Exit pupil/Eye relief

[Popeye (Bert)]

In the July edition of the Sky & Telescope there is an article on page 90 relating to eye aberrations and the use of EPs.
It has me confused as I was under the impression that, with age, we are not able to use the smaller aperture EPs because of our pupil loses its flexibility.
About a year ago I came back to astronomy after a break of a few years and (now aged 74) I have used a Barlow - with its drawback of loss of FOV- to help me with eye relief as I understood it. Recently I was advised to go for EPs that have a wide FOV but with a 20mm eye relief. However this article (in S&T) appears to me to refute the entire message about age and eye relief. (I have some understanding of the relationship between eye relief and exit pupil.)
The way I read it this expert is advocating the use of smaller apertures on EPs to counter eye aberrations.
I don't like using the Barlow because with eye at the EP I have to look around the edge of the view to take in only parts of the whole image at a time. Is this normal? Would the recommended EPs be the same?
If anyone has taken the time to read the subject article and has the knowledge to understand its meaning for us older blokes, I would be grateful for some understanding of its import to me as well as some guidance on EPs with long eye relief.
Cheers!
Bert

[Renato1 (Renato)]

Hi Bert,
I haven't read the article, but I do have eye aberration (astigmatism), and if the expert is advocating the use of smaller exit pupils on EPs to counter eye aberrations - he is correct. Stars and objects are much sharper for me with smaller exit pupils. Even when out in the city at night, and I am having a hard time reading an LED sign, I just put my two thumbs and index fingers together, forming a tiny exit pupil, and I can then easily read the sign.

As I understood it, using a Barlow or long eyerelief eyepieces is extremely handy, mainly because one can wear eyeglasses and correct the eye's aberration, while still maintaining a large exit pupil and seeing the brighter, wider image.

What you say about not saying all the eyepiece field of view happens to me with a few of my very shallow eyerelief eyepieces with rubber eyeguards attached. But when I remove the rubber eyeguard and get closer to the lens, the problem goes away.

You should also have the exact same problem of not seeing the entire field when looking through binoculars - do you?
Regards,
Renato

[astro744]

First of all I haven't got the article and so I haven't read it.

Eye relief is the distance from the surface of the eye lens of the eyepiece to the front of your eyeball. The eye lens is the bit you look through as opposed to the field lens which is at the other end inside the barrel. The greater the eye relief the further back you need to place your eye to see the entire field as the eyepiece was designed for. 20mm eye relief is great for eye glass wearers. Too much eye relief and you get the situation you describe whereby you are getting vignetting of the exit pupil common with long focal length eyepieces used with Barlows which extend the eye relief. A solution would be to use a Powermate which places the exiting light cone to a point the eyepiece designer intended.

Exit pupil is the diameter of the cone of light leaving the eyepiece. If small then this cone of light enters the centre of your eye and does not extend too much toward the edge of your eye. If the exit pupil is large you use more of your eye to see the cone of light and the outer portions of your eye will exhibit some aberrations that will affect the image.

If you have astigmatism and you observe without glasses you may notice that with low power you just cannot get a good image yet with higher power the image appears much better and this is because at higher power (smaller exit pupil) you get a much smaller light cone entering your eye.

exit pupil = eyepiece focal length divided by telescope focal ratio.

Now another adverse affect of growing old is that your eye wont dilute as much as when young and therefore you may not allow the entire light cone to pass into your eye at low powers. This however is unrelated to the advice given to observe with smaller exit pupils for sharper images.

[Popeye (Bert)]

Thanks to both of you for the time taken to answer my questions. It did not occur to me that there was a difference between eye lens and field lens. Must find out about Powermate. And now I see the use of the message about using small apertures for EXIT PUPIL as opposed to eye relief.
I was aware of the issue regarding the pupil not being able to shrink enough for small apertures but I was getting E pupil mixed up with ER.
I don't believe that I have astigmatism, Renato. I don't have any problem with binoculars. Your tip about taking the cup off the EP is one that I will try out next time we have a break in the weather (usually at full moon!) We desperately need the rain that we are getting now but its getting a bit wearing.
I will copy you inputs for further reading.
Very grateful.
Cheers!
Bert

[astro744]

See http://www.televue.com/engine/TV3b_p...k#.U6PNSJSSy5I regarding Powermates versus Barlows.

Powermates are not cheap but they perform very well. Note though that the 2x Powermate is only available in 2" size and is heavier than a 2" 2x Barlow (although not as heavy as the 4x). The 1.25" 2.5x Powermate would be ideal for 25mm eyepieces upward to avoid giving you eye relief that is too long. Can also be used for eyepieces under 20mm to give you quality high power viewing through eyepieces that have a larger eye lens making it much easier than those pin hole 6mm and under Plossl eyepieces.

Note the very early 2.5x Powermates were not threaded for filters but the current new ones are. I'm not sure which year the change was made.

[Renato1 (Renato)]

Originally Posted by Popeye

I don't believe that I have astigmatism, Renato. I don't have any problem with binoculars.
Bert

Then I strongly suspect the rubber eyecup.

I've had the problem in a few eyepieces. In some I could roll the rubber eyecup down, in others ranging from a 4mm plossl to a 2" 30mm wide angle, I've had to remove the eyecup altogether.
Cheers,
Renato

[Popeye (Bert)]

Thanks, astro.

Originally Posted by astro744

See http://www.televue.com/engine/TV3b_p...k#.U6PNSJSSy5I regarding Powermates versus Barlows.

Powermates are not cheap but they perform very well. Note though that the 2x Powermate is only available in 2" size and is heavier than a 2" 2x Barlow (although not as heavy as the 4x). The 1.25" 2.5x Powermate would be ideal for 25mm eyepieces upward to avoid giving you eye relief that is too long. Can also be used for eyepieces under 20mm to give you quality high power viewing through eyepieces that have a larger eye lens making it much easier than those pin hole 6mm and under Plossl eyepieces.

Note the very early 2.5x Powermates were not threaded for filters but the current new ones are. I'm not sure which year the change was made.

[Popeye (Bert)]

For the first time since the days before and just after the full moon we have had non-stop rain. Some of it torrential with wild winds too.
But tonight I there is but a few wispy clouds and I have the scope set up for darkness. I will use the Barlow and try your suggestion of rolling down the eyecup or removing it altogether.
BTW astro too, I now own an LX90 (second hand) and have sold the Dob and the ETX. I was a previous owner of the LX90 and glad to have it back.
Will change my particulars when I work out how I did it before.
Cheers!
Bert.

Originally Posted by Renato1

Then I strongly suspect the rubber eyecup.

I've had the problem in a few eyepieces. In some I could roll the rubber eyecup down, in others ranging from a 4mm plossl to a 2" 30mm wide angle, I've had to remove the eyecup altogether.
Cheers,
Renato