Lecture 11 - Telescopes 3

Question 1

3 Methods for compensating ametropia:

Answer 1

1. full correction for refractive error behind eyepiece (Fe)
2. partial correction for refractive error over objective (Fo) (not practical)
3. Focusing: changing the separation of Fe and Fo

Question 2

1. full correction for refractive error behind eyepiece (Fe)

Answer 2

• Simplest method: telescope clipped on to, or held over, spectacle correction
• Or correcting lens can be fitted into holder behind eyepiece if required :
• No effect on magnication because telescope still afocal (see Figure 30

Question 3

2. Partial correction for refractive error over objective

Answer 3

• Use partial correction to achieve some divergence or convergence of light entering telescope, which would then be amplified by telescope to the required amount
• Complex calculation to work out effect on magnification
• Impractical, and never attempted

Question 4

3. Changing the seperation of Fe and Fo

Answer 4

• shorten to correct myopia and lengthen for hypermetropia
• length change needed obviously depends on Rx
• effect on magnification

Question 5

Summary of management with Myopia vs Hypermetropia:

Answer 5

• Myopia:
- Galilean: Mtel Higher either correction lens behind eyepiece
- Astronomical: Mtel higher with focusing telescope (shorten)

• Hypermetropia:
- Galilean: Mtel Higher with focusing telescope
- Astronomical: Mtel higher with correction lens behind eyepiece

Practical points:
• small Rx → small change
• significant cyl (>2DC)→ behind eyepiece
• removing specs: short BVD → wider FoV

Question 6

General Types of telescopes:

Answer 6

• Generally 3 types: Hand held, Clip on and Spectacle mounted. You could further distinguish monocular versus binocular telescopes and focusable v fixed focus devices.

Question 7

Special variants of Telescopes:

Answer 7

• Autofocus
• Contact lens telescope (with CL or with IOL)
• Intra-ocular lens telescopes
• IOL VIP system - combination of negative and positive IOL
• BTLT - Behind the Lens Telescopes

• Bioptic or BITA (Bi-level Telemicroscopic Apparatus)
- For patients who fulfil certain Visual Field and Acuity requirements driving with BiOptics is legitimate in some States in the US
- Driving with BiOptics is not legal in the UK hence there is limited interest in the technology

Question 8

Bioptic lens:

Answer 8

• A BiOptic is a lins system with a telescope attached to a pair of glasses, above one’s normal line of sight.
• Allows a trained user the opportunity to detect objects or movement within his/her driving scene using the wide field of view available through the regular spectacle lens and to resolve fine details such as road signs and traffic lights by glancing briefly and intermittently into and out of the miniature telescopic unit.

Question 9

Contact lens telescopes:

Answer 9

• Galilean system
- Eyepiece created by (high-powered) negative contact lens
- Objective created by (high-powered) positve spectacle lens
- Vertex distance of spectacles is equal to length of telescope (sum of focal lengths)
- Field of view better than with conventional telescope because objective & exit pupil close to eye, but the FoV also depends on the diameter of the objective lens (use blended aspheric to avoid ring scotoma)
• Rarely used because of the many practical and cosmetic disadvantages

Question 10

Fitting of CL telescopes:

Answer 10

• Check that conventional telescope improves acuity as expected
• Check nystagmat for oscillopsia
• Fit contact lens, maximise BVD of trial frame, perform over-refraction
• Contact lens must be stable!
• Vergence amplification means separate reading specs are required
• Uncorrected ametropia changes the effective eyepiece power
- + 10 hypermetrope with -30 contact lens, effective eyepiece is -40
- -10 myope with -30 contact lens, effective eyepiece is -20

Question 11

Advantages of Cl Telescope:

Answer 11

• In theory patients would be allowed to drive under UK law if VA and field requirements were met
• Can get acuity much better than expected in congenital nystagmus
• Intra-ocular lens could be used instead of contact lens giving a longer vertex distance so better magnification but lOL can’t be changed if acuity worsens
• Wide FOV

Question 12

Disadvantages of Cl Telescope:

Answer 12

• Low magnification, so only works for moderate acuity loss
• Patient needs to be adapted to contact lens wear first
• cL difficult to insert-poor near acuity with highly negative lens power
• Need to wear system regularly for prolonged periods - Adaptation to spatial distortion required
• Poor cosmesis, specs with long BVD and high plus lenses

Question 13

IMT Lens disadvantages:

Answer 13

• Inflammatory deposits on device
• Pigments on device
• Guttae
• Posterior synechie
• Iris transillumination defects > 21 days
• Iritis > 30 days

Question 14

Verification of telescopes:

Answer 14

If the parameters of a telescope are unknown, its type and magnification can be estimated by direct observation or by locating and measuring the size of the entrance/exit pupil.

• Determination of Magnifying Power
1. Direct Comparison
2. Estimating based on exit pupil measurements

• Determination of telescope type by exit pupil method

Question 15

1. Direct comparison:

Answer 15

• The magnified view seen through the telescope with can be compared with the non-magnified view seen by fellow eye.
• Use tile pattern etc. as target (Figure 39)

Question 16

2. Estimating based on exit pupil measurements

Answer 16

• Measure size of exit pupil e.g. using mm scale on band magnifier (Figure 40)
• Measure size of objective lens (entrance pupil)
• M = entrance pupil/Exit pupil

Question 17

Determination of telescope type by exit pupil method:

Answer 17

• In general, Keplerian (astronomical) telescopes are longer than Galilean telescopes, but the use of erecting systems which incorporate mirrors or prisms may be misleading.
• The exit pupil movement can be used to distinguish telescope types: Looking into the eyepiece observe the apparent movement of the exit pupil while moving the eyepiece lens from side to side:
1. Astronomical gives Against
2. Galilean gives with

Question 18

Telescopic refraction with telescopes:

Answer 18

• When viewing distant object : Uncorrected myope will shorten length, while uncorrected hypermetrope will increase tube length.

Question 19

When can calibrated telescopes be best used to determine spherical refractive errors:

Answer 19

• may be applicable to populations in under-served areas where electricity and modern equipment are not available.
• alternative subjective refraction method in low vision population → patients can be refracted at a ‘normal’ 6m testing distance.
• may be applicable to patients with extreme Rx e.g >25 DS and/or patients where retinoscopy or autorefraction is difficult/inaccurate due to media opacities or extremely small pupils and a starting point for subjective refraction is needed