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Young's Photo Gallery
James W. Young, Professional Photographer
Image of the Week Archives
April 26, 2015
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I have decided to be different this week! What is this, you ask? This is a very
out-of-focus image of the star Vega taken with the 24-inch telescope at JPL's
Table Mountain Observatory on October 31, 2007. This particular image was
used to confirm the final mirror alignment of the telescope after the primary
mirror was removed for cleaning and re-coating of aluminum, then re-installed.
The dark circle in the center is the obscuration of the secondary (smaller) mirror
that reflects the starlight down to the camera at the base of the telescope tube.
Carefully note the small faint dot of light in the very center of the dark circle.
If the alignment of the optical mirrors was not perfct, this dot would not appear
in the center.
The four symetrically placed dark lines from the center circle to the outside larger
circle edge, are the refraction patterns of the four thin metal 'vanes' that hold
the secondary mirror and housing. The one at the bottom is darker (and wider)
because it has an additional thin wire cable to supply the necessary voltage to
control the motion of the mirror (in and out of focus).
Any question? Write to me here: astroyoung@verizon.net
The alignment of a telescope's optical system is one of the most critical factors
in making accurate astronomical observations.
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e-mail astroyoung@verizon.net
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