Astronomy FAQ for Dummies (like Me)

Overview of Questions | Answers | References

In progress

On this page, I collect some astronomy questions for dummies like me (I am a permanent beginner...). Please note that while many astronomy hobbyists may laugh at some of the questions and answers, people new to astronomy might be grateful that at least a few of their own questions will be/are dealt with...

See also Quick & Dirty Astronomy Glossary


Overview of Questions


Telescopes (General)

Dobson/Newton Telescopes

Maksutov-Cassegrain Telescopes


What Kind of Eyepieces Should I Buy?

An answer to this question would break this page, and moreover, I do not have the experience to provide a useful answer. So I just want to pick out a few things that I've come across:

Which Focal Lengths of Eyepieces Should I Buy?

When I looked into the literature and searched the Internet to answer this question, I found two recommendations based on the exit pupil of eyepieces, one for five, one for three application areas. Starting from these values, you can obtain the focal lengths and magnifications for your telescope(s). Another approach is based on recommendations for the use of magnifications. The magnification is given by the focal length of the telescope and of the eyepiece (see also below) so that the resulting eyepiece focal lengths can be derived from this. I went through all this for my telescopes and already my eyepieces and report on this on page Eyepiece Selection (Focal Length). By the way, if you have, like me, several telescopes, the whole thing becomes a bit difficult, and you have to compromise and / or choose more eyepieces to cover all needs.

And here is another example from "practice"! If you buy a cheap Sky-Watcher telescope, you typically get two eyepieces, one with a focal length of 25 mm as an overview eyepiece, and one with focal length of a 10 mm for details. Sometimes, a Barlow lens is also included,allowing you to achieve a focal length of 5 mm for even more details. Regardless of the quality of the supplied eyepieces and Barlow lens, such a selection of focal lengths (25 mm, 10 mm, 5 mm) appears to make sense, as a table (see there) suggests, in which I applied the above-mentioned recommendations for the use of magnifications to my telescopes as well as to the somewhat "extreme" Celestron C8.

Which "Apparent Field of View" Should I Select for my Eyepieces?

The apparent viewing angle (apparent field of view) is a measure of the angle that an eyepiece shows as a sky section. It depends on the design of the eyepiece and is usually specified by the manufacturer. See the glossary for more information.

Simple eyepieces (waiter, Plössl) have an (apparent) visual angle between 40 and 50 degrees. This appears to be a "view through a tunnel" compared to super-wide angle eyepieces with 100 and more degrees. If you look at the prices for such eyepieces, you can also see that there are hardly any limits to the top.

The dealer telescope service writes: Eyepieces up to 70 ° are very universal eyepieces and especially suitable for lunar and planetary observation. Eyepieces from 80 ° facial field are often taken for Deep Sky observation.
It is a vision angle of 70 degrees as "ideal for the human eye". These eyepieces are significantly cheaper than eyepieces with a greater visual angle. If it is a bit more, there are eyepieces with an angle of 82 degrees - which can be purchased between 100 and 200 EUR. It is then unfortunately really expensive ...

Dobsons: For Dobson telescopes, which have to be manually adjusted, eyepieces with a higher angle of sight can be used because the telescope has to be followed more frequently.

What Does the "True Field of View" Mean for Eyepieces/Telescopes?

The true field of view determines the size of objects that can be (completely) observed in a telescope. It is calculated from the apparent field of view and the magnification of the telescope according to the eyepiece used:

Examples: Sun and moon correspond to a visual angle of about 0.5 ° (30 '), Jupiter varies between 30 "and 50", and Venus can reach over 1'. Large spherical star clusters can reach 15 '(0.25 °), the Andromeda galaxy is 150' (2.5 °), the Pleyades / Seven Sisters (open start cluster, M 45) are 1.8 ° x 1.2 ° and the Hyades (open star cluster, Mel 25) are even 5 ° x 4 °.

Application: A Plössl eyepiece with 52 ° apparent visual angle has, at a magnification of

I have, however, to admit that my calculations of the true field of view rarely fit exactly in practice. Most of the sky objects appear smaller than expected ...

What Does "Long Eye Relief" Mean for Eyepieces?

Eye relief is simply the distance (in millimetres) you need to hold your eye from the outer lens of an eyepiece to see its full field of view. Short eye relief means that you have to hold your eye close to the lens. This is always a problem with lower-cost eyepieces like Plössls, especially when they have short focal length.

Without glasses, 10-20 mm of eye relief is fairly comfortable. But if you need to wear glasses at the eyepiece, look for eyepieces with at least 17-20 mm of eye relief, which are called long eye relief eyepieces.

Even if you don't were glasses, your eyelashes sometimes brush against the the top lens of an eyepiece and they can leave streaks of eyelash oils that have to be cleaned off regularly. Here, long eye relief eyepieces also come to your rescue.
(From Long Eye Relief Eyepieces by Brian Ventrudo, adapted:

What Is the Significance of the Exit Pupil for Eyepieces?

The exit pupil determines, how bright the image of a certain object, for example, the moon, will appear in the eye piece. For the same exit pupil it will appear with the same brightness, irrespective of the telescope, its aperture, and its magnification.

If the exit pupil of an eyepiece is too small, objects appear too dim (below 1 mm for deep sky objects, below 0.7 mm for planets, below 0.5 mm for the moon and bright double stars), if it is larger than that of the human eye (>7 mm), only part of the light hits the human eye. For galaxies, choose an exit pupil of 2-3 mm, not at all the maximum magnification (from the Internet).

Generally, you can use the exit pupil as a criterion to select appropriate eyepiece focal lengths for one's telescope(s). I did so for my equipment and report it on page Eyepiece Selection (Focal Length Selection).

For more informationen see the Glossary and Telescope Calculations.

Which Magnifications Are Useful for Which Purpose?

I found (and added to... source regrettably unknown...) the following recommendations for the use of magnifications:

In the following table, I apply these recommendations to the Sky-Watcher "kit eyepieces" (with Barlow lens) to my telescopes as well as to the "extreme" Celestron C8 to check to what extent the kit eyepieces represent a good selection of focal lengths. In the left part of the table, I calculate the magnifications for the telescopes and eyepieces. In the right part, I change the approach and check, which eyepiece focal lengths correspond to the recommendations for the use of magnifications for the individual telescopes. The specified eyepiece focal lengths are not always "exactly calculated," but are based on common focal lengths.

Telescope Focal Length of
Focal Length of Eyepiece
Very Low
5 mm
10 mm
25 mm
10-20 x
30-50 x
80-100 x
150-200 x
Heritage 100P
Explorer 150PDS
Suitable Focal Lengths of Eyepieces

*) There are no such "long" eyepieces; **) beyond maximum usable magnification

As you can see from the table, the Sky-Watcher eyepieces, together with a Barlow lens, make up for a suitable choice of focal lengths for the Heritage 100P (if Sky-Watcher would only deliver better eyepieces with its telescopes...). For the Explorer 150PDS the eyepiece focal lengths (25 mm, 10 mm, and 5 mm; here these eyepieces are not included, though) fit quite well, too. The Skymax-102 is also delivered with the 25 mm and the 10 mm eyepieces, but here they do not fit so well.

In general, the use of magnifications can, in addition to using the exit pupil as a criterion, be utilized to select appropriate eyepiece focal lengths for one's telescope(s). I did so for my equipment and report it on page Eyepiece Selection (Focal Length Selection).

Telescopes (General)

Why Do I Sometimes See a (Round) Shadow in front of My Eye when I Look Through the Eyepiece?

If you use a Newtonian or Dobsonian telescope, that is, a reflecting telescope, this is typically the shadow of the secondary mirror that you sometimes see, especially when you look straight into the eyepiece. For me, this effect is the stronger the longer the focal length of the eyepiece is. At a focal length of 25 mm, this effect is quite pronounced for me.

How Does the Image Appear in Different Telescope Types?

I keep asking myself this question, but I cannot remember the answer. I therefore put the answer together in a table:

∨View /Telescoep > Refraktor (Linsenteleskop) Reflektor (Newton-Spiegelteleskop) Schmidt-Cassegrain, Maksutov-Casegrain, Ritchey-Crétien (Astrograph) Sucherfernrohr (Linsenfernrohr)
Direct (The image in the eyepiece is turned 180°, that is, it is upside-down) The image in the eyepiece is turned 180°, that is, it is upside-down (The image in the eyepiece is turned 180°, that is, it is upside-down) The image in the eyepiece is turned 180°, that is, it is upside-down
Wit Zenit prism/mirror Typically, a 90° zenit mirror is used, which mirrors the upright image (left and right are reversed) --- Typically, a 90° zenit mirror is used, which mirrors the upright image (left and right are reversed) ---
With Amici Prism Upright and correct --- Upright and correct ---

Source: Orientierung am Teleskop: Bilddarstellung (

Dobson Telescopes

Which Are the Reasons in favor/against Buying a Dobsonian Telescope?

In preparation

Maksutov-Cassegrain Telescopes

Which Are the Reasons in favor/against Buying a Maksutov-Cassegrain Telescope?

In preparation

What About the Obstruction in Comparison with Newtonian Telescopes?

In preparation




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