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Frequently Asked Questions |
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Frequently Asked Questions |
Say you want to
look at the image of right ascension 05h30m on October 2, 1996. You will
find that image at this URL:
archive/jpg/1996/10/02/10020530.jpg
I want to study one region of sky. Any tips?
Yes. Say you want to study all the images of right ascension 05h30m in 1996.
Then you go to
archive/jpg/RA/0530/1996/
And there you will find all the images for that region of sky for that
year conveniently located in one list.
If you want the FITS format images (Hcompressed) rather than JPEG
format, then replace "jpg" in the example above with "fits". The files
will have the ".fth" extension, which allows them to be read directly
into the U. of Iowa's image display and analysis program, "Camera,"
which is activated on the Sun workstation network of the UIUC Astronomy
Department with the command "camera". Others may download "Camera"
from the U. of Iowa web site (see Stardial's
Links).
How do I line up star charts with a Stardial image?
You will first need to determine what region of sky is visible in your particular Stardial image in terms of right ascension and declination. A special help page will do this for you. On the help page, specify the image you are interested in and the image will appear with right ascension and declination labeled. Remember: North is at right. East is up.
Now that you are oriented with your image, you can find the corresponding region on a star chart. If you have your own star chart on paper or in a book, you simply find the proper right ascension and declination and then turn your chart so that north is at right and east is up. Remember:
Charts
are also available on-line. These charts are already restricted to a
declination range of -8 to 0 degrees, similar to Stardial's
images. This leaves you the task of finding the proper right ascension.
These charts are NOT oriented with north at right like Stardial images.
So remember to mentally rotate the chart 90 degrees clockwise so it matches
the Stardial images. Or electronically rotate the chart using a
graphics program, or print it out and then physically rotate the paper.
What is this very bright object with a vertical
spike of light protruding from it?
(see this example)
Although it may appear to be a comet or an exploding star, this is
actually an effect produced by the CCD camera.
Each Stardial image is made up of thousands of dots called "pixels" for
"pix-ture el-ement." Each pixel can have a certain intensity, ranging
from black (minimum) to white (maximum) with many shades of grey in
between. When the CCD gathers light from the sky, it is filling these
pixels with intensity. Brighter objects may fill pixels to their
maximum intensity. When pixels reach their limit and light continues
to shine on them, the pixels will overflow and "bleed" intensity on
nearby pixels. Bleeding is responsible for these spikes of light that
often accompany bright stars and planets.
Several things may cause streaks to appear in Stardial images. The most
likely candidates are satellites, airplanes, and meteors.
What is this long streak that extends across the image?
In general, the streaks are caused by anything that appears to move relative to the stars. Stardial is capable of tracking stars as they move across the sky (as the earth turns). Stars, therefore, remain fixed points of light through the duration of the exposure. On the other hand, an object passing though the field of view against the starry background will expose the path which it took.
Airplanes (especially those arriving and departing Willard Airport) are identifiable by their flashing lights. They often leave two or more parallel streaks. (example 1, example 2)
Satellites leave a faint, but very straight line. The satellite's path line may vary in brightness due to it rotation and reflecting characteristics. If a satellite is moving slow enough, it may be possible to see steps in it's path, where the CCD advanced one row of pixels (example). Those steps can be used to approximate the satellite's velocity. You can even make a good determination of what satellite you are looking at using satellite ephemeris data (see Links).
Meteors are more unpredictable in their appearance. They often leave a very short path line with varying magnitude. If you see a streak that has a definite beginning and end within the field of view, it is likely to be a meteor, because satellites and airplanes rarely appear out of nowhere and disappear just as fast. However, you may see the path of an airplane or satellite end abruptly if the camera's shutter closed while the object was in view.
Image processing errors (see this example) sometimes cause a streak. This streak has a regular pattern which is exactly aligned with a row of pixels - evidence that it is not something in the sky, but rather a problem that occurred somewhere between the CCD camera and the image archive.
Bleeding pixels cause very short streaks. Please refer to a previous question regarding bleeding pixels.
The sky is either cloudy or very hazy. Not only do clouds block the
light from stars, they also reflect the city lights below. This
saturates the CCD image with white light. In many cases, Stardial will
automatically dispose of such images and replace them with a pure white
image to save space.
Why is this image all white?
The image you are looking at may be an actual image of a cloudy sky, or it may be a false white image produced by Stardial to inform you that the image was cloudy. In any case, the original image has been preserved in the FITS archive, if you need to confirm that it was cloudy.
HINT: While you are browsing the Stardial archive, you can easily avoid these cloudy images by noting their file size. Any 2KB GIF image or any 6KB JPEG image is entirely white.
Extra-for-experts: sometimes when the sky is (or was) perfectly clear,
Stardial still records an all-white image. This can happen when the
bright moon overloads the CCD sensor. Astronomers prefer to observe
near the time of the month near new moon, because then the light from the moon
doesn't make the night sky so bright. Stardial observes every night
whether the moon is bright or not.
If you want to find a specific object in a Stardial image (for instance,
the star Mira, or the nebula M-42), you will first
need to determine the coordinates of that object. The necessary
coordinates are the right ascension specified as hours and minutes and
declination specified as + or - degrees. Coordinates can be obtained
from several sources:
How do I find a specific object?
