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Iris
2006-Aug-11, 06:50 PM
I have been considering to buy myself a few filters for my telescope. I have it for a while, and I didn't even buy any accessories for it yet!

Since there is just one store with telescope stuff around here and that it isn't easy to reach for me, I decided to glance at eBay for some stuff. I figured a nice first to get would be a moon filter. Therefore, I browsed around and found this:

Clicky (http://cgi.ebay.ca/Telescope-1-25-Moon-Filter-30-N-D-Filters_W0QQitemZ290018128431QQihZ019QQcategoryZ28 181QQrdZ1QQcmdZViewItem)

It's nice, it's cheap... I figured it wouldn't be bad for me to buy it! But then I read that line in his description of the item...



"VIEWING THE MOON, WHILE AT ITS BRIGHTEST,

FULL MOON, OR 3/4 MOON, WITH OUT THE USE OF A MOON FILTER,

CAN DAMAGE YOUR EYES PERMANENTLY !"

Wha, is that true??? I sure as heck did that a few times. What do you guys say?

Dave Mitsky
2006-Aug-11, 06:54 PM
It is most definitely NOT true. Does observing the Moon during the daytime hurt your eyes? No, it does not.

At night, your pupils are dilated and yours eyes are more sensitive to light due to dark adaptation, just as they are if you happen to suddenly exit a dark movie theater into bright sunlight. It takes a few minutes for your eyes to adapt and there is some concomitant discomfort.

Experienced lunar observers ususally eschew any filters and simply turn on a white light. The use of high magnification reduces the brightness as well.

http://jeff.medkeff.com/astro/lunar/obs_tech/index.htm

Dave Mitsky

Tog
2006-Aug-11, 07:15 PM
What Dave said. Even in a 16" scope where I get a very bright Moon image, it's unpleasant, but far from painful or damaging. The worst you can probably expect is very poor night vision just long enough to trip of your stool or eyepeice box. Or your car if it's a dark one. ummm not that any of that has ever happened. or anything.;)

George
2006-Aug-11, 08:15 PM
I also see no way for the eye to be damaged by looking at the moon with a scope.

Telescopes seem to be designed to match the light gathering ability of the scope's aperature with the magnification. This one to one match will not allow an increase to the surface brightness. Since the total quantity of light, however, varies, the moon and other large bright objects will be less comfortable with marginal magnifications.

For a closer look at this, here are the equations...

EP (Exit Pupil) = FL eyepiece / f-ratio = Aperature / mag.

Light Gathering = (Aperature/EP)^2 = mag^2

So, if your magnification is 50x, then your apparent size has increased 2500x and the light gathering is 2500x, also. Thus, the net brightness per unit area is never greater, regardless of which lens you choose. The unit area brightness would be the same as when you look at the moon without magnification, except for the discomfort that comes when you increase the total amount of light flux into your eye. [Actually, it will get dimmer per unit area due to light loss in lenses, I think.]

[The first equation may be an engineering design equation and not an optical law. That is my hope, at least.]

Glom
2006-Aug-13, 10:04 PM
Yeah, but it can't half screw up your night vision. Lousy moon! (I love it really. Hauntingly beautiful!)

Kaptain K
2006-Aug-14, 11:42 AM
When I was in college, we called the Moon the Great Astronomical Obstruction Number Two! Of coure, the Sun was the Great Astronomical Obstruction Number One!

George
2006-Aug-14, 04:22 PM
... Of coure, the Sun was the Great Astronomical Obstruction Number One!
It's ironic that this is true even for solar astronomy. For example, SOHO can not sit in the L1 spot as its signals would be overcome by the suns noise.

Dave Mitsky
2006-Aug-15, 08:43 AM
I also see no way for the eye to be damaged by looking at the moon with a scope.

Telescopes seem to be designed to match the light gathering ability of the scope's aperature with the magnification. This one to one match will not allow an increase to the surface brightness. Since the total quantity of light, however, varies, the moon and other large bright objects will be less comfortable with marginal magnifications.

For a closer look at this, here are the equations...

EP (Exit Pupil) = FL eyepiece / f-ratio = Aperature / mag.

Light Gathering = (Aperature/EP)^2 = mag^2

So, if your magnification is 50x, then your apparent size has increased 2500x and the light gathering is 2500x, also. Thus, the net brightness per unit area is never greater, regardless of which lens you choose. The unit area brightness would be the same as when you look at the moon without magnification, except for the discomfort that comes when you increase the total amount of light flux into your eye. [Actually, it will get dimmer per unit area due to light loss in lenses, I think.]

[The first equation may be an engineering design equation and not an optical law. That is my hope, at least.]

George,

I believe that you're mistaking the entrance pupil of the human eye, which is usually given as a maximum of 7mm (but is rarely achieved after the age of 40), for the exit pupil of a telescope in the formula for light grasp. That is to say, the light gain achieved by a telescope is equal to the square of the result of its aperture divided by the maximum human eye pupil of 7mm.

The formula for aperture gain is posted at http://www.astro-tom.com/technical_data/useful_formulas.htm

When a telescope is operating at a magnification of 50x, the apparent or angular size of an extended object is increased 50x as well, not 2500x. However, the the light is now spread over 50^2 (2500) times more retinal area than without the use of the telescope, so the available light per unit retinal area is decreased by 1/(50^2). Therefore, the light gain by the telescope aperture exactly cancels out the loss in light per unit area.

A telescope cannot increase the surface brightness of an extended (i.e., non-stellar) object. Doing so would violate the Second Law of Thermodynamics by allowing heat to flow spontaneously from a cooler to a hotter body. (In fact, since there is always light lost in the optics of any telescope, the fully dark adapted naked-eye view of an extended object is always the brightest one. This fact confounds many amateur astronomers.)

What a telescope can do is to increase the image scale. The telescope displays an object of equal surface brightness (actually a bit less given the light loss that occurs in all optical systems), only larger. This is the reason that fainter and fainter deep-sky objects can be seen through increasingly larger apertures. Large faint objects look brighter and more detailed than small ones of the same surface brightness because of the way the human visual system operates. The receptors in the retina will not operate if the image falling upon them is not large enough. It's a matter of biophysics, not optics.

The first formula you stated is a consequence of physics, not engineering.

The average albedo of the Moon is somewhere around 7, giving it all the brightness of asphalt.

Dave Mitsky

George
2006-Aug-15, 01:15 PM
I believe that you're mistaking the entrance pupil of the human eye, which is usually given as a maximum of 7mm (but is rarely achieved after the age of 40), for the exit pupil of a telescope in the formula for light grasp. That is to say, the light gain achieved by a telescope is equal to the square of the result of its aperture divided by the maximum human eye pupil of 7mm.
We agree in forumula, but do we differ with the 7mm diameter assumption? Do scopes, and binoculars, produce a fixed 7mm diameter beam at the eye?


When a telescope is operating at a magnification of 50x, the apparent or angular size of an extended object is increased 50x as well, not 2500x. However, the the light is now spread over 50^2 (2500) times more retinal area than without the use of the telescope, so the available light per unit retinal area is decreased by 1/(50^2). Therefore, the light gain by the telescope aperture exactly cancels out the loss in light per unit area.
That is what I attempted to say. There is no gain in light per unit area but the image is much larger and still just as bright (almost).


A telescope cannot increase the surface brightness of an extended (i.e., non-stellar) object. Doing so would violate the Second Law of Thermodynamics by allowing heat to flow spontaneously from a cooler to a hotter body. (In fact, since there is always light lost in the optics of any telescope, the fully dark adapted naked-eye view of an extended object is always the brightest one. This fact confounds many amateur astronomers.)
The second law of thermo is no restriction to increasing the brightness of an object. In terms of energy, taking flux over a large area and concentrating it into a small area will produce a brighter image as long as the projection is limited in magnification. Burning leaves with a magnifying glass demonstrates this ability. The second law will only demonstrate that the gain from the formula will be less than 100%. At the burn point, the leaf will not receive 100% of the light entering the lens, nor will our eye see 100% of the original brightness (as seen without a telescope) if the projection size on our retina has increased to match the gain in flux entering the eye (with a telescope).


What a telescope can do is to increase the image scale. The telescope displays an object of equal surface brightness, only larger. This is the reason that fainter and fainter deep-sky objects can be seen through increasing larger apertures. Large faint objects look brighter and more detailed than small ones of the same surface brightness because of the way the human visual system operates. The receptors in the retina will not operate if the image falling upon them is not large enough. It's a matter of biophysics, not optics.
Would it not be both optics and biophysics? With large aperature, wouldn't a point source be brighter due to the additional light gathered by the larger aperatue and concentrated at a point on the retina?


The first formula you stated is a consequence of physics, not engineering.
Maybe so, but I am doubtful, though I admit I haven't studied enough about this to question it too strongly. It looks more like a telescope design criteria equation to me.

There may be some evidence for this found in telecompressors as they concentrate an image into a slightly smaller area, I think.

My hope is to see someone make a colorscope with such ideas, though I am not all that sanguine about it just yet.

Tog
2006-Aug-16, 06:44 AM
We agree in forumula, but do we differ with the 7mm diameter assumption? Do scopes, and binoculars, produce a fixed 7mm diameter beam at the eye?

Exit pupil for a scope (or binos) is aperture dia/magnification. 7X50 Binoculars has about a 7.1 so they are a little big but still close.

Something like a 40mm ep in a fast Newt, say an 8 inch f/4 (fl=800mm for 20X) wold have an exit of 200/20, or 10 mm Exit. a 4mm EP would have an exit of 1mm.

Maksutov
2006-Aug-16, 12:09 PM
I fail to see how any of this would damage the Moon. I mean, check it out, even at 7X. It's already damaged beyond repair. It obviously needs some kind of celestial Clearasil.

George
2006-Aug-16, 02:08 PM
Exit pupil for a scope (or binos) is aperture dia/magnification. 7X50 Binoculars has about a 7.1 so they are a little big but still close.

Something like a 40mm ep in a fast Newt, say an 8 inch f/4 (fl=800mm for 20X) wold have an exit of 200/20, or 10 mm Exit. a 4mm EP would have an exit of 1mm.
Yes, and an interesting point here which you are demonstrating is how a 40mm eyepiece would produce an exit pupil much larger than the eye could receive. For lunar observation, I would assume its brightness would cause the eye to contract to ~2 or 3 mm. Of course, this would help lower the apparent surface brightness as a result, making the moon easier on the eye. I suppose in this case, going to shorter focal length eyepieces would make the moon become brighter in unit area compared to the 40mm ep; at least until you got down to about a 15mm ep.

Maksutov, it's Galileo's fault. It was fine till he messed with it. :)

Dave Mitsky
2006-Aug-16, 06:15 PM
"We agree in forumula, but do we differ with the 7mm diameter assumption? Do scopes, and binoculars, produce a fixed 7mm diameter beam at the eye?"

Again, you're confusing the exit pupil of a telescope or binocular, which for a given instrument depends upon magnification, with the eye's entrance pupil. The formula for aperture gain, light grasp, or light gathering, call it what you will, compares the aperture of the dark adapted young adult human eye (the entrance pupil of the observer), which is typically about 7mm, with that of the instrument in question. A 70mm refractor has an aperture ten times that of the observer's dilated pupil and a surface area 100 times as large, so it gathers 100 times more light.

"The second law of thermo is no restriction to increasing the brightness of an object. In terms of energy, taking flux over a large area and concentrating it into a small area will produce a brighter image as long as the projection is limited in magnification. Burning leaves with a magnifying glass demonstrates this ability. The second law will only demonstrate that the gain from the formula will be less than 100%. At the burn point, the leaf will not receive 100% of the light entering the lens, nor will our eye see 100% of the original brightness (as seen without a telescope) if the projection size on our retina has increased to match the gain in flux entering the eye (with a telescope)."

I'm sorry but that is not correct. It would violate the Second Law. A telescope cannot increase the surface brightness of an extended object. The Sun is a very special case and should not enter into this discussion.

For a discussion of the concept of surface brightness see http://mysite.verizon.net/vze55p46/id18.html

"Would it not be both optics and biophysics? With large aperature, wouldn't a point source be brighter due to the additional light gathered by the larger aperatue and concentrated at a point on the retina?"

Point sources (i.e., stars) are a completely different matter since they do not have "area". A telescope does, in fact, make stars brighter because they are point sources. Increasing the aperture causes more and more light to be concentrated and fainter stars will be seen.

However, extended objects are not point sources and cannot be made brighter. The two should not be confused.

Here's what the noted observer Jay Reynolds Freeman had to say on the topic in a sci.astro.amateur post:

"Image brightness of extended objects on the retina varies as the square of the exit pupil diameter (provided the exit pupil is no larger than the pupil of your eye), and is also proportional to the coefficient of transmission of the telescope as a whole (including the effects of secondary obstruction, poor coatings, et cetera).

Thus, images of extended objects on the retina as seen with a telescope can never be brighter than as seen with the naked eye. They can be larger, though, which can make them easier to detect."

The noted amateur telescope maker Mel Bartels discusses the topic at length at http://www.bbastrodesigns.com/visual.html

Keep in mind that what we see through a telescope is actually the result of an afocal system that ultimately depends upon the receptors in the retina. If an object is too small to activate those receptors, it simply can't be seen. Since a telescope magnifies objects as well as gathers light, it permits tiny galaxies to be seen because of the pecularities of the psychophysiology of the human eye.

http://hyperphysics.phy-astr.gsu.edu/hbase/vision/retina.html#c2

Roger Clark's now out-of-print book _Visual Astronomy of the Deep Sky_ explains these concepts quite well.

There's a discussuion at http://www.mapug-astronomy.net/AstroDesigns/MAPUG/VisualDet.htm

"Maybe so, but I am doubtful, though I admit I haven't studied enough about this to question it too strongly. It looks more like a telescope design criteria equation to me."

The formula applies equally to all telescopes. How does design enter into the picture?

"There may be some evidence for this found in telecompressors as they concentrate an image into a slightly smaller area, I think."

Yes, but again, this is just another case of basic optical principles. A Barlow lens does the reverse.

Dave Mitsky

hhEb09'1
2006-Aug-16, 06:36 PM
I fail to see how any of this would damage the Moon. I mean, check it out, even at 7X. It's already damaged beyond repair. It obviously needs some kind of celestial Clearasil.LOL. I got a call from my brother a couple days ago, apparently one of his friends had received the infamous Mars-big-as-the-moon email and had put it to practical use: a few days ago when a group of them noticed the moon low and on the horizon, with a deep red tint, he insisted that it was Mars not the moon, because of the color.

George
2006-Aug-16, 08:35 PM
Again, you're confusing the exit pupil of a telescope or binocular, which for a given instrument depends upon magnification, with the eye's entrance pupil. Apparently so. I'll study this more closely.



The second law of thermo is no restriction to increasing the brightness of an object.
I'm sorry but that is not correct. It would violate the Second Law. A telescope cannot increase the surface brightness of an extended object. I can't see any reason why. The energy out will always be less than the energy in, regardless of magnification. Increasing the surface brightness is not, by itself, a violation. Even the retina will always receive less power (ie flux) than the input power. Yet, by focusing the flux into a smaller area, the object would become brighter than normal. Why can't a scope be made to do this?

Your Mel Bartel's page states...
"If there is no magnification to the image, the surface brightness will increase by the ratio of the scope's aperture to the eye's aperture squared..."

Admittedly, he then points out...
"However, in order to fit all of the light from the 10" aperture into the eye's exit pupil, we must use at least 33x. 33x will dilute the image brightness by 33^2 =~ 1000x, so we are back where we started."

This is where I struggle because I do not see why this is mandatory. Why not limit it to, say, 20x here. My guess is that it is a design criteria and not an odd optical law. Why can't new optical elements be added to restrict the magnification, producing improved surface brightness? Again, the total energy output is less, keeping it in accord with the 2nd law.


The Sun is a very special case and should not enter into this discussion. Yes, increasing it's surface brightness is the last thing we need to do. Besides, it won't even reveal its color if we do. :)


For a discussion of the concept of surface brightness see http://mysite.verizon.net/vze55p46/id18.html Thanks, Dave. I'll sure read it.


Here's what the noted observer Jay Reynolds Freeman had to say on the topic in a sci.astro.amateur post:

"Image brightness of extended objects on the retina varies as the square of the exit pupil diameter (provided the exit pupil is no larger than the pupil of your eye), and is also proportional to the coefficient of transmission of the telescope as a whole (including the effects of secondary obstruction, poor coatings, et cetera).

Thus, images of extended objects on the retina as seen with a telescope can never be brighter than as seen with the naked eye. They can be larger, though, which can make them easier to detect."
This is restating the formula, and seems to be true if every telescope is designed to produce an exit pupil in accordance with the formula. Yet, I see no reason why a telescope could not be designed to offer less magnification than normal for any given aperature, though it would mean a different formula would be used.


The noted amateur telescope maker Mel Bartels discusses the topic at length at http://www.bbastrodesigns.com/visual.html

http://hyperphysics.phy-astr.gsu.edu/hbase/vision/retina.html#c2

Roger Clark's now out-of-print book _Visual Astronomy of the Deep Sky_ explains these concepts quite well.

There's a discussuion at http://www.mapug-astronomy.net/AstroDesigns/MAPUG/VisualDet.htm
Thanks, I'll take some time out to check these out.


The formula applies equally to all telescopes. How does design enter into the picture?
Design criteria by engineers can establish formulas as guidelines. These formulas, of course, are in accord with physics, yet they are not laws themselves. It is possible the formula is an optical law in order for the focal plane to be on the retina. But, obviously, I am questioning whether it is a law or not.


"There may be some evidence for this found in telecompressors as they concentrate an image into a slightly smaller area, I think."

Yes, but again, this is just another case of basic optical principles. A Barlow lens does the reverse.
Both of these are clues that the formula is not a law; maybe it is a design criteria. It makes some sense because why design a telescope that limits the clean magnification capability for any given aperature? How many people would be willing to pay the big bucks for large aperature (probably 20 inch or better) to get poor magnification only to see objects in color. It might even be dangerous for any terrestrial use; a product liability issue, no less.

I downloaded the demo dbOptic software, but it seems limited in the information I need. It has a nominal price, however.

Iris
2006-Aug-17, 12:22 PM
*Accidental double post, sorry*

Iris
2006-Aug-17, 12:25 PM
Thanks for all the answers! Though I got a bit lost when it comes to the current discussion (considering formulas ain't my forte. ;)

I didn't buy the said filter ... I don'T quite need it, if I read what you guys said... and I think that what stopped me the most is the principle of the thing: why would I buy something off someone that gives such gross information as an argument for his item?

I'll keep my money for a more useful filter... Do you have any suggestion??

George
2006-Aug-17, 01:56 PM
Thanks for all the answers! Though I got a bit lost when it comes to the current discussion (considering formulas ain't my forte. ;)
Sorry, but it is a big part of the fun. I never know just what I might learn as things go deeper. :)


I didn't buy the said filter ... I don'T quite need it, if I read what you guys said... and I think that what stopped me the most is the principle of the thing: why would I buy something off someone that gives such gross information as an argument for his item?

I'll keep my money for a more useful filter... Do you have any suggestion??
Good. A smart choice, IMO.

You may enjoy reading this site on filters (http://sciastro.net/portia/advice/filters.htm).

JohnW
2006-Aug-17, 04:01 PM
Here's another useful filter guide (http://www.belmontnc.4dw.net/filters.htm). What to get will depend on your scope and what you like to observe.

Also, you should consider Astromart rather than eBay when shopping for cheap/used gear. Unlike eBay, it's mainly people who know what they are talking about selling to people who know what they are talking about. There's less risk of ending up with junk.