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Image Quality: Focus |
Welcome to http://www.handsonuniverse.org/activities/Explorations/HOUtips/focus/focus.html
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Image Quality: Focus |
Open the eight images of Almaak. This star is also known as Gamma 1 Andromedae.
These images show a sequence of focusing the telescope/CCD system. Use log scaling on each of the images. What do you notice about this star as the images come into better and better focus?
Open jup010414_0132n.fts, jup010414_0138b.fts.
These are images of Jupiter and two of its moons. The first picture has ok focus but Jupiter is so bright, it is difficult to see any features. Notice the overflow of light into the image to either side of Jupiter. There was no filter on the CCD camera.
Thinking that a filter would help to cut down on the brightness of Jupiter, a
blue filter was added to the system. However, now the telescope is not
focused properly. Notice the irregular shape to the moons. Since Jupiter
was low in altitude, and in fact the moons are not point sources and are dim
compared to Jupiter, it is difficult to focus on an object like Jupiter or its
moons. It is better to focus on stars. (Compare these images of Jupiter to ones taken
on a night in February.)
Open
m38poorfocus.fts
The telescope system is still not in focus. Notice the consistent irregular shape of the stars. This object is not high in the sky on the night it was taken, April 14, 2001. The telescope is at a low angle because of the low altitude of the object. Any number of things interfere with good balance and alignment of the systems, including the telescope and the ccd camera.
After working with the telescope system for better focus, and using stars much higher in the sky towards the zenith as focus targets, a better focus was achieved. Objects higher in the sky are better targets for the season and better images are achieved. Open an image of the Black Eye Galaxy, m64n2m010414.fts and compare the results.
Another image was taken of M38 was taken four days later, April 18th. The telescope was focused at the zenith, but when it was pointed at a low angle to capture m38, the stars were still not round. The angle of the telescope is the problem here. Compare an image taken on April 18th of M38 with an image of M66 taken the same night. M66 was in a much better position in the sky for the telescope system.
One of our HOU teachers set up his telescope and CCD camera and took these images of Jupiter on two different nights.
jup001106_0924z_infocus.fts
On one night, the focus was sharp. The moons appear as points in this image.
jup001202_0300z_notinfocus
On another night, notice how the moons look like circles. The telescope system is out of focus. However, it is valuable to keep such an image because we can see the position of each of the moons, and notice how their positions change.
Open JupG1999Nov01.fts and JupV1999Nov01.fts
These images were taken with a telescope in the network of SKYWatch teachers participating in Illinois State Board of Education's SKYWatch project.
When the right combination of filters and focus and exposure time is achieved, one can see the bands of Jupiter. When log scaling is applied, one clearly sees the moons of Jupiter. This is a great combination for observing both Jupiter and its moons.
However, when the filter was switched from a green filter to a V filter, the system was not refocused. When the image is out of focus, the moons look more like little blocks of light rather than points.
Prepared by Vivian Hoette, vhoette@hale.yerkes.uchicago.edu. 08/01/2002
