Correcting lens add-on, where to buy?

[bojan]

Gets,
Does anybody use add-on corrective lens on their eyepieces?
Idea is to get (cheap) plastic lens blank (with appropriate optical properties - astigmatism for example) and then machine it to required diameter and attach to eyepiece.
I tried Alibaba and ebay, but not much luck there so far - obviously those items are not very popular, or I just didn't use the right keyword for search.

Where do opticians buy the material for prescribed glasses?
Thanks in advance for reply :-)


Edit:
Something like this.. or this (not quite what I had in mind but very close)

[JA]

Hi Bojan,

Would the lenses from reading glasses typically anything from +1.5 to 4 available at pharmacies be of any use? I have a few such lenses salvaged from broken frames if you need them.

Best
JA

[bojan]

Hi JA,

Yes, of course :-)
I also have tens of them (broken frames but lenses are OK) , however they are a bit difficult to center in the lathe.

Blanks (like on links) are already centered, so I though to try with them first.

Actually, my first reason for question posted here was, I have an old scope finder with cross hairs, and they can't be focused independently but with glasses (+~2) they are OK with my eyes.

[JA]

Hi B,

Maybe a simple close up lens used for macrophotography would work. they are available from +1 to several dioptres. Since you will be using them with eyepieces you might want them in smaller diameters, which is typically 37mm at the low end. You could then perhaps just use the 37mm lens with its own thread and then 3D print an adapter to suit you eyepiece if the eyerelief distances and magnification suit. It would be an experiment. Here are some links for eBay 37mm close up lenses....

Best
JA

https://www.ebay.com.au/itm/28497368...Bk9SR_bHjITLYQ

https://www.ebay.com.au/itm/38541896...Bk9SR_bHjITLYQ

[bojan]

Originally Posted by JA

Hi B,

Maybe a simple close up lens used for macrophotography would work...

Excellent advice, thank you!

[By.Jove (Jove)]

Originally Posted by bojan

Gets,
Where do opticians buy the material for prescribed glasses?

Bojan,

Suppliers like Hoya and Zeiss have catalogs of standard circular lens blanks spanning the whole range of whatever an optician might require. These are circular, approx 75mm diameter, big enough to accommodate being cut down to suit any frame. What the cheap opticians do is buy the appopriate blanks, cut to size and fit to the frame. The inter-pupil-distance (IPD) is also taken into account to determine where the lens centres should be as this varies quite widely from person to person.

The cheap $25 specs from chemists only deal with diopters (plus or minus) and optically are quite poor; since in daylight the iris of the eye is around 1mm aperture and working at f/15, there is no real need to attempt to correct for anything else.

However your post implies you want to correct for astigmatism, that's a whole different ball-game ...

a) chemists dont stock anything that corrects for astigmatism, and
b) the likes of Hoya or Zeiss will not sell lens blanks to an individual, only a qualified registered optician they recognise.

That leaves the practical alternatives as:

1. Cannibalise the lenses from a pair of old specs you have that have the required astigmatism correction, or

2. Buy a Televue Dioptrix - the only commercial astigmatism corrector you (or I) can actually buy.

NB I have "been there done this before" asked my optometrists before (they know I am into astro) and was given some blanks to play with from Hoya, and have discussed what's actually available from them - at the time I was aiming to make a really big RDF instead of the miserable little things we see. The blanks I got were OK for specs, but totally hopeless for astro. I ended up using a glass meniscus lens instead, from AliExpress.

[bojan]

Hi Jove, thank you for your reply..
My immediate intention is simple +2 (for me, I am just far-sighted), but one of my friends has astigmatism, so the solution to the issue should be inclusive..
I am now trying to determine the optical centre of one of my broken reading glasses lenses (I will try with laser by rolling the holder and lens lightly glued to it so I can adjust the centre as required)..

Then, I will machine it like this...

[bojan]

Gluing of polycarbonate proved to be tricky ( I tried to use "white glue" because it is water-based, not to damage plastic with solvent vapors. bu it doesn't stick hard enough).. so I ended with good, old grinder).

Result is shown in the attached image below.. quite good. I don't have to use glasses when aiming scope any more.

[hamishbarker]

you could make up a prescription and order a pair of glasses from zenni.com, selecting the old-style round wire frames to get nice round lenses for your use. zenni are quite cheap (US$25 incl. prescription and anti-reflection, plus 8-15$ for other special things like oil resistant (fingerprints) or anti-fog (might be interesting to see if it reduces dew?). you could get your exact prescription and have a pair of lenses, one for your finder and say one for a slip-on additional lens for eyepieces if you made an appropriate sleeve. I have ordered from them previously for my actual glasses (both single vision and bifocals). Don't know what the centering would be on these lenses but presumably it could be centred.

https://www.zennioptical.com/p/stain...1?skuId=450114

[bojan]

Originally Posted by hamishbarker

you could make up a prescription and order a pair of glasses from zenni.com,...

Thanks for that, I am OK now but this will be interesting for someone with astigmatism (I am far-sighted only, old age..).

[Boozlefoot]

Bojan, Try search in Aliexpress for "test lens kit", then follow various offers. 38mm dia round lenses in various magnitudes, can be had individually - about $2.80 each, and another couple of bucks postage.

https://www.aliexpress.com/item/1005...d=HdtLlN9TtlOz

[glend (Glen)]

Just switch to EAA and watch the view on your screen, no need for EPs, wear your normal glasses etc.