Hi all,
I was just wondering, what are the effects of astigmatism through the eyepiece? What will stars look like?
Effects of Astigmatism
Effects of Astigmatism
Jeremy Ng
C8, CR-150HD, TMB 80 f/6
Orion SVP Intelliscope, AstroSlew I
Minolta Activa 12x50WA
C8, CR-150HD, TMB 80 f/6
Orion SVP Intelliscope, AstroSlew I
Minolta Activa 12x50WA
-
Canopus Lim
- Posts: 1144
- Joined: Fri Jun 03, 2005 12:46 pm
- Location: Macpherson
Stars when focused looked a bit like a cross instead of a point at the centre of the FOV. For stars towards the edge of the FOV, the stars look like lines. Having astigmatism, means stars are not point like and that it is impossible to have best focus. You can check for your own astigmatism through the eyepiece.
Astigmatism can come from the telescope, the eyepiece and the user. I assume that your telescope and eyepiece do not have astigmatism. Now, to check if you have astigmatism, take out your glasses, place a star in the centre of the FOV of the telescope with the eyepiece inside. Defocus the star until you see the diffraction rings. If the defocus star in the centre of the FOV looks elliptical instead of circle, it may mean that you have astigmatism in your eyes. To verify, move your head and observe the ellipse. Does the ellipse move as you rotate your head? If it does, it means that you have astigmatism. You will notice that the angle of the ellipse will rotate 90 degree when you defocus it from inside to outside focus or vice versa. The angle of the ellipse corresponds to the astigmatism angle of your eye. Because the defocus stars are ellipse in shape, it is impossible to have a best focus unlike a circle.
To further check on how severe is your astigmatism, use different eyepieces and hence giving different exit pupil. Vary the exit pupil (by varying the eyepiece), until you get the largest exit pupil in which the centre defocus star is a circle instead of ellipse. That is the LIMITING EXIT PUPIL you can use without astigmatic corrected glasses or viewing without glasses.
From experiment, TV website state that astigmatism of 100 (1.0 diopter) is not visible in the eyepiece when the exit pupil is 2mm or less. I feel it is a correct result. It means that if your astigmatism is more than 100, you will see astigmatic stars at exit pupil of 2mm or maybe even less than 2mm depending on how severe the astigmatism. Of course above 2mm you will see astigmatism.
The bigger the exit pupil (the lower the magnification), the more obvious the astigmatism. Also, it may be necessary to have a higher correction for astigmatism when using telescopes in the dark because the eye tests by opticians are done not in very dark rooms and the eye pupil is not fully open and hence the correction of astigmatism is only for daylight and not at night. At night it could be worse. For my case it is probably 1.0 diopter at night and 0.75 diopter in the day time.
It is very necessary to have astigmatism correction at low magnification. For high magnification, it may not be required. For me, my most limiting eyepiece is my 11mm eyepiece giving me 2.2mm exit pupil. I do see slight astigmatism without my spectacles.
Astigmatism can come from the telescope, the eyepiece and the user. I assume that your telescope and eyepiece do not have astigmatism. Now, to check if you have astigmatism, take out your glasses, place a star in the centre of the FOV of the telescope with the eyepiece inside. Defocus the star until you see the diffraction rings. If the defocus star in the centre of the FOV looks elliptical instead of circle, it may mean that you have astigmatism in your eyes. To verify, move your head and observe the ellipse. Does the ellipse move as you rotate your head? If it does, it means that you have astigmatism. You will notice that the angle of the ellipse will rotate 90 degree when you defocus it from inside to outside focus or vice versa. The angle of the ellipse corresponds to the astigmatism angle of your eye. Because the defocus stars are ellipse in shape, it is impossible to have a best focus unlike a circle.
To further check on how severe is your astigmatism, use different eyepieces and hence giving different exit pupil. Vary the exit pupil (by varying the eyepiece), until you get the largest exit pupil in which the centre defocus star is a circle instead of ellipse. That is the LIMITING EXIT PUPIL you can use without astigmatic corrected glasses or viewing without glasses.
From experiment, TV website state that astigmatism of 100 (1.0 diopter) is not visible in the eyepiece when the exit pupil is 2mm or less. I feel it is a correct result. It means that if your astigmatism is more than 100, you will see astigmatic stars at exit pupil of 2mm or maybe even less than 2mm depending on how severe the astigmatism. Of course above 2mm you will see astigmatism.
The bigger the exit pupil (the lower the magnification), the more obvious the astigmatism. Also, it may be necessary to have a higher correction for astigmatism when using telescopes in the dark because the eye tests by opticians are done not in very dark rooms and the eye pupil is not fully open and hence the correction of astigmatism is only for daylight and not at night. At night it could be worse. For my case it is probably 1.0 diopter at night and 0.75 diopter in the day time.
It is very necessary to have astigmatism correction at low magnification. For high magnification, it may not be required. For me, my most limiting eyepiece is my 11mm eyepiece giving me 2.2mm exit pupil. I do see slight astigmatism without my spectacles.
AstroDuck
Thanks Canopus. I somehow felt you would be the first to reply regarding Astig. 
Anyway, when in-focus, does astig look like there are 2 very fine very short lines coming out of the star?
I checked at an optician once, but they said I;ve only got about 25deg of Astig. and didn't need any optical aid. Now maybe I need some.
Anyway, when in-focus, does astig look like there are 2 very fine very short lines coming out of the star?
I checked at an optician once, but they said I;ve only got about 25deg of Astig. and didn't need any optical aid. Now maybe I need some.
Jeremy Ng
C8, CR-150HD, TMB 80 f/6
Orion SVP Intelliscope, AstroSlew I
Minolta Activa 12x50WA
C8, CR-150HD, TMB 80 f/6
Orion SVP Intelliscope, AstroSlew I
Minolta Activa 12x50WA
-
Canopus Lim
- Posts: 1144
- Joined: Fri Jun 03, 2005 12:46 pm
- Location: Macpherson
-
Canopus Lim
- Posts: 1144
- Joined: Fri Jun 03, 2005 12:46 pm
- Location: Macpherson
I guess spectacles.. haa. Just make sure that when doing the eye test, you try and accurately determine the lines on the eye chart and get the best angle and value for the astigmatism correction for the spectacles. Also, get the spectacles MULTICOATED, you do not want your eyepieces to have 95 percent transmission only to drop to 91 percent transmission because of your uncoated spectacles. 
AstroDuck
Hmmm... So is there any way to overcome this problem? Like looking through the eyepiece while wearing contact lens? Because wearing spectacles and looking through the eyepiece is quite difficult...
Fascinating Nature - The Night Sky.
Interesting Fact - Planets exists.
Fascinating Nature + Interesting Fact = Astronomy
Interesting Fact - Planets exists.
Fascinating Nature + Interesting Fact = Astronomy
Contact lenses cannot correct astigmatism (that I know of). The only other common way is to use a Dioptrx by Televue.
Last edited by jermng on Mon Dec 11, 2006 8:21 pm, edited 1 time in total.
Jeremy Ng
C8, CR-150HD, TMB 80 f/6
Orion SVP Intelliscope, AstroSlew I
Minolta Activa 12x50WA
C8, CR-150HD, TMB 80 f/6
Orion SVP Intelliscope, AstroSlew I
Minolta Activa 12x50WA