Ramiro H. Rodriguez

[Homemade Telescope]

During the 1st lockdown at Paris on March 2020 I decided to put hands at work to retrieve the possibility to look at the sky in greater details, so I decided to build a telescope. This is how it went...

The starting point

The idea of building a telescope first started when I found a couple of lenses from a photograph camera at the trash.

Soon after, the pandemic outbreak arrived to Paris.

Being locked at home, I didn't have all the materials and tools I though I will need to assemble the telescope.

However being bored at home drove me to give it a try.

The magnification (M) of the telescope is easily defined as the ratio of the two focal distances: M=f2/f1

Working Principle

As a starting point, it is good to know that a very simple telescope can be made with only two lenses aligned such that they share the focal point.

A first lens, the objective lens of focal length f2, collects the light from the sky and converge it into its focal point to form an image. A second lens, the eyepiece lens of focal length f1, then collects the same compressed light from the focal point and, reversing the process, creates an image of the sky, that seems closer to the viewer, hence creating a magnified image.

Lenses Characterization

Therefore, one needs to know the focal distance of each lens to get the separation at which the lenses should be placed. That is easier to do at the lab with a laser, but since I didn't have one at home, I instead used the light coming through the window: part is coming from the blue sky and part is coming from the yellow-ish building in front that is seen on the picture at the right.

I placed each lens over a support (a book), one by one, pointing to the window. The lens collects the light and produce and image exactly at his focal point. So by placing a peace of paper over which the light is projected I could find the distance at which the image was net and well focused. That's a simple and cheap way of measuring the focal distance with millimeter precision: no laser needed ;)

Notice that the image projected over the paper sheet is the just the inverted image (upside down) of the view I have through the window: the blue sky and the yellow building.

Proof of Concept

Knowing the focal distance of each lens, I placed them directly over a table to form the arrange represented in the schematic picture of the working principle of a refractor telescope.

Then I compared the view with and without the prototype, taking two pictures with my phone with no zoom on the camera.

As seen in the pictures, the first light of the prototype was a yellow "something" hanging from a window on the building with the red curtains. That building is about 100 m away.

view without the prototype

view with the prototype

Second Prototype

While waiting for the night to fall I worked on assembling the part in a more convenient way such that I could point the telescope to the sky.

I placed the lenses over a piece of wood, fixing them with some scotch, to be able to move the ensemble without losing the calculated distance between the lenses.

Placing the piece of wood over a rolling table allows controlling the horizontal direction for pointing while a couple of books allows tunning the elevation angle.

Also I used a stick and an empty bottle of wine to fix the position of the phone to take some net pictures.

In the prototype I also included a Barlow lens I had, this type of lens, used in series right after the objective lens, effectively increases its focal distance, which improves the magnification without the need of having a longer telescope ;)

First Light: The Moon *u*

I waited for the moon to rise on the sky, it rises behind the yellow building I had in front of my window.

Here below, the image at the left is a picture I took of the moon only using my phone camera with maximum zoom. The moon is seen as a white disk on the sky.

The image on right shows the picture I took when using the telescope together with the phone on the same zoom configurations as before.

The improvements in clear and the view was delightful, specially for a lockdown period.

A simple comparison of the relative diameter of the moon in these two pictures gives the resulting magnification M= 12.4 x

(I took care of placing the images with the same size so no artifact is introduced displaying the images on this website)

Well, that's all I did for the moment.

I hope you enjoyed and I hope I inspired you to work on those projects that you may have but for which you feel you don't have all necessary tools to start. Having the motivation is probable the most important element that you need to start making progress.

Below are some other pictures I took reducing my phone zoom to avoid some of the chromatic aberration seen in the pictures: this is the blue/red halos at the edges of the moon. This is a typical defect of produce in telescope built with lenses.

¿Do you have any question?

Send me an email 19.rodriguez.89@gmail.com if you want to obtain more information about the project

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