Optical quality of glasses

[Jared]

I am among those who have to wear glasses when observing. I have heard a lot said about the need for long eye-relief, but I was wondering if the optical quality of glasses has any effect. :shrug:

Will all the obsessing about optical quality in our instruments be for nothing if at the last moment the light has to travel through a sub-standard lens in our glasses?

For example, when I got my glasses some years ago I had the choice of getting some kind of coating applied that reduced diffraction I think. I didn't get the coating and now when I look at bright lights I get radiating spikes bouncing all over the place.

Are there better types of glasses than others? What about contact lenses?



Thanks a lot.

Jared.

[citivolus]

The last time I got eyeglasses, I made the stipulation to them that I wanted the best optical quality that I could get. Zeiss provides an Abbe Number (http://en.wikipedia.org/wiki/Abbe_number) for their lenses so that you can see how much chromatic aberration they will have.

Here is one PDF file that has that information in it, on page 1.1:

http://www.vision.zeiss.com/4125682000258738/EmbedTitelIntern/SOLA_Specs/$File/SOLA_Specs.pdf

The higher the Abbe number, the better. Note that the trade-off is weight, as the best Abbe number available in eyeglasses can be had from crown glass, while resins tend to be lighter but not perform quite so well optically. While fluorite lenses would optically perform even better, I am not aware of anyone making them for eyeglasses.

Another thing you can see by looking at that chart is that, unless you are specifying your own lenses, you can end up with absolutly poor optical performance. The light weight, super high index of refraction lenses perform the worst visually, while they are the ones commonly used for stronger presciptions.

In my case, the CR-39 resin seems to have the best optical performance for its weight. Note that certain materials are not available beyond certain prescription limits, so if you have particularly bad eyesight you may need to make some tradeoffs.

Regards,
Eric

[Satchmo]

I'm fairly shortsighted and use high index lenses with AR coatings. I always notice a marked inprovement in my vision for naked eye astronomy if I wear my disposable contact lenses, but I get funny artifacts on bright stars looking through a telescope. I don't notice any differnece when looking through a telescope.

[Jared]

Thanks Eric and Satchmo! That's just the information I was after. Very interesting. :thumbsup:



Jared

[Geoff45]

If you are short or longsighted, surely just adjusting the focus of the scope will fix things without the need for specs. If you have astigmatism, then of course that won't work and you are stuck with glasses.