LV: Compensating Telescopes for Ametropia

Again there are three methods for this

1. Full correction for refractive error behind eyepiece

This is the simplest method in which the telescope is clipped onto or held over the spec correction. You could also fit a correcting lens into a holder behind the eyepiece if required. This method doesn't have any effect on the magnification of the system because the telescope is still afocal (unmodified).

2. Partial correction for Rx over objective lens

This achieves some divergence or convergence of light entering the telescope which is then amplified by the telescope to give the correct amount. This is complex to work out and rarely attempted.

3. HERE WE GO MORE F'IN CALCULATIONS: Changing the separation of Fe and Fo

• Shorten the scope to correct myopia and lengthen for hypermetropia - how much depends on Rx
• Changing the length has an effect on magnification

Astro Example

Work out the magnification and length of a 3x astro (Fe +60, Fo +20) used by a -10.00 myope with the correction behind the eyepiece. NB With a myope you want divergent light coming out of the eyepiece which can then bend into the patient's eye thru the specs

a. The magnification is as for an emmetropic user as the telescope is afocal so = -Fe/Fo = -3x
b. t = fo' + fe' = 50mm + 16.7mm = 66.7mm

Ok that was easy, but what about if the guy was uncorrected and the telescope was focussed to compensate?

t = fo' + fe' = 1/20 + 1/60-(-10) = 64mm

it's like a part of the Fe power has been borrowed to correct the ametropia leaving it as +70D
So to correct for myopia you need to decrease the sep between the lenses w/Astro
And to correct for hyperopia you need to increase the sep between the lenses w/Astro
the Fe in that case will go down to +50D and the length will increase to 70mm.

Galilean Example

3x Fe = -60, Fo = +20 fixed for -10 myope

t = fe' + fo' = 1/20 + 1/(-60+10) = 0.05 - 0.02 = 0.03m = 30mm
Mtel = Fe / Fo = 50/20 = -2.5x

Conclusion

Myopia

• Higher mag with focussing astro
  
• Increase length of astro telescope to correct
• Higher mag with correction behind eyepiece for Galilean
• Decrease length of Gal to correct

Hypermetropia

• Higher mag with corr.behind eyepiece with astro
• Decrease length of astro to correct
• Higher mag with focussing Gal
• Increase length of Gal to correct

These egs used large ametropias so effect less pronounced usually. Any big cyls >2.00 will have to be corrected behind the eyepiece. Removing spec correction = shorter v.d. and wider field of view but if the scope is going to be used for spotting then it may not be practical to remove specs. Telescopes are mainly used for near vision due to the increase in working space which is usually needed for manipulative tasks like writing or sewing or some ting. The problem is the fov is far smaller so this counteracts that whole thing a bit.