Observational Instruments.
The days are getting shorter, the temperatures are dropping. There's no doubt about it: Christmas is approaching. Children and grown-ups alike will soon be writing that famous letter to Santa Claus.
Some will have the brilliant idea of ordering a telescope, a chance to travel into space and escape from earthly worries.
With a little over 60 years of experience, may I offer some useful advice to these future astronomers? For years, I've organized practical evenings for the setup of telescopes sold by chain stores, whose salespeople, despite the inscription on the back of their T-shirts, are not necessarily experts in the field. What should you choose?
First of all, I'm clear: the most important thing in a refractor or telescope is its mount. It sounds strange, but that's how it is. Lightweight mounts, made of plastic or aluminum, should be avoided. We observe the sky OUTSIDE, therefore, exposed to the slightest breeze. If your instrument experiences even the slightest tremors, this refractor, this telescope, will soon end up in the back of the garage. Disgusted.
To avoid this failure, the ideal place to start is a motorized stainless steel mount such as the EQ3-2.
You will install a tube of either 114/900, a 150, or a 200 mm. All this depends on your budget. This mount is sturdy enough to support all these instruments.
The telescope will not move, even in the wind. The small "just-right" motor will allow the telescope to track objects in the sky, and your child will be able to invite his friends over without having to bother his dad to re-adjust the pointing. Dad, the Moon is gone!
What should you buy?
The Astronomical Telescope.
Suffice it to say that buying a small, cheap plastic telescope weighing a few grams is a devious and very harmful trap. The adjustment is miraculous, and the optics aren't always aligned. In short, it's strongly discouraged unless you're willing to pay an exorbitant price for a Fluorite telescope. However, it's not traditional to start astronomy with a Fluorite telescope.
Telescopes.
Three types of telescopes are available on the market.
Newtonian Type
Isaac Newton (17th century) designed this type of telescope. His was barely 3.8 cm in diameter.
The primary mirror (parabolic) is placed at the bottom of the tube, and a small, flat, secondary mirror extends the focal length perpendicular to the tube, near the aperture. If the mirror has a focal length of 2 m, the tube must be 2 m long. Tricky. I had built one, with a focal length of 2.39 m, at the 6th REG, in Mayotte and at the 4th RE. It was a monument. Incidentally, in Mayotte, as it was installed next to the mess hall, on the rock, we had a security inspection because some thought the DLEM had equipped itself with a missile aimed at the South Pass of Bendrelé.
So, the Newtonian telescope is bulky. But it's easy to assemble and use. Personally, I built seven of them after "cutting and polishing" the mirrors myself.
Schmidt Cassegrain telescope
Much more compact than the Newtonian, this telescope has a spherical primary mirror and a hyperbolic secondary mirror. A major advantage: its very short length. My 300 mm telescope, which has a focal length of 3000 mm, measures only 60 cm. With a Newtonian telescope, it would measure 3 m!
We're looking "at the back" of the telescope.
Dobsonian type
In recent years, another type of telescope has arrived on the market: the Dobsonian. These are large tubes, generally larger than 200 mm, Newtonian type, meaning you look from the side of the tube, near the aperture. The problems: bulk. A 300 mm diameter and 1500 mm focal length telescope measures 1.5 m. It's difficult to transport. But if you're staying at home, no problem. Second problem: most of the time, they are not slaved to the point of view being tracked. This means that you will have to constantly operate the telescope to track objects AND IN BOTH DIRECTIONS, RIGHT ASCENT AND DECLINATION.
Big advantage of the Dobsonian. Its aperture is around 5. So, for deep sky, it offers little magnification but lots of light. With a good eyepiece, it's also possible to do planetary imaging. Another advantage: the price. Much cheaper than a Schmit Cassegrain.
A quick reminder about the instrument's diameter and magnification.
Diameter
Let's start with the diameter and state the obvious. A 114/900 has a mirror with a diameter of 114 mm and a focal length of 900 mm.
The diameter of the mirror will affect the amount of light collected. The larger the mirror, the more light will enter the telescope. If you want to collect rainwater, you'll get more in a 50 cm diameter basin than in a cup saucer coffee.
So, if you want to do deep sky observation, that is, observe objects such as galaxies, star clusters, and nebulae, which are faint objects, you need to focus your search on a large diameter. Greater than 200 mm.
Magnification
Magnification. No secret. It's an arithmetic calculation. It's the ratio between the focal length of the mirror and the focal length of the eyepiece. Explanation:
You look through an interchangeable eyepiece. It has a number on it. This is the focal length of the inner lens. The higher the number, the less it magnifies. A 42x eyepiece will magnify much less than a 14x eyepiece.
Hence: how many times does my telescope magnify? With a 114/900 telescope and a 25x eyepiece, I have a magnification of 36 times. (900 divided by 25). With a 10x eyepiece, I have a magnification of 90x (900x divided by 10).
I switch to a compact Schmidt-Cassegrain telescope. Either a Meade or a Celestron with a diameter of 200mm. Since they have an aperture of 10x, they have a focal length of 2000mm.
I take my eyepieces again. If it's the 25x eyepiece, 2000x divided by 25 equals 80x. If it's the 10x eyepiece, 2000x divided by 10 equals 200x magnification. It's a no-brainer, especially since with the 200mm, you'll have more light entering.
A golden rule: the magnification required of a telescope should never exceed 150% of its diameter. For a 200mm, the maximum magnification is 300x. Concretely, in a 114/900, Saturn will appear very small, certainly with its rings, but... With a 200, it will be of a beauty that will mark you forever. The same goes for the Moon and its craters or Jupiter and its moons.
If the price makes you think twice, know that there is a remarkable second-hand market because we are dealing with astronomy enthusiasts, generally concerned about equipment, careful, and eager to buy a larger, more powerful telescope. They have no other option but to put their 200 up for sale.
This is not an attempt to advertise to anyone. But I will allow myself to mention a few organizations we contact and who have always given us satisfaction, both in terms of the quality of their equipment and technical advice: l'Astronome in Lorient; la Cléf des étoiles in Toulouse; Bresser in Brignolles; Médas in Vichy; Pierrot Astro in Béziers.
Just as if you feel the need for some advice, I can only encourage you to contact your nearest astro club or simply contact me, either by email or by phone. I will be happy to share a moment of passion with you.
Happy ordering from Santa Claus.
And above all, don't forget: with the naked eye or with a simple pair of binoculars, or better yet, with your eye fixed behind a telescope, you will discover the unsuspected beauties of the sky once isolated from the stray lights that our need for security multiplies in our cities.
Look at this celestial vault, alone or with others, and above all, listen to it. It will send you messages of peace, reverie, and above all, humility. Everything we need.
