Should I use my telescope with or without glasses? What if I have astigmatism? Is it my eyes or is it the eyepiece? In this article, we cover how best to use a telescope if you have eyesight conditions including astigmatism.
If your eyewear only corrects for spherical power, i.e. nearsightedness or farsightedness, you can use your telescope without glasses because you can compensate by adjusting the focus.
This is not so simple with astigmatism.
Focusing the instrument’s eyepiece will not compensate for astigmatism because this vision condition requires cylindrical correction.
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 1/3 of the population has some degree astigmatism. 1
Using A Telescope With Astigmatism
But…wearing glasses can cause extra light loss or scattering and unwanted reflections!
Using Eyeware Or Not
If you have a fair amount of astigmatism, you will need to wear your eyeglasses, when using a telescope (or binoculars).
There are exceptions.
When using a telescope to view planets (like Jupiter) or objects that are distant at very high magnifications, you mightn’t need glasses because the exit pupil of the telescope is small and the light entering your eye so tiny that interference from astigmatism is negligible.
If you wear glasses, for comfort choose eyepieces designed for long eye relief.
You could also look at getting a coating on your eyeglasses that will reduce reflection and flaring.
Let’s have a look at eyeglass-free if you have astigmatism.
Using A Telescope With Contact Lenses
Rather than glasses, you can wear contact lenses that correct for astigmatism while using a telescope or binoculars.
In general, using contact lenses with optical instruments, such as a telescope or binoculars, is really up to you, the user, and this might depend on your degree 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.
Long Eye Relief Eyepieces
If you prefer to wear glasses, look for eyepieces with long eye relief (at least 15 mm) so you can use the instrument comfortably.
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.
Astigmatism And Eyewear-free
You might get away with no eyewear and less expensive shorter eye relief types for eyepieces down to a certain power.
To work this out, you will need to find the largest exit pupil that does not reveal astigmatism. This will give you the longest focal length eyepiece you can use without wearing eyeglasses.
(Remember: The longer the eyepiece focal length the lower the power.)
So, these eyepieces will be in the high power end of the range.
Choosing Eyeglass-free Eyepieces
One way to work out the eyepieces you can use without wearing glasses is to increase the power of your eyepieces (from low to high) one by one (using individual eyepieces or by way of a Barlow lens) until you find the point where the image has no irregularities.
Of course, you’ll need to have a number of eyepieces on hand to do the above suggestion.
Another option is to use only 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.
You can do this by masking the aperture so it is theoretically reduced in size by 1/2, 1/4, etc.
For example, if you have an f/7 telescope and the eyepiece FL is 42mm, you have a 6mm exit pupil.
Then by using the paper cut-outs to mask the aperture, you can half the scope’s aperture and then half that again so the opening is ¼ of the scope’s aperture.
Making a mask with an opening ½ the telescope’s aperture yields a 3mm exit pupil; a mask with an opening yields 1.5mm, etc.
About Exit Pupil
Exit pupil= eyepiece focal length ÷ telescope focal ratio
This dark-adapted dilation can range from 5 mm to 9 mm, the expected maximum for a human eye, with the amount reducing with age.
I cover more about exit pupil in my article on choosing binoculars for astronomy.
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 your view, rather than in the center. And, you’ll see this more so with the lowest power eyepieces, resulting in 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.
Recommended Eyepiece For EyeGlass Wearers
If you wear glasses and are looking for a good eyepiece check out the Explore Scientific 92º 17 mm (also available in 12 mm).
It has long eye relief (20 mm). It is quality at lower price than the top rated and its ergonomics is excellent. It is weightier than short eye relief eyepieces, but that is to be expected.
You can get this at Amazon.com — See details.
So the options for using a telescope if you suffer astigmatism are to wear eyeglasses or toric contacts, or use TV Dioptrx correctors.
- Jenean Carlton. 2013. Basic and Beyond: Astigmatism. Eyecare Business