Davidgojr
member
Reged: 08/09/08
Posts: 58
Loc: San Antonio, TX
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It seems obvious that some larger scopes could collect enough moonlight during a full moon to cause eye damage during viewing. I could imagine this becoming a serious problem for some larger Dobsonian reflectors. Does anyone know at what aperture this would start becoming a concern? (Assuming that the moon was full and viewed through a low power eyepiece with no lunar filter.)
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San Antonio, TX
Orion XT6
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Mr Q
sage
Reged: 02/25/08
Posts: 351
Loc: N Central New Mexico
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I doubt that any available size mirror today will cause permenant damage to your eyes other that ruin your night vision. Prolonged exposure to such a bright light may cause damage such as "snow blindness" but I bet you would get a severe eye strain headache before any real damage occurs. I've looked at the full Moon many times with a 10" newt on low power and I can still see what I'm typing!
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What goes around, comes around, eventually.
Meade DS-10(10" newt)
10x50, 10x70 binos
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 6754
Loc: Beatrice, Nebraska
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Quote:
It seems obvious that some larger scopes could collect enough moonlight during a full moon to cause eye damage during viewing. I could imagine this becoming a serious problem for some larger Dobsonian reflectors. Does anyone know at what aperture this would start becoming a concern? (Assuming that the moon was full and viewed through a low power eyepiece with no lunar filter.)
Well, it won't really cause any damage to the eye. The moon is basically a day-lit landscape, so looking at it with a telescope is no more dangerous than looking at a bright landscape at noon here on Earth. In a large scope, the brightness can be a little hard on a recently dark-adapted eye (at least until you get adapted to the brightness), but that won't cause any damage (and is what moon filters are for). I would use as large an aperture as the seeing will permit for viewing the moon. Clear skies to you.
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
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rtomw77
sage
Reged: 10/06/04
Posts: 461
Loc: Deer Valley, AZ
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The 30" Obsession report just posted has a warning not to use it on the Moon. All I can say from personal experience is that the full Moon at low power through a 12" scope is uncomfortable to me.
Tom
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Celestron 8" Nexstar GPS XLT
Meade 8" f/6 newtonian on GEM-New GSO Mirror
StarBlast 4.5" Dob
XT12 IntelliScope Dob with COL
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Tim2723
The Moon Guy
Reged: 02/19/04
Posts: 5117
Loc: Northern New Jersey
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Yes, the unfiltered Moon through a large aperture can be very uncomfortable, but not permanently damaging. No matter the size of the scope, you are observing reflected sunlight. The intense radiation so well known to solar observers is absent. No matter how large the objective you cannot, for instance, start a fire with Moonlight.
There are a number of very informative threads on selecting apertures for lunar observing here in the archives, and a couple of very scholarly discussions can be found in 'The Best of the Lunie Bin' thread one level back.
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The crwth will set you free!
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Joe Bergeron
Vendor - Space Art
Reged: 11/10/03
Posts: 843
Loc: Upstate NY
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It may seem obvious, but it isn't true (like so many other things which seem obvious). What if the Moon came so close it looked 100X bigger? Would you worry about hurting your eyes if you looked at it? It's just about the same thing, except without the earthquakes, killer tides, and general global destruction.
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Seven telescopes of a diverse nature.
Multiple chums glittering in the sky. New friends await.
My Web Site
English Lessons for Amateur Astronomers
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star drop
Guilty as Charged
Reged: 02/02/08
Posts: 2708
Loc: Cattaraugus Co., NY
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It's been nineteen years that I have been viewing full moons with my 25" dobsonian at 99.5x without filters and I have suffered no eye injuries as a result. The full moon is less blinding when is is in a brighter sky near sunrise or sunset or in a large urban light dome and quite tame during the day as viewed in my reflector. I seem to remember reading somewhere that the average reflectivity of the lunar surface is around seven percent. It is also a diffuse reflection so the seven percent is reflected in many directions. I think the only way one could damage ones eyes in this manner is to stumble about after viewing during the night and walk into something.
   
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Ted
25" Tectron F/5 Original Naglers 9,13 & 20 mm T2, 30 mm Meade UWA, TV 32 mm Widefield , Ethos 25 mm
Orion 10x70 binoculars
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rtomw77
sage
Reged: 10/06/04
Posts: 461
Loc: Deer Valley, AZ
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Quote:
The 30" Obsession report just posted has a warning not to use it on the Moon.
Tom T. says in a follow up post to his review that the warning was just his warped sense of humor. I stand corrected.
Tom
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Celestron 8" Nexstar GPS XLT
Meade 8" f/6 newtonian on GEM-New GSO Mirror
StarBlast 4.5" Dob
XT12 IntelliScope Dob with COL
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deSitter
professor emeritus
Reged: 12/09/04
Posts: 733
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Quote:
It may seem obvious, but it isn't true (like so many other things which seem obvious). What if the Moon came so close it looked 100X bigger? Would you worry about hurting your eyes if you looked at it? It's just about the same thing, except without the earthquakes, killer tides, and general global destruction.
Funny..
What really matters is the focal ratio of the scope. Your eye is about f/10 when stopped all the way down. Any other optical system at f/10 will show the same surface brightness as your stopped down eye does. However a large reflector at f/4 will show it to be 6 times brighter - on the other hand the Moon is generally very dark, and were it sitting on the table, would appear something like a mouse roller ball smudged with grease. In short there is nothing to fear from the Moon, unless you live in a trailer park and are prone to being abducted by aliens. Because we all know that is where they really live..
It should also be pointed out that the eye is specially made to have a gigantic dynamic range. People hardly notice when the Sun has just gone down and the light level has decreased 50-fold.
-drl
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Davidgojr
member
Reged: 08/09/08
Posts: 58
Loc: San Antonio, TX
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Quote:
Quote:
The 30" Obsession report just posted has a warning not to use it on the Moon.
Tom T. says in a follow up post to his review that the warning was just his warped sense of humor. I stand corrected.
Tom
This is what most likely made me wonder about the lunar eye damage possibility. I had just read that review about the Obsession and was surprised to see the warning at the bottom.
I'm still new to astronomy but I am learning at a fast rate. Could someone please explain or post a link discussing focal ratio and how it relates to practical observing?
Also, can any scope available increase the level of surface brightness (or brightness per unit area) of the moon than what you would see with your naked eye? deSitter, doesn't it also matter not just on the focal ratio but also on the level of magnification?
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San Antonio, TX
Orion XT6
Edited by Davidgojr (08/24/08 03:09 PM)
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Joe Bergeron
Vendor - Space Art
Reged: 11/10/03
Posts: 843
Loc: Upstate NY
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No. No telescope can make the surface brightness of the moon (or any other extended object) look brighter than it does to the eye. If that were possible, we'd all have purely optical night vision goggles by now.
Focal ratio by itself has no effect on the surface brightness of things as seen through an eyepiece. It's an interaction between exit pupil size and the size of your own pupil. If your exit pupil diameter matches the pupil of your eye, then surface brightness is at maximum, regardless of the aperture or focal ratio of the telescope in use. The size of the image will of course vary depending on what you have, but not the per unit brightness. Naturally it's usually helpful to have a bigger image at a given surface brightness, which is one reason why bigger telescopes are good. The only thing that could change any of this is if your telescope is inefficient at light transmission, but that only makes things dimmer, not brighter.
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Seven telescopes of a diverse nature.
Multiple chums glittering in the sky. New friends await.
My Web Site
English Lessons for Amateur Astronomers
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Ptarmigan
Carpal Tunnel
Reged: 09/23/04
Posts: 1873
Loc: Arctic
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The Moon at an 8 inch telescope is too bright for me, especially if it is full and at low power. Higher power makes the Moon darker. It leaves a tadpole image after you see a camera flash. Also, you almost project the Moon through the telescope like the Sun.
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Ptarmigans=Cute and Cuddly
Edited by Ptarmigan (08/24/08 06:19 PM)
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Dave Mitsky
Postmaster
Reged: 04/08/02
Posts: 6089
Loc: Pennsylvania, USA
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I've used a 17" f/15 classical Cassegrain to observe the Moon for many years. I don't employ a moon filter, relying rather on higher magnifications and the use of a white light if needed.
http://jeff.medkeff.com/astro/lunar/obs_tech/index.htm#glare
As Joe stated, a telescope is not able to increase the surface brightness of an extended object. This means that no telescope, no matter how large, can damage the human eye during lunar observing. Bright moonlight certainly willl affect night vision and a dilated pupil but exiting a dark movie theater directly into bright sunlight causes a similar temporarily distressing effect.
Dave Mitsky
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Chance favors the prepared mind.
De gustibus non est disputandum.
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CESDewar
GorillAstronomer
Reged: 01/16/05
Posts: 1802
Loc: Morganton, GA, USA
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Quote:
The 30" Obsession report just posted has a warning not to use it on the Moon.
The key was the Smiley face at the end - I definitely read that as a joke 
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deSitter
professor emeritus
Reged: 12/09/04
Posts: 733
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All this being said, a neutral density filter is a great aid to comfortable viewing of the Moon, and essential if you want to use all your scope's resolution. If the glare of the Moon causes your eye to stop down to a size smaller than the exit pupil of the eyepiece, then you've effectively stopped down your scope to a smaller aperture.
-drl
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Centaur
Pooh-Bah
Reged: 07/12/04
Posts: 1112
Loc: Chicago
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A telescope can indeed make objects much brighter than they appear to the naked eye. The wider the aperture and the lower the magnification, the more this is true.
With a telescope I’ve never really feared blindness from a Full Moon, but I have gone to aperture reduction or a filter. A couple of years ago my pupils had been chemically treated to widen their aperture during a doctor’s examination. They were still rather wide when a Full Moon appeared that night. It seemed painfully bright and forced me to turn away.
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For astronomical graphics, including
monthly wallpaper calendar, visit:
www.CurtRenz.com/astronomical.html
Curt Renz - "Centaur"
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Bill Weir
scholastic sledgehammer
Reged: 06/01/04
Posts: 879
Loc: Metchosin (Victoria), Canada
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I love large aperture on the Moon. Using the 25" is a trip. Craterlets and rimae jump right out at you. One night I counted 7 craterlets in Plato.
I'm with Dave on the leaving the lights on while looking at the Moon. I hate using filters.
Bill
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6'' Orion SkyQuest
12.5'' f/5 Custom Truss Dob
William Optics 80mm ZenithStar II ED Doublet
f/5 25" newtonian on a giant GEM, any time I want
Observing sessions grand total for 2007, 171.
So far in 2008, 108
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Joe Bergeron
Vendor - Space Art
Reged: 11/10/03
Posts: 843
Loc: Upstate NY
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Quote:
A telescope can indeed make objects much brighter than they appear to the naked eye. The wider the aperture and the lower the magnification, the more this is true.
We're talking about surface brightness of extended objects (not stars). You cannot brighten that beyond the naked eye value, regardless of your aperture or magnification. If you try, you simply get an exit pupil bigger than your eye can accept, effectively stopping down the telescope. The surface brightness then peaks and remains the same at any lower power. If that weren't true you'd have people setting their eyes on fire by looking at a white wall with a 30" telescope at 1x. I realize this may not be intuitively obvious. I advise you to read a book about optics and telescopes to get the idea.
Note that when I say "naked eye value" I mean the optimum value for any given target, with appropriately dilated pupils and dark adaptation.
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Seven telescopes of a diverse nature.
Multiple chums glittering in the sky. New friends await.
My Web Site
English Lessons for Amateur Astronomers
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Centaur
Pooh-Bah
Reged: 07/12/04
Posts: 1112
Loc: Chicago
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Quote:
We're talking about surface brightness of extended objects (not stars). You cannot brighten that beyond the naked eye value, regardless of your aperture or magnification.
The point of larger aperture is to gather more light. Otherwise, why covet large aperture? Observe the Moon with constant magnification but different apertures and you will notice the difference in brightness. My first telescope fifty years ago was a 3-inch reflector with a single 30-power eyepiece. It came with a disk with a 1.5-inch hole in the middle to be placed in front of the scope to reduce the brightness of the Moon when near full. It worked.
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For astronomical graphics, including
monthly wallpaper calendar, visit:
www.CurtRenz.com/astronomical.html
Curt Renz - "Centaur"
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Jim Mosher
sage
Reged: 05/22/06
Posts: 228
Loc: Newport Beach, CA
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Curt,
A 1.5-in (38 mm) disk at 30x produces a 1.3 mm exit pupil. If that was actually placed on-axis around the secondary (as you seem to be suggesting) considerably less than the full pupil of your young eye would have been filled with moonlight. So its not surprising you would see less than the naked eye brightness. If you could have increased the eyepiece power to 60x, the image would have been dimmer still because, as Joe explains, the smaller exit pupil would have filled even less of your eye's pupil with moonlight. That's one of the many reasons for coveting larger apertures -- it permits increase of magnification without this loss of brightness. Another is it increases the angular resolution, and for a point source (like a star) it potentially stuffs more light into a smaller spot.
But none of this means that the image without the stop (when more of your eye's pupil is filled) exceeds the brightness of the naked eye Moon. To check that you'd have to use a plane mirror to simultaneously view the Moon without optical aid through one eye, and through the telescope with the other. The through-the-telescope view will certainly be larger, but I'm confident you'll find it no brighter. Most likely it will be dimmer.
-- Jim
P.S.: if you actually try the dual image experiment, a better target than the Moon is to point your telescope towards the zenith during the daytime, so that you're looking at a patch of blank sky. Holding a very small mirror at 45° in front of the other eye you can fill it with a view of the same part of the sky. With a high power eyepiece the sky seen through the telescope will be much dimmer. With a low power eyepiece it will approach the brightness of the direct view, but never exceed it.
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