The following is an alphabetical list of terms you’ll encounter throughout the documentation, some of which may be unfamiliar or have special uses particular to Observatory.
The Two Micron All-Sky Survey (2MASS) is a survey of the whole sky in three infrared wavebands around 2 micrometers.
The European Southern Observatory (ESO) is a intergovernmental research organisation for astronomy. ESO has built and operated some of the largest and most technologically advanced telescopes. These include the New Technology Telescope (NTT), and the Very Large Telescope (VLT), which consists of four individual telescopes, each with a primary mirror 8.2 meter across, and four smaller auxiliary telescopes.
Observatory can search and download images directly from the ESO Archive, including those obtained using the VLT (8.2 m), VST (2.6 m), VISTA (4.1 m), NTT (3.6 m) and the ESO 3.6 m telescope. If you have the right credentials, you can even download data that is still under the proprietary period.
The Digitized Sky Survey (DSS) is a digital version of several photographic atlases of the night sky.
DSS has two generations. Observatory can search and download both DSS-I and DSS-II images.
Gemini Observatory Archive. The Gemini Observatory consists of two 8.19 m telescopes, the Gemini North in Hawaii and the Gemini South in Chile.
The Hubble Space Telescope (HST) is a space telescope that was launched into low Earth orbit in 1990, and remains in operation.
The W. M. Keck Observatory is a two-telescope astronomical observatory near the summit of Mauna Kea in the U.S. state of Hawaii. Both telescopes feature 10 m primary mirrors. Through the Keck Observatory Archive (KOA), Observatory provides access to the images obtained with the NIRC2 instrument.
The Messier objects are a small set of astronomical objects first listed by French astronomer Charles Messier in 1771.
Observatory can overlay the objects in this catalog onto your images.
The NASA/IPAC Extragalactic Database (NED) is an on-line astronomical database for astronomers that collates and cross-correlates astronomical information on extragalactic objects (galaxies, quasars, radio, x-ray and infrared sources, etc.).
Observatory uses NED for its Virtual Observatory and Astrometric Matching.
The New General Catalogue of Nebulae and Clusters of Stars (NGC) is a catalog of deep-sky objects compiled by John Louis Emil Dreyer in 1888. It contains 7,840 objects. Dreyer published two supplements, known as the Index Catalogues (IC). They contain a total of 5,386 objects.
Observatory can overlay the objects in this catalog onto your images.
This is a catalog containing 983,261 galaxies, all brighter than magnitude B∼18. Observatory includes the PGC2003 catalog, and can overlay the objects in it onto your images.
Palomar Transient Factory Archive. A survey using the Palomar Samuel Oschin 1.22 m Schmidt Telescope.
Observatory can search and download the level 1 images of this archive.
The Sloan Digital Sky Survey (SDSS) is a multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5 m wide-angle optical telescope.
Observatory can search and download the imaging catalog data of Data Release 12 (DR12), which is the final data release of SDSS-III.
Spitzer Heritage Archive. The Spitzer Space Telescope is a 0.85 m NASA infrared-wavelength space telescope.
Observatory can search and download the level 2 images of this archive.
SIMBAD (the Set of Identifications, Measurements, and Bibliography for Astronomical Data) is an astronomical database of objects beyond the Solar System.
Observatory uses SIMBAD for its Virtual Observatory and Astrometric Matching.
The Tycho–2 Catalog is a catalog of more than 2.5 million of the brightest stars. The catalog is 99% complete to magnitudes of V11.0 and 90% complete to V11.5. The Tycho–2 positions and magnitudes are based on the observations collected by the European Space Agency’s Hipparcos satellite.
Observatory includes the Tycho–2 catalog and uses it for Astrometric Matching. It also has the ability to overlay the catalog onto your images. Although it is highly accurate, its use for Observatory’s Astrometric Matching is limited to wide field images only.
A catalog of 113,780,093 stars covering the entire sky, mainly in the 8 to 16 magnitude range in a single bandpass between V and R with positional errors of about 15 to 20 mas. UCAC4 can be downloaded and requires approximately 9GB of disk space.
Observatory can use this catalog for Astrometric Matching. It also has the ability to overlay the catalog onto your images. Although it contains less stars then the USNO-A2.0 catalog, it is much more accurate.
A catalog of 526,280,881 stars covering the entire sky, down to magnitude 19. USNO-A2.0 can be downloaded from ftp://ftp.nofs.navy.mil/pub/outgoing/usnoa/ and requires approximately 7GB of disk space.
Observatory can use this catalog for Astrometric Matching. It also has the ability to overlay the catalog onto your images. Although it contains more stars then the UCAC–4 catalog, it is much less accurate.
Wide-field Infrared Survey Explorer. WISE is a 0.4 m NASA infrared-wavelength space telescope.
Observatory can search and download the images of the WISE AllWISE Atlas.
In addition to thermal noise, a CCD camera generates some noise due to its electronics that doesn’t change with exposure time. A bias frame records this noise.
The image below is a bias frame taken with my ST–7XE NABG camera cooled at –20 °C. On this camera, the shortest possible exposure is 0.11 seconds, not 0 seconds, but it will do for a bias frame.
At first glance, you may think that this camera is defective. Let’s take a closer look at this image:
Although the image looks really noisy, it has very little noise in it. I have enhanced the contrast by setting the black and white points for displaying the image to 90 and 130 respectively. The very brightest pixel in the entire frame is 176 units, and the very darkest is 55 units. Since this camera produces 16 bit grayscale images with 65,536 available brightness levels, and it adds a pedestal of 100 to every pixel to prevent negative values from occurring due to noise, the overall average brightness value of 105 means that at this temperature the camera produces very little noise. The low-level mottling effect across the entire frame is very small.
Another notable feature in this image is a gradient from left to right. The magnitude of the variations are very small, and this gradient is typical of many cameras. It occurs because of the time delay in reading across the chip. The pixels at left have a little more time to build up thermal noise, so they are brighter. The pixels at the left edge of the frame have an average value of 116. Pixels at the far right have an average value of 102. The gradient will occur in all frames, and is automatically dealt with when you reduce the images with dark, flat-field, and bias frames.
As with all calibration frames, you will normally not apply a single one to your images, but combine many into a master bias and use that for calibration. This is because calibration also introduces some noise from the calibration frames themselves, and by using master calibration frames you minimize that. An example is a median combination of the above and 199 other bias frames taken in the same sequence:
I have enhanced the contrast by using the same black and white points as before. Clearly the master bias is much less noisy than the single bias frame. You can now see a fine scale structure which is inherent to the CCD. You can also observe a few Hot Pixels.
The process of applying dark, bias, and/or flat field frames to light frames.
A star’s light is spread across several pixels by air turbulence and diffraction. The center is referred to as a centroid.
Records the dark current (also called thermal current) that occurs during an exposure of a given duration and temperature. It is taken with the shutter closed. The dark frame is subtracted from the light frame of the same duration and temperature to remove the dark current. If the dark and light frame do not match in duration, a bias frame can be taken to scale the dark frame, and allow it to be applied to the light frame.
The image of a star has a typical brightness profile as depicted below. The curve shows an idealized picture of the brightness levels of the pixels across a star image. With the brightest values at the top, this curve has most of the values falling near a central value (the centroid of the star image). There is no definite edge, but the Full Width at Half Maximum (FWHM) is a way to characterize the width of the star image. To find the FWHM, take the highest value, divide it by two, and measure the distance across the curve at that point.
A bright pixel in an image caused by a pixel sensor with a higher than normal dark current.
An image which records the optical issues of your telescope and camera. Typically, the flat records vignetting as well as shadows from dust motes on your CCD and filters. By applying a flat frame, most of these optical issues can be cleaned up in your images.
Also called an image. Taken with the shutter open.
A type of container in the Observatory library that may contain images and stacks of images. Albums cannot be nested.
A small icon Observatory places on an image in the browser to indicate that an adjustment, tag, or other change has been applied to it.
The part of the Observatory interface that displays the contents of the currently selected Collection.
A Folder, Album or Smart Album.
An area in Observatory that displays the image currently selected in the Browser. You can use the Editor to perform adjustments on images as well as compare them with each other.
See Primary Selection.
A type of container in the Observatory library used to organize albums and smart albums. Folders can be nested.
A naming convention that you can apply to destination folders when images are exported. Observatory provides a number of preset folder formats, and you can also create and save your own custom folder formats.
See Image Version.
An image you see and change in Observatory is called a Version. Versions are based on the original master, and you can change, copy, and adjust them any way you want. In Observatory, only versions are ever changed. The masters are never touched. A Version is like a recipe that is applied to its Master. It is a representation of the Master.
Throughout Observatory and its documentation the term “Image” and “Version” actually is short for “Image Version”. Unlike a Master, a Version typically doesn’t take up much disk space.
A Layer Adjustment is a recipe applied to a Master. Each Layer has its own adjustments.
In Observatory, a container file that holds collections of images. The masters of these images may reside inside as well as outside this container file. You can have multiple libraries.
A master that is stored inside the Observatory library itself. After its creation, it is never modified. Any changes to an image based on a managed master are applied to a Version of the master. A managed master has no corresponding source folder. Unlike regular masters, a managed master is permanently removed if all its versions are removed.
You can export a Managed Master that was created by the Virtual Observatory by choosing File ▸ Export ▸ Managed Master…. This exports the original downloaded file.
You can also create a Managed Master from an existing image or stack by choosing Image ▸ New ▸ Master…. You would typically only do this for a stack, and only if you want to apply Layer Adjustments to it. You cannot export such kind of Managed Master though, only a Version of it.
The source image that is referenced by Observatory. The master is never modified, so that the original image is preserved and can be used again. Anytime a change is made to the image, that change is applied to a Version of the master.
When you import an image from your file system, it is not copied into the library, but a reference to it is stored in the library. Only when you download an image using the Virtual Observatory, it is copied into the library. The latter is named a Managed Master.
Immutable Version of the master image. It always exists if a master exists, but doesn’t count towards the number of versions of a master.
In Observatory, if an image is Matched, it can relate its image pixels to actual sky coordinates. For this it needs to know the center equatorial coordinates of the image, the projection used, and the transformation matrix. Observatory currently only supports Gnomonic (tangent plane, TAN) projections for Astrometric Matching.
Masters may already contain all required information. For images that do not contain this information, Observatory can add it by choosing Image ▸ Match…. To make best use of all Observatory’s features, it is recommended to Match all images, except those that are for calibration only.
A naming convention that you can apply to images as they are exported. Observatory provides a number of preset name formats, and you can also create and save your own custom name formats.
Images whose masters are currently unavailable to Observatory. Offline images appear in the Browser with a warning badge. Observatory currently does not support an offline workflow.
A single image in a Stack that is referenced by layer adjustments of other images in the same stack. It is usually the most representative or best image of the stack, e.g. the sharpest one. Every stack has a Pick.
The most important image in an image selection. The image in focus in the current editor. The primary selection is identified by a thicker blue border. There can only be one primary selection in an image selection.
In Observatory, the process of adding a value to an image to indicate its quality in relation to other images in a selection.
Dynamic album in Observatory used to organize images by gathering them based on search criteria. Smart albums cannot be nested.
A source folder represents a folder in the file system, outside of the Observatory library. Because the macOS Sandbox restricts access to the file system, you need to explicitly give Observatory permission to access the folders in which your images reside. Neither these folders, nor their contents, is ever modified by Observatory.
A set of images, all part of the same album and each with an associated Weight. Usually these are images of the same object, or the same calibration image type, with the intend to combine them. Every stack has an associated “Stack Image”, which has no master, but is the result of combining the images within the stack. To further process a Stack Image, one first has to create a Managed Master from it.
Creating a stack does not alter the original images. Use this to create master calibration images, or to combine images of the same object to reduce noise.
A descriptive word about the subject in a collection or image. Collections and images can have multiple tags, some of which are automatically added by Observatory.
See Image Version.
A property of each image version in the Observatory library that is used when combining multiple images. Images that are not part of a stack may only have a weight of 100% (accepted) or 0% (rejected). Images that are part of a stack may have any weight from 0% to 100%. The weights can be set manually or computed automatically.