# Astronomy FAQ for Dummies (like Me)

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...

## Overview of Questions

### Magnification and further Telescope Parameters

#### How Do I Calculate the Magnification of a Telescope?

The magnification of a telescope is calculated from the ratio of the focal length of the telescope and the focal length of the eyepiece:

• Magnification = (Focal length of the telescope) / (Focal length of the eyepiece)

Example Heritage 100P (focal length 400 mm) with 10 mm eyepiece: 400 mm / 10 mm = 40 x

#### What is the Minimum Useful (or Practical) Magnification of a Telescope and how Do I Calculate It?

The minimum useful magnification is defined as follows: the exit pupil of the telescope and the eye pupil are the same size, so no light is lost. Depending on the source, values between 5 mm and 8 mm are used for the eye pupil, the typical value that is used is, however, 7 mm.

The minimum useful magnification (normal magnification) is determined by the size of the exit pupil:

• Minimum Usable Visual Power (Magnification) = (Focal Length of Telescope) / (Maximum Usable Focal Length of Eyepiece) = (Focal Length of Telescope) / ((Focal Ratio) * (Exit Pupil)) = (Aperture of Telescope) / (Exit Pupil) = (Aperture of Telescope) / 7

#### What is the Maximum Useful (or Practical) Magnification of a Telescope and how Do I Calculate It?

The maximum practical visual power / useful magnification of a telescope is defined by the fact that the resolutions of the telescope and the eye match. It is the highest magnification at which there is still a gain in detail. Beyond that no further details become visible and one speaks of an "empty magnification." In addition, the exit pupil has a size of exactly 0.5 mm at this magnification.

The maximum practical visual power / useful magnification depends essentially on the aperture of the telescope (primary mirror diameter or the lens aperture):

• Maximum Practical Visual Power = Aperture * X

X amounts to:

• 1.5 for Newtonian telescopes (including Dobsonians), rich-field refractors, Schmidt-Cassegrain telescopes
• 2 for refractors from f/8, Maksutov-Cassegrain telescopes

Examples: Heritage 100P (Newton) > 150 x, Skymax-102 (Maksutov-Cassegrain) > 204 x

Stoyan (Deep Sky Reiseführer) speaks of the "beneficial" visual power, at which the airy disk is still not resolved and at which the magnitude limit of the telescope is reached. It is calculated as:

• Beneficial Visual Power = Aperture / 0.7

This corresponds more or less to a factor X of 1.5 (exact: 1.43) in the first formula of this question.

#### What is the Maximum Magnification of a Telescope and how Do I Calculate It?

For small-scale deep sky objects Stoyan (Deep Sky Reiseführer) proposes to go far beyond the beneficial visual power up to the maximum visual power, which is twice as high as the beneficial visual power (about 3 x the aperture value). Depending on the telescope type and the seeing (air turbulence), this is, however, not always possible. Smaller telescopes reach their maximum visual power easier because it is lower than that of large telescopes and thus, the seeing has less influence.

#### In which Cases Does It Make Sense to Go Beyond the Maximum Practical Magnification?

For small-scale deep sky objects Stoyan (Deep Sky Reiseführer) proposes to go far beyond the beneficial visual power up to the maximum visual power. For details see Maximum Magnification.

#### Why and when Does It Make Sense not to Go Beyond a Magnification of 200 x?

Seeing causes details below a certain size to be blurred. The higher the magnification, the closer you get to this range. In most areas of Germany, the best magnification you can use is about 150-200 times (I also found values of 180 and 200 on the Internet). Only rarely and only in areas with really good seeing (for example in high mountains) one can go beyond this. (From: Kleiner Ratgeber für den Teleskopkauf von Bernd Leitenberger: www.bernd-leitenberger.de/teleskop-entscheidungshilfe.shtml, adapted and translated)

#### Why is It, with Respect to the Resolution, not Worthwhile to Use Apertures Beyond 120 mm?

Normally, resolution is limited to about 1" due to air turbulence in the atmosphere. Only in rare exceptional cases, it is better. Thus, telescopes having an aperture of more than 120 mm* will not produce any real advantage with respect to resolution (Astroshop, Oden).

*) 120 mm seems to refer to Dawes criterion.

#### Which Telescope Parameters Depend Solely or together with other Parameters on the Aperture of a Telescope?

Advertisements typically list the aperture and the focal length of telescopes. So, these seem to be the two most important parameters of telescopes. But what is their impact on the perfomance of a telescope. Here I try to answer this question for the aperture. The term aperture refers to the diameter of the opening of a telescope. For mirror telescopes this is either the diameter of the primary mirror or a value that takes care of obstructions that limit the light receiving area.

Solely on the aperture depend:

• Resolution ("): α = 116 / (aperture in mm); Dawes criterion
• Light gathering power: (Light gathering power) = (aperture in mm)² / 49
• Maximum usable magnification: (Maximum usable magnification) = X * (aperture in mm); X = 1.5 or 2 (depending on the type of telescope)
• Maximum magnification: Maximum magnification = 2 x (Maximum usable magnification)

The aperture is included in:

• Aperture ratio: (Aperture ratio) = (Focal length of telescope in mm) / (aperture in mm)

#### Which Telescope Parameters Depend Solely or together with other Parameters on the Focal Length of a Telescope?

Advertisements typically list the aperture and the focal length of telescopes. So, these seem to be the two most important parameters of telescopes. But what is their impact on the perfomance of a telescope. Here I try to answer this question for the focal length.

The focal lenght may directly impact the lenght of a telescope:

• Length: For refractors about the same as the focal length, for Newtonian telescopes it is a little less (particularly for so-called photo Newtonians); Schmidt-Cassegrains and Maksutov-Cassegrains reflect the light rays several times and are therefore shorter by a factor of 3 and more.

The focal length also impacts the following parameters:

• Aperture ratio: Aperture ration = (Focal lenght) / (Aperture)
• Magnification: Magnification = (Focal length of the telescope) / (Focal length of the eyepiece)

#### What Does the Resolution of a Telescope Mean and what Parameters Does It Depend on?

Resolution (or resolving power) is defined as the ability to separate two closely spaced objects (e.g., binary stars in astronomy). There are two empirically found criteria for this ability:

• Rayleigh criterion: diffraction discs do not touch each other
• Dawes criterion: diffraction discs produce an 8-shaped image

Usually the resolution is calculated according to empirical formulas, and usually only the resolution according to Dawes is given, because it is considered as more "practical":

• Rule of thumb (Rayleigh criterion): Resolution of the telescope (") α = 138 / aperture (objective/mirror diameter in mm)
• Rule of thumb (Dawes criterion): Resolution of the telescope (") α = 116 / aperture (objective/mirror diameter in mm)

As the formulas show, the resolution of the telescope parameters depends only on its aperture (objective/mirror diameter).

### Eyepieces

#### 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:

• True field of view = (Apparent field of view) / Magnification = (Apparent field of view) * (Focal length of the eyepiece) / (Focal length of the telescope)

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

• 20 x (20mm eyepiece on the Heritage 100P with 400mm focal length), a true visual angle of 2.6 ° => about the size of the Andromeda galaxy,
• 50 x (8 mm eyepiece on the Heritage 100P with a 400 mm focal length), a true visual angle of about 1 ° => about half the size of the Pleyades,
• 100x (4 mm eyepiece at the Heritage 100P with a 400 mm focal length), a true visual angle of approximately 0.5 ° => the size of the sun / moon.

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: oneminuteastronomer.com/5820/long-eye-relief-eyepieces/)

#### 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, if it is larger than that of the human eye (>7 mm), only part of the light hits the human eye.

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).

#### Which Exit Pupil Size Can Be Used for Which Purpose?

I found the following uses for different values of the exit pupil on the Televue Website (adapted and translated):

"Normal Magnification"

• 2 - 4 mm exit pupil: Experience has shown that these eyepieces are used most frequently.
• 4 - 3.5 mm exit pupil is optimal for most large-area, faint nebulae.
• With 2 mm exit pupil, the eye already perceives 80% of the maximum theoretical resolution; for many objects the perceptibility is optimal, e.g. for most galaxies.

"Maximum Magnification"

• 1 mm (to min. 0.8 - 0.5 mm) exit pupil: With an exit pupil of 1 mm, 95% of the theoretical maximum possible resolution is perceived. Any further magnification only makes sense if the telescope and eyes are good.
• 0.8 mm exit pupil provides the maximum perceptibility of small, low-contrast details with perfect seeing and is the sensible maximum magnification for planets.
• 0.5 mm exit pupil is the maximum magnification, any further magnification does not improve the view. An exit pupil of 0.5 mm can only be used to separate close double stars, and at the utmost limit of the telescope to perceive the weakest details.

Furhter recommendations from the Internet:

• Objects appear too dim below
• 1 mm for deep sky objects
• 0.7 mm for planets
• 0.5 mm for the moon and bright double stars
• For galaxies, choose an exit pupil of 2-3 mm, not at all the maximum magnification.

I found further recommendations, all in all four of them, that suggest, which exit pupil is suitable for which (deep sky) objects / uses. The magnification and the focal length of the eyepiece can then be calculated from this. I consolidated these in my own proposals that I present in tabular format:

 Category Deep Sky Application Area Exit Pupil (mm) Minimum Magnification / Maximum/Large Field of View Search 7...10 Overview, large-area nebulae 4.5...5...6 (7) Normal Magnification Optimal for large-area, faint nebulae; nebulae, open star clusters 3.5...4 Perceptibility optimal for many objects, e.g. for most galaxies, and mid-size deep sky objects 2...3 Maximum Magnification / Maximum Resolution Actually, the "normal" upper magnification limit... Globular star clusters 1...1.5 With perfect seeing, achieves maximum perceptibility of small, low-contrast details; planetary nebulae, small galaxies; maximum magnification for planets that makes sense 0.6...0.7...0.8 Separation of narrow double stars, small planetary nebulae; at the extreme limit of the telescope, to perceive the faintest details 0.4...0.5

#### Which Magnifications Are Useful for Which Purpose?

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

• Very low (10-20 x): Search for objects, extended objects (large-area nebulae, Andromeda galaxy, ...) (added by me)
• Low (30-50): Star clusters, galaxies, and nebulae
• Medium (80-100): Craters and valleys on the moon, Saturn rings, Jupiter and moons
• High (150-200 x): Mountain peaks and fine details on the moon, surface details on Mars, separation of close double stars

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.

 Sky-Watcher-Eyepiece/Barlow Magnification Telescope Focal Length of Telescope Focal Length of Eyepiece Very Low Low Medium High 5 mm 10 mm 25 mm 10-20 x 30-50 x 80-100 x 150-200 x Heritage 100P 400 80 40 16 25 10 4-5 ---** Skymax-102 1300 260 130 52 ---* 25-32 12-15 6-9 Explorer 150PDS 750 150 75 30 40 16-25 7-10 4-5 C8 2032 406 203 81 ---* 40 20-25 10-15 Magnification 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 (www.robani.ch/Bilddarstellung.htm)

### Telescope Types

#### Which Are the Pros and Cons of a Refractor?

 Pros Cons Impressive contrast and sharpness Small diameter = less light collected Light and transportable Chromatic aberrations, blurriness if not corrected No obstruction, which might deteriorate the light gathering power Higher price, particularly for larger apertures and elaborate correction of color finges Closed tube = protection against humidity and dust, no thermical deteriorations Bulky if the reflector is large Maintenance and cleaning almost nonexistent

#### Which Are the Pros and Cons of a Reflector (Newtonian Telescope)?

 Pros Cons Good contrast Optical quality often disappointing (coma aberrations, spherical mirror instead of a parabolic one) No chromatic aberrations (colored fringes around stars) Obstruction (caused by secondary mirror) deteriorates the light gathering power and lowers the contrast in comparison to a refractor Large primary mirror = better light collecting capacity Open tube = high vulnerability to dust, humidity etc., thermal air currents possible, which might impair the imaging Relatively low cost Collimation and mirrors cleaning processes Dobson design possible Bulky and heavy for large apertures (from 8" on )

#### What is a Dobsonian Telescops and which Are its Pros and Cons?

The Dobsonian telescope was introduced by John Dobson in the 1950s and is more or less only used by hobby astronomers. It is a Newton telescope with a simplified mechanical design that is easy to manufacture from readily available components. Dobson's intention was to create a large, portable, low-cost telescope. The optical design is optimized for observing faint, deep-sky objects such as nebulae and galaxies. This type of observation requires a large objective diameter (that is light-gathering power) of relatively short focal length and portability for travel to relatively less light-polluted locations. (After Wikipedia, adapted)

According to Stefan Gotthold (Clear Sky-Blog; translated) "there is only one big difference between a Dobson and the Newtonian telescope, and this is the mount, also called rockerbox. Unlike other telescopes, the Dobsonian has a special mount (azimuthal mount) that does not stand on a tripod. The telescope is moved with a so-called rockerbox and aligned to the sky. The Dobsonian follows in an azimuthal movement and must be rotated in two axes to follow sky objects." In the end, a rockerbox is a special form of an azimuthal mount... So-called mini Dobson telescopes (for example, the Sky-Watcher Heritage 100P or the Heritage P130) have very simple rockerboxes, which, in my opinion, give away many advantages of the Dobsoian rockerboxes...

Advantages and disadvantages of Dobson telescopes at a glance (according to Clear Sky-Blog, Stefan Gotthold, translated and adapted):

 Pros Cons Low cost - a lot of aperture for little money Photography possible only with restrictions Transportability Observation with a Dobsonian telescope not suitable for balconies (except for Mini-Dobsonians) Quick setup Large telescopes are heavy - bulky and heavy for large apertures (from 8" on) Tube can be re-used on a parallactical mount Collimation and mirrors cleaning processes (as with Newtonian telescopes)

Source: Clear Sky-Blog (Stefan Gotthold): www.clearskyblog.de/2016/03/28/warum-ein-dobson-ein-gutes-einsteigergeraet-aber-nicht-fuer-jeden-geeignet-ist/

#### Which Are the Pros and Cons of a Maksutov-Cassegrain Telescope?

 Pros Cons Contrast very good Long cooling down times Almost no color aberrations, good correction of image deficits Higher obstruction (only for scopes from Asia) Low obstruction, no diffraction (radiation around bright stars) because no secondary mirror struts Light loss (because of the limited reflection of the mirrors) Closed tube = protection against humidity and dust, no thermical deteriorations Low aperture ratio > insensitive for light and only usable for bright DSOs High magnification due to long focal length > good for planets, moon, sun Small field of view Compact, short tube length (tube shorter than focal length), small apertures are compact and transportable Large apertures (> 8") are more on the expensive side Partly low cost (small apertures) High weight due to the meniscus lens

#### Which Are the Pros and Cons of a Schmidt-Cassegrain Telescope?

 Pros Cons Short tube length (tube shorter than focal length) Long cooling down times Closed tube = protection against humidity and dust, no thermical deteriorations Higher obstruction/secondary mirror than a Newtonian telescope Very practical with respect to handling Light loss (because of the limited reflection of the mirrors) Transportable Small field of view The look is always comfortable Focusing problem (mirror can flip) The SC thread allows to attach various accessories More expensive than a Newtonian telescope of similar aperture Tubes with a fork mount are set up fast

Source: astroshop.de (www.astroshop.de/beratung/teleskop/teleskop-wissen/vor-und-nachteile-der-bauarten-im-ueberblick/c,8690), translated and adapted

## References

 gerd (at) waloszek (dot) de About me made by on a mac!
 01.12.2020