Tracking and Guiding
An Equatorial mount has a motor that rotates the attached Optical Tube Assembly (or OTA) around the polar, or equatorial axis, at the same rate as the spin of the earth but in the opposite direction. The earth spins towards the east so the mount needs to rotate in a westerly direction in order to follow a point in the sky. The accuracy of the rotation speed will likely depend on the quality of the mount.
The rotational position around the pole in the sky is called the Right Ascension (or RA) and the rotational speed and be sped up or down to correct the rate by signals sent to the mount. Inaccuracies in motors and gears could lead to errors in maintaining sync with the earth's rotation.
Inaccuracies with the alignment of the mount's equatorial axis pointing to the Celestrial Pole will cause a drift in the pointing to the sky, either up or down, over time. This is why Polar Alignment is important, as the more accurately the alignment of the equatorial axis to the Celestrial Pole, the smaller the pointing drift up or down. This up or down angle in the sky is called the Declination (or Dec) and can be corrected by signals sent to the mount.
Tracking
Long exposure imaging requires accurate tracking of an object's apparent motion in the sky for the duration of each exposure.
Inaccurate tracking of the sky will show up as smudge like blurs of stars in an image in place of the expected pinpoint dot of a star.
- It should be noted that star streaks or smudges at the edge of an image will be from field rotational errors caused by poor Polar Alignment and can not be fixed with tracking. An Equatorial Mount is designed to rotate the OTA around the Celestrial Pole in the sky. The surface of the camera sensor must rotate in sync with the rotation of the sky. Tracking corrections can not keep all parts of a sensor following the sky. The larger the sensor, the larger the field rotation error the more pronounced the star streaks. Good Polar Alignment will allow the Camera to rotate correctly resulting in all parts of a camera sensor rotating evenly as stars appear to rotate around the Celestial Pole.
A Mount is responsible for tracking the sky. The quality of the tracking is usually reflected in the quality of the Mount. The more expensive Mounts tend to rely on more precise manufacturing, better technology and associated software to obtain accurate tracking of the sky.
A perfect mount does not exist. Even the most accurately manufactured Mount can still not correctly track an object due to it not being able to predict the changing atmospheric distortion in advance.
A way to reduce the inaccuracies in the tracking is for a Mount to make many measurements of the sky, compare the star positions to accurate star catalogues, and produce a mathematical model of the sky with the difference between the real and apparent position of stars in the sky. This model is called a tracking model. Some high-end Mounts create and use these models to improve tracking.
The mount quality, together with the tracking model corrections, can make tracking so accurate that depending on the camera's image sensor size, further constant corrections may not be necessary.
The constant correction of the mount's tracking is called guiding. Even with the best of astrophotographic systems, guiding could be considered a SafetyNet to compensate for mechanical changes in a non-perfect tripod and mount.
Guiding
Originally, guiding a telescope, keeping it pointing to the same spot in the sky was done by hand. A person constantly observed stars and manually adjusted the mount pointing direction as needed.
Computers and sensitive cameras can now make this tedious guiding task automatic.
Autoguiding
Automatic guiding, or Autoguiding, allows computers to replace a person monitoring the mount movement and making manual corrections with computer-controlled movements.
Autoguiding can also reduce tracking inaccuracies in a Mount. The basic idea of auto-guiding is for a computer application to continuously take an image of the sky where the OTA is pointing, select and remember a feature's position within that image, take another image, find the same feature in the next image and if the feature has moved, then send correction instructions to the mount to correct the Mount's pointing position. This cycle is constantly repeated so that the movement of the mount can be constantly corrected.
Autoguiding allows the Mount's tracking to be constantly monitored and corrected in real-time, improving the accuracy of tracking the sky and also improving the chances of obtaining pinpoint stars in long exposure images.
Types of Guiding setups
The following setups are used for Astrophotography Autoguiding.
- Guidescope - the most common setup for OTAs up to about a focal length of 1500mm.
- Off-Axis Guiding (OAG) - the most common setup for OTAs over about a focal length of 1500mm.
- On-Axis Guiding (ONAG)
- Image Autoguiding
1. Guidescope Guiding.
Guidescope guiding uses a separate telescope, called a guidescope, for guiding. A guidescope is smaller OTA and camera attached to the main imaging OTA and aligned along the same axis pointing to the same position in the sky.
2. Off-Axis Guiding (OAG)
Off-axis guiding is where the guiding camera is not aligned with the same axis as the OTA.
The light in the OTA used for imaging is sampled by positioning a small mirror to the side of the imaging sensor path (before any filters) to deflect a portion of the light 90 deg up to the guide camera. As the light is deflected 90 degrees, the guide camera is no longer pointing along the same axis as the OTA and thus, the setup is called off-axis guiding.
3. On-Axis Guiding (ONAG)
On-axis guiding is where the guiding information is collected using the same light from the OTA as used for imaging and the light for guiding is also along the same path as the imaging OTA.
Innovations Foresight ONAG
ON Axis Guiding (ONAG) is a registered trademark of Innovations Foresight with their particular technologies having pending patents.
The Innovations Foresight ONAG box that sits in the imaging light path has a dichroic beam splitter inside of it that reflects the visible light from the telescope 90 degrees to the imaging camera. The dichroic beam splitter passes the near-infrared light to the guiding camera which is on the same axis as the OTA (see Figure 3).
ONAG where the Imaging and Guide sensors mounted next to each other.
Companies such as SBIG use to make cameras with dual chips. The main imaging sensor had an additional small imaging sensor mounted next to it so both sensors were on the same camera. They were not popular as the guide chip had reduced sensitivity being behind filters and may no longer be available. (similar setup to Figure 4 Image Autoguiding)
4. Image Autoguiding
Image Autoguiding is the use of the main image camera data for guiding as well as imaging the desired object from the same camera. It's a form of on-axis guiding but without a dedicated guide camera.
Planetary imaging is an example of an application for guiding on an object in the image. The objects are typically planets or the moon.
The FireCapture planetary imaging application, the Sharpcap feature tracking and ASICap, part of ZWO ASIStudio Software package, are examples of Image Autoguiding.
Image Autoguiding that analyses the image using Fast Fourier Transformation mathematics. The objects could be anything from planets to stars. I don't know if any application, apart from a demonstration software, utilised this autoguiding technology.
Autoguiding Software
A very popular and free guiding application is PHD2.
Some PHD2 links: