Field of view simulation astrophotography

Types and principles of astronomical telescopes

General astronomical telescopes can be classified into refractive telescopes, reflective telescopes and folding reflective telescopes according to their structures.

refractor

The so-called refractive telescope is a lens that collects the light from distant objects and presents a real image as an objective lens, which makes it possible to see the distance from a distance.

Light refracts from the square to the focus. The advantage of refraction telescope is that it is easy to use, and it will not be seen even if it is slightly neglected.

I don't know, because the inside of the lens barrel is sealed by the objective lens and eyepiece, the air will not flow, so it is relatively stable. Furthermore, due to the optical axis,

The image deterioration caused by misalignment is also better than that of reflective telescope. Small-caliber lenses are spherical and can be mechanical.

The output of the mill is large, so the price is cheaper.

(1) galileo telescope

The first human telescope used a concave lens as an eyepiece, and the images seen through the telescope were the same as those directly seen with the eyes.

The sample is an upright image, and the ground surface is convenient but it can't expand the field of vision. At present, this design is no longer used for astronomical observation.

(2) Kepler telescope

Using a convex lens as an eyepiece, all refractive telescopes are of this type now, and the imaging is up and down, but I am like this.

Our celestial observation has no influence, because the eyepiece is a convex lens, and more than two lenses can be put together to form a group.

Expand the field of vision, and can improve aberration and eliminate chromatic aberration.

reflecting telescope

Reflecting telescope uses a concave mirror called the primary mirror instead of the objective lens. In addition, a small mirror called a secondary mirror will become the primary mirror.

The light collected by the reflector is reflected out of the lens barrel, and the light image reflected by the secondary mirror is amplified by the eyepiece, and the reflection type is the largest.

The advantage is that because the main mirror is a mirror, the light does not need to pass through the glass, so there will be no color difference at all, and it will not absorb ultraviolet rays.

Light or red light, so it is very suitable for physical observation such as spectroscopy. Although there is no chromatic aberration, there are other kinds of aberrations, such as concave reflection.

If you grind it into a parabolic shape, you can eliminate the spherical aberration. Because the lens barrel can't be sealed, the main mirror is easily affected by smoke and dust.

Ring, so it is difficult to maintain, at the same time, it is greatly affected by the air temperature and airflow in the lens barrel, and it is easy to move the primary mirror and the secondary mirror when handling.

The position of the mirror and the correction of the optical axis are also quite complicated and inconvenient to carry. In addition, the diffraction of the secondary mirror seat will make the star brighter.

There are cross or star diffraction patterns in the star image, which also reduces the contrast of the image, and the stability of the image is not as good as that of the refractive telescope.

Mirror.

At present, there are five well-known reflecting telescope designs ... I will only list two small and medium-sized reflective telescopes on the market.

(1) Newton formula (Newton)

1668 was invented and designed by Newton. It consists of a parabolic primary mirror and a plane secondary mirror, facing the optical axis at an angle of 45 degrees.

The simplest structure is to install the plane secondary mirror slightly in front of the focal point of the reflected light of the primary mirror (as shown above).

Single, high image contrast, most people choose, usually the focal ratio is between f4 and f8.

(2) Ka seglin or Ka seglin for short.

Using hyperboloid as the secondary mirror, the light gathers before the primary focus and passes through the primary mirror.

Focus a round hole behind the main mirror. Because of one reflection, the lens barrel can be shortened, but the field of view is narrow and astigmatic.

It's more serious than Newton, and it's a little crooked.

Fold reflecting telescope.

The kind that uses the advantages of reflection and refraction, basically like reflection, also has the disadvantages of reflective telescope. In order to eliminate the comet aberration whose field of view deviates from the optical axis, a lens is used, and the main mirror is a spherical mirror, which is easier to grind than the reflective one. ..

Only one kind of folding reflecting telescope which is widely used is introduced.

Schmidt card type

It was invented by Schmidt in 1930 for astrophotography. The spherical concave mirror is mainly used as the primary mirror to eliminate it.

In addition to coma, an aspheric lens is placed in an appropriate position in front of the main mirror as a correction lens.

(Corrector) Corrects the spherical aberration of the primary mirror. Therefore, a wide-angle (up to 40-50 degrees) field of view can be obtained.

Common spherical aberration and coma aberration in general mirrors are only slight chromatic aberration caused by correction mirrors. Application of photography

Mitte telescope, the focal ratio can be very small (generally between f 1 and f3, the lowest can reach ÷0.6), so it is very suitable.

Star field and nebula photography.