Discussion on Wide View Eyepieces

[Airconvent]

I've always been very unacademic as far as this hobby is concerned.
While everyone here relish in the gruesome details, I just want to peer through the eyepiece and look at the heavens!

anyway, quite curious on the difference between magnification and wide view eyepieces.
ok...I know fundmentally, the lower the magnification, the wider the view.
Hence my 32mm Plossl gives a wider view than my 15mm. Both are rated at 52deg FOV!

Now I also notice that for the Meade's new range of wide angle ep, the maximum focal length is 24.5mm for the 1.25" range with up to 68 def FOV.

These ep actually command a premium by virtual that they are "Wide angle" .
So why should I buy a 24.5mm eyepiece with 68 deg FOV when I can get a 32mm plossl which is cheaper?

I had a 68deg FOV 20mm generic eyepiece. When I peered through it, I found that the FOV from it is similar to the 26mm plossl, so why bother with the wide angle?

[ariefm71]

Given the same TFOV, it is more desirable to have higher magnification and smaller exit pupil. That's why if money is no object, ppl will choose 31mm Nag over 35mm Pan.

[Canopus Lim]

Yes smaller exit pupil is good as our pupil will decrease when we aged and also it will help mask away our aberrations in our eyes for example astigmatism. Also, having smaller exit pupil will not 'waste light' as a large exit pupil like a binoculars (around 6 to 7mm) effectively means reducing the aperture of the telescope (or stopping down the aperture). However, smaller exit pupil tends to mean narrower field of view.

[denebman]

...
So why should I buy a 24.5mm eyepiece with 68 deg FOV when I can get a 32mm plossl which is cheaper?

I had a 68deg FOV 20mm generic eyepiece. When I peered through it, I found that the FOV from it is similar to the 26mm plossl, so why bother with the wide angle?

When talking about the TFOV, 24.5mm ep with 68deg FOV is same as the 32mm/52 deg FOV one except that the wider one delivers higher mag. and helps you perceive more detail.

My experience: a good 2" super wide field ep gives you a feeling that there is no glass between you and the view.

[Canopus Lim]

I wasn't that clear with my previous statement. If the exit pupil is greater than the eye's pupil, there will be 'waste of light' and effectively means stopping down the telescope's aperture since exit pupil is found by the f ratio and f ratio is related with the focal length and diameter of the scope's objective. Having a smaller objective means longer f number and smaller exit pupil. Therefore, the effective telescope aperture is determined by what gets into the eye and that is the size of the eye pupil.
Lastly, the TFOV is determined by the field stop. Wide angle lens will provide higer magnification at the same field of view which is an advantage.

[elton]

Airconvent,

If your AFOV is large enough, you will experience the "porthole in space" effect which can be quite enjoyable. Viewing extended objects through a 50 AFOV ep even at low power is less fun because the narrow AFOV makes things feel more distant.

While it is true that you should generally use the highest mangification for the given TFOV, there are times when a larger exit pupil is required. Since the larger the exit pupil, the brighter the image, very dim objects are not best seen at very small exit pupils (high magnifications). Also, there is an optimum range of exit pupils when using UHC, OIII, and HB filters. My experience has been that an exit pupil of 2-4 works well for wide field deep sky views because the images will be pleasantly bright while light is not wasted.

[weixing]

Hi,
Everytime when I read about "waste of light", it always is about the size of exit pupil matching the pupil of human eye. But I just wonder will a very large AFoV also have the same "waste of light" issue???

For example, a 120mm scope will have an area of 11310mm squared, so the light it collected will be 11310unit of light (is there such unit?? ). Let's assume that the exit pupil of the eyepiece is smaller than your eye pupil, so no light is wasted via exit pupil. Now, all these light collected by the scope will go through 3 eyepieces of same focal length (so same magnification): a) 82degree AFoV, b) 60degree AFoV and c) 50degree AFoV.

The lighted collected will be spread out equally in the eyepiece, so it will give (something like the density of light):
a) 82degree AFOV: 11310/82=138unit of light per degree FoV.
b) 60degree AFoV: 11310/60=189unit of light per degree FoV.
c) 50degree AFoV: 11310/50=226unit of light per degree FoV.

So base on above, the image per degree FoV of 82degree AFoV will seem dimer than a 50degree AFoV... is that right??? Also, we know that our eye can only "see clearly" around 60-70degree FoV at any one time without moving our eye... that's why a 82degree AFoV eyepiece will give the "in space" feeling. So will the 82degree AFoV eyepiece actually waste a lots more of light when compare to a 60degree AFoV eyepiece at any one time??

If the above is correct, for the best view of an object, it is best to use a eyepiece with 60-70degree AFoV that will give a TFoV that just cover the object.

Anyway, the above are all my guessing work with some logical thinking, so the above may be correct (or part of it) or totally nonsense... I've totally no idea at all... I just let's my mind running wild... ha ha ha

Any comments??

Have a nice day.

[denebman]

Hi,
For example, a 120mm scope will have an area of 11310mm squared, so the light it collected will be 11310unit of light (is there such unit?? ). Let's assume that the exit pupil of the eyepiece is smaller than your eye pupil, so no light is wasted via exit pupil. Now, all these light collected by the scope will go through 3 eyepieces of same focal length (so same magnification): a) 82degree AFoV, b) 60degree AFoV and c) 50degree AFoV.

The lighted collected will be spread out equally in the eyepiece, so it will give (something like the density of light):
a) 82degree AFOV: 11310/82=138unit of light per degree FoV.
b) 60degree AFoV: 11310/60=189unit of light per degree FoV.
c) 50degree AFoV: 11310/50=226unit of light per degree FoV.

...

For three given eyepieces with the same focal length, the one possesses wider FOV is because its field stop circle is bigger (without sacrificing optical performance of the image field), the light loss has nothing to do with how much the AFOV is. What really counts is how good the anti reflection coating and the transparency the glass "sandwich" are. For example, a typical Ortho gives only 44 deg. AFOV, but when people arbitrarily increase its field stop size, some very old German ortho ep give 90 deg. AFOV.

[Canopus Lim]

Elton,
You are right, objects that are has low surface brightness should be seen with bigger exit pupil to get a brigher image. Anyway, those widefield eyepieces give the porthole space effect, it feels that you are very close to the object and that that you are not looking through a restricted narrow field of view of a typical plossl.

[weixing]

Hi,

For three given eyepieces with the same focal length, the one possesses wider FOV is because its field stop circle is bigger (without sacrificing optical performance of the image field), the light loss has nothing to do with how much the AFOV is. What really counts are how good the coating and the transparency the glass "sandwich" are. For example, a typical Ortho gives only 44 deg. AFOV, but when people arbitrarily increase its field stop size, some very old German ortho ep give 90 deg. AFOV.

Hmm... a few funny and silly question in my mind... hoping the expert in optics here will help clearing my doubt:
Q1) If I view at M43 using two same focal length eyepiece (assume quality of eyepiece is same)with different AFoV through the same scope. The eyepiece with bigger AFoV will show M43 and part of M42, but the eyepiece with the narrow AFoV will only show M43. Since both are at same magnification and using the same scope, each eyepiece will show the brightest and clearer image of M43 or both will be the same??

Q2) If I view at M101 (a spiral galaxy) using two same focal length eyepiece (assume quality of eyepiece is same) with different AFoV through the same scope. The eyepiece with a very large AFoV will cover M101 full, but you need to move your eye to see it all. The eyepiece with the narrow AFoV will only show part of M101. Since both at same magnification and using same scope, each eyepiece will show the brightest and clearer image of M101 spiral arm or both will be the same??

Have a nice day.