-white film is safe when two layers are used. And welder's glass shade No. 14 is also safe for visual use but it is not uniform enough for use over a telescope objective, and such a filter may crack if placed at the eye end. Less dense shades (lower numbers) are not suitable for direct solar observing. Under no circumstances should color film, smoked glass, or gelatin neutral-density filters be used for direct solar observation." Roger W. Tuthill, Inc. will send a copy of the article with their product literature (they sell the "Solar Skreen", a mylar sun filter), if you send them a stamped ($1.24 U.S.), self addressed, large envelope. The address is 11 Tanglewood Lane, Mountainside, New Jersey 07092. Another company to contact is Thousand Oaks Optical, Box 314, Wyandotte, Michigan 48192. They sell both mylar and metal-coated glass filters in a variety of sizes. Explicit Disclaimers: I am speaking for myself, not Tektronix. I am not affiliated with any of the companies mentioned in this article. Mark Mehall uucp: {ucbvax,decvax,ihnp4}! Tektronix Inc. tektronix!tekecs!markm P.O. Box 1000 60-770 CSnet: tekecs!markm@tek Wilsonville, Oregon 97070 ARPAnet: tekecs!markm.tek@csnet-relay Phone (503) 685-2275 FAX: (503) 682-3408 GRP III, II Auto
ugzannin@sunybcs.UUCP (Adrian Zannin) (10/27/86)
In article <2445@gitpyr.gatech.EDU>, cmpbsdb@gitpyr.gatech.EDU (Don Barry) writes: . . . > If in doubt, don't do it. Projections are always the safest way. And > *Never* use a filter material on the back of a telescope eyepiece, even > those little filters that come with telescopes. I have heard first-hand of > one such filter that *exploded* after several minutes viewing, fortunately > when no one was looking. The heat in this area of the telescope is most intense. [] Just to give you an idea of the care that should be taken with telescopes even in just normal handling: About two months ago or so, we decided it was time to clean out the garage. I had a little rinky-dink telescope out there that I still ocassionally used to view the moon, but more to view the action on the other shore during the local hydroplane races, the lens covers lost a long time ago. After most of the work was done, I left. When I came back, half the garage was charred and it was all wet. Heres what happened: After I left, my father was rearranging some of the stuff that we had moved from the garage. He needed to move the telescope, and since there was no convienient place to prop it up, he just set it up on its tripod just outside the garage door. However, the scope was aimed up onto the sky...and guess what...after a while the sun *just happened* to move across the section of sky where the telescope was pointed. Yup, you guessed it. It acted just like a magnifying glass. The other end of the scope was pointing down at some wood in the garage...and...*POOF*...it all went up. After the fire company left, the fire investigator came over and he was stumped...he said he had never seen anything like it. The total damage was about $2500 - $3000. Kind of expensive for just a little carelessness... Kind of bizzare, isn't it? People still don't believe me when I tell them... -- Adrian Zannin -- SUNY at Buffalo Computer Science UUCP: ..{bbncca,decvax,dual,rocksvax,watmath,sbcs}!sunybcs!ugzannin CSNET: ugzannin@Buffalo.CSNET BITNET: ugzannin@sunybcs.BITNET ARPANET: ugzannin%Buffalo@csnet-relay.ARPA
chip@vaxwaller.UUCP (10/29/86)
> Similarly, what harms a camera is different than what will harm your > eyes. A 5.0 or 6.0 neutral density photographic filter will let you > take a picture of the sun, but won't protect your eyes if you look > through it. Also, never look through your view-finder when trying to take a shot of the sun even when you've mounted the neutral density filter!! How do you line up the shot? Simple. With the lens cap on simply align your camera with the sun using the shadow of your lens as a guide. Move your setup around until you have minimum shadow around the lens...equal on all sides. Kind of like a sundial. Simple, easy, quick and SAFE. (Remember, however, to leave your lens cap on when not shooting...no sense asking for trouble.) Happiness; Chip -- ,, *** SOLIDARNOSC *** Chip Kozy (415) 939-2400 x-2048 Varian Inst. Grp. 2700 Mitchell Dr. Walnut Creek, Calif. 94598 {zehntel,amd,fortune,rtgvax,rtech}!varian!vaxwaller!chip