Generally, there are several kinds of Celestial Coordinate System. The major ones are Horizontal and Equatorial. The Equatorial Coordinate System is used to specify locations of objects in the sky when observed from Earth. The related article on Wikipedia is a long and is more a mathematical reading. I found the article about Declination (abbreviated “DEC”) a bit more practical for astrophotography. DEC is comparable to latitude in a simplified form. The second part of a coordinate is Right ascension (abbreviated RA) is comparable to longitude. Sky-and-Telescope has a good introduction article about all this.

“DEC” and “RA” will follow you all the time when you use the gear or read information about objects. Each object in the night sky has coordinates expressed in DEC and RA. DEC is expressed in degrees, whereas RA is expressed in hours, minutes, and seconds of time (0 to 24 hours). These coordinates are independent of your local time and and location. Still, the time and location are needed to calculate the direction in which your camera should point to get a given object in the field of view.

An equatorial mount has two rotation axis. They are named after the axis of the coordinates they are following. RA (Right Ascension) is perpendicular to the Equator plane and so coaxial to the axis of the rotation of the Earth. The DEC axis is perpendicular to the RA axis.

Today, you most probably do not need to twiddle with DEC and RA numbers in coordinates. Most of the time, these coordinates can be copied from a star navigation or planning software directly into the Go-To software of your mount or another control system.

Quite some mount control systems use have integrated catalogs. With this, there is no need to copy-n-paste coordinates. You can search for objects by name. Quite some manufacturers advertise their mounts by the number of object in the integrated catalogs. Do not treat these marketing numbers as a main choice criteria for a mount. They are only a nice-to-have. The ability to “Go-To” to a tiny stars by its name is still handy, if that star happened to be in the center of the area you want to photograph.

Catalogs

All, or better “most”, objects in the sky are listed in one of the Astronomical Catalog. Creation of these catalogs goes far back in the history. In absence of today’s modern equipment, this was a complex, and almost a heroic work. Due to historical fragmentation and different purposes of observations, there is a very large number of different catalogs with a lot of overlaps. The good news is that only 3-4 of them are used astrophotography most. An even better news is that all of them are meanwhile integrated in some software where you will need fetch data from a catalog.

In the first steps, you should only memorize haw catalogs are referred. For example, “M” in the beginning “M42” stands for Messier Catalog. M42 is the famous Orion Nebula. Since Mr. Messier was creating his catalog as a list of objects disturbing his observations of comets, many other astronomers spotted this prominent object in the sky in dedicated observations and named it in their own way: NGC 1976. NGC stands for “New General Catalogue”. Later on, NGC has been upgraded and became the IC catalog, which is even split in two parts. Since may folks did not find time and money to re-write names just because of a catalog upgrade, in many cases NGC names remained along with the the new IC names. In addition, the famous Orion Nebular is also known as LBN 974, and Sharpless 281. On the first run, all this sound confusing and it actually is. In praxis, it is much better. The confusion is hidden and is controlled by the modern software. Coordinates for the same object are the same, well, in most cases…

And just to make the confusion complete, it is worth mentioning that quite some attractive areas in the sky or nice groups and clusters of objects, like the magnificent Orion Molecular Cloud Complex are not listed in any catalogs. You need to find coordinates of the center of the FOV either on the Internet or compose this on your own a help of a star navigation software, e.g. Stellarium.

Emil Ivanov provides a nice list of major catalogs including a short description of a catalog and images from a very large number of objects sorted by catalog.

Major Object Properties

An object in the sky has a very large number of astronomical numeric properties. The three below are most important for astrophotography:

• Rise and set times as well the maximum altitude are the most important to know. They depended on date, time and location and thus must be calculated each time you need them. They are most important, since they de-faco pre-define the quality and the duration of visibility of an object. Certainly, rise and set times should fall into the dark time at your location on a given date. Low altitudes have many negative atmospheric effects and aggregate more light pollution. I set my lowest limit at 20 degrees. Telescopius does a great job in visualizing these values for a selected object at a given time of the date and date of the year. For some reasons such overview is hard to find elsewhere. It is very convenient for planning. Despite of the maximum altitude, you can also see if an object rises on your location at all.
• Angular size or diameter, basically the apparent dimensions. This is the second most important parameter if you have a limited selection of gear. The values are expressed in arc-minutes and arc-seconds. The size of the largest and a famous object on the sky – the  Andromeda Galaxy has to be expressed even in degrees: 3.167 x 1. Andromeda is around six times larger than the Moon. Angular size is important as it basically defines if and how it makes sense to photograph an object with your equipment. Despite of the Andromeda Galaxy there are around 20-30 other famous Deep Sky targets which you can photograph in a focal length of 200-400mm on an APS-C camera. Spend some time and add parameters about your equipment in a star navigation software to visualize the resulting FOV on a map of the sky. This will help to make decisions when and how to photograph.
• Apparent magnitude which is basically the visual brightness. The brightness is measured on a scale from -1 to 10, where 0 is the brightness of Vega. Apparent magnitude is not very important, rather just informative. You should use this value just for a rough judgment of what you are going to target and process later. For example, Whirlpool Galaxy is a small (11’x 7′) and faint object with magnitude of 8.4. It is a lower limit from my perspective what makes sense to target with an APS-C camera and a 400mm lens. The Andromeda Galaxy has magnitude of 3.44 and the Orion Nebulais listed to have 4, whereas the core of the Orion Nebula is so bright that you need to make an HDR composition to retain its details.

Finding Objects Easy Way

Unless you are interested in star constellations and in traditional navigation methods across the starry sky, finding an object and moving it into the FOV of your camera is just a necessary burden in astrophotography, especially in the beginning. Fortunately, today, it is completely automated while using any modern equatorial mount. If you use a star tracker, it is fare more challenging. You can either use a laser pointer, which is not allowed in many places, or slowly approach the final destination by trail-n-error.

The Sun Actually Sets at Three Different Times

Yes! A civil, nautical and an astronomical. The fun starts at the end of the astronomical twilight – “Astronomical dusk” which is the start of so called “dark time”. Most regular weather and information services do not specify which phase of the sunset and sunrise they refer to, but it looks like they use the beginning of civil twilight. Quite sometime time passes until from the first phase of the sunset and until the dark time. Each of the three twilight phases runs for 30-45 minutes (depending on the location, certainly). For the regular use, Telescopius provides a timeline with sunset, sunset and the dark time for each of your locations along with the moon rise and set times. I usually plan to arrive on my astrophotography spot latest one hour before the dark time starts. The time until the “dark time” is roughly the same the time I need to setup, calibrate the equipment and do some test shots.

Seasons

Since all the things in the universe rotate around some center, including Earth, the view into the sky from the Earth surface is very different in the nights through the year. Some objects are only visible from the South or from the North Hemisphere or or never rise hight enough. Despite of the weather and seeing this is the next largest factor which defines you can photograph in a given month. See the page Seasons of the Astrophotography.

Sun, Moon, Planets

The Sun and the Moon are large (just “large”, not “largest”) objects on the sky. They are roughly of the same visual size. They both can be interesting targets for astrophotography. You can photograph both of them with just a tripod and a super telephoto lens. As you know, you should be careful with the Sun. Be very careful! Photographing the Sun is still possible with special filters. It is also very tricky, since the best pictures can be made in seasons of rare and unpredictable strong solar storms. Photographing Sun requires special techniques which are very different from the deep sky astrophotography. The same applies to the planets of the Solar System, and on top, they are visually extremely small even though they are so close.

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Generally, there are several kinds of Celestial Coordinate System. The major ones are Horizontal and Equatorial. The Equatorial Coordinate System is used to specify locations of objects in the sky when observed from Earth. The related article on Wikipedia is a long and is more a mathematical reading. I found the article about Declination (abbreviated “DEC”) a bit more practical for astrophotography. DEC is comparable to latitude in a simplified form. The second part of a coordinate is Right ascension (abbreviated RA) is comparable to longitude. Sky-and-Telescope has a good introduction article about all this.

“DEC” and “RA” will follow you all the time when you use the gear or read information about objects. Each object in the night sky has coordinates expressed in DEC and RA. DEC is expressed in degrees, whereas RA is expressed in hours, minutes, and seconds of time (0 to 24 hours). These coordinates are independent of your local time and and location. Still, the time and location are needed to calculate the direction in which your camera should point to get a given object in the field of view.

An equatorial mount has two rotation axis. They are named after the axis of the coordinates they are following. RA (Right Ascension) is perpendicular to the Equator plane and so coaxial to the axis of the rotation of the Earth. The DEC axis is perpendicular to the RA axis.

Today, you most probably do not need to twiddle with DEC and RA numbers in coordinates. Most of the time, these coordinates can be copied from a star navigation or planning software directly into the Go-To software of your mount or another control system.

Quite some mount control systems use have integrated catalogs. With this, there is no need to copy-n-paste coordinates. You can search for objects by name. Quite some manufacturers advertise their mounts by the number of object in the integrated catalogs. Do not treat these marketing numbers as a main choice criteria for a mount. They are only a nice-to-have. The ability to “Go-To” to a tiny stars by its name is still handy, if that star happened to be in the center of the area you want to photograph.

Catalogs

All, or better “most”, objects in the sky are listed in one of the Astronomical Catalog. Creation of these catalogs goes far back in the history. In absence of today’s modern equipment, this was a complex, and almost a heroic work. Due to historical fragmentation and different purposes of observations, there is a very large number of different catalogs with a lot of overlaps. The good news is that only 3-4 of them are used astrophotography most. An even better news is that all of them are meanwhile integrated in some software where you will need fetch data from a catalog.

In the first steps, you should only memorize haw catalogs are referred. For example, “M” in the beginning “M42” stands for Messier Catalog. M42 is the famous Orion Nebula. Since Mr. Messier was creating his catalog as a list of objects disturbing his observations of comets, many other astronomers spotted this prominent object in the sky in dedicated observations and named it in their own way: NGC 1976. NGC stands for “New General Catalogue”. Later on, NGC has been upgraded and became the IC catalog, which is even split in two parts. Since may folks did not find time and money to re-write names just because of a catalog upgrade, in many cases NGC names remained along with the the new IC names. In addition, the famous Orion Nebular is also known as LBN 974, and Sharpless 281. On the first run, all this sound confusing and it actually is. In praxis, it is much better. The confusion is hidden and is controlled by the modern software. Coordinates for the same object are the same, well, in most cases…

And just to make the confusion complete, it is worth mentioning that quite some attractive areas in the sky or nice groups and clusters of objects, like the magnificent Orion Molecular Cloud Complex are not listed in any catalogs. You need to find coordinates of the center of the FOV either on the Internet or compose this on your own a help of a star navigation software, e.g. Stellarium.

Emil Ivanov provides a nice list of major catalogs including a short description of a catalog and images from a very large number of objects sorted by catalog.

Major Object Properties

An object in the sky has a very large number of astronomical numeric properties. The three below are most important for astrophotography:

• Rise and set times as well the maximum altitude are the most important to know. They depended on date, time and location and thus must be calculated each time you need them. They are most important, since they de-faco pre-define the quality and the duration of visibility of an object. Certainly, rise and set times should fall into the dark time at your location on a given date. Low altitudes have many negative atmospheric effects and aggregate more light pollution. I set my lowest limit at 20 degrees. Telescopius does a great job in visualizing these values for a selected object at a given time of the date and date of the year. For some reasons such overview is hard to find elsewhere. It is very convenient for planning. Despite of the maximum altitude, you can also see if an object rises on your location at all.
• Angular size or diameter, basically the apparent dimensions. This is the second most important parameter if you have a limited selection of gear. The values are expressed in arc-minutes and arc-seconds. The size of the largest and a famous object on the sky – the  Andromeda Galaxy has to be expressed even in degrees: 3.167 x 1. Andromeda is around six times larger than the Moon. Angular size is important as it basically defines if and how it makes sense to photograph an object with your equipment. Despite of the Andromeda Galaxy there are around 20-30 other famous Deep Sky targets which you can photograph in a focal length of 200-400mm on an APS-C camera. Spend some time and add parameters about your equipment in a star navigation software to visualize the resulting FOV on a map of the sky. This will help to make decisions when and how to photograph.
• Apparent magnitude which is basically the visual brightness. The brightness is measured on a scale from -1 to 10, where 0 is the brightness of Vega. Apparent magnitude is not very important, rather just informative. You should use this value just for a rough judgment of what you are going to target and process later. For example, Whirlpool Galaxy is a small (11’x 7′) and faint object with magnitude of 8.4. It is a lower limit from my perspective what makes sense to target with an APS-C camera and a 400mm lens. The Andromeda Galaxy has magnitude of 3.44 and the Orion Nebulais listed to have 4, whereas the core of the Orion Nebula is so bright that you need to make an HDR composition to retain its details.

Finding Objects Easy Way

Unless you are interested in star constellations and in traditional navigation methods across the starry sky, finding an object and moving it into the FOV of your camera is just a necessary burden in astrophotography, especially in the beginning. Fortunately, today, it is completely automated while using any modern equatorial mount. If you use a star tracker, it is fare more challenging. You can either use a laser pointer, which is not allowed in many places, or slowly approach the final destination by trail-n-error.

The Sun Actually Sets at Three Different Times

Yes! A civil, nautical and an astronomical. The fun starts at the end of the astronomical twilight – “Astronomical dusk” which is the start of so called “dark time”. Most regular weather and information services do not specify which phase of the sunset and sunrise they refer to, but it looks like they use the beginning of civil twilight. Quite sometime time passes until from the first phase of the sunset and until the dark time. Each of the three twilight phases runs for 30-45 minutes (depending on the location, certainly). For the regular use, Telescopius provides a timeline with sunset, sunset and the dark time for each of your locations along with the moon rise and set times. I usually plan to arrive on my astrophotography spot latest one hour before the dark time starts. The time until the “dark time” is roughly the same the time I need to setup, calibrate the equipment and do some test shots.

Seasons

Since all the things in the universe rotate around some center, including Earth, the view into the sky from the Earth surface is very different in the nights through the year. Some objects are only visible from the South or from the North Hemisphere or or never rise hight enough. Despite of the weather and seeing this is the next largest factor which defines you can photograph in a given month. See the page Seasons of the Astrophotography.

Sun, Moon, Planets

The Sun and the Moon are large (just “large”, not “largest”) objects on the sky. They are roughly of the same visual size. They both can be interesting targets for astrophotography. You can photograph both of them with just a tripod and a super telephoto lens. As you know, you should be careful with the Sun. Be very careful! Photographing the Sun is still possible with special filters. It is also very tricky, since the best pictures can be made in seasons of rare and unpredictable strong solar storms. Photographing Sun requires special techniques which are very different from the deep sky astrophotography. The same applies to the planets of the Solar System, and on top, they are visually extremely small even though they are so close.

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