Very basic thing I don't understand about scopes

[BrendanC]

Hi all,

So, having used my scope for nearly two years now, there's something I still don't 'get'.

I have a Skywatcher 130PDS so it has a mirror of 130mm. Apparently, a 200PDS, with a bigger mirror, should be able to capture 'more light', so that I can make out dimmer objects and more detail.

But how is this? Say I have a small hand mirror, and I look at a reflection of, say, the moon in it. Then I get a big wall mirror and do the same. The moon doesn't look any bigger in the big mirror!

So does 'more light' really mean a bigger field of view? As in, you can see more of the sky around your object of interest? Or is this more to do with the mirror in the scope being parabolic, so that more light is focused into the eyepiece? Or a bit of both?

Told you it was basic!

Thanks, Brendan

 

[johninderby]

Have a read of this for some in depth info on the subject.

https://starizona.com/tutorial/observing-theory/

Edited May 12, 2020 by johninderby

[miguel87]

4 minutes ago, BrendanC said:

Hi all,

So, having used my scope for nearly two years now, there's something I still don't 'get'.

I have a Skywatcher 130PDS so it has a mirror of 130mm. Apparently, a 200PDS, with a bigger mirror, should be able to capture 'more light', so that I can make out dimmer objects and more detail.

But how is this? Say I have a small hand mirror, and I look at a reflection of, say, the moon in it. Then I get a big wall mirror and do the same. The moon doesn't look any bigger in the big mirror!

So does 'more light' really mean a bigger field of view? As in, you can see more of the sky around your object of interest? Or is this more to do with the mirror in the scope being parabolic, so that more light is focused into the eyepiece? Or a bit of both?

Told you it was basic!

Thanks, Brendan

 

 

But if you reflected the light off your two different mirrors onto a wall, the larger one would would show a bigger patch of light.

Yes more light means more detail and dimmer objects.

You are right that it is to do with the shape of the mirror focussing light to a point.

Imagine positioning your eye at 100 different places on the mirror. You could see the same particular star from any of these positions. The starlight covers the whole mirror (this applies to every star within the view of the telescope). When focussed to a point you are therefore collecting a brighter image.

The same as a tea cup vs a saucepan catching rain.

Field of view is not dictated by aperture.

Hope that sort of makes sense?

Edited May 12, 2020 by miguel87

[BrendanC]

But it's not the same as a tea cup vs saucepan! That's the analogy I keep coming up against.

Put them both out in the rain, they both fill up to the same point. The saucepan however will get more 'diameter' of rain, which equates to a greater field of vision - which is the same as saying the larger mirror would reflect a bigger patch of light. The patch is bigger, which is more light, but it's the same amount of light per square cm as a smaller mirror, right?

I think the parabolic shape is key here. I'll follow that other link provided and get back to you when I have an answer!

Sorry, it's just like those basic questions that you suddenly realise you don't know the answer to, and feel you should, like why did that apple fall downwards...

[dweller25]

A bigger mirror has more surface area than a smaller one so will collect more light ( or water )

So at the same magnification the image in the bigger mirror will appear brighter

Edited May 12, 2020 by dweller25

[miguel87]

Yeah they would fill to the same level so the pan would have lots more water in.

Yes the shape is key, the light is being brought to focus. That's why you cant use your mirror as a telescope!

There is no debating the fact that a larger mirror collects more light and shows dimmer objects.

I upgraded from a 130 to 200 and the difference is VERY significant. Many objects that were invisible before are now easily locateable.

Its the whole reason that your pupils dilate in the dark, bigger aperture.

[Kev M]

As above ,

 

Think of capturing raindrops the same as capturing light photons, now if you poured the water ( or focused the photons ) into a container ( or a sensor ) which do you think would have captured the most ?

[miguel87]

1 minute ago, Kev M said:

As above ,

 

Think of capturing raindrops the same as capturing light photons, now if you poured the water ( or focused the photons ) into a container ( or a sensor ) which do you think would have captured the most ?

Nice analogy, with the container of water being the camera or eyepiece.

[andrew s]

Perhaps,  what you are missing is that the telescope mirror focuses all the light onto a small area while your flat mirror does not. If you look  down you eyepiece from say, 15cm behind it,  at the well illuminated mirror you will see on "image" of the mirror much reduced in size. All the light from the mirror is now in that tiny spot.

The ratio of the mirror diameter to the spot size gives you the magnification of the telescope/eyepiece combination. 

Regards Andrew 

Edited May 12, 2020 by andrew s

[Robindonne]

I jump in as a rookie, admitting at first i also didn’t know the whole theory about this mirrorcle phenomenon.   The rain falling in a cup of tea or pan is maybe a bit hard to understand and it actually doesnt describe it correct.   A bigger pan would catch more raindrops(stars).  

The way i started to understand is like a lightbulb.  It spreads light almost everywhere around it.  And that single source of light can be collected with various sizes “pans”.  (Edit** the collecting scope determines the amount of visible lightsources by its FL. The longer the FL the smaller the amount of objects.
Imagine yourself on the bottom of a 1 meter wide well, with a 1 meter depth vs 1 meter wide well on a depth of 20 m.. the 20 m shows an extreme smaller part of the sky)

Compared to the rain theory its more understandable because you describe a single source of light.  

Nice to test this is to put your hand in front of the telescope and block almost all light. You’ll find out the same amount of objects are visible only fainter or darker. Then you find out that from each lightsource you still collect the light, but a much smaller percentage.

Some newtonians have a dustcap with a small hole in it.  Its basically to view the moon in a tempered light.  Just put on that cap and open the small hole to see the phenomenon of “same amount but fainter or darker objects”

Edited May 12, 2020 by Robindonne

[miguel87]

16 minutes ago, Robindonne said:

I jump in as a rookie, admitting at first i also didn’t know the whole theory about this mirrorcle phenomenon.   The rain falling in a cup of tea or pan is maybe a bit hard to understand and it actually doesnt describe it correct.   A bigger pan would catch more raindrops(stars). 

It does describe it correctly. The point source of a star is so far away that the incoming photons are parallel (like raindrops and not like a lightbulb).

A bigger pan would catch more raindrops. Yeah...exactly, the bigger mirror catches more light.

[andrew s]

1 hour ago, BrendanC said:

But it's not the same as a tea cup vs saucepan! That's the analogy I keep coming up against.

Put them both out in the rain, they both fill up to the same point. The saucepan however will get more 'diameter' of rain, which equates to a greater field of vision - which is the same as saying the larger mirror would reflect a bigger patch of light. The patch is bigger, which is more light, but it's the same amount of light per square cm as a smaller mirror, right?

I think the parabolic shape is key here. I'll follow that other link provided and get back to you when I have an answer!

Sorry, it's just like those basic questions that you suddenly realise you don't know the answer to, and feel you should, like why did that apple fall downwards...

You are mistaken the larger diameter of the pan equates to larger aperture. What equates to larger field of view  would be the angle over which you can collect rain.

 Consider pans of different height but the same diameter. A shallow pan can collect rain coming in at at shallow angles as well as directly down. A tall pan mostly directly down.

Regards Andrew. 

Edited May 12, 2020 by andrew s

[Waddensky]

1 hour ago, miguel87 said:

There is no debating the fact that a larger mirror collects more light and shows dimmer objects.

Not trying to make matters more complicated, but a larger telescope shows dimmer objects not by increasing the surface brightness, but by allowing more magnification. In fact, the surface brightness of a deep-sky object is slightly less in a telescope than compared to the naked eye due to light loss in the optical path.

[Pompey Monkey]

If the pots and pans is confusing,  try to think the of funnels  with the same size small end (eyepiece  or camera) but with different top diameters.

Which funnel will fill a cup with water faster?

A telescope effectively funnels photons into a smaller area to make the photon flux (photons per unit area, per time) greater.

 

[vlaiv]

15 minutes ago, Waddensky said:

Not trying to make matters more complicated, but a larger telescope shows dimmer objects not by increasing the surface brightness, but by allowing more magnification. In fact, the surface brightness of a deep-sky object is slightly less in a telescope than compared to the naked eye due to light loss in the optical path.

Of course you are making things complicated - you are mixing absolute surface brightness and magnification. Eye responds to relative surface brightness - add magnification and things will become dimmer.

Aperture does not change absolute surface brightness but it changes relative surface brightness.

I'm also making things complicated - by using terms absolute and relative

What I mean is surface brightness per actual sky area vs surface brightness per apparent observing area - or something like that ... getting lost in terminology here, help!

 

[BrendanC]

Told you it wasn't that simple!

Sorry, I seem to have created a bit of a kerfuffle.

I'm sure the answer's simple. I just don't know what it is.

[Supernova74]

Firstly you can not compare an house hold mirror to a precision made telescope mirror and the purpose having a large mirror as possible is to collect much light as possible as a whole to reach your eye through the eyepiece however in theory you should be able to see more detail and yes the object your viewing should be brighter also but unfortunately you can have the largest telescope in the world it always goes down to seeing conditions 

[Tommohawk]

 The funnel analogy is probably the best, because it more or less corresponds to the light cone.

You have 2 scopes,

First is 130PDS which has a diameter of 130mm and focal length of 650mm. It is F5, thats the ratio of the diameter and the focal length, and 

Second is 200PDS which has a diameter of 200mm and focal length of 1000mm. It is also F5.

Imagine you point the scopes upwards to collect light, or in this analogy water. The rain drops are parallel, rather like the light rays (from a very distant object) so the bigger scope will collect more water/light.

The image from the second scope will actually by bigger, because it has a longer focal length, but it wont be dimmer because you have collected more light. The image brightness per square cm is the same, but its a bigger area, so more total brightness.

Hopefully that makes sense, and is accurate!

[Tommohawk]

To embellish that a bit, if the 200mm diameter scope had the same focal length as the 130, ie 650mm, it would be "faster" at F3.25. In that case the image size would be the same as the 130PDS, but it would be brighter....

...and to add a bit more... if you go for a bigger diameter scope with the same F value, you will get a reduced field of view - which isnt necessarily what you want. Remember some objects out there are huge, but faint. That's why we don't all just use the biggest scope we can get our hands on!

 

 

Edited May 12, 2020 by Tommohawk

[miguel87]

20 minutes ago, BrendanC said:

Told you it wasn't that simple!

Sorry, I seem to have created a bit of a kerfuffle.

I'm sure the answer's simple. I just don't know what it is.

Lots of people have given you the answer 🙂

Bigger area collects more photons and focuses them into an image.

That's the basic, short version.

[Waddensky]

32 minutes ago, vlaiv said:

Of course you are making things complicated - you are mixing absolute surface brightness and magnification. Eye responds to relative surface brightness - add magnification and things will become dimmer.

Aperture does not change absolute surface brightness but it changes relative surface brightness.

I'm also making things complicated - by using terms absolute and relative

What I mean is surface brightness per actual sky area vs surface brightness per apparent observing area - or something like that ... getting lost in terminology here, help!

 

I'm pretty sure we're on the same track but I'm struggling with terminology too 😄. Doesn't help that I'm not a native speaker of English 😅.

What I meant is, when the exit pupil is the same, the image produced by a telescope has the same brightness no matter what the aperture is. It's just that larger apertures allow for higher magnifications and therefore the projected image of the same surface brightness is larger, activating more light receptors. Our eyes detect larger objects more easily than smaller objects with the same surface brightness.

Edited May 12, 2020 by Waddensky

[Pixies]

OK - so as they say, "a picture paints a thousand words":

 

[miguel87]

11 minutes ago, Waddensky said:

 

What I meant is, when the exit pupil is the same, the image produced by a telescope has the same brightness no matter what the aperture is.

Wow, I never realised this.

So a 4mm exit pupil has the same brightness in every telescope, binoculars, monocular etc???

This would just provide high mag in a 'big' telescope and wide field views in a small telescope?

Interesting thanks!

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?

I guess we normally sacrifice some brightness for a bit of magnification.

Edited May 12, 2020 by miguel87

[johninderby]

Read the optimum magnification section of the tutorial I linked to.which covers the subject in detail.

A quote from the tutorial.

“For a given telescope, the following rules apply.  For small objects with low surface brightness (such as galaxies), use a moderate magnification.  For small objects with high surface brightness (such as planetary nebulae), use a high magnification.  For large objects regardless of surface brightness (such as diffuse nebulae), use low magnification, often in the range of minimum magnification “

Edited May 12, 2020 by johninderby

[Timebandit]

 

 

All things being equal , then more aperture has the ability to gather more photons.

I think BrendanC  you are trying to talk yourself out of aperture fever and trying to stick with your 130mm aperture and not upgrade to a 200mm aperture 😀

Give into the aperture fever a good 200mm is a great scope and will give you years of great visual images , especially at Dark sites then fainter DSO will pop to the eyes