I use a lot of jargon and unintuitive terminology in my posts. Hopefully these definitions can make reading a little easier.
- Mount: A mount is a piece of equipment that holds a telescope. The most common type used for imaging is called the German equatorial mount. This kind of mount has two driven axes. One of the axes, called Right Ascension, is pointed as accurately as possible at the celestial pole. When this is done, only this axis needs to rotate at a fixed speed to track the sky as it moves from the rotation of Earth.
- Celestial pole: The point in the sky that the earth revolves around. This point will stay static and the rest of the sky will appear to rotate around it. In the Northern hemisphere, Polaris is very close to the celestial pole.
- Polar alignment: This is the process by which the right ascension axis of a mount is pointed at the celestial. This can be done with polar scopes that are visually aligned with Polaris, or with cameras through software. The more accurate the polar alignment, the more accurate guiding will be while imaging.
- Guiding: A telescope mount needs to very accurately counteract the rotation of the Earth to take long exposure images. It’s similar to trying to take a long, sharp picture of a car speeding down a highway from a few miles away. Since a mount is made from metal using traditional machining techniques, it will never be perfect. Guiding uses some method, usually a separate telescope and camera, to watch the stars to see if they are moving relative to the image. Software watches the stars very closely, to within fractions of a camera pixel, every few seconds. If the star has moved from where it should be, the software sends a command to the telescope mount to attempt to move the star back to where it should be.
- Guide scope: A secondary telescope mounted to the main imaging telescope. Usually smaller in aperture and shorter in focal length.
- Angular resolution: Objects in the sky, being at so greatly varying distances, are typically measured in units of angle instead of by their absolute size. There are 360 degrees in a circle. There are 60 arcminutes in one degree. And there are 60 arcseconds in one arcminute. A star can typically appear from two to ten arcseconds wide in an exposure. At best, Jupiter is 47 arcseconds wide at opposition. The moon is about 31 arcminutes wide.
- Differential flexure: This is a term usually used to describe relative motion between the main imaging scope and the guide scope. If the two are not rigidly connected, they can move relative to each other throughout a night as gravity pulls on each scope. This can lead guiding to move the telescope away from the intended target.
- Seeing: The atmosphere can act like a lens and refract light that travels through it. This typically happens from abrupt changes in pressure in layers of the atmosphere. This is also the reason why stars twinkle when you look at them with the naked eye. Seeing can bloat a point source of light by up to several arcseconds, severally limiting imaging resolution on bad nights.
- Transparency: This is a measure of the ability of light to make it through the atmosphere to the observer. This mostly has to do with humidity.
- Light pollution: Sources of light on the ground, such as street lamps and car headlights, can be reflected by the atmosphere back down to the ground. You can see a city glow from many miles away on even a clear night. This added light is detrimental to imaging. The noise from the light pollution can drown out the signal from dim targets, requiring exponentially more imaging time to average out than in dark skies. The Bortle scale is a classification of the mean glow of the sky. I live in a high Bortle 6 zone.
- Diffraction: Since light acts as both a wave and a particle, it can exhibit some strange effects. Diffraction is the tendency of light to “bend” around objects. In particular, the cross spikes you see on bright stars in my images are an example of diffraction. There are four thin, orthogonal bars that hold a mirror in my telescope, and light diffracting around these causes the bright diffraction spikes.
- Integration: Commonly called “stacking,” this is the process of taking many exposures of the same target and averaging them together to create a new image with a better signal to noise ratio. Each pixel from each image is usually averaged together, with some kind of rejection scheme to remove outliers, such as satellite trails.
- Calibration: Single exposures can have many artifacts in them that are correctable. Calibration is the process of using other frame types to remove these artifacts from images. Calibration is typically done on each target exposure, known as a “light frame,” using bias, dark, and flat frames. This is done before all of the light frames are integrated.
- Bias frame: A bias frame is an image taken with a camera at the quickest shutter speed with the aperture blocked off from all light. This image will contain the read noise of the camera, or noise inherent in the process of reading the value from each pixel to output as an image. It will also contain the bias, or the “base” value that each pixel has. Generally, cameras do not output values of zero for pixels that receive no light.
- Dark frame: A dark frame is similar to a bias frame, except it is taken at the full exposure length. It contains the same bias data, but also shows the heat noise that each pixel might get from the ambient environment. Simply put, when things are hot, they are more likely to shed electrons, and these electrons can jump down into the pixel wells and register as a photon. This effect is the reason why many astronomy-specific cameras have methods to significantly cool the sensor.
- Flat frames: Light frames can suffer from three effects: vignetting, quantum efficiency inconsistencies, and dust.
Vignetting is an effect caused by different parts of the sensor seeing different parts of the aperture. Typically, the center of the sensor can see the entire aperture, and so gets the most light. A corner of the sensor may only receive light from, say, 80% of the aperture, and so will be darker than the center. A flat frame will mimic this, and so, can correct it.
Quantum efficiency is a measure of the efficiency of a single pixel well. A pixel doesn’t register every single photon that enters it. Modern CMOS cameras are quite efficient, with mine exhibiting an average of 80% efficiency over the pixels. However, every pixel in the image will not be equally efficient. A good average of many flat frames will show the differences in quantum efficiency between the pixels, and correct for it.
Dust can fall on any part of the optical system. The closer it is to the sensor, the more prominent it is. Recording the dust in flat frames can remove it in calibration. - EAA: “Electronically assisted astronomy” is similar to traditional imaging, but it is typically done with shorter exposure times and shorter total integration times. It’s more of a live experience, as frames are quickly coming in and and integrating in real time to produce a better and better image. In heavily light polluted environments, this can prove much more fruitful than visual astronomy.
- ASCOM: This is a standardized programming interface for astronomy equipment drivers in the Windows environment. Most equipment will have an ASCOM driver that will allow software to interface with and control it.
- Reflector: A telescope that uses mirrors to collect and focus light. My telescope is a Newtonian reflector, which uses one large parabolic mirror to focus light to a plane. A second mirror reflects this cone of light out of the side of the telescope to a sensor or eyepiece.
- Collimation: The alignment of mirrors within a reflecting telescope so that the optical axis of the primary mirror is aligned with the optical axis of the sensor or eyepiece. There are many tools available to do this, with a laser inserted into the focuser being the most common. In parabolic Newtonians, the farther you get from the optical axis of the primary, the worse the focused image becomes. You want this optical axis centered on your sensor to get the best image.
