Exit Pupil Sizes and Choosing Eyepiece Focal Lengths.

[CosmoSat]

Jumps in exit pupil sizes according to their size/area given in mm.

Exit Pupil D      D Sq         Sq Area

0.5    x 0.5       =0.25           0.2

0.7    x 0.7       =0.49           0.4

1       x 1          =1                0.8

1.4    x 1.4       =2                1.6

2       x 2          =4                3.2

2.8    x 2.8       =8                6.4

4       x 4          =16            12.8

5.6    x 5.6       =32            25.6

8       x 8          =64            51.2

As can be seen each exit pupil size will have twice the surface brightness of the

earlier one if we start from 0.5 mm and the image size will decrease gradually too.

The calculated focal lengths of eyepieces (mm) for various f ratios 

to achieve the above exit pupil sizes are given below.

Ex.P-    0.5        0.7        1         1.4        2        2.8         4         5.6           8

f/4.5 -   2.25      3.15     4.5     6.3       9      12.6       18       25.2        36

  

f/5 -      2.5       3.5       5        7         10     14          20       28           40

f/6 -      3          4.2       6        8.4      12    16.8         24       33.6        48

f/8 -      4          5.6       8      11.2      16    22.4        32       44.8

f/10 -    5          7        10      14         20    28           40       56

f/12 -   6          8.4     12      16.8      24    33.6        48

f/15 -    7.5     10.5     15       21       30    42

From the above table, just knowing the f/ratio of a scope will help one choose properly spaced eyepieces irrespective of aperture.

Needless to say, one does not have to stick to the exact given focal lengths, but the nearest/close rounded off figures would be as good...

So for an f/6 scope -      3     4.5     6      9      12     17 /18    24      34/36     and  48 .

The in between focal ratios can also be used to produce another range and have similar increments in area and brightness -

0.6     0.85     1.2     1.7     2.4     3.4     4.8     6.8.

The given exit pupil sizes (from both the ranges) can be grouped into 4 or 5 classes according to their magnification factors...

Exit Pupil           Magnification          Exit Pupil

0.5 & 0.7           Very High               0.6 & 0.85/0.9

   1 & 1.4           High                       1.2 & 1.7/1.8

   2 & 2.8           Medium                  2.4 & 3.4/3.6

   4 & 5.6 - 6      Low                        4.8 - 5

   6 +                 Very Low                   6 +

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Hope this guide proves useful to you help choosing eyepieces.

Clear Skies!

[John]

Hello and welcome to the forum ! 

Some interesting information there.

[CosmoSat]

The Pentax XW series eyepieces seem to correspond closely to the focal lengths calculated above for a F/5 scope..

Exit pupil-   0.5     0.7     1     1.4     2     2.8     4     5.6     8

FL    -          --      3.5     5     7       10   14       20   30      40

Clear Skies!

[CosmoSat]

Thanks John! 

Hope this would be useful, hve spent many a sleepless night to arrive this...

Clear Skies!

[Laurie61]

Hi and welcome  just to add to what you have covered above. The rule of thumb for max exit pupil (e.p) size is around 7 mm, this is generally regarded as the size the human pupil will dilate up to under very dark conditions. So this can be used to give a guide to the longest fl eyepieces for various f scopes, an f5 scope would need a 35 mm eyepiece and an f8 56 mm to generate a 7 mm e.p

The eye's ability to dilate reduces with age which might also influence your eyepiece choice.        

[CosmoSat]

Hi Laurie n thanks!

...n for bringing this up too, yes, I was expecting this question to come up that how come no FL sizes for the 7mm exit pupil. Infact I have had those calculated too but choose to leave them out. One reason is that the jump from the earlier size to the 7mm is by a factor of 1.5 and not 2 and so the brightness factor may not be noticeable. Secondly the average considered 7mm dilated eye pupil is probably under laboratory conditions and might not hold true for the average observer on the field. So one can calculate accordingly for the last size. But guess for exit pupils from 6mm to 8mm which fall under the very low magnification category, one should not really bother if the exit pupils exactly match our eyes entrance pupil. In all probability they may not. And these EP sizes are usually used to obtain wide field of views and not detection purposes so a bit stopped down aperture if at all should not matter. 

This ofcourse is my personal opinion and I may be right or I may be wrong, so any further views on this subject will be highly appreciated.

We can as well include the column for the 7mm size, but I dont see an edit option here, wanted to do some other editing too.

Clear Skies!

[great_bear]

I feel your pain CosmoSat  - I too spent many, many, many sleepless nights over eyepiece formulas before noticing something rather important:

Experienced observers don't actually seem to use them.

They talk about them of course - we all do - and debate endlessly whether eyepiece formulas should be exit-pupil based, mag-ratio based, or Don Pensack's highly-pragmatic mag-step method. At the end of the day though, ask most observers why they've kept hold of the particular ones in their collection, and they'll likely reply "Erm... I just like these particular eyepieces..." - they may even struggle to put their finger on the reasons why. It's not at all unusual to discover no particular focal-length relationship between the individual eyepieces in an experienced observer's collection.

There's really only two things to remember:

1. As magnification rises above 150x, atmospheric conditions play a dramatically increasing role in image degradation
2. As exit pupil size drops below 1mm, equipment and eye limitations play a dramatically increasing role in image degradation

As mentioned elsewhere, for all practical purposes there is no lower limit on magnification; the image just stops getting brighter that's all.

[ronin]

I agree with GB, all the figures in the world mean little, if you get good results out of an eye piece then use it.

Exit pupil is generally one of the last consideration on my mind (below last - I don't bother with it), and in just about 100% of the posts asking what eyepiece no-one asks about it and no-one in a reply mentions it either.

This hobby is generally about getting out and observing, forget the figures, go observe.

AlsoI have 5 scopes, the exit pupil alters from scope to scope, no point in thinking about all options that arise. The eyepiece either works or doesn't.

[John]

Exit pupil is often mentioned on here when discussing the very lowest and (less often) the very highest power eyepieces to use but rarely other than that. There are a few folks on another forum who base their eyepiece choices with exit pupil as the highest priority. I'm not one of them, although it's a factor included in my eyepiece calculator, but there may be something in it that I'm missing  

[Mike73]

If I was really interested in observing galaxies and seeing as much detail as possible then I'd solely base my EP's on exit pupil.

Matching the exit pupil to the surface brightness of the galaxy would be the most important factor, magnification would already be linked to the aperture scope you are using and FoV just wouldn't matter because 99% of galaxies are small objects.