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Old 18-06-2014, 04:10 PM
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Wavytone is offline
Join Date: Jul 2008
Location: Killara, Sydney
Posts: 4,147
Originally Posted by samgibbs View Post
Recently i have discovered that my 50ish eyesight has been adversely affecting my ability to focus the eyepieces i use for my astronomy sessions.
Sam, you have passed the age where you need either contact lenses, or specs. The lens in your eye continues to grow, and after about age 45 the lens is so thick and stiff your eye muscles can no longer focus it. If you have slight myopia or hypermetropia (short or long sightedness) this won't trouble you during the day when your pupil is small (1-2 mm in bright light, means your eye is working at f/10-f/15 which is very tolerant of small optics defects), but at night your pupil opens up to 5 mm or more and at f/3, any defocus due to myopia or hypermetropia will be very evident. When you focus a scope, the eyepiece position that suits you clearly doesn't suit most others.

Alternatively, if both eyes are good, you could have laser surgery to correct one eye for long distance (i.e. infinity) and use that for focussing telescopes.

FWIW I am 57 and have always been quite long-sighted, and wear specs to correct this - I will won't switch to contacts let alone resort to laser surgery for various reasons.

If you are at the telescope where you will share with others the trick is (a) buy long eye-relief eyepieces (20mm or more is needed) so you can wear your specs and see through the eyepiece, and (b) while wearing your specs, set the focus. This will be the correct "infinity" focus for other people.

As for long-eye-relief eyepieces there are several series that offer 20mm or so across the whole range from short to long focal lengths. Ultra wide eyepieces (>68 degrees aFoV) are useless in this situation so settle for around 65 degree fields of view.

Premium: Televue Delos or Panoptics above 27mm;
Mid range: Vixen LVW, Baader Hyperion (inferior to LVW);
Cheap: Vixen LV (obsolete but superb if you can find a set), Vixen NLV, SLV (not plossl), Orion Stratus or Edge-On.

The extreme wide-angle designs like the Ethos or the ES eyepieces all have very short eye relief and are not suited to wearing specs.

FWIW I have a set of Vixen LVW's for the above reason, and some time ago sold a set of Vixen LV's (regrettably) as IMHO the LV's were superb.

Last edited by Wavytone; 18-06-2014 at 04:22 PM.
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