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A question about exit-pupil and eyepieces

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#26 Jon Isaacs

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Posted 21 November 2019 - 02:27 AM

As long as the eye, the pupil, is fully illuminated they will be equally bright, at least to the extent that you can likely notice.





This is true of galaxies, nebulae and planet's, "extended objects". 


It is not true for stars which are point sources. They are dimmer when the exit pupil is larger than that your pupil.  Stars act as points of light at anything but small exit pupils/high magnifications so that light that does not enter your eye, dims the star.


With an extended object, this loss of light is compensated for by the fact that the object is smaller, but a point cannot get smaller.


Close yourself up in a bathroom with a towel under the door as necessary and wait with a camera and a set of calipers preset to 7mm for 20 - 30 minutes. Take a pic in the mirror with the calipers held near your eye.



It only takes your pupil a short time to fully dilate. One minute is sufficient.


I do not recommend using calipers in the dark, they're sharp, too risky to hold near your eye.


When I've done this, I've used a thin strip of wood about 7 mm wide. I just need to measure it's width with the calipers.


Several attempts will be necessary to get the focus correct. 


When you have a sharp photo, transfer the photo to a computer, zoom in on the image and measure the diameter of your pupil and the width of the stick on the computer screen with the calipers.


Your pupil diameter diameter is the ratio of the zoomed in measured diameters times the width of the stick. For example, if the stick is 9.1 mm wide and on screen, you measure your pupil to be 52.6mm and the stick to be 71.0 mm then your pupil diameter would be:


D = (52.6mm/71.0mm) x 9.1 mm = 6.74mm.


Even with a 6.7 mm dark adapted pupil, I'd choose a quality 30mm-31 mm 82 degree eyepiece for F/5 over a longer focal length eyepiece. 


My dark adapted pupil is 7.7 mm and while I have very high quality 35 mm (7 mm exit pupil) and 41 mm (8.2 mm pupil), both 68 degree Panoptics, I rarely use them, the 31mm 82 degree Nagler provides nearly the widest view but with signifantly more magnification and is almost always my preference. In a 12 inch with a 1500mm focal length:


31 mm 82 degree = 1.60 degrees at 48.4x, 6.2 mm exit pupil.


35 mm 68 degree = 1.48 degrees at 42.9x, 7.0 mm exit pupil


41mm 68 degree = 1.75 degrees at 36.6x. 8.2 mm exit pupil.


It's unlike that your dark adapted pupil is as large as mine but even for me who can nearly take in that 8.2 mm exit pupil, the 48.4 x versus the 36.6 x is the important factor.  I even choose the 35 mm over the 41mm for this reason.


This is based several hundred nights under dark skies with these three eyepieces. Under brighter skies, I'd forego these three for an even shorter focal length eyepiece.


I should add, these are expensive eyepieces, the least expensive the 35 mm Panoptic, currently on sale for $351. Explore Scientific offers very similar eyepieces that are not quite as perfect as these TeleVues but still quite sharp across the field and significantly less expensive. Their 34 mm 68 degree is $290.


There are eyepieces that cost less than $100 that are workable as finder eyepieces. Ed's 38 mm 70 degree eyepiece is one of them. It does offer essentially the widest field of view at the expense of the large exit pupil and a significant loss of sharpness away from the center of the field of view, stars towards the edge are far from tight, round points at F/5.



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#27 aeajr


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Posted 21 November 2019 - 08:29 AM

Awesome tips as usual smile.gif


My first scope was a TS Photon 6" f/5. I still have it, but am currently selling it. My new and only scope is a Taurus 300 12" truss dobsonian. I got it about a month ago (had to wait like 4 months for it). I am hoping to write a review about it one day. For now, I can only say that it is an very well thought scope.


I use a Telrad to navigate the night sky and a TS Optics 50/204mm finder-telescope with a 17mm 50° Plössl, which gives me a FoV 4.17°, about the same as the outer circle from my Telrad.


One day I will beat my laziness and update my signature too! smile.gif

That looks like a very nice scope.  Best of luck with it.


My suggestions for your eyepiece plan would be to start with a 34 or 35 mm for your low power, 7 mm exit pupil.   Something in the 66 to 70 degree AFOV to provide your wide view. A 30 mm 82 degree would also be a good choice. 


Just as 7 mm EP is considered the lowest power recommendation, 1 mm EP is often suggested as the high power limit, which equates to an eyepiece equal to the focal ratio of the scope.  In your case that would be 5 mm.


Now that we have defined the ends of the initial set you can fill in.



Here is my eyepiece set for my 12" Dob.  This was carried over from the 8" F5.9 it replaced. 


Apertura AD12 12”/305 mm Dobsonian/Newtonian, 1520 mm FL F5 FR

Two speed focuser and flocked tube.

8X50 RACI Finder and Telrad


Two low power wide view. 


Agena   38 mm/70     40X and   1.75 degrees FOV     EP 7.6 mm  2”

Meade  20 mm/82     76X and   1.07 degrees              EP 4.0         2”


The AA 38/70 is a fairly low cost eyepiece, but I am satisfied with it. If I were to replace the AA 38 with a higher quality eyepiece it would likely be an ES 68 40 mm, but I would want to test it first to be sure it did not introduce a shadow of the secondary.  If that produced a shadow, then I would opt for the ES 82 30 mm, but I would give up a little FOV with this one.  My primary use of my lowest power is to maximize FOV.    



This is my preferred midrange - a zoom


Baader Hyperion 8-24  zoom    63X to 190X and .79 to .35 degrees

Used mostly in the 16-8 mm part of the range


These two are only used if I am on a target I plan to observe for a while where these magnifications would be appropriate and I would prefer a wider FOV than the zoom provides.  I don't use them often but if I find I am using them more and more with the 12" I might add the ES82 11.


ES          14 mm/82   108X and    .75 degrees             EP 2.8
ES          8.8 mm/82  172X and    .47 degrees             EP 1.7



My high power set, above the 8 mm of the zoom


ES          6.7 mm/82  226X and    .36 degrees             EP 1.3
Meade    5.5 mm/82  276X and   .29 degrees              EP 1.1
ES          4.7 mm/82  323x and    .25 degrees              EP .94

               (4.7 mm was added after I got the 12" Dob)



I have had the 12" since April and have not had a night were I could push past the 4.7 mm eyepiece.  However, if I care to try going higher I add a 1.5X barlow to the zoom or a 2X barlow for the others in order to explore higher magnifications.


Baader Hyperion 8-24+1.5XB   94X to 285X


ES  82        8.8+2XB      344X and    .24 degrees
ES  82        6.7+2XB      452X and    .18 degrees
Meade 82   5.5+2XB      552X and    .15 degrees



As you can see, I am a little longer with the 38 than the standard low power guideline and a little shorter with the 4.7  than the recommended for high power.  They work well for me. 


When I went from the 8" Dob to the 12" I found I could add about 40X to 70X more magnification to each target than what the 8" would allow.  A clear benefit of the extra aperture.  


I hope you find this helpful.    

Edited by aeajr, 21 November 2019 - 04:29 PM.

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