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Using Field of View Indicators to Find a Guide Star

The purpose of this page is to provide tips on using TheSky to help you locate a candidate guide star when using a dual-CCD camera (such as the SBIG ST-7 and ST-8). Many of the concepts can also be applied to other cameras (CCD or film) used with an off-axis guider.

To begin, we have set our Sky Display to show a complex field of view indicator (FOVI), which is actually a combination of four individual FOVIs. (See How do I Add a Dual-CCD Field of View Indicator for background information.)

The central, large rectangle (labelled "Imaging Frame") represents the primary, or imaging, detector's field of view. The inner circle (labelled "ID"), and outer circle (labelled "OD") represent the boundaries of the arc which the secondary, or guiding, detector can cover, through rotation of the CCD camera body. The narrow, vertical rectangle is used to indicate the portion of this arc actually occupied by the guiding detector, represented by the small "square" just beneath the North-indicating arrow's head.

From the above view, we can see that—while the galaxy is framed nicely within the imaging detector—there are no suitable guide-stars within the guiding detector's field of view. Furthermore, no such guide-stars can be found within a small region immediately surrounding the guiding detector. However, there are two candidate stars located in the bottom portion of the guiding detector's rotational arc.

We'll first check the appropriateness of the brightest of these, the magnitude 8.4 star. By clicking first on the galaxy, then on this star, we find the relative position angle to be +160. This means we would rotate the camera 160 counter-clockwise (clockwise if the optical system yields a mirror-image) to place this star in the middle of our guiding detector. We can simulate this within TheSky before we actually move the camera.

In this view, we can see that—while the candidate guide star can certainly be used for guiding the image—it requires the camera be oriented such that the ends of the target galaxy are uncomfortably close to the edges of the imaging detector. If this were the only guide-star candidate, we could probably use it adequately. However, let's check the second candidate, the magnitude 9.3 star.

By continuing the rotation, we have placed the second candidate guide star in the middle of the guiding detector. Notice that the camera orientation results in the galaxy being ideally framed within the imaging detector's field of view. This appears to be the best guide star to use for this target and imaging system configuration.

By first clicking on the galaxy, and then the chosen guide star, we find the relative position angle to be a little over +197. This means we should rotate the camera 197 counter-clockwise (clockwise if the optical system yields a mirror-image) from North, to place this star in the middle (left-to-right) of our guiding detector when the galaxy is centered on our imaging detector.

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Last modified: Friday December 13, 2002.