Jeffrey Baltus Astro

Resources/FAQ

On this page are answers to FAQ, useful websites/software/applications, and acronyms/abbreviations/terms to know for all things related to astronomy. Note that there is a LOT of information on this page, so it is highly recommended to use the “Find in page…” function within your browser (e.g., using CTRL + F with Windows computers to search for a specific term).

FAQ

General Astronomy

Visual Astronomy

  • What telescope should I buy?
    • This is a question that is often asked with, unfortunately, unrealistic expectations in terms of budget available and what can be achieved with telescopes. the best possible option(s) for visual and/or astrophotography uses. Fortunately, there is a great Quick Guide on the r/telescopes subreddit that can help you decide. In general for beginners, I recommend a dobsonian (tabletop or 8″ aperture) or a refractor on a steady tripod. Furthermore, the folks at Telescopic Watch have numerous reviews, so be sure to visit their website to see whether a particular telescope is recommended or not.
  • What eyepieces should I buy?
    • If you just purchased (or are currently purchasing) your first telescope and it came (or will come) with eyepieces, then my recommendation is nothing. You won’t want to spend $1,000 USD or more on eyepieces if you don’t know what you’ll even use. The eyepieces that came with your telescope will almost assuredly be “serviceable” at least to get you started. The best thing to do is to get familiar with the night sky given the equipment that you have for a number of observing sessions. Once you have a better grasp for what objects you like to see – whether it is the planets, our moon, nebulae, or galaxies – and you know how to use your equipment (including knowing how to focus and, if needed, collimate your reflector properly), then you’ll know what eyepiece focal lengths you’re missing or would like to upgrade. Only once you get to that point should you look into purchasing additional/new eyepieces; at that point, here are some good guides:
  • What are some beginner objects to look at?
    • The most obvious objects to start looking at are the moons and planets. Beyond that, if your telescope and location allows, check out these additional resources:
      • Splitting Double (or more) Star Systems, such as Albireo
      • Messier Objects (a list of 110 objects catalogued by Charles Messier in the late 1700s)
      • CloudyNights user WillR put together and ranked a list of 100 objects “observed multiple times over 2 years in a 5” reflector from a Bortle 4-5 site and latitude 40°N”; the direct link to his PDF list is here.
  • How do I find certain objects in the sky?
    • Beyond the Moon and perhaps some bright planets, it can be difficult for beginners to find objects, especially if their telescope doesn’t have StarSense or similar go-to/push-to technology. I learned how to find objects by having Stellarium on my phone (with the red light night mode to preserve my dark adapted vision) and using it to star hop to what I want to view. (Personally, I greatly enjoy finding these objects myself with this method rather than having a true “go-to” solution with no manual work.) Another great resource for learning to find objects in the sky is the book “Turn Left at Orion”. It can be found on eBay or other online book stores for ~$15.
  • Why doesn’t the object I’m looking at have the same amount of color and detail of images online?
    • The images seen online are the result of very long exposure times (often >=15-20 hours in total) and post processing techniques to extract as much detail and color as possible. Our eyes do not work in nearly the same way, as they only let in so much light and effectively only see in grayscale in the dark. This can sometimes lead to disappointing results for unfamiliar visual observers, so be sure you have realistic expectations for what you can see visually with a telescope.

Astrophotography

  • How can I get into astrophotography and what equipment should I buy?
    • Astrophotography can quickly become very expensive very quickly. The first thing to do is determine what types of objects you want to take pictures of; unless your budget is >$5000 or so total, you’ll want to pick to do either 1) lunar and planetary; or 2) deep sky objects like galaxies and nebulae.
      • For lunar/planetary imaging, you’ll want a high focal length telescope with a camera that has an appropriate pixel size (for the telescope’s aperture) and high frame rate because you’ll do what is called “lucky imaging”. For more information, follow this guide from Cloudy Nights that should have all the information you need.
      • For deep sky objects, there are two options:
        • For something simple that you can set outside on a clear night and not have to put in too much effort, then look into getting a smart telescope; Seestar and Dwarf are two of the most popular brands, and they can take great images straight “out of the box”.
        • If you want to build your own rig, see my astrophotography equipment guide for the various equipment components that you will want to purchase slowly over time.
  • When is the best time to image the planets?
    • Because the orbits of the planets make them change positions relative to Earth, the best time to image the planets will differ. For the interior planets Mercury and Venus, these planets always appear close to the Sun so you’ll have to balance two considerations: 1) only part of the planet will be illuminated as seen from Earth, so you’ll need to check Stellarium or another source to see how much is visible; and 2) you’ll likely want to try imaging them around their respective Greatest Elongations, which is when they are furthest east or west, respectively, away from the Sun (meaning you’ll likely have more time before they set to the west in the evening or the Sun rises and illuminates too much of the sky in the morning). For the other (superior) planets, this becomes much simpler: check for when they are at opposition. For imaging (and visual observation), this is when they will appear the biggest and brightest in our night sky and is the best time to get pictures of them.
  • How should I process my lunar/planetary images?
    • There are a number of good, free software options that can provide very good results (with good input data/images to work with). Some example software includes PIPP, AutoStakkert!, Registax, and/or AstroSurface. There are plenty of excellent tutorials online that show possible ways to use them for processing images, including this video by Naztronomy for AstroSurface.
  • What is the correct way and workflow to process my deep sky images?
  • What did I capture in my image?
    • This is a common question I see online from people who are curious if they captured a certain constellation or a particular deep sky object. The easiest way to know what you captured is to upload the image on nova.astronomy.net to have it plate solved and annotated.

Useful Websites, Software, and Applications

Here are a list of websites, software, and applications that I often use for astronomy (visual and/or astrophotography) or which others regularly recommend to beginners, listed in alphabetical order:

NameInformation
Agena AstroOnline astronomy store to purchase equipment.
ASIAirAll-in-one computer control ecosystem (and application) to automate astrophotography session(s). Produced by ZWO, numerous hardware versions exist of this product.
ASIStudioZWO computer astronomy software, for image acquisition and processing.
astro.vanbuitenen.nlVery useful website for current comets, planets, and minor planets. I use this frequently to identify bright comets visible now or soon.
AstroBinImage hosting site for astrophotographers.
AstronomicsOnline scientific and astronomy store to purchase equipment.
Astronomy.netWebsite to plate solve and annotate astronomical objects in images. Very useful for beginners to upload and see what they captured.
Astronomy.ToolsWebsite with a collection of tools, including determining field of view with your equipment for visual observing and astrophotography, and also determination of [over/under]sampling for astrophotography.
AstrophericWeather-related website and application to help determine cloud cover, seeing, and transparency.
AstroSurfaceAll-in-one image stacking and processing software, most useful for planetary and lunar astrophotography.
AutoStakkert!Lunar and planetary image stacking software.
AutoStitchFree software to create mosaics from individual images, useful for close images of the moon (to get the entire moon in one image) or widefield/panorama nightscapes.
B&H PhotoOnline photography store, which also features some astronomy equipment.
Beginner’s Guide Double StarsUseful guide to binary star systems from the Milwaukee Astronomical Society, especially what magnification is needed to split the stars.
ClearoutsideWeather website and application that is useful to determine potential viewing or astrophotography sessions, especially for cloud cover.
Cloudy NightsForum for astronomy topics and questions. Two notes: 1) be careful trying to set-up an account, as many people often get autobanned if they do it wrong and then they need to reach out to the moderators to have it fixed; and 2) there is a classifieds section, but you must be a member for 180 days (and other requirements) before you can list any gear for sale.
DeepSkyStacker (DSS)Registration and stacking software for deep sky images.
Deep-Sky Watch: What Can You See With a TelescopeWebsite that visually displays how different objects would appear under varying conditions, including by telescope size and location (e.g., dark site vs. light polluted city).
European Space Agency (ESA)Official website of the European Space Agency.
FirecaptureSoftware used with a dedicated astrophotography (planetary) camera to control exposure settings. Useful for planetary and lunar imaging.
GraXpertFree AI tools that can be used within Siril for astrophotography image processing, including steps for background extraction, deconvolution, and denoising.
High Point ScientificOnline astronomy store to purchase equipment.
If the Moon Were Only 1 PixelAn interactive website to visualize how large the solar system is, to the scale of the Moon being 1 pixel large.
My Aurora Forecast & AlertsApplication to identify likelihood of aurorae at your location. (This is also available on the Apple Store.)
NASAOfficial website of the National Aeronautics and Space Administration.
NASA APODAstronomy Picture of the Day from NASA, updated daily.
Next SpaceflightWebsite and application that shows information regarding past and upcoming rocket launches.
Nighttime Imaging ‘N’ Astronomy (NINA)Free and open source software to automate astrophotography session(s).
PHD2Free autoguiding software for astrophotography session(s).
Photons to PhotosUseful website for camera sensor information, including DSLR read noise vs. ISO settings for most widely used/available cameras.
Planetary Imaging Pre-Processor (PIPP)Lunar and planetary pre-processing tool that can be used to separate individual image frames from a video (to then be stacked with other software) when doing “lucky imaging”.
PixInsightAll-in-one paid software for deep sky image stacking and processing. There is also a Forum section on the website.
RegiStaxProcessing software for lunar and planetary imaging. Very useful for wavelet sharpening.
SharpcapSoftware used with a dedicated astrophotography (planetary) camera to control exposure settings. Useful for planetary and lunar imaging.
SirilFree all-in-one software for deep sky image stacking and processing.
Sky and Telescope MagazineOnline magazine for astronomy news. They feature newsletters that include what upcoming events are visible/happening in the sky.
SkyAtlas (ZWO)ZWO application used to remotely control their mounts (this is best used when you don’t have an ASIAir to control and plate solve for astrophotography).
Skyviewer.appInteractive website to view images from the Vera C Rubin Observatory.
Spot the StationOfficial NASA application used to track the International Space Station. Alerts can let you know when it is visible at your location.
StarStaXFree stacking software used primarily for star trail astrophotography.
StellariumWebsite, software, and mobile application used as a personal planetarium. It is free, but the mobile application has a paid version that includes additional features.
Stelvision Telescope SimulatorVisual telescope simulator to realistically “see” what is visible with your equipment.
SVBonyPopular manufacturer of telescopes and astrophotography gear from Hong Kong.
SynScan ProApplication from Sky-Watcher used to remotely control go-to tracker mounts.
Telescope Limiting Magnitude CalculatorWebsite that can help identify the limiting magnitude visible for your equipment and location (used for visual astronomy).
Telescopic Watch: Telescope ReviewsExcellent website that reviews telescopes and makes recommendations (or lack thereof) for various models.
TelescopiusWebsite with lots of great information, including a telescope simulator for both visual and astrophotography.
WinJUPOSFree software that corrects for rotation during planetary imaging. This allows more images to be stacked for a cleaner/sharper final photo.
ZWOPopular Chinese astrophotography manufacturer.

Acronyms, Abbreviations, & Terms to Know

Astronomers and astrophotographers often use abbreviations and jargon to describe things that may be confusing to beginners. The following table (sorted alphabetically by the Acronym/Abbreviation/Term) with many of these common abbreviations/terms and their high-level definition. Finally, with so much information included in this table, it is designed as a quick reference/reminder rather than as a starting point for learning astronomy.

Acronym/Abbreviation/TermDefinition/Description
500 RuleSimple (but outdated) calculation to determine maximum exposure length for untracked astrophotography. Often, this will be an “overestimate” and result in star trailing. A better method is the NPF Rule. The “500 rule” formula is: maximum seconds of exposure = 500 / focal length in mm.
AberrationOptical imperfection(s) in the lens that cause varying wavelengths of light to not focus at the same point. Often this will appear as “chromatic abberation”.
Absolute MagnitudeThe measure of actual brightness/luminosity of a celestial object. A lower number means the object is brighter, due to the exponential nature of how this is calculated.
AchromaticA type of lens that fixes some but not all aberrations present in the optical system, typically resulting in “chromatic aberration” in the image.
Altitude & AzimuthThese form a fixed celestial coordinate system from the viewpoint of the observer. Altitude refers to the height angle relative to the horizon, while azimuth refers to the circular angle along the horizon relative to North (e.g., East is 90°).
Amp GlowA bright, hazy noise pattern on the side of an image during long exposures caused by heat and near-infrared light. This can be calibrated out of a final stacked image through dark frames.
Angular SeparationThe apparent distance between objects for the observer, regardless of their actual distance. This is typically measured in degrees, arcminutes, and/or arcseconds. One arcminute is 1/60 of a degree and one arcsecond is 1/60 of an arcminute.
Annular Solar EclipseThis eclipse occurs when the Moon passes in front of the Sun, but its orbit is further out from the Earth such that the entire Sun is not completely blocked (like during a total solar eclipse). An “outer ring” of the Sun is still visible around the Moon, so it is unsafe to view this without eclipse glasses or specialized filter(s).
APAstrophotography acronym
ApertureThe diameter of the opening (or mirror, for reflector telescopes) of a telescope or lens that determines how much light is able to be collected.
AphelionThe point at which an object’s orbit places it farthest from the Sun.
ApochromaticA type of lens that fixes most (if not all) aberrations present in the optical system. This results in the best possible image or view of the object of interest.
ApogeeThe point at which an object’s orbit places it farthest from the Earth (for objects orbiting the Earth, such as the Moon).
Apparent MagnitudeThe measure of apparent brightness/luminosity of a celestial object from an observer (e.g., from Earth). A lower number means the object is brighter, due to the exponential nature of how this is calculated.
AsterismA recognizable pattern of stars that is not a recognized/official constellation, such as Orion’s Belt or the Big Dipper.
Astronomical Unit (AU)A unit of length that was conceived as the average distance between the Earth and Sun. This is defined as 49,597,870.7 km or 92,955,807.273 miles.
AutoguidingAn automated electronic camera/tool used to ensure the object of interest remains precisely in the field of view during long exposures (for astrophotography) or during observing (for visual astronomy).
BacklashTerm used to describe the very small gaps in gear teeth of a tracking mount for astrophotography.
BackspaceThe amount of space needed between the telescope or camera lens and the camera sensor for astrophotography. The industry standard for this is 55mm.
BarlowA lens that effectively doubles the focal length (and thus the magnification) for a telescope. This comes with a tradeoff that the view becomes dimmer (due to a higher focal ratio) and for visual observing some believe having a barlow produces a worse view than using a smaller focal length eyepiece to achieve the same magnification.
Bayer Matrix [Filter]On color (OSC) cameras, this is a filter that only allows certain wavelengths to pass through to a given pixel. Often this pattern will follow a Red-Green-Green-Blue 2×2 pattern, meaning half of the pixels in the camera will only capture green light. Further processing of an (astronomical) image allows for a full color image to be created.
Bahtinov MaskA simple device placed in front of the telescope or camera lens that is used to assist in achieving proper focus.
Bias FrameA type of calibration frame in astrophotography that is used to remove read noise within a camera from a final, stacked image.
BinningThe process of effectively “combining” pixels in a camera sensor. This can be done during image acquisition or during post-processing, and is commonly done to combat significant oversampling or increase signal-to-noise ratio.
BortleA scale created to measure the impact of light pollution, with lower numbers meaning less light pollution. A “Bortle 1” sky means the darkest skies possible while a “Bortle 9” sky would be a large and very light polluted city.
Broadband FilterA filter that is used with monochrome cameras to capture a wide of light wavelengths, such as capturing all “red” light (e.g., 600 – 700nm light).
Calibration FramesThese are additional images taken that will remove (or “calibrate out”) things that are not wanted in astrophotography images. See this information from High Point Scientific for more information.
Celestial PoleThe imaginary line extending through the middle of the Earth, from North Pole to South Pole. The Earth rotates around this axis.
Charged-Coupled Device (CCD)An older type of astrophotography camera that was very popular prior to CMOS astrophotography cameras.
Chromatic AberrationAn optical imperfection that appears as color fringing on the side of celestial objects, either during visual observation or in astrophotography images.
CollimationThe process of aligning the mirrors in a reflector telescope.
ComaAn optical imperfection that appears as elongated stars toward the edge of an astronomical image.
Coma CorrectorAn optical device that corrects the coma present in the optical system, fixing elongated stars.
Complementary Metal-Oxide Semiconductor (CMOS)A newer and most common dedicated astrophotography camera used by astronomers.
ConjunctionFor an observer, this is when two celestial objects appear close to each other, such as if Jupiter and the Moon are within 1° of each other in the night sky.
ConstellationAn area in the sky, as designated by the International Astronomical Union (IAU) or other organization. Note that these areas are not defined solely by the stars found within the constellation borders (though they are commonly treated and drawn as such on sky maps).
Coronal Mass Ejection (CME)A burst of magnetic fields and plasma material from the Sun that, if aimed toward Earth, can cause geomagnetic storms and auroras on Earth.
Dark FramesA type of calibration frame in astrophotography that is used to remove dark current within a camera from a final, stacked image.
Dawes LimitA theoretical limit for the resolving power of a telescope. The formula is given as: R = 4.56/D, where R is the resolution in arcseconds and D is the aperture in inches.
Declination (DEC) and Right Ascension (RA)These form an unchanging celestial coordinate system from the Earth. Declination refers to the height angle (listed in degrees, arcminutes, and arcseconds) relative to Earth’s equator, while Right Ascension refers to the angle relative to the eastward position along the celestial equator from the Sun at the March equinox (listed in hours, minutes, and seconds). For example, the Andromeda Galaxy is at RA 00h 44m 09s and DEC +41° 24′ 49″.
DeconvolutionAn astrophotography postprocessing technique to recover detail lost/blurred due to atmospheric disturbances, tracking errors, and/or imperfect optics.
DenoiseAn astrophotography postprocessing technique to remove some noise present in the final, stacked image.
Dew ShieldA device added to the end of a telescope to prevent dew from forming on the optics. This also helps prevent stray light from entering the optical train.
Digital Single-Lens Reflex (DSLR)A mirror system camera to capture images which can be used by beginners (and some experts) for astrophotography.
Deep Sky Object (DSO)Any astronomical object that is not a star and is outside of our solar system. Common examples include nebulae and galaxies, which are frequent targets of interest for astrophotographers.
DeepSkyStacker (DSS)Software used to integrate and stack deep sky astrophotography images.
DitherAn astrophotography acquisition technique that moves the camera’s field of view slightly after every X number of frames. This eliminates walking noise in a final, stacked image.
DrizzleAn astrophotography image integration technique that was originally developed for the Hubble Space Telescope to overcome undersampled data.
Dual-Band FilterA filter that allows two specific wavelengths of light to pass through, such as Hydrogen-Alpha and Oxygen-III, while blocking all other light wavelengths.
EclipticThe path that the Sun, Moon, and other planets appear to move across the sky.
Electronically Assisted Astronomy (EAA)A combination of visual astronomy and astrophotography where a camera aids in the viewing of celestial objects by showing the (sometimes “live stacked”) image on a screen.
Electronic Auto Focuser (EAF)A device used to automatically focus a telescope, often used in complex, automated astrophotography set-ups.
EphemerisThe calculated position, velocity, and trajectory of a celestial object, such as planets, asteroids, and comets.
EpochA moment in time used as a reference point. A commonly used epoch is J2000, which is the reference frame of the equinox and ecliptic as of January 1, 2000 12:00 Terrestrial Time (TT).
EquinoxThe time at which the Sun is aligned directly above the equator. This occurs twice per calendar year (March and September).
Equitorial (EQ) MountA telescope mount that aligns with the northern or southern celestial pole and tracks the apparent movement of the night sky (due to the Earth’s rotation).
Exit PupilThe size of the image in the eyepiece. The formula is: eyepiece focal length / telescope f-ratio.
Extra-low Dispersion (ED)A type of lens that fixes many aberrations present in the optical system.
Eye ReliefThe distance your eye should be from the eyepiece to see the full field of view, given in millimeters.
Field of ViewThe amount of sky visible through the telescope and eyepiece, given in degrees.
FIT FileAn astrophotography specific file type that contains the image and metadata.
Flat FieldThis refers to an uniformly-illuminated view, most often required/desired in astrophotography images and can be obtained through a field flattener optical device, if needed.
Flat-Dark FramesA type of calibration frame that matches the exposure time of a flat frame to most accurately eliminate the dark current noise in a “master flat” image.
Flat FramesA type of calibration frame that corrects for dust in the optical train and vignetting.
Focal LengthThe measure of how a telescope or camera lens refracts/reflects light to converge at a focal point, often given in millimeters.
Focal Ratio (F-Ratio)The ratio of focal length and aperture of a telescope or camera lens. A lower number (e.g., f/2) is typically called “fast”, meaning it lets in a lot of light. A higher number (e.g., f/10) is typically called “slow”, as not much light comes into the optical train and causing dimmer views/images.
Focal ReducerA device which reduces the focal length of a telescope, the opposite of a barlow lens.
Full-Width Half Maximum (FWHM)An astrophotography measure of where the amplitude of a light source reaches half its maximum value. For example, a star will in theory be very bright in exactly one pixel and have some brightness in surrounding pixels; the FWHM measures how many nearby pixel(s) it takes to reach a brightness of half that one very bright pixel.
GegenscheinFaint nebulous light from dust that reflects sunlight, located on the opposite side of Earth from the Sun.
Greatest ElongationThe point at which an inferior planet (e.g., Mercury and Venus, from Earth’s perspective) reaches the maximum angular separation from the Sun. This occurs at both a Greatest Eastern Elongation (during which the inferior planet is furthest away from the Sun as seen during the evening) and a Greatest Western Elongation (during which the inferior planet is furthest away from the Sun as seen during the early morning).
GuidingAn astrophotography acquisition process that uses short exposures on a separate camera to identify small tracking imperfections and sending short “pulses” to correct the mount’s position.
Harmonic DriveAlso called a strain wave gear system, this is type of tracker mount used to counter the Earth’s rotation for visual and (especially) astrophotography purposes. This uses a different technology than traditional gear mounts which does not always require counterweights and typically has higher payload capacity.
Hot PixelBright and sometimes discolored pixels caused by an overheated sensor. These can sometimes cause issues with image stacking (such as walking noise) if the underlying images are not dithered properly.
Hydrogen-Alpha (Ha)A specific deep-red visible light wavelength given off by the hydrogen atom, often found in emission nebulae.
InferiorThis refers to the orbital position of one celestial object being inside the orbital position of another. For example, Mercury and Venus are inferior planets to Earth because their orbits are closer to the Sun.
Integration (or Stacking)The astrophotography technique of combining multiple images to improve the signal and (effectively) reduce the noise to make a better a final image.
International Space Station (ISS)A manned space station jointly operated by multiple space agencies, launched in 1998 and continually occupied since late 2000.
KP IndexA measure of the global auroral activity on Earth, with values ranging from 0 (no activity) to 9 (significant activity).
Lagrange PointOrbital positions at which an object may be at equilibrium. There are five points in the Sun-Earth orbit which allow for an object to remain in its own stable orbit. At Lagrange Point 2 (i.e., the stable orbital location superior to Earth from the Sun) is where the James Webb Space Telescope is permanently located.
Light FramesThe most important astrophotography image type, as this is the celestial object of interest that is being captured. For example, these would be the images of the Andromeda Galaxy (that would often then be calibrated and stacked to form a final image).
Light LeakAn optical/imaging train may have a light leak if there are any small openings between components through which unwanted light may be introduced. This can lead to odd artifacts in the image, including strange colored streaks that appear in the background.
Light Year (ly)A unit to measure the distance light travels in one Earth year, defined as 9.46 trillion kilometres or 5.88 trillion miles.
LinearThe original state of an astronomical image that shows the true number of photons (and thus electrons released) by each pixel without alteration. Often this will be a very dark image, which will then be stretched to show the contrast in the imaged object of interest.
MeridianThe imaginary line through the sky from the north celestial pole, overhead to the observer’s zenith, and to the south celestial pole.
Messier ObjectA collection of deep sky objects cataloged by French astronomer Charles Messier. These are some of the brightest and biggest deep sky objects in the (northern hemisphere) sky, and are often some of the first targets of interest for amateur astrophotographers and visual observers.
Narrowband FilterA filter that is used with monochrome cameras or visual astronomers to allow a small range of light wavelengths, such as only allowing 3 or 7 nanometers of Hydrogen-Alpha light (around 656nm).
Near-Earth Object (NEO)An asteroid or comet that passes within 1.3 astronomical units of Earth.
New General Catelogue (NGC)A commonly referenced astronomical catalog featuring 7,840 celestial objects.
NoiseUnwanted “stuff” that occurs in astrophotography images resulting from anything other than the object(s) that are being imaged. Sources for this include camera read noise, camera thermal noise (dark current), and light pollution.
NonlinearThe result of an astrophotography image after it has been stretched out of its original linear form. This allows for better contrast in the photographed object(s) of interest.
NPF RuleA more complicated formula than the “500 rule” to calculate the maximum exposure time for individual untracked astrophotography images. (Online calculators can help you with this.)
OccultationThis is when one celestial object passes in front of another celestial object, such as if the Moon passes in front of and obscures the view of another planet (e.g., Mars).
Off-Axis Guiding (OAG)A guiding technique in astrophotography that uses a prism to “prick” light from the primary optical system for guiding rather than requiring a separate guide telescope. This is very useful when using long focal lengths.
OppositionThe point at which a superior celestial object is exactly opposite in the sky than the Sun. For example, when Jupiter, Earth, and the Sun are perfectly aligned in their orbits, Jupiter is said to be at opposition and is at the closest point in its orbit to Earth, making it appear the biggest and brightest possible.
One Shot Color (OSC)This is a typical acronym to describe cameras that natively capture color images (using a bayer matrix filter).
Optical Tube Assembly (OTA)This is a common acronym used to describe the telescope tube, including any lenses or mirrors within said tube.
OversampleThis describes when the pixel scale is smaller than focal length and astronomical seeing allows, making images often appear soft.
ParallaxThe apparent shift in an objects position compared to other more distant objects due to the orbit of the Earth.
Parsec (pc)A unit to measure the distance of objects outside the solar system, defined roughly as 3.26 light years or 206,265 astronomical units.
ParigeeThe point at which an object’s orbit places it closest to the Earth (for objects orbiting the Earth, such as the Moon).
PenumbraThe lighter portion of a shadow where light is partially obstructed. This is most apparent during lunar eclipses.
Periodic Error (PE)The tracking error that happens periodically in the Right Ascension direction of a tracking mount.
PerihelionThe point at which an object’s orbit places it closest to the Sun.
Pinched OpticsThis occurs when the clips/screws holding a telescope objective lens or mirror is too tight, causing a distortion in the image most notable on large stars.
Pixel ScaleHow much of the field of view covered by one pixel in an image, denoted in arcseconds. For example, a well-sampled image is likely to be between 1-2 arcseconds per pixel with OK seeing.
Plate SolveThe method/process used to determine what portion of the sky is represented in an image.
Polar Alignment (PA)The process of lining up a telescope mount with either the North or South celestial pole. This is done to allow the mount to counter the Earth’s rotation to keep an object of interest in the desired field of view.
PrecessionThe change in orientation of an object’s rotational axis. Earth “wobbles” every 26,000 years, meaning that Polaris (or the “North Star”) has not and will not always be at Earth’s North celestial pole.
Point Spread Function (PSF)The description for how a light source’s shape appears in an image.
Rayleigh LimitThe minimum angular separation at which an optical system can resolve (or distinguish between) two points of light, such as a binary star system.
RedshiftThe increase in wavelength due to the distance between two objects increasing. For example, due to the expansion of the universe, far objects appear redder than similar closer objects. This can be used to determine how far away objects are and at what rate the universe is expanding.
ReducerA lens that reduces the focal length of an optical system. This is the opposite of a barlow lens.
Remote ObservatoryHosting location for astrophotography equipment (for a fee). Usually has excellent seeing/transparency and mostly clear skies (250-300 nights of the year).
RetrogradeThe apparent “backwards” motion of a superior planet (or object orbiting the Sun) due to Earth seeming to overtake the other object.
Root Mean Square (RMS)Referring to the accuracy of astrophotography guiding, this is the distance that guide stars move averaged between the right ascension and declination axes. The is dependent upon seeing conditions and equipment (particularly the mount).
SamplingThis refers to how sharp and how much detail can be seen in an astrophotography image, as determined by pixel scale and seeing conditions. A well-sampled image will be sharp, as opposed to over-sampled (appearing “soft”) or under-sampled (lacking detail) images.
SeeingA categorization/measurement of the turbulence (or unstable air) in the atmosphere, leading to blurring and distortions in visual astronomy or astrophotography images.
Shot NoiseThis defines the randomness of light photons (quanta) reaching the camera sensor. Sources of “shot noise” include both desired signal (i.e., the light from the celestial object being photographed) and undesired signal (i.e., noise) most commonly in the forms of light pollution and dark current. Both signal and noise components increase with greater integration (i.e., longer individual exposures or more stacked exposures), though the noise component is equal to the square root of the total signal such that the signal-to-noise ratio increases with the greater integration.
SiderealThe time scale related to the rotation of the Earth in respect to the fixed stars in the night sky. One Earth sidereal day equals 23 hours 56 minutes 4 seconds long.
SignalIn astrophotography, this is the desired light (and thus electrons in the camera sensor) that come from the desired object of interest being imaged. Everything else that affects the camera sensor is “noise”.
Signal-to-Noise Ratio (SNR)The amount of Signal compared to Noise in an astrophotography image. Best images have a high SNR and can best be achieved by longer total integration/exposure times. Other things can help increase SNR, such as having a cooled dedicated astrophotography camera to limit dark current or imaging from a dark site location.
Smart TelescopeAn all-in-one astrophotography device that can go-to an object of interest, plate solve, take numerous short exposure images, and (internally) stack the images to produce a final result. This can be controlled by a smart phone or tablet. (Note: this device is not used at all visually, meaning looking through an eyepiece.)
SlewThe term for a mount moving the telescope to a celestial target in the sky.
SolsticeThe time when the sun reaches the “highest point” in the Northern or Southern skies. These occur in June and December, respectively.
Speed of Light (c)The universal physical constant at which no object or information can move faster. This is defined as 299,792,458 meters per second (or ~186,000 miles per second).
Star RemovalThe astrophotography post processing technique of removing stars from an image in order to separately edit them and the nebula or galaxy that was imaged.
Strong Thermal Emission Velocity Enhancement (STEVE)Purple and green light in a straight line in the sky that is believed to be caused by hot gases in the atmosphere. (Example.)
StretchThe astrophotography post processing technique of making the dim object of interest have greater contrast (and the bright stars technically having less contrast).
SuperiorThis refers to the orbital position of one celestial object being inside the orbital position of another. For example, Mars, Jupiter, Saturn, Uranus, and Neptune are superior planets to Earth because their orbits are further from the Sun.
TerminatorThe line separating the illuminated and dark hemispheres of an object, such as the moon.
Tidal LockTerm used to describe when an orbiting body’s rotation is synchronized with it’s orbit. For example, the Moon is tidally locked to Earth such that it’s rotation matches it’s orbit, meaning the same side of the Moon is always facing the Earth.
Total Lunar EclipseAn event when the Moon passes through the Earth’s shadow, covering the entire Moon so it doesn’t receive any direct sunlight.
Total Solar EclipseAn event when the Moon passes between the Earth and the Sun, covering the entire Sun from certain location(s) on the Earth. It is only safe to view this with an unaided eye during totality, when the Sun is completely blocked.
Tracking MountThe device used to counter the rotation of the Earth so an object of interest is kept in the desired field of view.
TransitAn event when a smaller celestial body passes in front of another larger celestial body. For example, Mercury or Venus sometimes pass directly across the face of the Sun.
TransparencyA categorization/measurement of the amount of particles, smoke, moisture, etc., in the air that affect how “clear” the air appears.
UmbraThe innermost and darkest portion of a shadow where light is fully obstructed. This is most apparent during total lunar eclipses when the moon turns red (which is due to how light refracts in the Earth’s atmosphere since no direct sunlight is hitting the Moon at that time).
UndersampleThis describes when the pixel scale is larger than focal length and astronomical seeing allows, making images lack detail.
Untracked AstrophotographyAstrophotography imaging done without a tracking mount. The object of interest is photographed with very short exposures and the field of view is constantly moved back in frame after drifting across the camera sensor.
VignetteThe darkening of an image around the edges due to less light reaching that area of a camera sensor. (In astrophotography, this is calibrated out through flat frames).
Walking NoiseThe result of fixed noise patterns in a camera sensor, such as hot pixels, causing a consistent “streaking” across an image. (In astrophotography, this is best handled through dithering during image acquisition.)
WaveletsA representation of an image at different scales. Adjusting wavelets in post processing – in particular with lunar and planetary imaging – can help sharpen an image and bring out more detail.
ZenithThe point in the sky directly overhead an observer.
ZodiacThe region of the sky that extends approximately 8° north and south of the ecliptic. There are 13 zodiac constellations (despite what astrology incorrectly says), which are: Capricornus, Aquarius, Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, and Ophiuchus.
Zodiacal LightFaint glow from dust in space reflecting sunlight along the ecliptic.