Astigmatism of the eye

Canopus Lim · Post by **Canopus Lim** » Thu Jan 12, 2006 11:01 pm

I have been taking note at the shape of the stars using my telescope and have found out a big problem with my eyes. I wear glasses that correct for short sightedness and astigmatism but experimenting with my scopes and different eyepiece combination I come to the realisation that my astigmatism is not fully corrected by my glasses. At first I thought it got something to do with my eyepieces (those long focal length ones) but I realised that it is my spectacles and my own eyesight at fault.

I went for another eye check and the prescription the optician says is the same and that my eye did not grow worse. However, it is definitely not the case as astigmatism is really so evident.

I will explain clearly how to identify astigmatism of the eye through using the telescope. Perhaps this will enable others to identify if they have astigmatism that is not corrected or in the first place for those who do not even know they have astigmatism.

The best way to test if astigmatism is there is through the star test. The amount of astigmatism present in the telescope system due to your eyes is directly related to the exit pupil. The exit pupil can be calculated: focal length of eyepiece (in mm)/ f ratio. A F/6 scope and 30mm eyepiece will give a exit pupil of 5mm in diameter. Firstly, astigmatism impact on what you see increases with increasing exit pupil size. The reason is the exit pupil illuminates a bigger area of the eye and hence the optical aberrations (errors. In this case astigmatism) in the eye becomes more pronounced. You can try this at home. Get a telescope combination to enable as big exit pupil as possible (about 6 to 7mm.) Point your scope at the star and centre it in the eyepiece. If you do it at home, it is highly possible that your pupil has not fully dilated to 6 to 7mm. However, it still can be used to test. Defocus the star that is centred, you will see the defocused rings. If you cannot see the entire defocused star (seeing only some part of it), move your head around and actually you can tell about how large is your pupil dilated to (by comparing how much of the defocused star you can see) Observe the defocused star carefully, is it CIRCLE IN SHAPE? OR ELLIPTICAL? If it is elliptical, rotate your eyepiece in the focuser, if the elliptical shape does not rotate, it means that the astigmatism is not due to your eyepiece. To confirm if your eye has astigmatism, rack the inside and outside focus of the star and if the elliptical shape IS ROTATED 90 DEGREE WRT TO EACH OTHER when you rack the focuser in and out of focus it means that your eye has astigmatism. DO MAKE SURE THAT THE STAR IS AT THE CENTRE OF FIELD AS EYEPIECES DO HAVE THEIR OWN ASTIGMATISM BUT IT IS NOT PRESENT AT THE CENTRE OF FIELD.

A good test is to test the star without spectacles and with spectacles and observe the difference. I found that my spectacle is most likely undercorrected for astigmatism; it does reduce the astigmatism though. Also, interestingly, the elliptical shape follows my astigmatism axis. This is true if you are viewing the star upside down or upright. Also I found out that my dob has an overcorrected mirror.

Lastly what is the effect of astigmatism on the stars? Basically, the stars will look hairly and elongated. That means the star is supposed to look like a circle but now looks like Saturn. Also for slight astigmatism of 100 or less, exit pupil of 2mm and below will not show any astigmatism, in fact the stars look perfect; because only the paraxial (near the centre) of the eye is illuminated slightly. Astigmatism of the eye is a problem for long focal length eyepieces and fast telescopes.

So.. just try the star test and check your eyes!

I will go to other opticians and check if they get the same eye result as the one I went. I may want to get the TV dioptric as it is better than spectacles due to the ability to rotate the cylindrical (for correcting astig) lens and it is fully multicoated and has higher transmission than my spectacles.

starfinder · Post by **starfinder** » Thu Jan 12, 2006 11:17 pm

A very interesting write up, Canopus Lim. I too have quite a fair bit of astigmatism, though I usually prefer to look through eyepieces without glasses bec I feel closer to the image.

BTW, your article on eye-induced aberrations reminds me of a verse from Shakespeare:

"The fault, dear Brutus, is not in our stars,
But in ourselves,..."
Julius Caesar (I, ii, 140-141)

Post by **weixing** » Thu Jan 12, 2006 11:43 pm

Hi,

The best way to test if astigmatism is there is through the star test. The amount of astigmatism present in the telescope system due to your eyes is directly related to the exit pupil. The exit pupil can be calculated: focal length of eyepiece (in mm)/ f ratio. A F/6 scope and 30mm eyepiece will give a exit pupil of 5mm in diameter. Firstly, astigmatism impact on what you see increases with increasing exit pupil size. The reason is the exit pupil illuminates a bigger area of the eye and hence the optical aberrations (errors. In this case astigmatism) in the eye becomes more pronounced. You can try this at home. Get a telescope combination to enable as big exit pupil as possible (about 6 to 7mm.) Point your scope at the star and centre it in the eyepiece. If you do it at home, it is highly possible that your pupil has not fully dilated to 6 to 7mm. However, it still can be used to test. Defocus the star that is centred, you will see the defocused rings. If you cannot see the entire defocused star (seeing only some part of it), move your head around and actually you can tell about how large is your pupil dilated to (by comparing how much of the defocused star you can see) Observe the defocused star carefully, is it CIRCLE IN SHAPE? OR ELLIPTICAL? If it is elliptical, rotate your eyepiece in the focuser, if the elliptical shape does not rotate, it means that the astigmatism is not due to your eyepiece. To confirm if your eye has astigmatism, rack the inside and outside focus of the star and if the elliptical shape IS ROTATED 90 DEGREE WRT TO EACH OTHER when you rack the focuser in and out of focus it means that your eye has astigmatism. DO MAKE SURE THAT THE STAR IS AT THE CENTRE OF FIELD AS EYEPIECES DO HAVE THEIR OWN ASTIGMATISM BUT IT IS NOT PRESENT AT THE CENTRE OF FIELD.

Hmm... is this the correct way to test for astigmatism of your eye?? Also, may be it your telescope that got the astigmatism. I think you should ask someone to look at the same star pattern?? If they see the same pattern, then may be it's your telescope.

Have a nice day.

Canopus Lim · Post by **Canopus Lim** » Fri Jan 13, 2006 9:49 am

Yup it is the correct way; I have also read internet (TV website and others) and books (optics books, telescope optics books and telescope making books) regarding this. It is optically sound too. I tested with my nagler 7mm with exit pupil of 1.8mm with my own eyes and the star is perfectly round. Therefore, it shows that my telescope is not astigmatic at all. However for larger exit pupil of about 3mm and above, my eye astigmatism shows up and this is confirmed with the elliptical defocused shape that rotates 90 degree at in the in and outside focus. Also, I found out that the star test is very sensitive. I read that people who have only 25 astigmatism can also see the effect. My astig is 75 and my eyesight can never focus my 40mm Konig (with 7mm exit pupil) or my binoculars without my glasses on! NOTE THAT THE EFFECT IS PROMINENT ON BRIGHT OBJECTS LIKE BRIGHT STARS. TERRESTRIAL OBJECTS HAS NO PROBLEM UNLESS YOU ARE FOCUSING ON A SMALL POINT LIKE A STAR. With my glasses it can be focused but it still looks quite hairy and elliptical even at the centre of field. I tested it on Mars last night and Mars look slightly elongated at focus. However, with my other eyepieces of focal length much shorter and hence much smaller exit pupil, Mars look perfectly round and looks good. Therefore I come to the conclusion that the problem is with my eyes.