The main function of telescopes in astronomical observation and research is to magnify celestial objects several times (characterized by the magnification of the telescope) and to increase the ability to receive light from celestial objects (characterized by the magnification of the telescope). The uses of telescopes are ultimately intended to help us get a “bigger, clearer” view of celestial objects.

In addition to the above two features, telescopes also have a number of other parameters that regular KTV users should know.

**1. Opening D**

This is the diameter D of the lens. Generally, refracting telescopes will have a D of around 50mm, 60mm, 70mm, 80mm or up to 150mm. Refractive lenses or combinations will be larger, usually 114mm to 300mm (glasses less than 114mm will not be good).

The larger the D, the larger the aperture, the more light the glass can absorb, and the higher the resolution of the glass. Units of measurement are generally millimeters; in some cases, the diameter may be expressed in units of inches (e.g., 8″ or 8 inches), with each inch equal to 25.4 millimeters.

**2. Opening index F: **

Is the ratio between the focal length f of the lens and D.

Recipe: **F = f / D**

-D: lens diameter

-f: focal length of the lens

For example:

- A normal telescope configuration would be a D70F700. This means that the lens diameter is 70mm and the focal length f is 700. And F will now be 10. This is a slow lens.
- Websites that sell KTVs often refer to their glasses as D f/F, like 6″ F/8, which means this KTV has a mirror diameter D of 6 inches, the aperture rating F is of 8, so the focal length f will be: 6 × 8 = 48 inches.

If F ≤ 5.0, we have a “fast” telescope. Fast telescopes are telescopes with a smaller focal ratio. It’s called fast because it produces a brighter image with more photons per pixel, resulting in a shorter (or faster) exposure than needed to capture the image.

On the contrary, if F ≥ 8.0 we have a “slow” telescope. Slow telescopes are those that have a larger focal ratio. It’s called slow because it produces a darker image with fewer photons per pixel, requiring a longer (or slower) exposure to capture the image.

This means that when shooting astrophotography, to achieve the same brightness for the image, the “fast lens” will take less exposure time than the “slow lens”.

Additionally, faster telescopes are more suitable for observing larger deep-sky objects (DSOs), such as galaxies and nebulae, because they capture more light during observation. Conversely, slow telescopes allow us to observe the planets better than fast telescopes, with sharper images and fewer aberrations.

Choosing a telescope is always a question of compromise.

Whether a fast or slow telescope is more important to you depends on what you want from your astronomy. The more you specialize, the more important this choice will be.

If you are a beginner and want to admire the whole sky, a medium focal ratio will be perfect.

Planet observation enthusiasts or moon hunters benefit from higher magnification and better image quality. It is therefore recommended to look more slowly through the telescope.

Faster telescopes have brighter and smaller images, but can’t handle magnification very well and are prone to color distortion, making them perfect for pointing at galaxies and nebulae.

Ultimately, if you want to venture into the world of astrophotography, a fast telescope with an apochromatic lens to handle color distortion is the way to go.

**3. Magnification**

The magnification of KTV is calculated by the formula

**M = f / fe**

-f: focal length of the lens

-fe: focal length of the eyepiece

For example: A KTV 10″ F/6 used with a 16mm eyepiece. So, what is the magnification of the glasses now?

Solution: Lens focal length f = FxD = 10×6 = 60 inches = 1524 mm

-> Magnification M = f/fe = 1524/16 = 95x

Or a normal telescope like the D80F900 with an eyepiece of around 10mm

then magnification M = f/fe = 900/10 = 90x

The magnification of the eyepiece is actually not a very important parameter, because changing the magnification is done very simply by simply changing the eyepiece to a different focal length. Another more important parameter depends on the aperture of the lens, namely the resolution capacity of the KTV.

**4. Glass resolution (resolution)**

Not all practical optical systems obey the fundamental laws of geometric optics exactly. Even seemingly perfect optical systems will produce an image of a point light source (a star can be thought of as a point light source) that is an Airy disk, which has dimensions to measure. This phenomenon is caused by the diffraction of light, which also has a wave character. According to the Rayleigh standard, the critical angular resolution is calculated by:

where λ is the wavelength of light.

For example: With yellow light with λ = 5.5*10-5 cm (which is most sensitive to the human eye), the angular resolution in arcseconds will be:

The above formula is applicable to both optical KTV and radio KTV.

Thus, it can be seen that the resolution ability of KTV depends entirely on the diameter of the lens. The larger D, the smaller the resolution angle, which means the higher the resolving power of the glass.

Meaning: Telescope resolution plays the most obvious role when using the telescope to observe binary stars. With binary stars where the two stars are too close together, if the glasses do not have sufficient resolution, looking at these stars through the glasses they will only appear as a single star, even if we increase the magnification to the maximum.

**5. Eye circle size h (Exit pupil)**

The exit pupil is the term designating the light beam leaving the eyepiece, the size of this light beam depends on the diameter of the objective and the magnification used:

In the example above, the eyering size would be h = 10/95 = 0.105 in = 2.67 mm.

The larger the eyepiece circle, the brighter the image will be, but the lower the magnification used, there will be a point where the eyepiece circle exceeds the diameter of the pupil, then we will only be able to see the bright part. . At this point, the observed image is the same as if we masked the outer part of the lens.

**6. Minimum and maximum magnification:**

-Minimum magnification:

As mentioned, the size of the eye circle in general should not exceed the pupil size of the human eye in complete darkness (generally about 7mm), otherwise part of the KTV light will be wasted because our eyes do not can’t absorb everything. . Because as the technician’s magnification decreases, the size of the eye circle increases, and at some point it will exceed the size of the pupil. Each technician therefore has a lower limit of magnification and is calculated by

m = 1.33D with D (cm)

m = 3.62D with D (in)

Taking the example of the 10 inch lens above, the minimum magnification for this lens will be m = 3.62*10 = 36.2x, so the eyepiece used should not have a focal length exceeding:

-Useful magnification (maximum magnification)

There is a lower limit of magnification, so there must also be an upper limit and that is the useful magnification of the KTV. In theory, a technician’s magnification can be increased infinitely, but in reality, many factors do not allow this to happen, and one of these factors is the resolving power of the glasses (already mentioned above). -above). To satisfy the resolution of the image (that is, when looking at the image we can still distinguish details), the magnification of the glass can only increase up to a certain limit, and when this limit is exceeded, we can see the details of the image. will no longer be recognizable.

The two images comparing Jupiter are still in good resolution and vice versa.

A rule for calculating the useful magnification is to take **D** objective lens diameter (in inches) multiplied by 50 or objective lens diameter (in mm) multiplied by 2, however, the above formula is only correct when the viewing conditions are perfect.

For example, with the scope initially given at 10″, the useful magnification would be **D*50** =10*50=500x.

In reality, there are many external factors (viewing conditions) that cause the useful magnification value to not reach the calculated number.

**7. Field of vision**

**– Apparent field of view of the eyepiece (Apparent field of view – AFV)**

AFV is the width of the angle observed when looking through the eyepiece (not plugged into the KTV), in degrees. Each type of eyepiece will have a different apparent field of view, from small like the Huygen AFV eyepieces with just 25 to 35 degrees to super huge like Tele Vue’s high-end Ethos line of eyepieces with AFV up to 100 degrees .

Apparent field of view of certain types of eyepieces:

**– True field of view (FOV)**

Different from AFV, FOV is the field of view when the eyepiece is installed in the KTV. Simply put, the actual field of view is the area of the sky you view through the KTV, also measured in degrees. The calculation formula is as follows:

**FOV = AFV / M (degrees)**

-FOV: field of view via KTV (degrees)

-AFV: apparent field of view of the eyepiece (degrees)

-M: magnification

Continuing to take the 10″ f/6 lens and 16mm eyepiece as an example, assuming it is a Huygen eyepiece with an AFV of 35 degrees, then FOV = 35o/95x = 0, 37o, at this point, if we use Ms. Hang’s viewfinder, we only see about ¾ (because the moon has a viewing angle of about 0.5 degrees)

**8. Eye relief**

Eye placement distance is the distance between the eyepiece and the viewing eye position to maximize the visible field. The position of the eye is usually taken by the focal length of the eyepiece (actually a bit smaller than the focal length of the eyepiece).

**9. The lowest star level visible via KTV:**

mt = 6.8 + 5logD with D (cm)

mt = 8.8 + 5logD with D (in)

For example, supernovae generally have an apparent brightness of level 11 or higher. If you have an opening of 15 cm, can you observe it? By applying the formula above, we calculate that the most open star level that can be seen with this lens is 12.7 > 11 -> visible, so you can rest assured to wear the glasses for supernova hunting:: ) (can you still hunt or not? So maybe it mostly depends on your luck :).

Source: Thienvanhanoi.org (According to HAAC – thegioithienvan.com)