Quote:
Originally Posted by kinetic
As per John's suggestion.....
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Hi John, all,
Sorry to sidetrack from the topic but you seem to have a good knowledge of eyepieces all round....
On the subject of wearing glasses, what exactly, in layman's terms, would
having to wear glasses mean to using eyepieces in general?
I have just entered my forties and recently found that what I thought was
exceptional 20/20 vision is now on a slow, gradual, typical deterioration
of a 40 plus person.
This is obviously frustrating at times and especially so when observing.
I find I have to sway between about 1.5 diopter cheapie $2 shop glasses
to read the monitor and see stars as pinpoints, to taking them off to
look through the eyepiece.
I have recently had my eyes tested and was told I have the raw deal out of
the two typical diagnoses.
I need glasses now for reading, and I also need a slightly different diopter
for distance vision.
Steve B.
Hi Steve,
If you care to start a new thread in the eyepieces section I will be happy to respond there. This is quite a complex topic which warrants a detailed reply and I don't think it entirely appropriate to divert this thread.
Cheers,
John B
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Hi Steve,
Before I begin let me explain
eye relief and
exit pupil. You asked about eye relief. The eye relief of an eyepiece is the distance from the front surface of the eye lens of the eyepiece that the image forms. In simple terms, how far away from the eye lens you need to place your eyeball to hold the exit pupil and comfortably view the image. The exit pupil is the diameter of the column of light that projects from the eyepiece and forms the image that you view. This is a function of the focal length of the eyepiece and the F-ratio of the telescope. You calculate the exit pupil by dividing the focal length of the eyepiece by the F-ratio of the telescope. eg. A 20mm eyepiece in an F5 telescope gives a 4mm exit pupil. A 10mm eyepiece in the same F5 telescope gives a 2mm exit pupil.
As Leon alluded to above,
sometimes, having a need to wear glasses doesn't matter. If a person has no astigmatism, then any correction can be accomodated with focuser adjustment alone. Both for shortsighted people and longsighted people. If this is the case then the easiest option is to just remove your glasses and adjust the focuser to make the required focus adjustment. Alternatively, you can use long eye relief eyepieces combined with your glasses, or use contact lenses, which work well in the absence of astigmatism. Unfortunately, a lot of myopic people (shortsighted) over 40 years of age (me included), also suffer from astigmatism. This cannot be corrected with focuser adjustment alone. It manifests when you are seeing stars that you cannot focus to a point. They always have some degree of flaring. Many people attribute this to an eyepiece aberration, when in fact it is caused by the astigmatism in their own eye. The worse the astigmatism of ones own eye, the worse the flaring. The difficult thing for some people to understand is why the perceived flaring changes with different eyepieces. As the width of the exit pupil that is created is reduced, the amount of astigmatism observed is reduced. As the eyepiece focal length is reduced in any given telescope, the created exit pupil is reduced. As you use progressively shorter focal length eyepieces in any given telescope the amount of observed astigmatism is reduced because you have a smaller exit pupil and are using a progressively narrower light cone. Consequently, as the light cone narrows you are using a progressively smaller section at the center of your eye lens which introduces less aberrations (including astigmatism) than are introduced when you use the outer areas of the eyes lens. The degree of astigmatism in the observers eye will determine at which sized exit pupil it begins to manifest and at which point they need to wear their glasses. I have .75 diopters of astigmatism correction and my astigmatism manifests at exit pupils over 3mm. I have no problem removing my glasses for eyepieces which create an exit pupil of less than 3mm.
There are several solutions to the problem and all of them have inherent issues.
1) Leave glasses on. The issue here is that you need to use long eye relief eyepieces to accomodate the glasses and take in the full field of view and also to avoid scratching your glasses and your eyepieces. I will expand further on this option below.
2) Use a Televue Dioptrix. I don't like this option. I do a lot of outreach work where other people share the view through my scope. No one else can use the eyepiece with the dioptrix fitted unless they have an identical astigmatism correction to me.
3) Use contact lenses. I have tried this option on and off over many years and it isn't the answer for me. The issue here is that contact lenses are weighted at the bottom of the lens to correctly orientate the lens on your eye and correct for your astigmatism. Unfortunately when you observe you often get your head on funny angles which means the contact lenses don't align correctly to the right position on your eye every time.
I have found over many years that using glasses combined with long eye relief eyepieces (16mm to 20mm) is the best way to go. The convenience here is that you never have to remove your glasses when you alternate between, eyepiece, star chart, laptop display, argo navis display and naked eye viewing of the sky. It also leaves both hands free all the time. This may not sound important but when you are up the ladder of a 25" or larger scope you want to hang on with at least one hand, while you adjust the focuser with the other. It is important when you get glasses for observing to get good quality ones with high grade anti reflective coatings. If your glasses do not have good anti reflective coatings you will get reflections back into the eyepiece from the glasses themselves and off your own eyeball. You will think you have "moths" flying around inside your eyepieces. Moreso, if you combine cheap poorly coated glasses with cheap poorly coated and internally baffled eyepieces.
I think I have covered most things. If not ask away. I have suffered astigmatism and observed with glasses for over 30 years and tried every available option known to modern (and ancient) science
Cheers,
John B