CN FORUMS ARCHIVE Telescope Reviews: Glasses, astigmatism, and EPs

Dear pmesquita ,

Hi, my first ever prescription for glasses when I was 30 years old (my first ever pair of glasses) was exactly the same as you (astigmatism of -1.75 left eye, -1.5 right eye, note the negative figures, this is because my eye lens is the correct shape vertically but does not have enough curve horizontally, so my glasses for this prescrption had a HORIZONTAL expansion / magnification only).

When I was 18 I could see the classroom blackboards 20 feet away or more quite well I thought. However with my astigmatic eyesight of being 30 years old I was NOT safe for driving (only found out when I needed to learn to drive in 2005). When I looked at TV aerials on roofs across the street I could see TWO aerials, 1 above the other, but there was only one there when I checked in binoculars. Also in the dark, pinpoint LED's on electrical equipment were NOT poinpoints, they had arcs of light several times the size of the dot shape of the LED surrounding the images, a bit worse my in left than right eye. Stars were horrid looking. It was VERY obvious that I needed spectacles. I went to the local optician and mentioned about that I had been reading up on astigmatism and worked out that I had it and he verified this and said "yes you are right, you have moderate astigmatism" (not mild, not severe).

So in your case, and mine, I needed to wear glasses at all times. They were single vision (not progressives/transistions, not bi/trifocal) lenses, and 1.5 index plastic (Abbe number 58, low dispersion!) I made sure to get Anti Reflective coating (not just the standard hard coat) on them (green multilayer AR coating).

For reading and driving I needed them at all times. I am still wearing them today.

However 5 years have passed and I was shocked to see that when I went for a new eye test last month I had gone to -3.5 in left eye and -3 in right eye for astigmatism (look at the MIDDLE column of figures that you can ask your optician to give you as a print out of your eye test, the CYL / CYLINDER value).

I had suspected that my eyes had gone a lot worse, but didn't realise how bad (as it had only gone 5 years). Now I am 35.

I have been for the last several months raising my glasses up at an angle (looks most odd) for driving and being at home and observing through binoculars, as in effect this means that I am looking through more curve (think about it) and as a result, with this I could read down to the bottom of the smallest letters on the eye test chart quite easily, and infact could also see the 3 sets of letters on the extra-small additional special eye test chart (super human vision LOL). I got the same sharpness when I was looking through the test spectacles when I was having the eye test, so I knew that my prescription was spot on.

My new glasses are ready on the 19th July and have a lot more astigmatism correction as I mention above. I chose frameless (rimless) this time with 1.6 high index plastic (Abbe number 42, some chromatic abberation, cannot be avoided). Try not to get polycarbonate as the Abbe is 28 or so (terrible for chromatic red / blue halos around the moon). The local small opticians had the lenses made by a 3rd party and would not allow anything other than polycarbonate for frameless design due to possibility of stress fracture of lens. So I used a large opticians with in-house lens making, who were able to offer 1.6 High index plastic (42 Abbe) for less chromatic aberration than polycarbonate. I had to go with high index this time around instead of 1.5 index (which has the lowest chromatic problems) as otherwise with my eyes having double the astigmastism now versus 5 years ago the thickness would make them too heavy.

Believe me, 1.75 dioptres of astigmatism is a medium amount, and NOT small, and YOU WILL need spectacles for driving and everyday use, or instead you'll have to have contact lenses.

For the record I also have +2 dioptres of far sightedness in my left eye, +1.75 in my right eye. 5 years ago with my old prescription I only had +1.25 and +1.00 respecively. However this is only an issue for daytime / driving / everyday use of glasses, and ANY optical instrument (binoculars / telescope) will completely remove this simply by refocussing the instrument.

As a side note, I imagine that for a person with severe short or long sight, of 6 to 9 diopteres, the refocuss would be substantial, which would I assume take the mirror spacing off it's sweet spot for correction of Spherical Aberration and make an otherwise great star test in an SCT only so-so?!

In answer to your question (@pmesquita ), you will find that you with your 1.75 dioptres of astigmatism you can observe without glasses when the exit pupil is around 1mm or less, approx.(found by dividing the eyepiece FL in mm by the telescope F ratio) so when above 177x (15mm eyepiece ) in a 7" F15 Maksutov-Cassegrain, for example. If doing planetary viewing then really you should be wearing glasses or using Dioptrx AT ALL TIMES as any optical abberations really hurts low contrast surface detail objects like planets.

For reading you should be wearing your normal distance / everyday glasses. When you are over about 40 years old (I am 35) you will need an additional set of reading glasses. Do NOT buy off the shelf reading glasses. You will need them to suit your personal prescription, as you have moderate astigmatism which affects text sharpness badly.

I hope that helps?

Cheers,

Alistair G.

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19.5" F4.1 Dob (too big, unused LOL), Meade SN102 (very much gets used), 8-24x25 Nikon zoom bino's. TV 8mm Rad., TV 32mm Pl. Meade SP26mm (cut down barrel!), MA25, MA9. Also use a friend's LX90 8" a lot LOL with Revelation ep set.

Edited by Live_Steam_Mad (06/08/09 08:02 PM)