Post by BillW on Feb 10, 2015 11:08:58 GMT
Hi,
Warning this is a (perfectly reasonable) rant!
Despite a rather dreadful journey the astrofest was ok. I was only there to browse specific items. I wanted to see these for real before splashing the cash.
People seem to rave about the event but I didn't go to any of the lectures so can't comment on that. It was busy and people seemed to be having an enjoyable time.
BUT what I heard several times, and found myself having to walk away from, were conversations between dealers and customers about the merits of various eyepieces.
I know these guys are in business and it's their job but the drivel and patter was astonishing at times. I came to the conclusion that either a: they know they're talking rubbish but a sale is a sale or, far more alarming, b: thay actually believe what they say is true.
This is the myth of the size of exit pupil and wasted light....
It has become so entrenched it is taken as truth, gospel, the word of the great eyepiece god himself/herself/itself (choose your own gender)
Let me be blunt, this is a lot of b*ll*cks. There is no such thing as "wasted light", really there isn't. Just because your pupil diameter doesn't magically match the exit puil of an eyepiece DOES NOT MEAN a thing. People (from the evidence of the weekend, generally salesmen) will argue blind that it does, I say again it DOES NOT!!!!
To really understand what is going on is pure physics, physics that has been usurpted by the neferious.
The only concesssion I will make is that having an exit pupil match the eyeball pupil is one of observing convenience. Just once more, it has nothing to do with wasted light.
As a starting point, a pair of 7 x 50 bins produces an exit pupil of ~7mm, but oh no I've only got an eyeball pupil of 6mm at night (MUCH smaller if you use your binoculars during daytime, but that never seems to be mentioned...) I must smash my binoculars and buy another pair. 10 x 50's will be much better, only a 5mm exit pupil and all will be well in the observing world. PAH!
The REAL issue is image brightness. Magnification comes around due to the way lenses "handle" the light passing through them. Generally in astronomy the largest possible light collector is wanted. To obtain the brightest image possible the light just wants to go from collector to detector. The bigger the collector the more flux ergo the brighter the image. Now this is where things go silly. The image SIZE is dictated by the focal length. Long focal length, large image scale at the focal plane,that's the laws of physics. However if the detector is the eye which has a pesky lens of it's own another lens is needed to "transfer" the image from the collector to the eyeball. Anyone with an enquiring mind has no doubt stuck their eye at the focal plane of their telescope without an eyepiece and seen a nothing sensible. The "eyepiece" is a necessary evil for visual use. The problem is that for convenience and it is nothing but convenience, eyepiece focal lengths tend be be much shorter than the focal length of the objective. Every book on astronomy describes how when high magnification is used the image gets dimmer, so what do we do, we use a longer focal length eyepiece but the image now gets smaller (but, more importantly in a low light level environment, brighter). Why does it actually get brighter? It is again the laws of physics and is quite a subtle point.
Taking it to the extreme, a thought experiment is now needed. OK everyone accepts that a lower magnification gives a brighter image and brighter images give better signal to noise ratio on the detector. That is, it looks smaller but it will look brighter. So where does that process end? Well if you have a telescope optic of focal length of 1000mm to get a "magnification" of unity (one) you need an eyepiece of 1000mm focal length eyepiece. This will give you the brightest image, barring optical losses, that it is physically possible to see. Doing the quick sum will reveal the exit pupil will be 1000mm across. Imagine a telescope exit pupil you could actually stand in front of an be truly surrounded by. Wait a minute, a 1000mm exit pupil certainly doesn't fit my eyeball pupil. No it does not BUT the laws of phyics dictate the image brightness is the most it can be, absolutely, it is the most dazzling a view of the universe that you're ever going to see. This is the point. The WHOLE image is the brightest it can be, it doesn't matter where you are actually looking within this massive exit pupil you will be seeing the brightest image that can be seen.
For convenience it is easier and more practical to use much shorter focal lengths. Remember, it is absolute, the more magnification the dimmer the view. Also for practical reasons we want to enjoy a good view of Jupiter so we need to magnify the image to make it appear larger. But as we make it larger it gets dimmer and dimmer so we have to work somewhere in the middle. We now get to the stage where the physics get married to the sales patter.
If you happen to have a telescope that when combined with a particular eyepiece ends up giving you an exit pupil that is larger than your eyeball pupil, so what?! The image your retina sees isn't magically diminished in intensity by this fact. That is due to the other fact that the light coming from a telescope is exiting in a parallel collimated beam which in turn is collected and focussed by the eye. If it were converging or diverging then a "stop", your eyeball pupil, WOULD have an effect but it isn't and it doesn't! In reality you are NOT wasting anything. The image brightness is dictated by the laws of physics not a salesman. The image you are seeing is the brightest PER UNIT AREA PER UNIT MAGNIFICATION that it can be.
Just move your head a little and soak up the view.
If you still don't believe me, check out Observational Astronomy by J Sidgwick, eyepieces and magnification section. He proves beautifully why a mag of 1 is the best and forget about the exit pupil and wasted light myth.
END OF RANT.
Warning this is a (perfectly reasonable) rant!
Despite a rather dreadful journey the astrofest was ok. I was only there to browse specific items. I wanted to see these for real before splashing the cash.
People seem to rave about the event but I didn't go to any of the lectures so can't comment on that. It was busy and people seemed to be having an enjoyable time.
BUT what I heard several times, and found myself having to walk away from, were conversations between dealers and customers about the merits of various eyepieces.
I know these guys are in business and it's their job but the drivel and patter was astonishing at times. I came to the conclusion that either a: they know they're talking rubbish but a sale is a sale or, far more alarming, b: thay actually believe what they say is true.
This is the myth of the size of exit pupil and wasted light....
It has become so entrenched it is taken as truth, gospel, the word of the great eyepiece god himself/herself/itself (choose your own gender)
Let me be blunt, this is a lot of b*ll*cks. There is no such thing as "wasted light", really there isn't. Just because your pupil diameter doesn't magically match the exit puil of an eyepiece DOES NOT MEAN a thing. People (from the evidence of the weekend, generally salesmen) will argue blind that it does, I say again it DOES NOT!!!!
To really understand what is going on is pure physics, physics that has been usurpted by the neferious.
The only concesssion I will make is that having an exit pupil match the eyeball pupil is one of observing convenience. Just once more, it has nothing to do with wasted light.
As a starting point, a pair of 7 x 50 bins produces an exit pupil of ~7mm, but oh no I've only got an eyeball pupil of 6mm at night (MUCH smaller if you use your binoculars during daytime, but that never seems to be mentioned...) I must smash my binoculars and buy another pair. 10 x 50's will be much better, only a 5mm exit pupil and all will be well in the observing world. PAH!
The REAL issue is image brightness. Magnification comes around due to the way lenses "handle" the light passing through them. Generally in astronomy the largest possible light collector is wanted. To obtain the brightest image possible the light just wants to go from collector to detector. The bigger the collector the more flux ergo the brighter the image. Now this is where things go silly. The image SIZE is dictated by the focal length. Long focal length, large image scale at the focal plane,that's the laws of physics. However if the detector is the eye which has a pesky lens of it's own another lens is needed to "transfer" the image from the collector to the eyeball. Anyone with an enquiring mind has no doubt stuck their eye at the focal plane of their telescope without an eyepiece and seen a nothing sensible. The "eyepiece" is a necessary evil for visual use. The problem is that for convenience and it is nothing but convenience, eyepiece focal lengths tend be be much shorter than the focal length of the objective. Every book on astronomy describes how when high magnification is used the image gets dimmer, so what do we do, we use a longer focal length eyepiece but the image now gets smaller (but, more importantly in a low light level environment, brighter). Why does it actually get brighter? It is again the laws of physics and is quite a subtle point.
Taking it to the extreme, a thought experiment is now needed. OK everyone accepts that a lower magnification gives a brighter image and brighter images give better signal to noise ratio on the detector. That is, it looks smaller but it will look brighter. So where does that process end? Well if you have a telescope optic of focal length of 1000mm to get a "magnification" of unity (one) you need an eyepiece of 1000mm focal length eyepiece. This will give you the brightest image, barring optical losses, that it is physically possible to see. Doing the quick sum will reveal the exit pupil will be 1000mm across. Imagine a telescope exit pupil you could actually stand in front of an be truly surrounded by. Wait a minute, a 1000mm exit pupil certainly doesn't fit my eyeball pupil. No it does not BUT the laws of phyics dictate the image brightness is the most it can be, absolutely, it is the most dazzling a view of the universe that you're ever going to see. This is the point. The WHOLE image is the brightest it can be, it doesn't matter where you are actually looking within this massive exit pupil you will be seeing the brightest image that can be seen.
For convenience it is easier and more practical to use much shorter focal lengths. Remember, it is absolute, the more magnification the dimmer the view. Also for practical reasons we want to enjoy a good view of Jupiter so we need to magnify the image to make it appear larger. But as we make it larger it gets dimmer and dimmer so we have to work somewhere in the middle. We now get to the stage where the physics get married to the sales patter.
If you happen to have a telescope that when combined with a particular eyepiece ends up giving you an exit pupil that is larger than your eyeball pupil, so what?! The image your retina sees isn't magically diminished in intensity by this fact. That is due to the other fact that the light coming from a telescope is exiting in a parallel collimated beam which in turn is collected and focussed by the eye. If it were converging or diverging then a "stop", your eyeball pupil, WOULD have an effect but it isn't and it doesn't! In reality you are NOT wasting anything. The image brightness is dictated by the laws of physics not a salesman. The image you are seeing is the brightest PER UNIT AREA PER UNIT MAGNIFICATION that it can be.
Just move your head a little and soak up the view.
If you still don't believe me, check out Observational Astronomy by J Sidgwick, eyepieces and magnification section. He proves beautifully why a mag of 1 is the best and forget about the exit pupil and wasted light myth.
END OF RANT.
