May be a silly question....

[ScubaMike]

.....but why do people not hook up their guiders through the main imaging telescope? (ie if using a CCD/converted webcam, use the image produced to guide the scope).

Is it because the scope is guided *during* the exposure of the imaging sensor? and that image would therefore be useless to guide by and the sensor would be saturated by the guide star?

[Helen]

Yes, you take short guide images with another camera to monitor movement and then adjust the scope so that the picture taken by the main camera over a long period of time doesn't show trailing. You can use just one scope but two cameras by using an off-axis guider. This uses a prism to take a small part of the light from edge of the field of view of the telescope and send this to the guide camer.

Helen

[ScubaMike]

Thanks Helen,

It seemed a dumb question, one that I thought I knew the answer to, but I couldn't confirm my thoughts with a search through the forums. (my searching skills may not be very good).

[NickK]

Current CCD chips are read in totality in one go. This means it's impossible to read a portion of the CCD at a different speed.

As Helen has said - an OAG uses a prism to act like a periscope in the main field of view. Luckily the image circle 'light cone' is usually larger than the imaging CCD sensor and big enough to cover the OAG prism with the overspill.