Why Do I See A Black Dot in my Telescope?

A telescope is one of mankind’s greatest inventions. Telescopes allow us to see and understand the world around us, by letting us examine the night sky and the universe that surrounds our planet.

This incredible device forms magnified images of distant objects, and can therefore be described as one of the most vital investigative tools of astronomy. 

So, what happens when it does not seem to work quite right, and you cannot quite see things so clearly?

You will never be able to analyze celestial objects in the far reaches of the universe if you cannot see through your telescope properly. Luckily, here at Wired Cosmos, we can help. 

If you can see strange marks or dots on your telescope, we can help you understand what this means and what you can do to fix it with this simple guide! 

Why do I see a black dot in my telescope?

You may have noticed a small black dot or spot in your telescope when you are using it. Unlike legend and mythology, this does not mean bad luck is coming your way.

All it means is that you can see the shadow of the secondary mirror in your telescope, and it is creating the illusion of a black spot when you look down the eyepiece.

If this is the case, then your telescope is not at the correct focus, and you will need to rectify this. 

To remove the black spot from your vision, all you have to do is turn the knob on your telescope, and refocus it until the black shadow becomes smaller and smaller until it disappears.

If it seems to grow larger as you refocus the telescope, then simply turn the knob the other way in the opposite direction to find the perfect focus, and be able to see images in the night sky clearly.

If you can still see the black dot when looking down your telescope, then it may be time for collimation. This is the process of aligning all of the components in the telescope to ensure enough light is able to pass through, and you get the right focus to be able to see things clearly. 

Why can I see the spider in my telescope?

Spiders in a telescope have thick, rounded bodies with legs that work as a means of supporting the mirrors inside of the telescope. The hub can be slightly tensioned to provide rigid support for the secondary mirror inside the telescope.

You may be able to see the spider of the telescope with the eyepiece, however it will be so thin that it should not disrupt your view. However, when you try to look at a star, you will see lines emanating from it.

These are called diffraction spikes, and they are caused by the spider inside of the telescope. You can alter the focus of your telescope until this is no longer visible if it bothers your viewing pleasure. 

If you can see the spider vanes on your telescope, then it will also need to be focussed. Seeing spider vanes in your telescope usually means that you have not achieved a full and accurate focus, so you will need to make some adjustments to your telescope, in the same ways as mentioned above. 

Why is there a cross in my telescope?

Sometimes, you may gaze through your telescope, aligning it with the stars and examining the image in front of you to find that the stars seem to have crossed spikes or crosses emanating from them, like some sort of glow.

Whilst these may look like it, they are not actually parts of the stars or created by the stars, but created by the telescope itself. These crosses are called diffraction spikes. 

A diffraction spike is the light that you are seeing through the telescope, as it extends from the star.

This is caused by the light bending or diffracting around an object, which is in this case, the support beams for the secondary mirror in your reflecting telescope. You will not see these crosses with a refractor telescope because they do not have secondary mirrors.

You can use optical tubes with optical lenses for the front end of your telescope, to hold up the secondary mirror, which will eliminate the diffraction spikes that you are seeing. 

You may also see a cross every time you look down the telescope, this could be if you are looking through the finder scope, which will have crosshairs by design.

Again, if you are not in complete focus, then the shadow of the spider vanes and the secondary mirror will reflect a bright circle, with black shadows and spider vanes when you look down the telescope. 

How to focus a reflector telescope

Most telescopes will have focus knobs on the side. These can be used to lengthen or shorten the distance between the lens of the telescope and the eyepiece, which will bring the image into greater focus for you. By altering this distance, you will be able to focus on a particular object of interest.

To do this, all you need to do is adjust the knobs one way or the other, until you begin to see an object in greater focus. You can also do this in order to remove the image of a black dot or spider by adjusting the focus until it disappears.

You can also change the eyepiece of your telescope to increase the magnification of an image, or to refocus it altogether.

Summary

To summarize, if you see a large black dot, spider vanes or what looks like crosses when you look down your telescope, then there is most likely an issue with the focus. Simply re-adjust until you no longer see the black dot or shadows, and you can enjoy using your telescope once more.

Gordon Watts
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