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Glossary | Telescope accessories | Capacity

Under the heading capacity you can find the technical properties of an accessory.

More information:

Active cooling

For long exposure times, many CCD cameras are cooled to reduce thermal noise and achieve higher image quality. CCD cameras are grouped into three categories: - no cooling - passive cooling via cooling fins - active cooling (e.g. using a fan and Peltier element)

Active cooling

An active cooling system dissipates the thermoelectric heat from the camera. A fan and/or Peltier device usually provides this. Planetary cameras, on the other hand, often do not have active cooling, as this is not necessary due to the shorter exposure times.


Prevents light from overflowing into an adjacent pixel in the event of overexposure. If the light from a star were to run into another pixel, a vertical spike could appear on the image.

Aperture ratio

This refers to the optimum telescope optic aperture ratio for this camera.

Apparent field of view

This is the angle of aperture of an eyepiece.


A camera that takes over the automatic tracking control during exposure, and corrects it independently.

Bayer matrix

An image sensor with 50% green and 25% each of blue and red colour filters. This creates a colour image. Just like the human eye, this sensor is most sensitive to green.

Bit depth

The bit depth is also referred to as the dynamic range of a chip. The number of bits in a picture also determines how many different shades of grey can be displayed. The depth of the image information depends directly on it. In an image taken with 1 bit depth, you will only see black and white dots. In this case, you can hardly call it a picture. Many details that the naked eye can discern are missing. 16 bits means the presentation of more than 65.000 shades of grey.

Chip size

The chip size comprises the outer dimensions of the chip. The larger the chip, the larger the image field produced.

Coating of optical system (3)

The optical elements are treated with ultra-thin coatings. Among others, there are anti-reflective coatings for reducing, and dielectric coatings for increasing reflection.

Colour camera

Colour cameras produce quick results, monochrome cameras capture more detail.

Connection (to the telescope) (5)

Specifies the product's connection or connection thread on the telescope connection end. The same adapter dimension must always be available on the telescope.

Connector (at camera end)

The connector or thread found directly on the camera. With this information, you know whether the camera already fits the telescope, or whether an adapter is required.

Connector (other end) (1)

Eyepieces, camera adapters or other accessories can be connected at the accessory end.


The counterweights are not taken into account when specifying the total weight, as the number of weights required depends on the optics used.

Dark current

Electrons are released in each pixel, whether they are illuminated by photons or not. Each CCD chip generates a certain amount of thermal energy and thus an amplifying signal. As soon as the image of the CCD chip is read out, you will see a noisy image.

Eye relief

Is the distance between the eyepiece and eye, within which the entire eyepiece field of view can still be seen. Longer eye reliefs are recommended for relaxed observing.

Filter thread

A filter thread is a fine thread into which 1.25" or 2" astronomical filters can be screwed.

Focal length

The smaller the focal length of an eyepiece, the higher the magnification when used with a telescope.

Frame (3)

The frame allows the filter to be attached to a connector, a filter holder or in the housing of a camera.

Full well capacity

The full well capacity indicates how many electrons a single pixel can hold. The larger the chip, the more sensitive it is to light, and the more electrons it can absorb. But what does that mean? The noise is reduced at a large full well capacity and the ability to reproduce very bright and dark objects simultaneously increases.

Image format

The image format describes with which type of file the captured image can be saved.

Images per second

This camera takes not just one picture, but a certain number per second. Often this value is given the abbreviation "fps" or frames per second. With a high number of images, you can use the advantage of "freezing" the image and the seeing and thus later creating the perfect image.


The interface is the type of connection or cable between the device (e.g. a camera) and computer, such as: USB, RS232, firewire or analogue video.

Load capacity

The accessory can be added up to a certain load weight, at which it is still sufficiently stable and serviceable.

Max. exposure time

The maximum exposure time in minutes that is possible with this camera.

Max. payload

For observing, the tube weight should not exceed the mount’s maximum payload. For a stable photographic combination of tube and mount, it is wise to also pay attention to the tube length and dimension, where appropriate, to stay below the maximum load, as the leverage effect on the mount must be taken into account.


Specifies the total number of pixels. With this handy value you can compare the resolutions of different cameras.


Microlenses increase the light extraction efficiency of a sensor.

Min. exposure time

The minimum exposure time, in seconds, that is possible with this camera.

Operating temperature

The range of outside temperatures at which the instrument optimally operates.

Optical length

The optically effective length of the component.

Pixel size

Specifies the edge length of the mostly square pixels that make up a camera sensor.

Quantum efficiency at 500 nm (green range)

Quantum efficiency describes what percentage of the photons that fall on a chip generate energy in the form of electrons. In concrete terms, this means how effective a chip is, for example, in comparison to a chemical film. Incidentally, these had a quantum efficiency of about 5%. So a modern CCD chip can gain more information in a shorter period of time.

RA worm wheel diameter

The worm wheel is an essential part of a mount’s worm gear.

Read noise

Each CCD camera tends to produce image noise, an interference that appears as a kind of unattractive grain on the image. The issue is especially important with longer exposure times. In addition to the general noise, there is also a noise generated by the process of reading the image: the read noise.

Resolution photo

The resolution of a sensor is expressed as the total number of pixels. In digital cameras it is usually specified in megapixels. However, the specification of the number of single pixels in a vertical and horizontal row is more precise, for example 2048x2048.

Right ascension axis diameter

The diameter of the right ascension axis is fundamental to the load-bearing capability of a mount.

Ring clamp

Clamps the tube evenly and protects from damage.

Sensor type

Indicates the type or construction of the chip.

Software (2)

Software included with the accessory.

Suitabe for... (1)

Describes the item for which the accessory is suitable.

Surface reflection

The amount of light that is reflected after striking the optical surface.


The remaining amount of light that reaches the eye.

Type (23)

Describes the type of accessory in more detail.

Type of build (3)

Type of camera (2)

Specifies the type of camera, for example: CCD camera, SLR, webcam or planetary camera.

Viewing position

The position in which an observer looks into the telescope or through the accessory. Usually the position is at an angle of 90°, 45°, or is straight.