Comparison Levenhuk LabZZ D1 vs Celestron FirstScope 76

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.

The type of focuser (mechanical unit responsible for focus the image) provided in the design of the telescope. The focus procedure involves moving the eyepiece of the telescope relative to the lens; different types of focusers differ in the type of mechanism that provides such movement.

— Rack. As the name suggests, these focusers use a rack and pinion mechanism that is moved by turning a pinion gear; and this gear, in turn, is connected to the focus knob. The main advantages of rack systems are simplicity and low cost. At the same time, such mechanisms are not very accurate, moreover, they often have backlashes. In this regard, focusers of this type are typical mainly for low-cost entry-level telescopes.

— Crayford. Focusers of the Crayford system use roller mechanisms in which there are no teeth, and the movement of the eyepiece is carried out due to the friction force between the roller and the moving surface. They are considered much more advanced than rack and pinion — in particular, due to the absence of backlash and smooth focus. The only serious drawback of "crayfords" can be called a certain probability of slippage; however, due to the use of special materials and other design tricks, this probability is practically reduced to zero. Due to this, this type of focuser is found even in the most advanced professional-level telescopes.

— Threaded. The design of the threaded focuser is based on two tubes...— one is inserted into the other and seated on the thread. The movement of the eyepiece necessary for focus is carried out by rotation around the longitudinal axis — similar to how a screw moves in a thread. Such focusers are extremely simple and inexpensive, but they are subject to noticeable backlash and require regular lubrication. In addition, they are rather inconvenient for astrophotography: when adjusting the focus, you have to rotate the camera connected to the eyepiece. Therefore, this kind of focus mechanisms is quite rare, mainly in small and relatively inexpensive telescopes.

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.

The total weight of the telescope assembly includes the mount and tripod.

Light weight is convenient primarily for "marching" use and frequent movements from place to place. However, the downside of this is modest performance, high cost, and sometimes both. In addition, a lighter stand smooths out shocks and vibrations worse, which may be important in some situations (for example, if the device is installed near a railway where freight trains often pass).