3.4 Problems
37
instance, the exposure time is likely to have been very short. Increase the exposure
time until you can see something other than noise.
4. Display software can stretch an image or display only a certain range of pixel values.
This is a very useful feature, but if you are looking at a completely different object
from those observed previously, it may result in nothing being displayed. Check
the settings.
5. At this point, there might be a problem with the camera or the control software or
even a cable. Check whether there is power to the camera and that all the cables are
seated correctly, as it is easy for them to be pulled out accidentally without being
seen in the dark of an observatory dome. If everything is OK, you will likely have
to power cycle the camera and the control software. Unless you have experience
doing this, talk to the observatory support staff first.
3.4.1 Optical Problems
When in focus, a star, which is a point source, should have a Gaussian, or bell-curvelike, profile in all directions, as the light from the star should be distributed circularly
around the central pixel. If it is not, then more than likely there is a tracking error.
However, it might be an indication of something more serious that needs addressing.
We have already discussed chromatic aberration, the appearance of coloured halos
around sources, caused by refraction bringing different wavelengths to different
focal points. As in all likelihood you will be using a reflector, it is unlikely you
will encounter this unless you are using poor eyepieces or you are using a low cost
refractor, such as might be the case with your telescope’s finder scope. Chromatic
aberration manifests as a brightly coloured halo, most often blue, around objects. If
your optical system suddenly develops chromatic aberration, you could be experiencing condensation on the optics or be using a poor eyepiece. This is illustrated in
Fig. 3.9.
In telescopes that use parabolic mirrors such as Newtonians, an effect known as
coma can sometimes be seen. Coma is a form of optical aberration which makes
point-like sources, such as stars, appear comet-like, in that they appear to have a
tail, hence the name. Coma is caused by the fact that parabolic mirrors bring only
parallel rays to focus at the same point, and coma may be more pronounced away
from the centre of the field. Precise collimation of the optics may help reduce coma,
as may the introduction of additional optics. However, in general, coma is caused
by the design of the optics. Telescopes using spherical mirrors, correcting optics, or
both, such as Schmidt and Ritchey–Chrétien telescopes, suffer from little or no coma
(Fig. 3.10).
All optical systems use curved surfaces to bend and focus light. This is not a
problem for the human eye, as the retina itself is a curved surface, but for a camera,
the projection of a curved image onto a flat surface can cause a problem known as
field curvature. The result of field curvature is that objects near the edge of the
37
instance, the exposure time is likely to have been very short. Increase the exposure
time until you can see something other than noise.
4. Display software can stretch an image or display only a certain range of pixel values.
This is a very useful feature, but if you are looking at a completely different object
from those observed previously, it may result in nothing being displayed. Check
the settings.
5. At this point, there might be a problem with the camera or the control software or
even a cable. Check whether there is power to the camera and that all the cables are
seated correctly, as it is easy for them to be pulled out accidentally without being
seen in the dark of an observatory dome. If everything is OK, you will likely have
to power cycle the camera and the control software. Unless you have experience
doing this, talk to the observatory support staff first.
3.4.1 Optical Problems
When in focus, a star, which is a point source, should have a Gaussian, or bell-curvelike, profile in all directions, as the light from the star should be distributed circularly
around the central pixel. If it is not, then more than likely there is a tracking error.
However, it might be an indication of something more serious that needs addressing.
We have already discussed chromatic aberration, the appearance of coloured halos
around sources, caused by refraction bringing different wavelengths to different
focal points. As in all likelihood you will be using a reflector, it is unlikely you
will encounter this unless you are using poor eyepieces or you are using a low cost
refractor, such as might be the case with your telescope’s finder scope. Chromatic
aberration manifests as a brightly coloured halo, most often blue, around objects. If
your optical system suddenly develops chromatic aberration, you could be experiencing condensation on the optics or be using a poor eyepiece. This is illustrated in
Fig. 3.9.
In telescopes that use parabolic mirrors such as Newtonians, an effect known as
coma can sometimes be seen. Coma is a form of optical aberration which makes
point-like sources, such as stars, appear comet-like, in that they appear to have a
tail, hence the name. Coma is caused by the fact that parabolic mirrors bring only
parallel rays to focus at the same point, and coma may be more pronounced away
from the centre of the field. Precise collimation of the optics may help reduce coma,
as may the introduction of additional optics. However, in general, coma is caused
by the design of the optics. Telescopes using spherical mirrors, correcting optics, or
both, such as Schmidt and Ritchey–Chrétien telescopes, suffer from little or no coma
(Fig. 3.10).
All optical systems use curved surfaces to bend and focus light. This is not a
problem for the human eye, as the retina itself is a curved surface, but for a camera,
the projection of a curved image onto a flat surface can cause a problem known as
field curvature. The result of field curvature is that objects near the edge of the