Black Circle Viewing Planet Telescope

Why does there appear to be a black circle in the middle of my image when I look at a star or planet?

The telescope has been subjected to a period of very progressive evolution. The development was motivated by several problems.

These limitations challenged the designs of the conventional telescope. But, quite moderately, it also helped modify the techniques used.

These inconsistencies can be categorized into two categories. These categories include- the theoretical dilemmas and the technical problems specific to the user.

In this article, we are troubleshooting one such problem with your astronomical endeavours. We are talking about a particular issue that you might have come across too often.

What is a reflector telescope?

The reflector telescope employs a mirror to accumulate the incoming light rays. The large distance between the earth and the other celestial bodies is the basic principle here.

Since the celestial objects are so far away, the incoming light rays are always parallel. Due to this phenomenon, your telescope’s mirror has a parabolic contour.

With the help of its design, the mirror can focus the light rays better over a single region. Astronomers, both amateur and expert, prefer the reflector scope better.

Advantages of a reflector telescope

Reflector telescope seldom experiences chromatic aberrations. This mainly because all the wavelength similarly reflects off the mirror.

The objective mirror is located all the way to the back of the scope. This means the objective’s size can be enlarged significantly.

The manufacturing cost of the reflector telescope is relatively less than that of the reflector. The light reflects the objective instead of passing through the scope. Due to this fact, only one side needs to be perfectly tuned.

Is there a black circle in the middle of the image when you look at the star?

If you are using a reflector telescope, you must have come across this quandary quite often. The Newtonian and the Schmidt-Cassegrain are the two most used reflector telescopes.

The dark circle that you see in the middle of the image is an obstruction caused by the secondary mirror. You will come across this problem when your telescope is not focused accurately.

Troubleshooting the imminent problem

You can fine-tune the knob until the shadow decreases in size. Once the size begins to diminish, you can turn the knob further until the circle disappears.

If you turn the knob more than it is necessary, the shadow will increase in size yet again. In this case, you need to rotate the same knob in the opposite direction.

You need to employ a high-powered eyepiece. This is true when If you want the shadow to disappear but also want better magnification.

The eyepiece for your telescope

The eyepiece or the ocular lens determines the field of view and the magnification of your scope. Different types of astronomy require eyepieces with different specs.

The star clusters and the nebulae might appear fantastic under low magnification. But the celestial planets require higher magnification for better viewing.

The working of an eyepiece in a telescope

A telescope is an optical device; it creates images similar to the lens of a camera. As an observer, you need a unique visual aid besides the scope to view the object.

Why? Because your optical biology is not tuned to focus on a real image. It will instead focus on the virtual image, which can only be created by an eyepiece.

Understanding the concepts of an eyepiece

The focal length

The focal length is an essential characteristic of an eyepiece. You will see this calibration written in millimetres on the edge of every eyepiece.

In compliance with your telescope, the eyepiece’s focal length determines its magnification. Here is the basic formula for the magnification of your optical device:

Small, the focal length, higher will be the magnification of the eyepiece. For most scopes, the normal range for the extent ranges between 50x to 250x.

The field of view of the eyepiece

The overall scope of the sky that you view is called the true field of view. The true field of view depends on magnification and eyepiece’s apparent field of view.

A particular design of an eyepiece determines its apparent field of view. The wider the apparent field of view, the broader is the true field.

For viewing larger objects, you need a broader field of view. This means you will need a lower magnification.

The field of view of the eyepiece is essential for the best viewing in astronomy. Both magnification and the field of view are easy to alter. Hence, you can change both as it suits you best.

Recommended Reading:


  1. Celestron
  2. Starizona
  3. Astronomy Notes
  4. Sky and Telescope
  5. Medium