Comparison National Geographic Refractor 90/900 EQ3 vs Celestron AstroMaster 90EQ

The focal length of the telescope lens.

Focal length — this is the distance from the optical centre of the lens to the plane on which the image is projected (screen, film, matrix), at which the telescope lens will produce the clearest possible image. The longer the focal length, the greater the magnification the telescope can provide; however, keep in mind that magnification figures are also related to the focal length of the eyepiece used and the diameter of the lens (see below for more on this). But what this parameter directly affects is the dimensions of the device, more precisely, the length of the tube. In the case of refractors and most reflectors (see "Design"), the length of the telescope approximately corresponds to its focal length, but in mirror-lens models they can be 3-4 times shorter than the focal length.

Also note that the focal length is taken into account in some formulas that characterize the quality of the telescope. For example, it is believed that for good visibility through the simplest type of refracting telescope — the so-called achromat — it is necessary that its focal length is not less than D ^ 2/10 (the square of the lens diameter divided by 10), and preferably not less than D ^ 2/9.

The highest useful magnification that the telescope can provide.

The actual magnification of the telescope depends on the focal lengths of the objective (see above) and the eyepiece. Dividing the first by the second, we get the degree of magnification: for example, a system with a 1000 mm objective and a 5 mm eyepiece will give 1000/5 = 200x (in the absence of other elements that affect the magnification, such as a Barlow lens — see below). Thus, by installing different eyepieces in the telescope, you can change the degree of its magnification. However, increasing the magnification beyond a certain limit simply does not make sense: although the apparent size of objects will increase, their detail will not improve, and instead of a small and clear image, the observer will see a large, but blurry one. The maximum useful magnification is precisely the limit above which the telescope simply cannot provide normal image quality. It is believed that, according to the laws of optics, this indicator cannot be more than the diameter of the lens in millimetres, multiplied by two: for example, for a model with an entrance lens of 120 mm, the maximum useful magnification will be 120x2 = 240x.

Note that working at a given degree of multiplicity does not mean the maximum quality and clarity of the image, but in some cases it can be very convenient; see “Maximum resolution magnification"

The highest resolution magnification that a telescope can provide. In fact, this is the magnification at which the telescope provides maximum detail of the image and allows you to see all the small details that, in principle, it is possible to see in it. When the magnification is reduced below this value, the size of visible details decreases, which impairs their visibility, when magnified, diffraction phenomena become noticeable, due to which the details begin to blur.

The maximum resolving magnification is less than the maximum useful one (see above) — it is somewhere around 1.4 ... 1.5 of the lens diameter in millimetres (different formulas give different values, it is impossible to determine this value unambiguously, since much depends on the subjective sensations of the observer and features of his vision). However, it is worth working with this magnification if you want to consider the maximum amount of detail — for example, irregularities on the surface of the Moon or binary stars. It makes sense to take a larger magnification (within the maximum useful one) only for viewing bright contrasting objects, and also if the observer has vision problems.

The luminosity of a telescope characterizes the total amount of light "captured" by the system and transmitted to the observer's eye. In terms of numbers, aperture is the ratio between the diameter of the lens and the focal length (see above): for example, for a system with an aperture of 100 mm and a focal length of 1000 mm, the aperture will be 100/1000 = 1/10. This indicator is also called "relative aperture".

When choosing according to aperture ratio, it is necessary first of all to take into account for what purposes the telescope is planned to be used. A large relative aperture is very convenient for astrophotography, because allows a large amount of light to pass through and allows you to work with faster shutter speeds. But for visual observations, high aperture is not required — on the contrary, longer-focus (and, accordingly, less aperture) telescopes have a lower level of aberrations and allow the use of more convenient eyepieces for observation. Also note that a large aperture requires the use of large lenses, which accordingly affects the dimensions, weight and price of the telescope.

The penetrating power of a telescope is the magnitude of the faintest stars that can be seen through it under perfect viewing conditions (at the zenith, in clear air). This indicator describes the ability of the telescope to see small and faintly luminous astronomical objects.

When evaluating the capabilities of a telescope in terms of this indicator, it should be taken into account that the brighter the object, the smaller its magnitude: for example, for Sirius, the brightest star in the night sky, this indicator is -1, and for the much dimmer Polar Star — about 2. The largest magnitude visible to the naked eye is about 6.5.

Thus, the larger the number in this characteristic, the better the telescope is suitable for working with dim objects. The humblest modern models can see stars around magnitude 10, and the most advanced consumer-level systems are capable of viewing at magnitudes greater than 15—nearly 4,000 times fainter than the minimum for the naked eye.

Note that the actual penetrating power is directly related to the magnification factor. It is believed that telescopes reach their maximum in this indicator when using eyepieces that provide a magnification of the order of 0.7D (where D is the objective diameter in millimetres).

The resolution of the telescope, determined according to the Dawes criterion. This indicator is also called the Dawes limit. (There is also a reading of "Daves", but it is not correct).

Resolution in this case is an indicator that characterizes the ability of a telescope to distinguish individual light sources located at a close distance, in other words, the ability to see them as separate objects. This indicator is measured in arc seconds (1 '' is 1/3600 of a degree). At distances smaller than the resolution, these sources (for example, double stars) will merge into a continuous spot. Thus, the lower the numbers in this paragraph, the higher the resolution, the better the telescope is suitable for looking at closely spaced objects. However, note that in this case we are not talking about the ability to see objects completely separate from each other, but only about the ability to identify two light sources in an elongated light spot that have merged (for the observer) into one. In order for an observer to see two separate sources, the distance between them must be approximately twice the claimed resolution.

According to the Dawes criterion, the resolution directly depends on the diameter of the telescope lens (see above): the larger the aperture, the smaller the angle between separately visible objects can be and the higher the resolution. In general, this indicator is similar to the Rayleigh criterion (see "Resolution (Rayleigh)"), however, i...t was derived experimentally, and not theoretically. Therefore, on the one hand, the Dawes limit more accurately describes the practical capabilities of the telescope, on the other hand, the correspondence to these capabilities largely depends on the subjective characteristics of the observer. Simply put, a person without experience in observing double objects, or having vision problems, may simply “not recognize” two light sources in an elongated spot if they are located at a distance comparable to the Dawes limit. For more on the difference between the criteria, see "Resolution (Rayleigh)".

The resolution of the telescope, determined according to the Rayleigh criterion.

Resolution in this case is an indicator that characterizes the ability of a telescope to distinguish individual light sources located at a close distance, in other words, the ability to see them as separate objects. This indicator is measured in arc seconds (1 '' is 1/3600 of a degree). At distances smaller than the resolution, these sources (for example, double stars) will merge into a continuous spot. Thus, the lower the numbers in this paragraph, the higher the resolution, the better the telescope is suitable for looking at closely spaced objects. However, note that in this case we are not talking about the ability to see objects completely separate from each other, but only about the ability to identify two light sources in an elongated light spot that have merged (for the observer) into one. In order for an observer to see two separate sources, the distance between them must be approximately twice the claimed resolution.

The Rayleigh criterion is a theoretical value and is calculated using rather complex formulas that take into account, in addition to the diameter of the telescope lens (see above), the wavelength of the observed light, the distance between objects and to the observer, etc. Separately visible, according to this method, are objects located at a greater distance from each other than for the Dawes limit described above; therefore, for the same tel...escope, the Rayleigh resolution will be lower than that of Dawes (and the numbers indicated in this paragraph are correspondingly larger). On the other hand, this indicator depends less on the personal characteristics of the user: even inexperienced observers can distinguish objects at a distance corresponding to the Rayleigh criterion.

This item indicates the eyepieces included in the standard scope of delivery of the telescope, or rather, the focal lengths of these eyepieces.

Having these data and knowing the focal length of the telescope (see above), it is possible to determine the magnifications that the device can produce out of the box. For a telescope without Barlow lenses (see below) and other additional elements of a similar purpose, the magnification will be equal to the focal length of the objective divided by the focal length of the eyepiece. For example, a 1000 mm optic equipped with 5 and 10 mm "eyes" will be able to give magnifications of 1000/5=200x and 1000/10=100x.

In the absence of a suitable eyepiece in the kit, it can usually be purchased separately.

The magnification of the Barlow lens supplied with the telescope.

Such a device (usually, it is made removable) is a diverging lens or lens system installed in front of the eyepiece. In fact, the Barlow lens increases the focal length of the telescope, providing a greater degree of magnification (and a smaller angle of view) with the same eyepiece. In this case, the magnification factor with a lens can be calculated by multiplying the “native” magnification with a given eyepiece by the magnification of the lens itself: for example, if a telescope with a 10 mm eyepiece provided a magnification of 100x, then when installing a 3x Barlow lens, this figure will be 100x3=300x. Of course, the same effect can be achieved with an eyepiece with a reduced focal length. However, firstly, such an eyepiece may not always be available for purchase; secondly, one Barlow lens can be used with all eyepieces suitable for the telescope, expanding the arsenal of available magnifications. This possibility is especially convenient in those cases when the observer needs an extensive set of options for the degree of magnification. For example, a set of 4 eyepieces and one Barlow lens provides 8 magnification options, while working with such a set is more convenient than with 8 separate eyepieces.