moon eye damage

[Ptarmigan]

The reduction of effective aperture does lead to a reduction in resolvable detail too. My 12" dob will show some very fine detail on the lunar surface but those are lost when the scope is stopped down to a smaller aperture.

Good point

One of these days I must get myself a 12" !

[ollypenrice]

If you want the resolution of large aperture but not the brightness then you could, in principle, achieve this by cutting two holes in an aperure mask as far apart as possible. How much difference this would really make I don't know, though.

Olly

[jnb]

The moon is about 15 magnitudes fainter than the sun, about 10⁶, so looking at the moon with a 6" scope is equivalent to looking at the sun if your pupil aperture could go down to 0.15mm, in practice the human pupil might go down to about 3mm so effectively that makes the moon through a 6" scope about 400 times fainter than the sun, add say 50x magnification and it's a million times fainter. It's dazzling but not damaging.

Another way to look at it is that a neutral density filter might let about 13% of light through so a 6" scope filtered would be about as bright as a 70mm unfiltered, or the other way around a 6" unfiltered would about the same as a 14" filtered. I wouldn't worry about it but I would use a filter purely to allow comfortable viewing.

[Ptarmigan]

In my post #25 my "usually" was the operative word.

I dont, did not, mean to doubt the users of big apertures that with patience and experience moments of great clarity can be grabbed, so, John, dont despair of me ;)

I've thought of an experiment, what we need is someone with a 24" and a web cam,

to chart the number of rejected frames from sequences with apertures of various sizes,

in the uk of course,

and see how often the max resolution can be used for each size scope.

if you see what I mean ?

Am I making sense ?

Or maybe not cos this aperture mask thing has been going on for years

[John]

If you want the resolution of large aperture but not the brightness then you could, in principle, achieve this by cutting two holes in an aperure mask as far apart as possible. How much difference this would really make I don't know, though.

Olly

Interesting idea - I'll give it a go sometime although I don't find any problem obseving the moon at full aperture with my 12" dob.

[swamp thing]

The moon is about 15 magnitudes fainter than the sun, about 10⁶, so looking at the moon with a 6" scope is equivalent to looking at the sun if your pupil aperture could go down to 0.15mm, in practice the human pupil might go down to about 3mm so effectively that makes the moon through a 6" scope about 400 times fainter than the sun, add say 50x magnification and it's a million times fainter. It's dazzling but not damaging.

Another way to look at it is that a neutral density filter might let about 13% of light through so a 6" scope filtered would be about as bright as a 70mm unfiltered, or the other way around a 6" unfiltered would about the same as a 14" filtered. I wouldn't worry about it but I would use a filter purely to allow comfortable viewing.

Have a read of this

[Ptarmigan]

Iachieve this by cutting two holes in an aperure mask as far apart as possible.

Interesting idea, but wouldnt that turn it into an interferometer ?

I think that was how the diameter of Beetljuice was measured way back when.

Never heard of it being used visually, whole image wise , , ?

Are you pulling a leg here ?

Your middle names Scheiner or Hartmann by any chance!

I think we need to keep things in focus

[ollypenrice]

Heh heh, I haven't a clue whether or not a kind of Hartmann Mask aperure mask would really improve resolution over a single hole one or not but multiple hole aperture masking is used to enhance resolultion in professional telescopes. It reduces the effects of turbulence as I understand it (which is not much!) As ever, the thing would be to try it.

http://en.wikipedia.org/wiki/Aperture_masking_interferometry

Olly

[ollypenrice]

No

The moons surface brightness doesn't change regardless of aperture. Its no brighter through my 20" Dob than it is when i look up at it with my unaided eye.  

But looking at it with the unaided eye is not uncomfortable whereas looking at it in a large ufiltered instrument patently is uncomfortable. Why is this? Genuine question. I understand the surface brightness optical argument but feel that it is missing something which may lie in the biological end of the setup!

Olly

[jnb]

The moon is about 15 magnitudes fainter than the sun, about 10⁶, so looking at the moon with a 6" scope is equivalent to looking at the sun if your pupil aperture could go down to 0.15mm, in practice the human pupil might go down to about 3mm so effectively that makes the moon through a 6" scope about 400 times fainter than the sun, add say 50x magnification and it's a million times fainter. It's dazzling but not damaging.

Another way to look at it is that a neutral density filter might let about 13% of light through so a 6" scope filtered would be about as bright as a 70mm unfiltered, or the other way around a 6" unfiltered would about the same as a 14" filtered. I wouldn't worry about it but I would use a filter purely to allow comfortable viewing.

Have a read of this

Dammit you're right, I always forget that as it's not usually a concern. But I stand by the practical comment that I will use a neutral density filter for comfortable observing.

[Ptarmigan]

 but multiple hole aperture masking is used to enhance resolultion in professional telescopes.

And apodizing screens are just multi-hole taken to an extreme,

dont know if we want to go down that old discussion !

Speckle did occur to me earlier and I was going to look up exposure needs and wonder if our humble CCDs were up to it,

but then I got caught by u know who for some domestic

so I am no further down that route yet.

Thanks for the ref. I'll go read.

Meanwhile I thought Swamp might volunteer his 20" for my webcam experiment, :)

[John]

But looking at it with the unaided eye is not uncomfortable whereas looking at it in a large ufiltered instrument patently is uncomfortable. Why is this? Genuine question. I understand the surface brightness optical argument but feel that it is missing something which may lie in the biological end of the setup!

Olly

I think could be to do with the contrast. The inside of an eyepiece is intentionally very dark indeed. The field of view is framed by the field stop which is totally black. The eye therefore has to deal with the massive apparent contrast variation as you peer around taking in both the blackness of the eyepiece internals and the bright lunar surface directly adjacent to it.

When we look at the moon with the naked eye even on a dark night there is some ambient light in the periphery of our vision and the moon is far from filling the field of view.

I tend to view the moon with 100 degree eyepieces so the illuminated field is almost as far as the eye can see and the eye has a more uniformly bright view to adjust to. Maybe thats why I don't find I need filters ?

I'm sure there is a scientific reason - I'm just kicking ideas around here !

[swamp thing]

But looking at it with the unaided eye is not uncomfortable whereas looking at it in a large ufiltered instrument patently is uncomfortable. Why is this? Genuine question. I understand the surface brightness optical argument but feel that it is missing something which may lie in the biological end of the setup!

Olly

Theres nothing missing Olly it makes perfect sense.

looking at the moon with the unaided eye is not uncomfortable and looking at it with a large unfiltered instrument obviously isn't uncomfortable either or else I wouldn't do it.........but i do,  because its plainly exactly the same brightness

[LukeSkywatcher]

I spent the first 6 months of owning my first scope observing through small middle inset cap. I didnt know the whole cap came of. When i found out that the whole lens cap was removable, i nearly died (or went blind) when i observed the Moon for the first time with the whole cap removed. 

Then the first time i observed the Moon with my 20x90 bins, i felt like the light was searing two holes in the back of my head.

I have a Moon filter, but never use it. I did find that it can make for a nice light pollution filter, but i have one of those also. 

[ollypenrice]

Theres nothing missing Olly it makes perfect sense.

looking at the moon with the unaided eye is not uncomfortable and looking at it with a large unfiltered instrument obviously isn't uncomfortable either or else I wouldn't do it.........but i do,  because its plainly exactly the same brightness

I struggle to accept this. If I look at the full moon unaided I feel no discomfort and do not need to allow my eyes to adapt before I can use them to do things outside. When I look at the full moon in a large instrument I certainly feel discomfort and, afterwards, have a kind of visual blankness and confusion which means I can't look at a moon map or see to change an eyepiece. I never look at the moon unaided and think 'ouch,' but that is exactly what I think at the scope sometimes. I'm at a loss to account for our different experiences.

What about the sun? Wouldn't your interpretation of the surface brightness rule oblige you to say that the sun is no more dangerous in a telescope than it is to the unaided eye? I realize that we should not observe the sun naked eye but I find it hard to believe that a telescope doesn't increase the danger. I won't be putting this to the test.

Our eyes are not photometers, they respond non-linearly to light but how this might play in the story I simply don't know.

There is also the question of magnification. The naked eye moon image does not strike many of our rods and cones but a magnified image can stike a majority of them. How does this affect us physiologically? I don't know but surely this can't be ignored?

Olly

[Charic]

I spent the first 6 months of owning my first scope observing through small middle inset cap. I didnt know the whole cap came of. When i found out that the whole lens cap was removable, i nearly died (or went blind) when i observed the Moon for the first time with the whole cap removed. 

Then the first time i observed the Moon with my 20x90 bins, i felt like the light was searing two holes in the back of my head.

I have a Moon filter, but never use it. I did find that it can make for a nice light pollution filter, but i have one of those also. 

that tickles my humour!......you won' be the last one either? 

The aperture 'filter 'hole is there for a purpose,  and only one cap is removeable, and stored on the other raised 'cap'  Its generaly used with the correct filtration for Solar work, but for visual observations, especially of the Moon, its not so drastic that its not worth viewing this way. It clearly does work for me   I  find that things can get so bright that one eye works and the other has night blindness for a while, and  I dont use my left eye on the telescope, does'nt feel right. 

[swamp thing]

I struggle to accept this. If I look at the full moon unaided I feel no discomfort and do not need to allow my eyes to adapt before I can use them to do things outside. When I look at the full moon in a large instrument I certainly feel discomfort and, afterwards, have a kind of visual blankness and confusion which means I can't look at a moon map or see to change an eyepiece. I never look at the moon unaided and think 'ouch,' but that is exactly what I think at the scope sometimes. I'm at a loss to account for our different experiences.

What about the sun? Wouldn't your interpretation of the surface brightness rule oblige you to say that the sun is no more dangerous in a telescope than it is to the unaided eye? I realize that we should not observe the sun naked eye but I find it hard to believe that a telescope doesn't increase the danger. I won't be putting this to the test.

Our eyes are not photometers, they respond non-linearly to light but how this might play in the story I simply don't know.

There is also the question of magnification. The naked eye moon image does not strike many of our rods and cones but a magnified image can stike a majority of them. How does this affect us physiologically? I don't know but surely this can't be ignored?

Olly

I Believe its to do (The moon) with total light. 

The surface brightness remains the same but the total light is vastly more when viewed through a scope.

The telescope makes a much larger image at the same surface brightness. I guess its a bit like looking at a light bulb from across the room then walking up close and looking at it. its no brighter but because more of the eye is copping the light it may seem brighter. 

Some (yourself included) find this uncomfortable others (me included) don't

Sorry my explanations not too good 

The sun is a different matter prof wilkinson explained it in a thread a while back but I can't seem to find it. He also explained all this a lot better than me. After all he's a teacher and I'm a builder

I think the sun is to do with UV or something like that. Best not to go near it with your dob though

[ollypenrice]

I Believe its to do (The moon) with total light. 

The surface brightness remains the same but the total light is vastly more when viewed through a scope.

The telescope makes a much larger image at the same surface brightness. I guess its a bit like looking at a light bulb from across the room then walking up close and looking at it. its no brighter but because more of the eye is copping the light it may seem brighter. 

Some (yourself included) find this uncomfortable others (me included) don't

Sorry my explanations not too good 

The sun is a different matter prof wilkinson explained it in a thread a while back but I can't seem to find it. He also explained all this a lot better than me. After all he's a teacher and I'm a builder

I think the sun is to do with UV or something like that. Best not to go near it with your dob though

Walking near a light bulb is easy to explain and Newton got there by pure thought. It's the inverse square law. A metre out from the bulb there's an imaginary spherical shell of light of radius one metre and surface area x. 2 metres out there's another imaginary spherical shell with surface area 4x. And so on. Your eye samples a certain tiny fragment of that surface but it samples the same area, so at 2 metres out it recieves a quarter of the light that it samples at a metre.

Total light is, no doubt, at the bottom of this but I don't understand it.

Olly

[Kropster]

No

The moons surface brightness doesn't change regardless of aperture. Its no brighter through my 20" Dob than it is when i look up at it with my unaided eye.  

Does the brightness depend on the amount of light captured (aperture) and image size (magnification).

If you use a big aperture to produce the same size image as a smaller aperture, will it not be brighter? 

If not, where do the extra photons go?

[ollypenrice]

Exactly, where do they go? The rogue here is exit pupil. They can go outside the observer's eye, of course, but do they?

Olly

[Ptarmigan]

, , ,

Actually, I think I'll withdraw that post,

I wasnt adding anything erudit or enlightening :)

but I use a filter cos otherwise I am discomfited

[Scooot]

Does the brightness depend on the amount of light captured (aperture) and image size (magnification).

If you use a big aperture to produce the same size image as a smaller aperture, will it not be brighter?

If not, where do the extra photons go?

I'll have a stab at this but someone please tell me if it's hogwash.

The same size image on the retina is produced if the exit pupil is the same. So a 10mm in an f5 10" scope gives a 2mm exit pupil and mag of 127. The same eyepiece in a 20" also gives a 2mm exit pupil but double the mag, so the image looks half the size, but brightness of the two images is the same. At 127 mag in the 20" ie 20mm eyepiece the exit pupil is 4 so the brightness is double the 10" at the same magnification ??

[cutepetgroomer]

The only exception would be during an eclipse I guess. Then it would get dangerous.

we talking solar eclipse ? Because lunar eclipses I know to be relatively safe to so

[Scooot]

I'll have a stab at this but someone please tell me if it's hogwash.

The same size image on the retina is produced if the exit pupil is the same. So a 10mm in an f5 10" scope gives a 2mm exit pupil and mag of 127. The same eyepiece in a 20" also gives a 2mm exit pupil but double the mag, so the image looks half the size, but brightness of the two images is the same. At 127 mag in the 20" ie 20mm eyepiece the exit pupil is 4 so the brightness is double the 10" at the same magnification ??

Hmm not quite right, a 4mm exit pupil is 4 times brighter than a 2mm because the area is 4 times bigger.

This is interesting

https://www.astronomics.com/eyepiece-exit-pupils_t.aspx

[John]

....but I use a filter cos otherwise I am discomfited

Thats a good reason to do it - it's a hobby after all and it's not supposed to be painful