How to focus a telescope?

You got a new telescope and you can't see through it? You should probably focus it, in this article we will explain the steps to follow, about how to focus a telescope, read carefully.

How is a telescope made?

In order for you to get a sharp image in the eyepiece, you need to rule out basic problems. The telescope cover is located at the mouth of the tube, to observe through the telescope you must remove it, some covers that are larger inside them include another small one, so make sure to remove the other smaller size cover.

Wherever we look, the wheel or ring is found, try turning it until it focuses, preferably do it during the day without focusing on the sun. The eyepiece holder is the opening through which you insert the eyepiece, make sure that it is not obstructed by the screw that is under the eyepiece, since it must move freely both inside and outside the tube.

The eyepiece is a lens that can be removed and inserted into the eyepiece holder to observe, these eyepieces are identified by numbers, the one with the highest number is in turn the one with the lowest magnification, after having focused you can change it for another higher magnification lens and thus focus again.

Sometimes the combination of lenses does not allow the correct focus of the object, for which it is suggested to change the lenses of the eyepiece holder, until obtaining the perfect combination. You may require a tube that extends the distance between the lenses, to be able to perform this action.

Also, temperature variation can affect focus, so you should wait a bit for the telescope to get used to refocusing. There are also filters that make it difficult to see, if you are using them, try focusing without them.

If you intend to observe through an eyepiece that has not been modified, you may have certain problems. I suggest you correct it as necessary and re-test the focus, you should also be aware that the mirrors are positioned properly, this happens in reflecting or Schmidt-Cassegrain telescopes, refracting telescopes, on the other hand, do not need mirrors.

The importance of focusing a telescope

By not focusing correctly in the telescope, it could happen that the image is blurred, affecting the sense of sight. It is also possible that the photo sensor cannot capture the image and this would not allow to obtain the details that are required in the photograph.

There are a number of aspects that can affect the focus, such as: the type of eyepiece, or perhaps the camera model you are using is not the most appropriate, the use of Barlow lenses, the use of filters and the variation in the position of the tube, specifically in reflecting telescopes.

In turn, it can affect the depth of focus or the temperature, the colder it is you have to refocus the telescope.

How to focus a telescope?

These steps are essential to be able to focus your telescope optimally. First of all, allow the tube to ambient until adequate thermal stability is obtained, avoid using the filters in the first approach.

Insert the eyepiece or the camera on top of the eyepiece holder and remember to remove the cover of the telescope, some telescopes have a clamping screw on the base, you must loosen it, check the focus mechanism again, and from this way, the eyepiece should move from the inside to the outside of the tube.

Now, direct the tube towards a celestial body very close to the object you want to photograph, make a first observation, if you cannot see, it is because it is out of focus, change the focus again using the focuser, if you are using a reflecting telescope instead of observing a celestial body you will be able to see a donut because it is out of focus

There is a disk of light in the mouth of the telescope with a shadow in the middle which passes through the secondary mirror, this happens through the spider axes that hold it and the tabs that hold the tube, you must move the focus wheel and look again until you spot a small star.

You need to place a Bahtinov or Hartmann mask over the mouth of the telescope, observe again using the mask, try to move the focus wheel and repeat again until you can see according to the mask you are using.

Remove the mask from the mouth of the telescope after focusing is complete, you should check the focus every hour, as the temperature will continue to drop as it gets darker.

The ideal techniques for focusing a telescope

There are several techniques that allow you to focus to a lesser or greater degree the precision, it may be that the visual focus does not have adequate precision for astrophotography, so you must move the focus wheel until you can see the image.

The focus differs for each eyepiece, or the combination of Barlow lenses that you are going to attach to the tube, you must be very careful if you suffer from any visual disease such as astigmatism, eye fatigue or allergies in particular that it doesn't allow you to focus.

You need to use a right angle viewfinder, which helps your body posture by allowing it to magnify the image 15x-25x, but it can also narrow the image, decreasing the brightness of objects, when using the camera should be aware that it is aligned with the viewfinder and try to wear an eye patch to avoid fatigue.

You must pay attention to the photographic focus, so that you can increase the image with the zoom of the LCD screen, I suggest you adjust the focus, until the stars you are seeing have the minimum size, use the image of the sensor to focus, if you're using a laptop it's much easier.

If you are using a reflecting telescope which is out of focus it will look like a doughnut, you should check the mouth of the telescope, or you may need to align the mirrors, you can also use a Hartmann's mask, which is a opaque sheet with perforated geometric figures.

One of them is the one designed by Scheiner, you must place the mask over the mouth of the telescope, the mask will darken the image, but if it is well placed, you will observe how geometric patterns are made in the image because of light diffraction.

The focus will be adequate when these geometric figures are placed on top of the image, although perforating these two circles that the mask brings would modify the lens opening and at the same time affect the depth of focus, I suggest you use the mask of Bahtinov that mitigates this problem.

The Bahtinov mask is a sheet but its design is a grid, it can be purchased or you can make it yourself in a simple way, by placing the mask in the mouth of the telescope, the stars will be observed in an image of diffraction, when the figure is symmetrical, the telescope is already focused, before buying a mask you should check that the size is adequate for the opening of your telescope.


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