Very basic thing I don't understand about scopes

[cloudsweeper]

Lenses and mirrors focus light.  The bigger they, the more light is gathered and focused.

For point objects, brightness increases with aperture.

For extended objects, surface brightness increases with exit pupil, but that alone is not enough - a small 'scope can give large exit pupil figures.  What is also needed is magnification, and for a given exit pupil, mag increases with aperture.

So whichever way you look at it - point or extended sources - aperture increases the brightness.

Doug.

[Waddensky]

2 hours ago, miguel87 said:

Does this mean that when looking for faint fuzzies, my lowest mag eyepiece should always be my first choice? But then naked eye would be at least as good?

There are two mechanisms taking place when magnifying with a telescope:

• the image becomes darker (for object and background sky equally, the contrast doesn't change). This makes the object harder to observe, because our eyes tend to detect the same contrast better on a light background than on a dark background.
• the image of the object becomes larger. This makes the object easier to observe because our eyes detect larger objects with the same contrast better than smaller objects

As you can see, these two effects lead to an optimum, in which darkening and object size are 'in balance'. This is sometimes called Optimum Detection Magnification (ODM). If the brightness of an object is such that the contrast of the object and the background sky is larger than the threshold contrast (the minimum contrast the eye is able to detect given the circumstances), you are in theory able to see the object. The reason you can't see a faint fuzzy with the naked eye is because the object is too small, the reason you can't see a faint fuzzy at high magnifications is (among other factors) because the image is too dark.

[miguel87]

34 minutes ago, Waddensky said:

There are two mechanisms taking place when magnifying with a telescope:

• the image becomes darker (for object and background sky equally, the contrast doesn't change). This makes the object harder to observe, because our eyes tend to detect the same contrast better on a light background than on a dark background.
• the image of the object becomes larger. This makes the object easier to observe because our eyes detect larger objects with the same contrast better than smaller objects

As you can see, these two effects lead to an optimum, in which darkening and object size are 'in balance'. This is sometimes called Optimum Detection Magnification (ODM). If the brightness of an object is such that the contrast of the object and the background sky is larger than the threshold contrast (the minimum contrast the eye is able to detect given the circumstances), you are in theory able to see the object. The reason you can't see a faint fuzzy with the naked eye is because the object is too small, the reason you can't see a faint fuzzy at high magnifications is (among other factors) because the image is too dark.

Interesting thanks, makes sense too. 

Thinking about the limitations and strengths of the human eye. I wonder if our average daytime aperture (maybe 3mm or so) is a sweet spot for detail? Surely we have evolved so that the part of the retina most often in use is the most effective. I guess it might be very task specific though and movement is probably more important that detail.

[BrendanC]

I'm not even contemplating upgrading! It genuinely is something I've recently been thinking about.

I mean, obviously a larger mirror means more photons, like a bigger bucket. But going back to my original analogy, the moon in a small mirror will be the same size as the moon in a big mirror, but the big mirror would have more peripheral detail than the smaller mirror. So, more photons, yes, but not more photons of the moon. Just the same number of photons of the moon, plus the extra photons of the additional stuff the bigger mirror reflects.

Also, I don't think I've been 'told' the answer! I'm quite surprised such a thread ensued. 

Aaaaanyway, it was just a thought. I'll go through the answers and see which one makes the most sense. I mean, I'm sure bigger mirrors are better, but I'm not absolutely certain it's as simple as 'because it collects more light'. 

Right, so, next question: how do we REALLY know the earth's not banana-shaped...?

[John]

When we can travel around again, come along to a star party and have a look through some larger aperture telescopes. The views can be quite startling

The difference between, for example, the globular cluster Messier 13 observed with my 100mm scope at 150x and my 300mm aperture scope at 150x is really marked in terms of both brightness and resolution. A completely different experience.

 

 

 

[Alien 13]

I hate optics, for instance when observing you will get more photons in your naked eye than any telescope no matter how big it is, what changes is the field of view you see. Imaging is just as bad, a large scope collects more photons but they are useless if the arc/sec per pixel is all wrong as is a 10 meter diameter scope with an APS-C sized sensor. Remember the old Mt Palomar images with a 200 inch scope with huge photographic plates and the similarity to images taken with a small frac and small sensor...

Alan

[miguel87]

31 minutes ago, BrendanC said:

I'm not even contemplating upgrading! It genuinely is something I've recently been thinking about.

I mean, obviously a larger mirror means more photons, like a bigger bucket. But going back to my original analogy, the moon in a small mirror will be the same size as the moon in a big mirror, but the big mirror would have more peripheral detail than the smaller mirror. So, more photons, yes, but not more photons of the moon. Just the same number of photons of the moon, plus the extra photons of the additional stuff the bigger mirror reflects.

Also, I don't think I've been 'told' the answer! I'm quite surprised such a thread ensued. 

Aaaaanyway, it was just a thought. I'll go through the answers and see which one makes the most sense. I mean, I'm sure bigger mirrors are better, but I'm not absolutely certain it's as simple as 'because it collects more light'. 

Right, so, next question: how do we REALLY know the earth's not banana-shaped...?

Ok, this will be my last post I promise!

Going right back to your original analogy of a small handheld mirror and a wall mirror.

Let's imagine shining the moon off them onto a blank wall. Same brightness but one bigger patch of light.

NOW, change the big mirror for 10 hand held mirrors, arrange them next to each other but angles so that they all reflect the moonlight onto the same patch of wall. Now both the single mirror and collection of mirrors are creating the same sized patch on the wall. BUT the collections of mirrors will create a brighter patch.

Turn the collection of closely arranged mirrors into one smooth curved mirror and you have a telescope mirror. Collecting more light from the same object. Moonlight is falling all around you, on your telescope, on the floor, on your house and your entire street. You can scoop up as much of it as you like with a big enough mirror.

You can't catch ALL the light off the moon in a handheld mirror! Even if you can SEE the whole moon.

If that isn't the answer then I dont know what is.

✌

Edited May 12, 2020 by miguel87

[BrendanC]

I like that answer!

So, it's as I first thought: it's not the size of the mirror, it's the parabolic effect of that mirror that is important.

I think.

This page is interesting - gives some of the maths behind resolving power and size: https://www.astronomynotes.com/telescop/s6.htm. Makes it very obvious that the amount of light is non-linearly related to the size of mirror, and how that relates to the distance of an object.

Happy for this thread to stop now. Thanks for all the suggestions!

[Stu]

58 minutes ago, BrendanC said:

I'm not even contemplating upgrading! It genuinely is something I've recently been thinking about.

I mean, obviously a larger mirror means more photons, like a bigger bucket. But going back to my original analogy, the moon in a small mirror will be the same size as the moon in a big mirror, but the big mirror would have more peripheral detail than the smaller mirror. So, more photons, yes, but not more photons of the moon. Just the same number of photons of the moon, plus the extra photons of the additional stuff the bigger mirror reflects.

Also, I don't think I've been 'told' the answer! I'm quite surprised such a thread ensued. 

Aaaaanyway, it was just a thought. I'll go through the answers and see which one makes the most sense. I mean, I'm sure bigger mirrors are better, but I'm not absolutely certain it's as simple as 'because it collects more light'. 

Right, so, next question: how do we REALLY know the earth's not banana-shaped...?

I think you may be missing a few points here. There is complicated physics at play that I don’t fully understand, but can bumble my way around.

Resolution is related to the diameter of the mirror, so a larger mirror will offer a more detailed view of the Moon for instance. It may be the same size but it will show more detail in good conditions and will maintain detail at higher powers than a smaller scope. The airy disk size of stars reduces with the increasing aperture of the mirror, so larger scope can ultimately split tighter doubles stars too.

The field of view is not directly related to the mirror diameter, it is related to the focal length (which determines magnification and ultimately the field of view) via the focal ratio, but assuming the same eyepiece, a 100mm aperture, 1500mm focal length scope will show the same level of magnification and the same field of view as a 300mm aperture 1500mm scope.

Finally as has been said, the main benefit of a large mirror is that it allows extended objects such as galaxies and nebulae to be magnified more whilst maintaining their brightness. Larger objects are perceived more easily by your eye as they can detect the contrast better. Galaxy hunters chase small, faint galaxies with large Dobsonians because they can use high powers (x150 and above) to enlarge them enough to be seen more easily; do that with a 4” scope and the exit pupil is too small for them to be visible.

As said, it is a complex area to do with the physics of light, the scope and also physiology of your eye’s response. Every day is a school day for me, I keep learning. You will pick it up quickly enough I’m sure.

[Louis D]

1 hour ago, John said:

When we can travel around again, come along to a star party and have a look through some larger aperture telescopes. The views can be quite startling

The difference between, for example, the globular cluster Messier 13 observed with my 100mm scope at 150x and my 300mm aperture scope at 150x is really marked in terms of both brightness and resolution. A completely different experience.

This is why I bought my 15" Dob.  Not for hunting faint fuzzies, but for gaining higher resolution and contrast at higher powers.  Planets start to show real detail, globulars start to resolve into pinpoints of light, bright nebula start to resolve into filaments, etc.

[davhei]

Considering the hand drawn picture by @Pixies a few posts back made it clearer to me.

Note how the same amount of rays reflect to your eye off both the small and large flat mirror. It is the size of your pupil that determines the brightness of the image in this case, since the mirror reflects most rays to the side of your eyes, not into them. For all intents and purposes it is like looking at the moon/object directly. There being a mirror inserted into the optical line doesn’t make a difference, assuming 100% reflection and a perfectly flat mirror.

The parabolic mirrors however focus many more rays to your eyes, with the larger reflecting more than the smaller.

No expert on this but it seems logical.

Edited May 13, 2020 by davhei