Telescopes

Issues of Concern

A telescope is made up of an objective and eyepieces. The objective gathers light from objects to form an image at its focal point. The size and quality of the objective lens are major determining factors for image clarity. The function of the eyepiece is to magnify the image that the objective lens forms. The eyepiece's magnification and the telescope's focal length allow the overall magnification possible by a particular telescope.

The telescopes can be classified into types based on the following:

• Eyepiece 
• Orientation: clip-on, handheld or spectacle-mounted
• Monocular or Bilateral
• Focus: fixed focus, autofocus, or focusable

Types of Telescopes

Galilean Telescope

The Galilean telescope has 2 lenses: the objective lens (closer to the object of regard) and the eyepiece (closer to the eye).[7] The convex lens converges the ray of light that enters the system. The eyepiece in a Galilean telescope is a concave (minus) lens positioned closer to the focal length of the objective lens to ensure that the converging rays diverge when exiting the system.

The focal lengths of the objective and eyepiece determine the distances between them to produce a real and erect image. The Galilean telescope is cheaper and lighter than the Keplerian telescope and thus tends to be the first choice for children. In cases where the individual has lost peripheral vision, the lens order is designed in reverse form (ie, the objective lens is placed closer to the eye to provide a wider field).

The telescope length is determined by the focal lengths of the lenses that are added. Given that the eyepiece is a minus lens, the telescope length can be calculated as:

t = f_{obj -} f_eye,

where t= telescope length, f_obj = focal length of the objective lens, and f_eye = focal length of the eyepiece.

Keplerian Telescope 

This is also known as an astronomical telescope. This instrument uses 2 convex (plus) lenses in which the objective has the lesser dioptric power. The first lens converges the light so that the rays diverge when they exceed the focal length of the first lens, which is the objective lens in the Keplerian telescope.[8] The second convex lens converges the rays that diverge after passing the focal length of the objective lens. When exiting the entire system, the rays become parallel. The sum of their focal lengths determines the distance between the lenses and produces a real and inverted image. This requires a prism to reverse the image, which leads to longer and heavier instruments. This type of telescope has a greater visual field and superior optical quality than the Galilean type, making this instrument much more expensive.

The length of a Keplerian telescope is calculated by the sum of the foci lengths of the objective and eyepiece lenses:[9]

t = f_obj + f_eye 

Focusable and Afocusable Telescopes

Focusable telescopes are telescopes that can be adjusted to focus on varying distances. These types of telescopes have applications in photography, medicine, and astronomy.[10][11] Afocusable telescopes have a fixed focus. A fixed-focus telescope can be considered for children, especially those with dyspraxia. A focusable telescope reaches far, near, and intermediate distances, making them the type usually preferred by children. The autofocus telescopes weigh more and are not usually given to children. The lenses must be of high power to limit the length of telescopes. The higher the power, the greater the aberration and the smaller the field of view.

Clinical Significance

Telescopes are excellent optical instruments for improving the quality of life for individuals with poor vision. They can help correct near, intermediate, and long-distance visual deficits. Telescopic and spectacle corrections improve students' contrast sensitivity, visual acuity, and reading rates for those with oculocutaneous albinism.[12]

Nursing, Allied Health, and Interprofessional Team Interventions

Telescope Enlargement

The enlargement given by the telescope or a magnifier can be calculated by dividing the image's size by the object's size. Given that the value of the angle subtended gives the size, the following formula can be used:

Enlargement = angle subtended by Image / Angle subtended by the object.

Regardless of whether a telescope is Keplerian or Galilean, the following formula can be used for afocal telescopes:

Enlargement = -F_eyepiece / F_object 

The power of the eyepiece must be greater than the objective lens to produce magnification.

To minimize aberrations and reach 12x magnification, astronomical telescopes can have up to 4 components. These telescopes have the negative effect of producing a loss in image clarity and tend to be heavy. The same cannot be said for the Galilean telescope, which is limited to only 3x distance enlargement, with a significantly reduced image quality with higher enlargements.

Concept of Exit Pupil

The exit pupil can be thought of as the circle of light seen when one looks through the telescope's eyepiece and is the image of the objective lens seen through the eyepiece. The diameter of the objective lens/eyepiece magnification gives the size of the exit pupil.

The exit pupil mustn't be too large so light is not wasted. The image clarity is not maximized, especially in bright illumination. The same can be said in dim illumination when the exit pupil is smaller, and the eye doesn't receive as much light. Ideally, the exit pupil in the telescope matches or is slightly smaller than the pupil size in different lighting scenarios for the best viewing. 

Vergence

In telescopes, the emergent vergence of light is given by the formula:

V = E²I / (1 - oEI),

where E is the enlargement of the telescope, I is the actual incidence of light, and o is the optical path length of the telescope. The formula can be simplified as V = E²I. 

Given the above formula, if a distance telescope is to be used to view near work, the accommodation required (V) would be given by the multiplication of the expected accommodation (I) and enlargement squared (E²). This influences the maximum distance an object can be moved without initiating blur.

Refractive Error Compensation

Refractive error can be compensated for by:

• The addition of the total refractive correction to the eyepiece lens or instructing the individual to use the telescope while putting on their glasses.
• Lengthening or shortening the telescope as needed. Myopes can shorten the telescope length, diverging the parallel rays and leading to less enlargement in a Galilean telescope and more in a Keplerian telescope. In comparison, the hyperope can lengthen the telescope length, which results in convergence of the parallel rays that would lead to more enlargement in a Galilean telescope or a reduced enlargement when using a Keplerian telescope.

Consideration when Using Telescopes for Near Work

When employing telescopes for near work, there are a couple of techniques that should be considered to get the best visual outcomes, which include:

1. Adding refractive correction to the eyepiece: The divergent light from the objective lens is amplified when it goes through the telescope. This is due to the amplification of divergent light. A powerful positive lens can be placed over the eyepiece or worn in a spectacle frame to improve near vision. This is practical only if the patient already uses a high-powered spectacle magnifier, which would ultimately increase the working distance.
2. Telescope length alteration: When the telescope is used for near work, the telescope length can be increased. This is dependent on the length of the telescope. The Astronomical telescope allows for a greater range of focus when compared to a Galilean telescope. The lengthening of the telescopes (Astronomical and Galilean) only affects the objective lens. The telescope's enlargement and length are said to have been altered in the same fashion.
3. Adding a lens cap to the objective: When an object is at a distance of about 40 cm, neutralization of the divergence of this object's working distance can be obtained by adding a +2.50D lens, which keeps the telescope afocal. When a reading cap is added to the telescope, it is known as a telemicroscope. The enlargement produced by this system is the product of its components:

Enlargement = Telescope Enlargement x (Lens Cap Power/4)

A telemicroscope can allow the working distance to be larger relative to adding a plus lens, which gives the same enlargement as the telemicroscope. This leads to the formula WDT = ET x WDP, where WDT is the working distance of the telemicroscope, ET is an enlargement of the telemicroscope, and WDP is the working distance of the addition plus lens. Additionally, a telescope's length is greater than a plus lens addition, which further increases the working distance.

Prescribing a Telescope

When prescribing a telescope, certain factors should be taken into consideration, which include:

• Patient goals: The patient's desired visual acuity influences the power of the prescribed telescope.

The power of the telescope can be calculated as:

Power = Best-corrected Visual Acuity/Goal Visual Acuity. 

If the patient's Best-corrected Visual Acuity is 6/36 and the goal Visual Acuity is 6/18, then the power = 36/18 = 2X. One must remember that as power increases, the visual field also decreases; opting for a higher power is not always the best option.

The intended use of the telescope plays a vital role in the type of telescope that should be prescribed. Assuming the patient wants to use the object for terrestrial viewing, it would be advisable to give the Galilean telescope as its reduced magnification would display the object as it appears and is more suited for tracking moving objects. At the same time, giving an astronomical telescope to an individual who intends to use it for viewing astronomical objects would be advisable.

• Nature of patient's condition: If the patient's condition is progressive, it would make sense to consider giving a slightly stronger telescope so that it would continue to be adequate as the vision gradually reduces, especially given the expense of telescopes.

Patients with conditions such as Stargardt's disease, retinitis pigmentosa, glaucoma, and age-related macula degeneration (ARMD) would benefit from bioptic telescopes attached to the lens to improve their distance vision.[13][14][15][16][17] Digital telescopic aids, which use screens or cameras to enlarge objects and provide magnification, can also be considered a viable option for these patients.[18][19]

• Age: The patient's age plays a strong role in the type of telescope the clinician should consider. Telescopes requiring less coordination and not too heavy would be better suited for children.

• Refractive error: The refractive error plays a role in the recommendations given to the patients who use the telescopes. Myopes usually need to shorten the telescope length. Hyperopes tend to lengthen the telescope to create clear images.

Nursing, Allied Health, and Interprofessional Team Monitoring

Telescopes are specialized devices and are different from regular glasses. Patients need appropriate education to use them properly and to achieve the best possible outcomes. Eye care practitioners and health care staff need to be competent in educating patients about using telescopes. Furthermore, most people who require telescopes fall into the spectrum of people with low vision. These patients have altered vision due to different causes, from acquired to inherited pathologies. Some of these conditions can be progressive. Healthcare professionals must be trained on how to manage the emotional stress of patients with debilitating ocular disease, in addition to possessing adequate skill in the area of visual rehabilitation.[20]