andrew s

Some background to start off with. The best theory of light and matter is called Quantum Electrodynamics a subset of Quantum Field Theory. I am not an expert on this topic but I have spent many happy hours trying to get a grasp of it. The unfortunate thing is that almost everything you have read about it in popular science books and articles is wrong! This is even true when written but eminent scientists. About the best introduction is "QED: The Strange Theory of Light and Matter" by Richard Feynman but even this can give you the wrong impression about what is physically happening rather than how things are calculated. The key thing about QED is that it is a field theory. It is about how the electromagnet field behaves and interacts with matter. It is a wave theory and particles are secondary. A photon is an excitation of the EM field and can be created (emission) or annihilated (absorbed). However, apart from these two events they are hard to pin down. Photons don't have a position operator so you can't even ask where they are (only where they are created or annihilated) . The concept of a light beam as a stream of photons is completely wrong. Most states of the electromagnetic field don't have a well defined number operator so you even can't ask how many photons are there! Most of the time the excitations of EM field are delocalised. Its best to think of photons only at the act of creation and annihilation. So the OP's question is difficult to address directly. But, to try anyway lets take the example of the image of a narrow slit illuminated from behind and the image captured on a screen (or CCD etc.) If we reduce the intensity of the beam so there is only one photons worth of energy in the beam at a time the image on the screen will be built up one detection at a time as a photon is annihilated at that spot. If you go on long enough you will see a well formed diffraction pattern emerge (the image of the slit). So it comes down to how well you want the pattern defined to be clear what it is. In other words what signal to noise ratio is acceptable in the image. That's my mind dump for now. Not sure if it helps or if it is the type of discussion you wanted @jetstream. Regards Andrew

I will get back to this but of for a bike ride! Regards Andrew

You need to be careful with MTFs. The graph shows the amplitude but not the phase. If you image black and white stripes you can get the back and white swapped round due to the phase shift. I had a book with a good example in it but I lent it out have as of now not got it back.. Regards Andrew

@jetstream it has long been known that linear features can be seen more easily than say separating double stars. The Dawes and similar criteria are conventions or definitions. They were set either by observations in the past or by diktat. They have a useful role for comparing system but they are not hard and fast limits. Regards Andrew

If you obstruct the telescope with your hand you dim the object and add diffraction effects. These would be best modeled by geometric optics for the obstruction and classical wave theory for the diffraction. I don't think light packets or photon contribute any insight and would open up a whole new can of worms. Photons the hardest quantum objects to pin down there are whole books on the subject. Regards Andrew

If the secondary mirror is properly sized it should not impact the resolution if you ignore any aberrations. One of my final exam question was " at what distance can you resolve the headlights of a car". That was it. I did two calculations. One as you indicate assuming the rods and cones size limited it and the other assuming the size of the iris did. Can't recall which was smallest. I am sure it would be the cells with an astronomical telescopes. Regards Andrew

I thought I had responded but it has gone missing. Yes. We normally view at a magnification where the Airy disc is not resolved and so the star image is as if it were from a point source. Regards Andrew

Look at your blue diagram. Take the yellow rays as being from an on axis star. They will have uniformly filled the telescope objective and will uniformly fill the exit pupil of the telescope/eyepiece. I don't know what you mean by the "exit pupil Airy disk" the exit pupil and the Air disk are two different things. I don't think it worth me continuing this exchange as we are clearly not understanding each other. Regards Andrew

I don't disagree with this as stated up to the point you can resolve the Airy disk. If you continue to magnify further the disk will appear bigger and dimmer. Regards Andrew

@miguel87 the top blue diagram shows exactly what I have been saying. Note the rays going through the exit pupil (those of the same colour) are parallel they are not going to a focus. This is very obvious for the lines that miss the eye in the lower part of the diagram. The focus of the lens for the rays shown is at infinity a long way from the field stop. The final focus is of course on the retina. There would have to be a lens to the left of the one in the diagram if it were drawn for an object other than at the focal point of the lens shown. That, of course, is how a telescope/eyepiece works. The telescope forms a real image and this is where the eyepiece focus is put to deliver parallel light to the eye. Normally this is where the field stop is placed. The exit pupil is as in the diagram. Where do you think the prime focus is on the diagram? I can't see the lower diagram clearly enough to comment. Regards Andrew PS the curved bule line is not the focus of the lens shown. It ,ooks like it is drawn as if the where a small entrance pupil, not shown, to the left which limits the width of the ray bundles shown. It is where you would place a field stop.

I would quote Macbeth but I don't have the time. Regards Andrew

"Any sufficiently advanced technology is indistinguishable from magic" Arthur C Clarke . Regards Andrew

I think you have the basic idea but you continue to misunderstood what the exit pupil is. As the exit pupil is an image of the objective/aperture stop formed by the eyepiece, all the light that goes through the objective goes through the exit pupil and as at the objective the light rays from a star are parallel. There is no real image your eye forms the real image. A camera at prime focus is at the focal point the exit pupil is at infinity. Regards Andrew

We don't use the exit pupil to view the image. The image from an eyepiece is at infinity ! We align the exit pupil of the telescope/eyepiece with the eyes entrance pupil and the cornea and lens then focus the light on the retina. With eyepiece projection ths eyepiece is reposition to form an image and the exit pupil will shift but it will not be at the focal point. It will be close as the objective focal length is much longer the thst of the eyepiece. Light is not evenly distributed at the focal point if it were there would be no image! The definition of the exit pupil is "The image of the entrance pupil formed by the optics of the system in question. " it is completely relevant. One more time. At prime focus the objective forms an image on the DSLR sensor which is at the focal point/plane. Telescopes are normally designed so the the objective is the entrance pupil. The exit pupil will then be at infinity. What limits the field of view is called a field stop. It could be the edge of the sensor, baffles in the telescope tube or the dew shield or your 7mm hole. As you say it just limits the field not the intensity across the field. In some cases you get vignetting where other elements than the objective are limiting the aperture as you go off axis. Regards Andrew

Quit right, but the focal point is not the exit pupil. That's what you are misunderstanding. The exit pupil is where the entrance pupil is imaged not the object e.g the star. For a telescope used at prime focus ( telescope plus CCD) the exit pupil is at infinity! As the objective (as entrance pupil) can't image itself. Your 7mm aperture is acting as a field stop. It a complex subject. Regards Andrew

Light is not focused at the exit pupil @miguel87 that's were you analogy fails. With a telescope/eyepiece the rays from a star are parallel through the exit pupil. You need to look at some ray diagrams to really understand what is going on. Regards Andrew

Simple bit first. If the entrance pupil of the eye is smaller that the exit pupil of the telescope/eyepiece it will limit the effective aperture and hence light grasp. The effect on resolution is more complex. You normally need quite a high magnification to see a diffraction limited image. I suspect t there is a crossover from the eye limiting the resolution to the telescope/eyepiece/seeing limiting the resolution. I don't know exactly where this is as the exit pupil reduces and probably changes person to person. Regards Andrew

So basically your threatening me with tooth paste, marbles and a rouge lipstick. Trembling Andrew

@Datalord I had read what you posted before but that was not what I was asking. If you save the two crops @vlaiv posted and looked at them in the future having had time to forget them do you think I you could pick which was which in an unbiased test? If so what about the final images would enable you to do this? Regards Andrew

@Datalord do you think one is significantly better than the other and if so how? Regards Andrew

Apart from slight differences in contrast they look the same to me. Regards Andrew

@vlaiv sorry I was not clear enough. However, on your point about noise adding clarity some character recognition software adds noise to aid in the recognition process. I certainly agree we enforce structure on images. Any sharp edges looks like clarity even if they are due to noise. The point I was trying to make was does software render images in different ways as they zoom in? If so it could explain some of the different perceptions about images. Regards Andrew

I was just having fun with the Tak lovers society on SGL. I am sure refractors can give nice views. However, for sheer simplicity, perfectly achromatic (IR to UV), and diffraction limited across a planetary field what's not to like about a Newtonian. Regards Andrew

In my opinion a slow ( above f5) good quality Newtonian is the perfect planetary scope. Far superior to those expensive toy telescopes by Takahashi 🤪 Regards Andrew

@vlaiv excellent description. Your ability to put these complex ideas across is very impressive you clearly have a good grasp of the subject. I do wonder if some of the differences seen by others is down to the way different software renders the image on to the monitor. Do you have any thoughts on this? Regards Andrew