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Astigmatism in Refractor Telescopes: Causes and Solutions

The image you’re seeing through your refractor is not the best. Could it be due to a minor aberration such as astigmatism?

When we look through a telescope, we want the image to be clear and sharp. Astigmatism is a type of off-axis aberration that interferes with this.

Astigmatism in refractor telescopes and its effect on image quality

An imperfect lens is the main cause of astigmatism in refractors. This is an optical aberration causing light to spread in an area rather than concentrate to a point. It’s not chromatic, but rather monochromatic (single color).

The light rays form an image that’s distorted – tending to stretch along one plane.

Astigmatism here relates to a telescope lens versus astigmatism I refer to in Using a Telescope with Glasses, relating to sight. Both can affect the image you see through your telescope (see diagnoses below).

If you have astigmatism in your refractor telescope, you may notice when observing objects in the night sky through your telescope that they appear distorted.

Causes of astigmatism in refractor telescopes

Telescopes feature the tangential and sagittal planes, which are perpendicular to each other. When these planes have distinct focal points, it results in astigmatism.

This leads to the formation of two focal surfaces, causing light to focus on one axis ahead or behind the other.

Primary reasons for this in refractors:

  • Defective instrumentation – lens imperfections, manufacturing defects that cause poor homogeneity of the glass
  • Mis-collimation (field astigmatism) – lens titling

Astigmatism in refractor telescopes vs reflector telescopes

The image quality problems you need to worry about are astigmatism and chromatic aberration in refractor telescopes (see my article on refractors vs reflectors).

Astigmatism affects refractors differently from how it happens in reflectors.

In reflectors, mirrors ( cf. lens) are responsible for light transfer. And aberrations common with them are spherical aberrations where spherical mirrors are used.

How to diagnose astigmatism in refractor telescopes

It’s best to make sure that the ‘astigmatism’ you see isn’t due to the telescope’s star diagonal, or eyepiece, or to do with your vision (do you need to wear glasses).

How do you know your problem is astigmatism? What you do is observe the image quality and look for certain characteristics.

It might be characterized by what you see when you view a bright object, for example, is it a…

  • Shape like a potato
  • Ellipse shape (when you go out of focus)
  • Triangular

Here are some steps you can follow to diagnose astigmatism in your refractor telescope:

  1. Choose a bright object, such as a star or planet, and make sure the object is in focus and centered in the eyepiece of your telescope.
  2. Next, observe the object. Look for any distortion or stretching in one direction, like any of the three listed above.
  3. Rotate the eyepiece.
    If the distortion or stretching remains the same regardless of the position, it may be a sign of astigmatism. If it moves, it could be the eyepiece that is the problem.
  4. Take out the diagonal.
    If you have a diagonal in the focus, take it out and check again. If the astigmatism goes then the diagonal was the problem.
  5. Observe the object at different magnifications. Does it get worse with higher magnifications?
    Astigmatism can become more pronounced at higher magnifications, so this can help you diagnose the problem.
  6. Use a collimation tool to check the alignment of the optics.
    Aberrations can sometimes be caused by misaligned optics, so checking the alignment with a collimation tool can help you identify the problem.

Some of these characteristics can be a sign of astigmatism in your refractor telescope. However, it’s important to note that other problems can also cause similar symptoms, e.g. coma or some other image defect.

Solutions for astigmatism in refracting telescopes

So you’ve diagnosed astigmatism as the reason for your imperfect image.

Just be aware that you may not be able to fix astigmatism in a refractor diy at home. Astigmatism of the telescope lens is generally due to a lens imperfection that requires replacement.

Thanks to Valery Deryuzhin’s input in the conversation on sci.astro.amateur for these solutions:

Object in viewer is potato-shaped:
The simplest approach is to have the lens replaced because the issue is likely due to the glass lacking homogeneity. Refer to the manufacturer in this case.

Object in viewer is ellipse-shaped:

  1. Two possible solutions. If both lenses are affected then relative rotation may help partially or fully alleviate the problem. If one lens only then the approach is to have the lens replaced as per above.
  1. Some say you can compensate by ‘mis-collimation’ or tilting the objective. Or, the objective may have a tilt already and need re-aligning to fix the ‘astigmatism’. Use a collimation tool or Cheshire eyepiece to check.
  1. Assuming you’ve taken it out in the diagnosis, but if not, a star-diagonal could be your problem if its mirror is tending to bow, i.e. concave or convex. Take it out and look through the viewer to check.

Object in viewer is triangular:
The problem might be due to pinching or stress of the optics. Try loosening centering screws or other likely culprits and see if it makes a difference.

Otherwise, the lens has a defect, likely damaged in the polishing stage of manufacture. You’ll need to replace it.

Don’t forget to always minimize possible imperfections by storing your telescope somewhere secure to avoid dropping or bumping the sensitive parts – see my article on storage ideas for your telescope. This will also help prevent impacts due to changes in temperature or humidity expanding and contracting the lens.