The following effects are likely to occur when the seeing is bad.
Characteristics of Bad Seeing:
The guide star is jumping around due to atmospheric turbulences. It may appear to change its shape, and, in bad cases, start to look like a wavering nebula. In the best seeing conditions, stars appear pasted on the guider camera screen, hardly moving at all.
Stellar images in the frame are spread out uniformly (they are not elongated in any direction). They appear circular but "fat". This can be contrasted to focus and guiding problems (see below). Click here for examples.
Using IRAF or other image processing packages, radial profile plots of stars in the image do not necessarily display more scatter around the fit, just a larger FWHM (because the image is circular). Click here for examples.
The detectability of fainter objects will decrease. In a given field of view, you will detect many more stars when the seeing is good than you will when the seeing is bad. Click here for examples.
Focusing might become difficult. A common consequence of bad seeing is that the FWHM will not substantially change for different foci. Similarly, guiding might be affected by the bad seeing because the guider will have problems keeping the guide star in the center of the `box' (see below).
What to do:
In general, there is not really a whole lot one can do about bad seeing. However, some things might be worth trying.
Seeing can change quickly for many reasons, such as a temperatuure change in the dome, clouds, or changing humidity. Opening all doors, windows, louvers, etc in the dome can improve the conditions if the turbulences affecting the images occur in or just outside the dome. If the primary mirror is air-condition equipped, its temperature should always be kept just below the ambient temperature. If, for some reason, there is a drop in the outside temperature, cooling the mirror might help.
The seeing will always be worst in the bluest filter, best in the reddest,
thanks to Debye scattering. If the weather conditions are stable, a good
strategy is to observe the targets in the bluest filter when they are at
a low airmass.
The following effects are likely to occur when an image is out of focus.
Characteristics of Bad Focus:
Stellar images in your frame may be elongated in direction of astigmatism (the aberration intrinsic to many optical systems of astronomical telescopes): not necessarily E-W or N-S (in contrast to guiding; see below). This natural astigmatism of the telescope/instrument combination will be enhanced as a consequence of bad focus. Also, parts of CCD may be affected worse than others (in contrast to guiding). With increasingly bad focus, the size of point sources on the image will increase and eventually turn into doughnuts. Click here for some examples.
A radial profile plot of stars in the image shows scatter, especially
near base of the stellar image. This scatter is due to the fact that the
function fit to the profile is averaged over all directions (but the images
are elongated along an axis). Click
here for examples.
Astigmatism and Coma
Astigmatism is an optical aberration which causes point sources away from the center of the field of view to appear elongated, frequently in a direction at a ~45 degree angle with respect to the N-S and E-W axes of the image. It is especially apparent in wide field imaging. There is nothing one (as an observer) can do against the natural astigmatism of many telescope/instrument setups, but its effects are emphasized when an image is taken out of focus. For an example of what astigmatism looks like, click here to access an out of focus image.
Coma is the aberration which causes point sources in the outer
regions and corners of the CCD to appear comet-like.This appearance is
due to the fact that rays of light do not all focus on the same image plane.
For
an example of coma, click here.
What to do:
The focus can change as the telescope slews to different positions,
or if the temperature of the truss arms is changing as the ambient temperature
changes. It is therefore advisable to take focus frames every once in a
while (or monitor the focus another way).
The manner in which these corrective motions are performed is governed by a set of parameters for the computer algorithm responsible for the guiding of the telescope. Example of such parameters are the time interval inbetween corrections, the velocities of the telescope motions in N-S and E-W directions, etc.
Here are some effects one might encounter when the guiding is bad.
Characteristics of Bad Guiding:
The guide star may make relatively large regularly-spaced jumps on the guider TV screen, especially if the guiding parameters are not set correctly.
Stellar images in the frame may be elongated in E-W or N-S direction, especially if the guider is having significantly more trouble for one direction that the other. Click here for examples.
The image is uniformly affected.
What to do:
One should keep in mind that guiding is a second order correction for the location on the sky in which is telescope is pointing. The first order correction is, of course, tracking. If the tracking is not working well, it will be next to impossible to obtain good images, regardless of how the guider parameters are set.
If the seeing is bad, it is very likely more difficult for the guider to guide, so there is no point in wasting too much time fooling with the guider parameters. In addition, if the seeing gets worse during the course of a night, a good guideline is to reduce the frequency in which corrections are made, i.e., increase the time interval inbetween corrections. Otherwise, the telescope might overcorrect.
If the seeing is good, however, decent guiding can only be obtained by setting the guider parameters correctly. This best setting unfortunately depends on a lot of things, such as the telescope setup, the conditions in the atmosphere, etc. Checking the telescope manual or getting advice from previous observers is a good first step.