An autoguider is an automatic electronic guidance tool used in astronomy to keep a telescope pointed precisely at an object being observed. This prevents the object from drifting across the field of view during long-exposures which would create a blurred or elongated image.
Imaging of dim celestial targets, usually deep sky objects, requires exposure times of many minutes, particularly when narrowband images are being taken. In order for the resulting image to maintain usable clarity and sharpness during these exposures, the target must be held at the same position within the telescope's field of view during the whole exposure; any apparent motion would cause point sources of light (such as stars) to appear as streaks, or the object being photographed to appear blurry. Even computer-tracked mounts and GoTo telescopes do not eliminate the need for tracking adjustments for exposures beyond a few minutes, as astrophotography demands an extremely high level of precision that these devices typically cannot achieve, especially if the mount is not properly polar aligned. [1] [2]
To accomplish this automatically an autoguider is usually attached to either a guidescope or finderscope, which is a smaller telescope oriented in the same direction as the main telescope, or an off-axis guider, which uses a prism to divert some of the light originally headed towards the eyepiece.
The device has a CCD or CMOS sensor that regularly takes short exposures of an area of sky near the object. After each image is captured, a computer measures the apparent motion of one or more stars within the imaged area and issues the appropriate corrections to the telescope's computerized mount.
Some computer controlled telescope mounts have an autoguiding port that connects directly to the autoguider (usually referred to as an ST-4 port, which works with analog signals). [3] In this configuration, a guide camera will detect any apparent drift in the field of view. It will then send this signal to a computer which can calculate the required correction. This correction is then sent back to the camera which relays it back to the mount. [4]
An autoguider need not be an independent unit; some high-end CCD imaging units (such as those offered by SBIG) have a second, integrated CCD sensor on the same plane as the main imaging chip that is dedicated to autoguiding. Astronomical video cameras or modified webcams can also serve as an autoguiding unit when used with guiding software such as Guidedog or PHD2, [5] or general-purpose astronomical programs such as MaxDSLR. [6] However, these setups are generally not as sensitive as specialized units.
Since an image of a star can take up more than one pixel on an image sensor due to lens imperfections and other effects, autoguiders use the amount of light falling on each pixel to calculate where the star should actually be located. As a result, most autoguiders have subpixel accuracy. In other words, the star can be tracked to an accuracy better than the angular size represented by one CCD pixel. However, atmospheric effects (astronomical seeing) typically limit accuracy to one arcsecond in most situations. To prevent the telescope from moving in response to changes in the guide star's apparent position caused by seeing, the user can usually adjust a setting called "aggressiveness". [6]
Amateur astronomy is a hobby where participants enjoy observing or imaging celestial objects in the sky using the unaided eye, binoculars, or telescopes. Even though scientific research may not be their primary goal, some amateur astronomers make contributions in doing citizen science, such as by monitoring variable stars, double stars, sunspots, or occultations of stars by the Moon or asteroids, or by discovering transient astronomical events, such as comets, galactic novae or supernovae in other galaxies.
A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are a major technology used in digital imaging.
A camera is an optical instrument that captures a visual image. At a basic level, cameras are sealed boxes with a small hole that allows light through to capture an image on a light-sensitive surface. Cameras have various mechanisms to control how the light falls onto the light-sensitive surface. Lenses focus the light entering the camera, and the size of the aperture can be widened or narrowed. A shutter mechanism determines the amount of time the photosensitive surface is exposed to light.
Astrophotography, also known as astronomical imaging, is photography or imaging of astronomical objects, celestial events, and areas of the night sky. The first photograph of an astronomical object was taken in 1840, but it was not until the late 19th century that advances in technology allowed for detailed stellar photography. Besides being able to record the details of extended objects such as the Moon, Sun, and planets, astrophotography has the ability to image objects invisible to the human eye such as dim stars, nebulae, and galaxies. This is done by long time exposure since both film and digital cameras can accumulate and sum light photons over these long periods of time.
A camera lens is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.
Photometry, from Greek photo- ("light") and -metry ("measure"), is a technique used in astronomy that is concerned with measuring the flux or intensity of light radiated by astronomical objects. This light is measured through a telescope using a photometer, often made using electronic devices such as a CCD photometer or a photoelectric photometer that converts light into an electric current by the photoelectric effect. When calibrated against standard stars of known intensity and colour, photometers can measure the brightness or apparent magnitude of celestial objects.
Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, in microscopy, optical fabrication and in retinal imaging systems to reduce optical aberrations. Adaptive optics works by measuring the distortions in a wavefront and compensating for them with a device that corrects those errors such as a deformable mirror or a liquid crystal array.
In astronomy, a guide star is a reference star used to accurately maintain the tracking by a telescope of a heavenly body, whose motion across the sky is primarily due to the rotation of the Earth.
Lucky imaging is one form of speckle imaging used for astrophotography. Speckle imaging techniques use a high-speed camera with exposure times short enough so that the changes in the Earth's atmosphere during the exposure are minimal.
An equatorial mount is a mount for instruments that compensates for Earth's rotation by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras. The advantage of an equatorial mount lies in its ability to allow the instrument attached to it to stay fixed on any celestial object with diurnal motion by driving one axis at a constant speed. Such an arrangement is called a sidereal or clock drive. Equatorial mounts achieve this by aligning their rotational axis with the Earth, a process known as "polar alignment".
A zenith camera is an astrogeodetic telescope used today primarily for the local surveys of Earth's gravity field. Zenith cameras are designed as transportable field instruments for the direct observation of the plumb line and vertical deflections.
An astrograph is a telescope designed for the sole purpose of astrophotography. Astrographs are mostly used in wide-field astronomical surveys of the sky and for detection of objects such as asteroids, meteors, and comets.
Image noise is random variation of brightness or color information in images, and is usually an aspect of electronic noise. It can be produced by the image sensor and circuitry of a scanner or digital camera. Image noise can also originate in film grain and in the unavoidable shot noise of an ideal photon detector. Image noise is an undesirable by-product of image capture that obscures the desired information.
Image stabilization (IS) is a family of techniques that reduce blurring associated with the motion of a camera or other imaging device during exposure.
The Dark Energy Survey (DES) is an astronomical survey designed to constrain the properties of dark energy. It uses images taken in the near-ultraviolet, visible, and near-infrared to measure the expansion of the Universe using Type Ia supernovae, baryon acoustic oscillations, the number of galaxy clusters, and weak gravitational lensing. The collaboration is composed of research institutions and universities from the United States, Australia, Brazil, the United Kingdom, Germany, Spain, and Switzerland. The collaboration is divided into several scientific working groups. The director of DES is Josh Frieman.
Polar alignment is the act of aligning the rotational axis of a telescope's equatorial mount or a sundial's gnomon with a celestial pole to parallel Earth's axis.
Afocal photography, also called afocal imaging or afocal projection is a method of photography where the camera with its lens attached is mounted over the eyepiece of another image forming system such as an optical telescope or optical microscope, with the camera lens taking the place of the human eye.
In lunar photography, the Looney 11 rule is a method of estimating correct exposures without a light meter. For daylight photography, there is a similar rule called the Sunny 16 rule. The basic rule is: "For astronomical photos of the Moon's surface, set aperture to f/11 and shutter speed to the [reciprocal of the] ISO film speed [or ISO setting]."
The Nikon D810 is a 36.3-megapixel professional-grade full-frame digital single-lens reflex camera produced by Nikon. The camera was officially announced in June 2014, and became available in July 2014.