Am I seeing the light? Is it an exit pupil thing? Can someone help?

[Dreamweaver]

Just trying to get my head around exit pupil and what it means.

Question 1.

When the exit pupil is greater than 5mm does this mean that a person (depending on their age) is not getting all the light coming from an object as their pupil will only dilate to 5mm (I'm over 50), but if you are much younger then perhaps 7mm dilation. This is where I get a bit lost...I think.

Question 2.

How does this translate to what is seen at the eyepiece?

Example.

A scope with 150mm objective lens with a 750mm FL has exit pupil of 6.4mm when used with a 32mm eyepiece giving a magnification of 23.44. Does this mean then that the image, according to the exit pupil calculation, is too big or too bright for a person’s eye as it is over 5mm? Does it mean that you are not getting the whole amount of light from the object because 1.64mm of light is shooting past the edges of the pupil and not going into the eye to be registered by any rods and cones? Is it therefore that 1.64mm of light is wasted because you don’t actually see it. Would this make the image appear a bit darker than if you had 7mm pupil dilation?

Question 3.

Does this mean that a DSO, if observed through a 150mm objective with 750mm FL & 25mm eyepiece giving exit pupil 5mm, would this image be twice the brightness of the image observed through a 150mm objective with 1800mm FL & same eyepiece? The latter scope yields an exit pupil of 2.08mm which is less than half the exit pupil size of the former image but of course with highier magnification. What does this mean in real terms assuming that everything else is equal and the seeing is exactly the same for both scopes? Would the image in the latter scope be bigger but half the brightness?

Confused...

[LukeSkywatcher]

Question 1.

When the exit pupil is greater than 5mm does this mean that a person (depending on their age) is not getting all the light coming from an object as their pupil will only dilate to 5mm (I'm over 50), but if you are much younger then perhaps 7mm dilation. This is where I get a bit lost...I think.

That is my understanding of it. The other questions i will leave to those that know the right answers. I personally do not tend to think about or worry about the mechanics/science of EP's or scopes.

I am happy with what works for me.

[barkis]

If your dark adapted eye has a dilation limit of 5mm, and as ones age certainly has a bearing on the size of the users dilated pupil,

then simply choose eyepieces that compliment your eyes capability. No point in having 7mm exit pupil impinging on a pupil only 5mm in diameter.

That may seem an over simplification from me, so perhaps someone more expert in this area will join the thread.

Ron.

[John]

I'm a bit over 50 too and try and aim for a maximum exit pupil of around 5mm. Having something a little larger isn't a big deal but, as far as I'm aware, it means that you are not getting the full benefit of your scope aperture as a % of the light gathered is not making it into the eye.

The effects of an overlarge exit pupil might not be noticeable unless you were able to directly compare the view of a faint object with a scope / eyepiece combination that delivers the same magnification but a smaller exit pupil.

Using higher magnification does tend to darken the background sky and make faint objects a little easier to view.

[Merlin66]

In the new book "Telescopes Eyepieces Astrographs" by Hallock Smith, Ceragioli and Berry, the whole chapter on visual telescopes revolves around the function of the exit pupil and the need to match it to the optics of the eye. (See pages 7-26)

The summary:

Exit pupils larger than the eye's pupil result in the effective aperature of the scope being "stopped down"

The optimum exit pupils and magnifications are:

Lowest usable magnification = telescope aperture/largest eye pupil

Highest usable magnification = telescope aperture/ 0.5mm exit pupil

[umadog]

It doesn't matter if you use an eyepiece with an over-sized exit pupil. Plenty of people do this to get a wider true field of view. You're not getting all the light, but so what? With many telescopes going to an over-sized exit pupil will be the only reasonably priced way to get the largest available true field. Once the exit pupil becomes over-sized the image won't start to get dimmer. It just will cease to get brighter. No big deal.

Remember that brightness is proportional to the square of the exit pupil diameter. So a 5 mm pupil is 6.25 times brighter than a 2 mm pupil.

[Merlin66]

You can only see what goes through the exit pupil of the scope and the entrance pupil of the eye.

You won't see a wider field of view with a larger exit pupil than that of the eye.

The eyeball works at around f3 (fully dilated) - if the eyepiece design can mirror this then a wider field of view is seen - through the exit pupil.

[Dreamweaver]

I need to read the book then. Thanks people for all your helpful responses.

So the answer to Question 3 as far as I can make out - is more or less 6 times as bright, rather then twice as bright? Anyway I had a fantastic view of Jupiter last night, colours started to become more discernable after a while, one of the moons was just peeping out over the north eat edge I think. A great sight. Got up at 4am to catch the last bit of dark sky. First view of Orion nebular too. Wonderful. Weren't too worried about exit pupil at 4am!

[acey]

To answer Question 3: Assume that your eye pupil is indeed 5mm, and for simplicity assume that both telescopes have perfect transmission, i.e. no light loss. A 150mm aperture scope at x30 gives exit pupil 5mm and the surface brightness of a DSO seen through this scope will equal the surface brightness of the object itself. If the magnification is then multiplied by a factor F the surface brightness will be reduced by a factor F^2. So when the magnification is raised to x72 the surface brightness will be reduced by a factor 5.76, i.e. the image surface brightness is approximately 6 times greater at x30 than at x72. The total magnitude is the same in either case, it's just that one image is larger than the other (it has 5.76 times the apparent area), so the light is more spread out. What determines the total magnitude seen through the telescope is aperture alone.

If you use an exit pupil larger than your eye pupil then you're only using a proportion of the available light cone - in effect you are using a smaller light-gathering area than the full objective lens or mirror. For example if my eye pupil is 5mm and I use a 10mm exit pupil then I am using a quarter of the area of the mirror. If the clear aperture were 150mm then I would effectively be using a 106mm scope (=150/sqrt(2)).

But you don't know the size of your eye pupil. All sorts of formulae or rules are offered, but everyone's eye is different and the only way to find out is by measuring. Flash photography in a darkened room is simplest, or you could try Herschel's method, which was to look at stars through holes of various sizes until he found the smallest hole that made no difference to the star's brightness.

Over-sized exit pupil was routinely used on 19th-century observatory telescopes in low-power finder eyepieces. With relfectors you might get problems with the shadow of the secondary becoming obtrusive, and with some eyepieces it may become difficult to get the correct viewing position, but in general terms there's nothing wrong with over-sized exit pupil, if you want the benefits of low magnification.

[Stu]

I have never got too hung up on exit pupil size, although I do understand the principles. The times I would normally use an exit pupil potentially larger than my pupil are with open clusters where I just want to be able to frame the whole object and am less bothered about contrast on very faint fuzzies. If I am looking at those, I make sure I use an ep which gives under 5 mm exit pupil.

For instance, my 106mm with a 41mm panoptic gives my maximum fov of 4.04° with an exit pupil of 6.3 mm, I will use this if I want to see most sky and am in dark conditions so the sky background is controlled.

Merlin66, could you clarify what you were saying about not seeing the whole fov if the exit pupil is larger than your pupil. My understanding in my example above is that I will be seeing the whole 4.04° but will be losing some brightness because some of the light is not entering my eye. Is that not correct?

Cheers

Stu

Stu

[rwilkey]

According to the pupil dilation calculator at: http://www.stargazing.net/naa/scopemath.htm

my pupil dilation is about 5.6mm at age 62, to be honest I have never found this a problem or noticed it, but I do avoid ep's that are known to give a large exit pupil such as the 32mm PanaView on an f/5 newt (mine being 1000/200 mm). I find a maximum focal length of 32mm (exit pupil 6.55mm) for an ep to be just right, and the safe bet for any low power eyepiece.

[rwilkey]

Sorry, re-reading my post I meant to say I find the 38mm PanaView gives too much exit pupil in an f/5 newt. I have the 32mm and love it.

[umadog]

You won't see a wider field of view with a larger exit pupil than that of the eye.

Not sure it works like that. Brightness is defined by exit pupil size. Field of view is independent of it. Here's the ray diagram that shows why: http://www.telescope-optics.net/images/46e.PNG

The exit pupil is labelled and is a disk of light. The apparent field (and so also true field) is defined by the angle epsilon. Light rays from all angles are constricted equally if the exit pupil is over-sized.

[acey]

The thing that dictates field of view is the field stop of the eyepiece and the focal length of the telescope. What dictates brightness (for a given eye pupil size) is aperture and magnification. Exit pupil is aperture divided by magnification and in that sense exit pupil dictates brightness.

[Stu]

Thanks both. That was my understanding of how it worked, thanks for the clarification.

Stu

[Astrobits]

I think that all the posts are correct. The answer is to know your own eye pupil diameter. To get your own pupil diameter approximately, take a piece of aluminium cooking foil and a small needle. Make pairs of pinholes with diatances of 4mm, 5mm, 6mm and 7mm separation. Try looking at a bright star through the pairs. the closest pair where you CANNOT see the star when your pupil is between the holes shows that your pupil is less than that separation. You can, of course try to refine the measurement by making holes every 0.5mm if you want. Using flash photography may or may not work. Many cameras now use a pre-flash or two to deliberately reduce the pupil size so as to avoid "red-eye".

The final answer to using all of the aperture of the telescope is to keep the exit pupil dia below that of your eye's pupil. To keep the largest field of view go for the biggest AFOV eyepiece you can afford. Instead of using a 32mm Plossl with an AFOV of 50 deg, use a 16mm eyepiece with an AFOV of 100 deg. They will have the same actual field of view, but the 16mm will have half the exit pupil dia.

If the exit pupil is larger than your eye's, the maximum aperture you will be able to use is found by multiplying your eye pupil diameter by the magnification. If you have an eye pupil dia of 5mm ( like me ) then at 30x mag you will use 150mm of aperture regardless of the actual aperture of the telescope. The appearance and brightness of objects viewed will be as if you were actually using a 150mm telescope.

[Merlin66]

I stand corrected!

The FOV is not affected by the telescope exit pupil being larger than the entrance pupil of the eye. Only the effective aperature is affected.