But it does NOT apply to the brightness of stars, for a very interesting and curious reason… At any magnification below this, the brightness of the image is a constant value which is exactly the same as you would see unaided.ġ) This analysis and frustrating conclusion apply to relatively large objects like nebulae, galaxies and (most) planets. (Young people have a pupil which can dilate to 7mm, older folk can’t dilate more than about 5mm). So this is WORSE than before.Īnd so – in summary – for any telescope / eye combination, you can divide the size of the object lens, by the size of your pupil to give the maximum magnification you can apply before the image starts to get fainter. So what if we tried a 200x magnification? Well that makes the exit pupil 2.5mm across (which is fine – all of the light energy now goes through the 5mm pupil of your eye) but the larger image is now 4x fainter because of the 2x increase in magnification. Only a quarter will pass through into your eye.
Most of the light energy is spilling uselessly onto your iris. So what if we magnify only 50x? Won’t this make the image 4x brighter? Well yes, it would EXCEPT that the column of light leaving the eyepiece is now 10mm in diameter – which is too wide to fit through your 5mm wide eyes.
This assumes perfect, lossless telescope optics of course. At this magnification the brightness of the image is reduced 10,000 fold – so the image brightness is then just the same as seen with the naked eye. Obviously, the objective lens is 100 times wider than a human pupil, and so collects 10,000 times more light than the human eye (as the light collecting area is 10,000 times bigger)īut to shrink that 500mm diameter column of light into a 5mm exit pupil requires a magnification of 100x. Now for the maths, which I’ll keep as simple as possible. More magnification means a bigger image on the retina, but with a smaller exit pupil, and vice versa. Hopefully, it will already be obvious that this ‘spreading’ makes an image appear dimmer.Ģ) The telescope has to shrink the wide, cylindrical column of light entering the objective lens into a far smaller column of light exiting the eyepiece (the width of this narrow column of light is actually known as the exit pupil), The role of the eyepiece is to focus and magnify the image, and telescopes often have a choice of eyepieces providing different magnifications.Īny telescope has to do at least the following two basic things…ġ) The telescope has to magnify the image – spreading the collected light over a wider area of your retina. The role of the objective lens is to collect as much light as possible, and the larger it is, the more light it can collect. There are other designs using mirrors and combinations of mirrors and lenses too, but they all suffer from the same fundamental limitation. The reason why is a bit difficult to explain but bear with me, and I shall try.Īstronomical telescopes typically have a large lens at the front called the objective lens, and an arrangement of smaller lenses at the back (or side) which you look through called the eyepiece. Most people are very surprised by this statement, and many assert that it cannot possibly be true. No telescope makes distributed objects such as NEBULAE, GALAXIES and (most) PLANETS appear brighter than they can be seen with the naked eye.
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Sadly, the truth is that the full beauty of many of these images is only revealed using long exposure photography, and what can be seen through a telescope with the eye has some fundamental limitations, which apply no matter how big and expensive your telescope is. We are all used to seeing fabulous, colourful images from space, taken by the Hubble Space Telescope or from some of the world’s best observatories, and I know this prompts many people to want to buy their own telescopes with expectations of seeing these delights first hand, only to result in disappointment and disillusionment.
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