{ Practical astronomy | Optics and imaging | Telescope mount }

Telescope mount

Since we often need high magnification in astronomy, it is often not possible to hold the optics free-hand. This is particularly true for astrophotography, where long exposure times require the use of a mount. In astronomy, the mount carries the telescope and is standing on, or is rooted in the ground. The mount will include two axes around which the telescope can be turned so that it can be pointed anywhere in the sky.

Alt-az mount

A photo tripod is a simple version of an altitude-azimuth mount.

The simplest case of a mount is a common photo tripod on which a camera or long tele lens can be mounted with a standard screw thread. The simple tripod allows us to turn the camera around a vertical axis (the azimuth axis) and around a horizontal axis (the altitude axis). A heavier telescope mount of this type is called an alt-az mount, for altitude and azimuth. Azimuth is the angle along the horizon that indicates whether the optics is pointing north, east, southwest, etc. Altitude is the angle above the horizon.

Maximum exposure

We will normally use an alt-az mount or photo tripod for stationary photography. The mount helps us avoid shaking the camera during exposures. However, due to the rotation of the Earth, the stars move across the sky and will make trails in exposures that last too long. The speed of this movement varies by a tiny amount depending on the object; and more generally, it is slower close to the celestial poles. The distance from the pole can be expressed by the declination δ, which is 0° on the celestial equator and +90° or −90° at the celestial poles. The Earth rotates in a time slightly less than 24 hours. The apparent movement of stars across the stationary camera's field of view during a time period t is

α = 15.04" t/s cos(δ)

The factor is correct for stars. For the Sun the factor is 15"/s, for the Moon about 14.5"/s. For photography, only the formula for stars is relevant. The other objects are bright enough that their small speed difference from the stars has no significant effect during the short exposure times used for those objects.

The image resolution is determined by either diffraction or pixels size. With a stationary mount, it is important for us to know how long our exposure can be before the stars become trails in the image. The time it takes for a star to move by one resolution element (Δα_A, the radius of the Airy disc) is

t = 0.0665 s (Δα_A/") / cos(δ)

If the pixel size or the seeing (typically 3" for long exposures, 1" in good conditions and for short exposures) is larger than the Airy disc, the exposure can be proportionally longer.

Longer exposures will cause stars to become trails in the image. A countermeasure is to use a motorised mount that compensates the stars' motion across the field. Although some alt-az mounts can do this, there would still be the problem that the star field rotates with respect to the camera. Professional alt-az mounted telescopes have field rotation motors, whereas amateurs are more likely to use an equatorial mount.

Equatorial mount

A German mount is an equatorial mount, i.e. the first axis is aligned with the celestial pole.

The equatorial mount is inclined against the vertical such that its primary axis is aligned on the celestial pole. Stars can be tracked with a right ascension motor that turns the primary axis by one revolution in just under a day (in one sidereal day). The required revolution speed is constant, and the star fields will not rotate with respect to the camera.

There are two major designs for equatorial mounts. The classical design from the 19th and early 20th century is the German mount. The primary and secondary axes form an actual cross, and the optics is offset from the axis cross. For balance, a counterweight is required on the declination axis' end opposite to the optics.

During the 20th century – as professional and amateur telescopes became larger and tended to be Cassegrain reflectors – the fork mount became prevalent. In this design, the two axes cross on the axis of the optics inside the telescope tube. The secondary axis is defined by the two bearings where the optical tube rests on the fork. The primary axis is the axis of the fork.

Recently, amateur astronomers and astrophotographers have been returning to the German mount. This design is more difficult to computerise, as there is a danger of the optical tube colliding with the mount. Affordable, computerised, German mounts have appeared on the market in recent years. The German mount is sturdier than the fork mount, and hence more suitable for photography with long focal length.

Tracking and guiding

A camera with telephoto lens mounted piggyback on a telescope. The observer will use a cross-hair eyepiece in the telescope for a guided exposure. This is a fork mount, by the way.

We call it tracking when a motor turns the primary axis to compensate the movement of the stars. However, the gears that mediate between the motor and the axis itself are not machined to absolute perfection. As a result, there are periodic tracking errors. Typically, over a period of ~10 min the stars may move back and forth by perhaps 10" to 40". Some computer controllers for equatorial mounts can be trained to compensate the periodic error. However, there tend to be different periods superimposed on each other, and in practice training may make hardly any difference.

The classic method around this problem is to guide the optics. We would use two sets of optics aligned in parallel. One would be used visually at high magnification and with a cross hair eyepiece, the other would be a camera with moderate focal length. The observer would use electronic controls to adjust the motor speed such that a star remains on the cross hair throughout the exposure. Often, this arrangement consists of a telescope on its mount as guiding optics, plus a camera with tele lens mounted piggyback on the telescope.

Today, guiding can be automated. That is to say, a second camera replaces the observer's eye, the computer checks its images frequently and sends adjustment signals to the mount. Because of the high resolution of such a guider camera and the high precision of the guiding algorithm, the guider optics can be quite moderate; the old paradigm of "big scope for guiding, small scope to take the image" does no longer apply. Some very advanced CCD cameras can also make the guiding optics obsolete: The computer or camera can preview the images. Guiding is then accomplished either by sending adjustment signals to the motors or by shifting the image across the detector to follow the movement of the guide star.

If you cannot guide your exposures and have to rely on tracking alone, then you will have to experiment with your mount to find out the longest non-trailing exposure for any given focal length. At short focal length, if the amplitude of the periodic error cannot be resolved, then the tracked exposure time is unlimited. At long focal lengths, the quality of the images will depend on the phase of the periodic error cycle: At times when the tracking error changes from too fast to too slow you can take longer exposures than at times when tracking is far from the average speed.