Mount Requirements
A motorized mount with good stability and excellent tracking are essential for deep sky viewing with a camera since exposures will likely be in the range of tens of seconds to more than a minute. Otherwise the rotation of the earth will cause elongated stars after only a few seconds. Like the choice of optics, there are many options for mounts that can be used for EAA. These are the key requirements:
A motorized mount with good stability and excellent tracking are essential for deep sky viewing with a camera since exposures will likely be in the range of tens of seconds to more than a minute. Otherwise the rotation of the earth will cause elongated stars after only a few seconds. Like the choice of optics, there are many options for mounts that can be used for EAA. These are the key requirements:
- Accurate and stable tracking is a must
- Alt-Az mounts work, but will have limitations on exposure time due to field rotation
- Equatorial mounts are an excellent choice
Software Bisque MX EQ Mount with Celestron 9.25" SCT
TRACKING
Visual observing is very forgiving of the motion of the sky, but astrophotography and EAA are not. Tracking is essential because star elongation due to the rotation of the earth is perceptible in a camera image after only a few seconds (see my page on Field Rotation for details). Assume we are using a camera with 3.8micron pixels and are trying to image an object at 60 degrees declination, due south (Azimuth 180deg) at a latitude of 37degrees. Using an 8" SCT at f/5 (1000mm focal length), a 10sec exposure produces a star trail 3.7 pixels long on a camera with a 12mm diagonal. This is enough to create noticeable star trails and cause blurring of the deep sky object. Even an 80mm f/5 refractor produces a star trail 3.6 pixels long with a 2sec exposure. Since many exposures will be longer than 5sec, a mount that will track the motion of the stars is an absolute necessity.
GOTO
All tracking mounts have a GoTo feature which will locate and center deep sky objects quickly and allow you to spend more time observing instead of searching. A GoTo alignment procedure is required to allow the mount to make a model of the night sky. This can be done with a single star or planet but for a more accurate alignment model at least 3 stars, and often more, are required. An alternative method to locate and center faint deep sky objects is to use Plate Solving whereby the camera takes and image, the SW compares the stars in the image to a database to determine exactly where in the sky the telescope is centered. If it is not centered on the object of interest the SW will command the mount to move to the right location, another image is taken and compared to the database to refine the location, and the process is repeated until the target is centered within a pre-specified tolerance. With Plate Solving only a good Polar Alignment is needed and a GoTo alignment can be skipped.
Visual observing is very forgiving of the motion of the sky, but astrophotography and EAA are not. Tracking is essential because star elongation due to the rotation of the earth is perceptible in a camera image after only a few seconds (see my page on Field Rotation for details). Assume we are using a camera with 3.8micron pixels and are trying to image an object at 60 degrees declination, due south (Azimuth 180deg) at a latitude of 37degrees. Using an 8" SCT at f/5 (1000mm focal length), a 10sec exposure produces a star trail 3.7 pixels long on a camera with a 12mm diagonal. This is enough to create noticeable star trails and cause blurring of the deep sky object. Even an 80mm f/5 refractor produces a star trail 3.6 pixels long with a 2sec exposure. Since many exposures will be longer than 5sec, a mount that will track the motion of the stars is an absolute necessity.
GOTO
All tracking mounts have a GoTo feature which will locate and center deep sky objects quickly and allow you to spend more time observing instead of searching. A GoTo alignment procedure is required to allow the mount to make a model of the night sky. This can be done with a single star or planet but for a more accurate alignment model at least 3 stars, and often more, are required. An alternative method to locate and center faint deep sky objects is to use Plate Solving whereby the camera takes and image, the SW compares the stars in the image to a database to determine exactly where in the sky the telescope is centered. If it is not centered on the object of interest the SW will command the mount to move to the right location, another image is taken and compared to the database to refine the location, and the process is repeated until the target is centered within a pre-specified tolerance. With Plate Solving only a good Polar Alignment is needed and a GoTo alignment can be skipped.
Celestron AVX EQ Mount
EQUATORIAL MOUNTS
Just as an equatorial mount is an excellent choice for astrophotography, it is also an excellent choice for EAA. With a good polar alignment and a quality EQ mount, one can take exposures of one to several minutes and not have to worry about field rotation because an equatorial mount tracks the sky motion by rotating its right ascension axis at the same rate. As with astrophotography it is important to balance the scope carefully and make sure to stay within the advertised weight limits of the mount. An equatorial mount with tracking and GoTo is an excellent choice for EAA. Alt-Az mounts on a wedge will also eliminate the problem of field rotation and have the added advantage of not needing to do a meridian flip. However, this setup is not as stable as an equatorial mount and may not track quite as well. For Dobsonians (assuming one can achieve focus with the camera) an equatorial platform will provide tracking for sufficient time for typical exposures, but this setup is not ideal due to the constant need to re-set the platform.
Just as an equatorial mount is an excellent choice for astrophotography, it is also an excellent choice for EAA. With a good polar alignment and a quality EQ mount, one can take exposures of one to several minutes and not have to worry about field rotation because an equatorial mount tracks the sky motion by rotating its right ascension axis at the same rate. As with astrophotography it is important to balance the scope carefully and make sure to stay within the advertised weight limits of the mount. An equatorial mount with tracking and GoTo is an excellent choice for EAA. Alt-Az mounts on a wedge will also eliminate the problem of field rotation and have the added advantage of not needing to do a meridian flip. However, this setup is not as stable as an equatorial mount and may not track quite as well. For Dobsonians (assuming one can achieve focus with the camera) an equatorial platform will provide tracking for sufficient time for typical exposures, but this setup is not ideal due to the constant need to re-set the platform.
Sky-Watcher AZ-GTi Alt-Az Mount
Alt-Az Mounts
Alt-Az mounts are also popular for EAA. They can be a good starting mount for EAA because of their lower cost and simplicity of setup compared to EQ mounts. They do not require a polar alignment - in fact they cannot be polar aligned since their axes of rotation are aligned with the local horizon and meridian, not the axis of rotation of the earth (unless at the north or south poles). This means that the apparent rotation of the sky limits the maximum exposure possible with an Alt-Az mount for live views. The field rotation rate depends upon the latitude of the observer, as well as, the Azimuth angle and Altitude of the object being observed. For most North American latitudes it is reasonable to expect to be able to use exposures of 30 seconds or less without noticeable field rotation. This is long enough to view many deep sky objects of interest. Exposures longer than that are possible at higher latitudes and/or at certain locations in the sky. See my page on Field Rotation for more details.
When used with software like the free program Sharpcap which can translate and rotate frames while stacking them live, I have been able to stack frames for more than 5min without any problem with field rotation. Obviously, the stars will rotate during such long exposures, but the software takes care to adjust each frame so that the images from each frame line up. You will notice, however, that the edges of the stacked frame will show some black where the images do not all line up.
Another issue with Alt-Az mounts arises when trying to view objects at the zenith. Because of their design, many cameras are large enough that they will crash into the mount base when the mount points at the zenith. To avoid this, all but a very few cameras, like the ZWO ASI224, need to be connected to the scope with a diagonal.
Alt-Az mounts are also popular for EAA. They can be a good starting mount for EAA because of their lower cost and simplicity of setup compared to EQ mounts. They do not require a polar alignment - in fact they cannot be polar aligned since their axes of rotation are aligned with the local horizon and meridian, not the axis of rotation of the earth (unless at the north or south poles). This means that the apparent rotation of the sky limits the maximum exposure possible with an Alt-Az mount for live views. The field rotation rate depends upon the latitude of the observer, as well as, the Azimuth angle and Altitude of the object being observed. For most North American latitudes it is reasonable to expect to be able to use exposures of 30 seconds or less without noticeable field rotation. This is long enough to view many deep sky objects of interest. Exposures longer than that are possible at higher latitudes and/or at certain locations in the sky. See my page on Field Rotation for more details.
When used with software like the free program Sharpcap which can translate and rotate frames while stacking them live, I have been able to stack frames for more than 5min without any problem with field rotation. Obviously, the stars will rotate during such long exposures, but the software takes care to adjust each frame so that the images from each frame line up. You will notice, however, that the edges of the stacked frame will show some black where the images do not all line up.
Another issue with Alt-Az mounts arises when trying to view objects at the zenith. Because of their design, many cameras are large enough that they will crash into the mount base when the mount points at the zenith. To avoid this, all but a very few cameras, like the ZWO ASI224, need to be connected to the scope with a diagonal.
© 2024 Curtis V. Macchioni