Length of dew shield

Dimensioning of dew shields without vignetting

The purpose of dew shields is to prevent dew from forming on the optics of a telescope and to prevent stray light (e.g. from street lamps) from entering the beam path. To avoid vignetting on the image, the dew shield must not be too long. The maximum length of the dew shield is limited by the area of the sky projected onto the chip and plays an important role in systems with short focal lengths that capture a large section of the sky (e.g. telephoto lenses).

The projected area of the sky is given by the angle of view achieved by the telescope-camera system. The angle of view depends on the focal length of the telescope and the diagonal of the camera chip.

Edge beams with a refractor / telephoto lens

Principle sketch of the edge beams for a vignetting-free dew shield for a refractor

Schematic illustration of image angle and length of dew shield

As can be seen in the schematic illustration, half the image angle is required for the calculation. This results from half the chip diagonal 'c' and the focal length 'f'.

Furthermore, the outer diameter 'D' of the telescope (or also the inner diameter of the dew shield) is required in order to determine the wall thickness of the telescope via the telescope aperture 'a' and then, in turn, the length 'L_T' of the dew shield.

The values of the angles are reduced from the formula, and only the wall thickness, the focal length and the chip diagonal remain.

Example calculation for the dew shield length for a refractor

Given:

Telescope aperture: 80 mm
Outer diameter telescope: 89 mm
Focal length: 480 mm
Chip diagonal: 23.2 mm

The dew shield should therefore not protrude more than 180 mm measured from the main plane of the lens system. As the main plane is usually not known, the front lens is selected here for simplicity.

With a refracting telescope, the measurement of the dew shield starts at the lens mount. With a Newtonian or Schmidt-Cassegrain telescope, on the other hand, the dimensioning starts at the main plane of the primary mirror. The tube is already the first part of the dew shield.
However, this does not change the basic formula. But since the dew shield in a Newtonian or Schmidt-Cassegrain telescope is not mounted at the height of the primary mirror, but slightly behind the front edge of the tube, the tube length is also subtracted in the formula as a substitute. (The distance from the main plane of the primary mirror to the front edge is usually not known).

Edge beams with a Newtonian telescope

Principle sketch of the edge beams for a vignetting-free dew shield for a Newtonian telescope

Example calculation for the dew shield length for a Newtonian telescope

Given:

The mirror diameter is decisive for a Newtonian telescope.

Telescope aperture: 285 mm
Mirror diameter: 250 mm
Outer diameter telescope: 298 mm
Tube length: 915 mm
Focal length: 1000 mm
Chip diagonal: 23.2 mm

A tube without a dew shield may therefore have a maximum length of 1500 mm.

When calculating the length of the dew shield, the maximum length of the tube including the length of the dew shield from the main mirror level is calculated and then the tube length is subtracted.

The dew shield should not protrude more than 1150 mm from the front edge of the tube.

Edge beams with a Schmidt-Cassegrain telescopePrinciple sketch of the edge beams for a vignetting-free dew shield for a Schmidt-Cassegrain telescope (Source for the concept of focal length: https://www.cloudynights.com/topic/777522-beginner-question-about-focal-length/?p=11197866)

Example calculation for the dew shield length for a Schmidt-Cassegrain telescope

Given:

Since the position of the primary mirror in the tube cannot be determined so easily, the tube length is used here in simplified form, as with the Newtonian telescope.

Telescope aperture: 203 mm
Focal length: 2032 mm
Tube length: 425 mm
Outer diameter telescope: 233 mm
Chip diagonal: 23.2 mm

The dew shield should not protrude more than 2200 mm from the front edge of the tube.