Contact Lens Experiment !

[Damo636]

I realise that by posting this everyone is going to think i'm barking mad but what the heck, here goes. I have been on hol's from work for the last couple of weeks and have been seizing every opportunity to get out under the stars. I wear contact's during the day at work but remove them in the evenings to let the old eyeballs breath and wear my specs till bedtime. At the telescope i wear my specs (need them to use the Telrad) but remove them while at the eyepiece. One night last week i wore the contacts while observing as i hadn't worn them all day and thought it would nice and convenient not to have to pfaff about with the glasses for once. Jupiter was my main target having not seen the big guy in a while. Now this is where it gets really strange. With my contact lens in i have 20/20 vision during the day but when observing Jupiter i have convinced myself that i can pick out more detail without them I went to the extent of removing one and switched back and forth and i found the planets bands more pronounced in the unaided eye. Maybe there are some opticians on here can shed some light on this but i am convinced my contacts degraded the image

[Carbon Brush]

Worse with the lens is not unexpected.

A complicated answer unfortunately. The content of which is in part determined by the eyepeice, your lens presciption and contact lens type.

I'm not an optician. But have worn glasses for 50 years and contact lenses about 5. I have looked at various visual enhancements for everyday, astronomy, photogrpahy and diving.

If you let me know (PM if you prefer) about your visual correction, then I can put in my 2p worth.

David.

[pete_l]

One thing my optician told me (we were chatting, I mentioned astronomy and we started talking about dark adaptation) was that people can need a different prescription for daytime and nighttime vision. He reckoned that for me, a separate pair of glasses an extra 0.25 dioptre stronger would adjust my vision (I'm short sighted) and make it easier to see the sky - just "eyeballing" not at a telescope.

Whether that was a ploy to get me to order another pair of glasses, or a genuine fact I can't say. I later ordered a stronger pair from an online seller for £15 rather than the £200 they wanted, using the prescription he gave me and adding a bit - but it's doubtful that it made any difference.

One thing we astronomers do know is that as the focal ratio gets smaller, the focusing becomes more critical as the light cone converges quicker. It's probably true with eyes, too. In darkness the pupil is larger, but the eye's size remains unchanged so the F-ratio is smaller. Therefore I can see that it's harder for eyes to focus precisely in the dark. If you can see properly in daylight but less well at night, it could be because it's harder to focus and your eyes can't quite adapt.

[acey]

If your lenses correct solely for long or short sight then they aren't needed when viewing through a telescope, since the focusser will achieve the required correction. If the lenses correct for astigmatism then this is something the telescope can't correct for, though if the astigmatism is slight it might not be noticeable. I wear spectacles which correct for short sight and astigmatism, and prefer to use the telescope without them.

Given the better light transmission when not wearing lenses, and the possibility of inferior optics in the lenses, it's not surprising that you might get a better view without contact lenses, especially if you have a fairly mild prescription.

[Carbon Brush]

Acey is right that long/short sight just means moving the focusser a little.

Astigmatism is a more problematic subject. Depending on the contact lens type, you may have to accept compromises.

A spectacle lens can be made with the amount and angle of the astig correction as close as the optician can measure it. Thus giving very good correction.

With contact lenses you can get astig correction. But not always to the angle that suits you. I have been given monthly lenses with the correct astig correction. But offered daily lenses with the angle somewhat off as that was all that was available in the type.

You can check for astigmatism by going to high magnification and taking a star through focus. If it goes round-point-round, then all is fine. If it goes oval - smaller -oval different axis, then there is astig in there somewhere. Either scope, or eyepiece or you. By rotating the eyepiece through 90deg, or changing eyepiece, you can take this out of the equation. If you can rotate the OTA through 90deg, you can see if the pattern changes. Changes you see with/without contact lens with reference to your prescription indicate correct lens (or not). Not quick or easy. sorry.

We are of course assuming here that the lens you have been given is actually the right one for your correction requirement. Over years I have rejected several spectacle lenses because of the optician's errors, or over-enthusuiasm. Then there was the manufacure defect, but that's another story.

[Damo636]

Cheers guys, at least now i know its not my imagination. David, i am short sighted and my prescription is -3.75 and i don't have any astigmatism that i am aware of. The contacts are the soft breathable type.

[Damo636]

I also wear the monthly disposable type and have a check up every 6 months. I am due another visit next month and will have a good old chin wag with the optician about this.

[Carbon Brush]

A -3.75 correction without astigmatism will only cause a problem if you have limited focusser travel. But it will be obvious.

Might be interesting to compare new lens and month-old lens performance. In handling, I can tell that an old lens has hardened. This suggests it may not have the same correction. Also an old lens can get a bit of scratch type damage, or become clouded by cleaning fluids.

In daylight our brains fill in a lot of visual defects. Looking at a very specific target and looking for fine detail, you will pick up on any optical problems.

You might find your optician offers you a freebie different lens or solution to try if you are reporting a trasparency issue.

I have a similar distance correction, but also some astig. I find best performance from a newish contact lens. No correction at all leaves a bit of astig, which only becomes an issue at high mag. Using specs means stray light gets in and I have to position the varifocal lens in exactly the right place.

Hope there is something useful in there.

[Damo636]

Thanks David, i'll try out a new set tonight if the weather permits and see does it make any difference. Where i work is quite dusty and as you say the lens could be degraded. If only i hadn't sat so close to the tv as a kid