Download Bad Seeing Bad Focus Bad Guiding

¾ Bad Seeing
9Characteristics of Bad Seeing
¾ Bad Focus
9 Characteristics of Bad Focus
9 Astigmatism and Coma
¾ Bad Guiding
9Characteristics of Bad Guiding
The 2.1 m Telescope,
OAGH - Sonora, Mexico
Many types of direct imaging projects depend on achieving the best spatial resolution possible with your
telescope and instrument. Three elements of observing which most commonly degrade the quality of direct
imaging are poor seeing, bad focusing (image out of focus), and bad guiding. If you suspect that your
image quality could be better, determining which of these three are affecting your images can be difficult,
especially since you might encounter several of them at the same time.
poor seeing
image out of focus
bad guiding
good focus
good seeing
good guiding
A.1. Seeing
The phenomenon known as seeing is caused by rapid, small scale turbulences in the atmosphere. These
scintillations along the line of sight cause images to vary slightly in intensity (due to changing airmass) as
well as in position (due to changing index of refraction along the line of sight). The variations in position
cause a point source (star) to be smeared out into a seeing disk a few arcseconds in diameter.
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. In the best seeing conditions, stars appear
pasted on the screen, hardly moving at all. Furthermore, it
may appear to change its shape, and, in bad cases, start to look
like a wavering nebula.
Stellar images in the frame are spread out uniformly. They
appear circular but "fat". This can be contrasted to focus and
guiding problems.
Problem: The seeing in this image is bad. Note how the stars appear round and big. The same image was
taken during good seeing conditions, and the number of detected stars is substantially higher in that image.
Fix: If the project allows seeing conditions as bad as the current ones, fire away. If not, wait until the
conditions are within the seeing boundaries set for the project in question.
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).
Problem: The seeing in this image is bad. The FWHM of
the curve is fairly large. Note, however, that there is not
much scatter around the fit.
Fix: Unless the seeing occurs inside the dome or very
close to the observing site, no fix is possible. Wait until
the conditions improve.
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.
Problem:
The seeing in this image is good. Note how the stars
appear clear and well-defined.
Fix:
Not necessary.
Problem: This images suffers from ridiculously bad seeing. The
FWHM in arcseconds is greater than 6! The full image is about
14' square. The section shown here is about 3.5' square and
only ~ 5 (large) stars and a couple smudges are visible. This is
obviously bad seeing because the stars all appear perfectly
round (but solid as opposed to out of focus donuts) and have
huge FWHM's.
Fix: Stop observing! Wait for the seeing to improve to
something much more reasonable.
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.
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 temperature 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.
A.2. Bad Focus
If the focal point of the secondary mirror lies either in front of
or beyond the focal plain (CCD), an image obtained with this
configuration will be out of focus. For a point source, the light
rays will thus form a circular image (doughnut), if the real focus
is sufficiently above or below true focus.
Problem:
This image is so much out of focus that the stellar images
appear as "doughnuts".
Fix:
Change focus setting by a comparatively large number of units in
either direction in order to try to get stars to appear as "filled
out circles" (may have to iterate several times). Then, do a focus
sequence for the finer tuning.
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.
Problem: Out of focus image. Note the stellar profiles are
elliptical as opposed to round and oriented in the direction of
astigmatism as opposed to EW or NS which would likely be
caused by bad guiding.
Fix: Take focus frames, or focus with whatever method is
appropriate for that telescope.
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)
Problem:
Radial profile of out of focus star. Note the large
scatter around the profile fit, especially around the
base of the profile.
Fix:
Change focus until the profiles show the smallest
FWHM with the least scatter.
Problem:
Contour plot of star in out of focus image. Note the
elongation along the direction of astigmatism as opposed
to along the x or y axes.
Fix:
Adjust focus until star has a round profile.
Problem:
Radial plot of out of focus star.
Fix:
Adjust the focus.
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
Problem: Out of focus image. Note the stellar profiles are elliptical as opposed to round and oriented in
the direction of astigmatism as opposed to EW or NS which would likely be caused by bad guiding.
Fix: Take focus frames, or focus with whatever method is appropriate for that telescope.
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.
Problem:
This shows the upper left-hand corner of a 17'x 17' image with
coma. The corners of images using the MDM 1.3m with large
CCD chips producing a wide field of view typically will have out
of focus stars. The focus further from the center of the image
is different and therefore nothing can be done while observing
to get an image everywhere in focus. The larger the chip and
larger the field of view, the greater the problem.
Fix:
No fix possible. Just make sure the center of the image is in
focus, or the region of interest.
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).
A.3. Bad Guiding
The principle on which most auto-guiders of astronomical telescopes are based is attempting to keep a
particular guide star at a fixed point in the field of view of the guider TV camera, frequently in the center
of a guide box on the guider TV screen. The telescope makes small, corrective motions to keep the guide
star at its fixed position, thus always pointing towards precisely the same position in the sky.
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.
Problem:
Terrible guiding causes the drawn out appearance of these
stars. The profile is elliptical and the major axis lies along the
x-axis of the chip. Bad guiding usually causes objects to be
elongated along either the x or y axis depending on whether the
guider is having trouble guiding in DEC or RA.
Fix:
Fiddle with guider parameters.
The image is uniformly affected.
What to do:
9 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.
9 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. (it is to say), increase the time interval in
between corrections. Otherwise, the telescope might overcorrect.
9 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.