8.4 Guiding
113
8.4 Guiding
In most cases, the tracking on the mount you are using will not be good enough to
undertake very long exposures without some kind of trailing. Many CCD cameras
come with a secondary CCD built in or an accessory that consists of a small, lowcost CCD and a small mirror or prism to guide and focus a portion of the image
onto the CCD. These are known as off-axis guiders. Alternatively, your telescope
may use a separate coaxial telescope and CCD for guiding. With your camera control
software, you should be able to access the guider CCD. Either this will automatically
pick a star on which to guide, or you will have to pick a star. If your field is very
crowded or contains a large galaxy or nebula, the autoguider may have problems
identifying a suitable candidate star, in which case you may have to select one for
it. The functionality of your guider will depend on the software you are using, so
once again, check with the observatory staff before using this feature. In some cases,
you will need the autoguider. For very short exposures, the amount a star moves
may be less than one pixel. In this case, autoguiding may be unnecessary, whilst for
longer exposures, the need for autoguiding will depend on the tracking quality of
the mount. Since telescope mounts are designed for sidereal tracking, when you are
imaging solar system objects, which do not move sidereally, you will be able to take
only short exposures before you see the object’s trail, even with autoguiding.
If an autoguider is unavailable or you do not wish to use it for some reason or it is
struggling to identify a suitable guide object, an alternative to tracking is stacking.
We will be discussing stacking in more detail later. However, broadly speaking, it
is a technique whereby a single exposure is generated by the summation of several
shorter exposures.
For each of your targets, you will have either to set up the autoguider or, if
it automatically picks a guide star, to check that it is indeed guiding. Generally,
changing a filter should not be a problem for the guider. Most telescope control
software reports guider data as it exposes, and you may wish to abort the exposure
if guiding is lost for than a few tens of seconds.
If you can adjust the exposure time of the guide CCD, it is often useful to increase
it in order either to make the guide stars more prominent or to increase the rate of
correction. Beware of setting the exposure time too short, as that can cause problems
with the control computer.
8.5 Target-Specific Problems
In this section, we will discuss some of the challenges specific to certain target types.
Remember that there is a fundamental difference between science images and the
images you see on the covers of books and glossy science magazines. The processing
used to make those images pleasing to the eye often means that the pixel values are
no longer linear. Aside from applying calibration frames, you should not perform
113
8.4 Guiding
In most cases, the tracking on the mount you are using will not be good enough to
undertake very long exposures without some kind of trailing. Many CCD cameras
come with a secondary CCD built in or an accessory that consists of a small, lowcost CCD and a small mirror or prism to guide and focus a portion of the image
onto the CCD. These are known as off-axis guiders. Alternatively, your telescope
may use a separate coaxial telescope and CCD for guiding. With your camera control
software, you should be able to access the guider CCD. Either this will automatically
pick a star on which to guide, or you will have to pick a star. If your field is very
crowded or contains a large galaxy or nebula, the autoguider may have problems
identifying a suitable candidate star, in which case you may have to select one for
it. The functionality of your guider will depend on the software you are using, so
once again, check with the observatory staff before using this feature. In some cases,
you will need the autoguider. For very short exposures, the amount a star moves
may be less than one pixel. In this case, autoguiding may be unnecessary, whilst for
longer exposures, the need for autoguiding will depend on the tracking quality of
the mount. Since telescope mounts are designed for sidereal tracking, when you are
imaging solar system objects, which do not move sidereally, you will be able to take
only short exposures before you see the object’s trail, even with autoguiding.
If an autoguider is unavailable or you do not wish to use it for some reason or it is
struggling to identify a suitable guide object, an alternative to tracking is stacking.
We will be discussing stacking in more detail later. However, broadly speaking, it
is a technique whereby a single exposure is generated by the summation of several
shorter exposures.
For each of your targets, you will have either to set up the autoguider or, if
it automatically picks a guide star, to check that it is indeed guiding. Generally,
changing a filter should not be a problem for the guider. Most telescope control
software reports guider data as it exposes, and you may wish to abort the exposure
if guiding is lost for than a few tens of seconds.
If you can adjust the exposure time of the guide CCD, it is often useful to increase
it in order either to make the guide stars more prominent or to increase the rate of
correction. Beware of setting the exposure time too short, as that can cause problems
with the control computer.
8.5 Target-Specific Problems
In this section, we will discuss some of the challenges specific to certain target types.
Remember that there is a fundamental difference between science images and the
images you see on the covers of books and glossy science magazines. The processing
used to make those images pleasing to the eye often means that the pixel values are
no longer linear. Aside from applying calibration frames, you should not perform