Should I use my telescope with or without my glasses? It’s a dilemma most eyeglass wearers have when using a telescope. The answer is it depends on your need for glasses or and if you have an astigmatism that causes distortions. I cover what you should know about both…using a telescope with glasses and using a telescope with astigmatism.
Can you look through a telescope with glasses?
If you are far-sighted or near-sighted, you don’t need to wear eyeglasses when using a telescope. You simply adjust the focus.
If you have astigmatism, adjusting the focus mightn’t provide the cylindrical correction that you need and so you may need to compensate with eyewear (or not) as covered in this article.
So, if your eyewear only corrects for spherical power, i.e. nearsightedness or farsightedness, there’s no need to be using a telescope with glasses because you can compensate for these by simply adjusting the focus.
This may not be as simple a solution if you have astigmatism.
What is astigmatism?
Astigmatism is when your eye shape deviates from what is considered the normal spherical curvature. Rather than a round ball, it’s shaped more like a football.
With astigmatism, what you see is blurry or distorted because the deviated shape prevents light rays from meeting at a common focus. So what you see is out of focus.
There are a few types of astigmatism and it is estimated that about a third of the population has some degree astigmatism.1
Sometimes you can get away without using eyewear because your eye muscles compensate for the distortion. I managed to do this for many years…no longer.
Using a telescope with astigmatism
If you have a fair amount of astigmatism, you will need your prescribed eyewear when using a telescope (or binoculars)…generally speaking.
There are exceptions and limits.
Where the exit pupil of the telescope is small and the light entering your eye tiny, you might find negligible irregularities from your astigmatism.
You might get away with no eyewear and the less-expensive shorter eye relief types for eyepieces up to a certain focal length limit.
Astigmatism and eyewear-free limit
To find your eye-wear free limit, you can try this technique. You increase the power of your eyepieces from low to high (long to short in length) one by one using individual eyepieces or you can use a Barlow lens or a combination of these.
With your eyepiece in place, look at a bright star centered in your view, and see if there is any distortion. If there is no distortion, try the next eyepiece and repeat until you find the limit of focal length…where distortion occurs.
Alternatively, you can use the one eyepiece (with the longest FL = largest exit pupil) and mask the telescope’s aperture with a series of paper circular cut-outs to reduce the telescope’s focal ratio in increments… theoretically reducing the exit pupil in size by 1/2, 1/4, etc.
For example, if you have an f/7 telescope and the eyepiece FL is 42 mm, you have a 6 mm exit pupil (42÷7=6).
Reducing the opening to ¾ of the aperture will yield a 4.5 mm exit pupil. An opening that’s ½ of the telescope’s aperture will yield a 3 mm exit pupil, and one that’s ¼ of the aperture will yield a 1.5 mm exit pupil.
The trick is to find the largest exit pupil that offers no interference from astigmatism. Exit pupil is the round frame of light emitted from the telescope eyepiece (see image).
The exit pupil at this point indicates the upper eyepiece focal length limit you can use without glasses.
- Eyepiece focal length = Exit pupil x telescope focal ratio
For example, if a 3 mm exit pupil is the limit without distortion, and you have a telescope with a f/7, a 21 mm eyepiece is your upper focal length limit without glasses. That is, with an eyepiece longer than 21 mm you’ll need to wear eyewear for best views.
Using a telescope with Glasses or contact lenses
Rather than glasses, some folk opt to wear contact lenses that correct for astigmatism while using a telescope or binoculars.
Using contact lenses with optical instruments, such as a telescope or binoculars, will depend on your degree or type of astigmatism and how satisfied you are with wearing contacts.
Some users with enough astigmatism swear that contact lenses have dramatically improved their experience of using binoculars and telescope.
The downside of wearing contacts is that your eyes can feel tired at the end of the day. If you intend viewing at night, an option is to take them out a few hours before dark and wear glasses until its time for viewing the night sky.
If you’d rather wear glasses than contact lenses or prefer to go without eyewear, read on.
It’s worth noting that wearing glasses can cause extra light loss or scattering and unwanted reflections! Eyepieces with long eye relief help with this.
If you wear glasses, eyepieces with long eye relief might suit you better as they are designed to give you more comfort when using a telescope.
Having a coating on your eyeglasses to reduce reflection and flaring is also a good idea.
Long eye relief eyepieces
If you prefer or need to wear glasses, eyepieces with long eye relief of at least 15 mm are meant for eyeglass wearers, to give comfort while viewing.
You might find these eyepieces more expensive than the standard type. They are also larger and weightier.
To save on costs, one option for eyeglass wearers is to incorporate a Barlow lens with eyepieces fitted for long eye relief and designed for low power (longer focal lengths).
An added Barlow lens to these will increase the power while retaining the comfort of long eye relief with fewer eyepieces needed.
Eye relief starts at 10 mm. For eyeglass wearers look for at least 15 mm. Though, many choose 20 mm for a comfortable fit.
About eye relief
This applies to telescopes, binoculars, and microscopes.
The instrument’s eye relief is the distance (mm) between your eye and the instrument’s eyepiece that gives a full field of view comfortably.
The measure is taken from the eyepiece’s last eye lens surface to the plane where all the light rays of the exit pupil come to a focus and the image is formed.
Is it your eyes or the eyepiece?
Most people know it if they have astigmatism. Without correction, astigmatism will make stars appear spiky or fuzzy around the edges.
Then again, you may not know you have it, if is not major or if your eye muscles compensate for it.
If you are not sure whether the distortion you see through the eyepiece is because you have astigmatism or because your eyepiece is faulty or instrument needs collimating, you can do the following…
To find out if the distortion is because of your eyes here is a simple test.
- View a bright night sky object with the lowest-power eyepiece in your telescope (to give you the largest exit pupil).
- Rotate your head and see if any irregularities move with your change in direction.
If this happens, you have astigmatism or another eyesight condition affecting the view rather than a faulty eyepiece or telescope.
If you do have astigmatism, you are likely to see irregularities around the perimeter of the object your are viewing. And, you’ll see this more so with the lowest power eyepieces, the largest exit pupils.
Small exit pupils won’t necessarily reveal eyesight astigmatism and this is why users sometimes think that the problem is the eyepiece without considering their eyesight.
Using telescope with glasses
Some eyepieces are better than others when it comes to using a telescope with glasses.
Recommended eyepiece for eyeglass wearers
If you are telescope viewing with glasses and want a good eyepiece, then consider the Explore Scientific 92º 17 mm (also available in 12 mm).
It has long eye relief (20 mm), is lower in price than the top-rated, but users claim it is of good quality with excellent ergonomics. It is weightier than short eye relief eyepieces, but that is to be expected.
You can get this at Amazon.com — See details.
Summary on telescope viewing with glasses
There’s no need to wear glasses if you have them for farsightedness or nearsightedness.
However, you may need to wear them if you have astigmatism.
So the options for using a telescope if you suffer astigmatism are to wear eyeglasses or toric contacts (or otherwise use TV Dioptrx correctors).
Long eye relief eyepieces are best if you need to wear glasses.
- Jenean Carlton. 2013. Basic and Beyond: Astigmatism. Eyecare Business.