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New Off-Axis Guider Adapter for C5 Cameras
 The C5 cameras are equipped with particularly large sensors, which impose quite big demands for the field of view — the used optics must be able to create high quality picture far from the optical axis. Off-axis guider adapters (OAG) further increase the demands for the size of the field. OAG incorporates a small diagonal mirror, reflexing the incoming light to the guider port, where is the guiding camera attached. And this mirror must be located far enough from the optical axis not to shield the imaging sensor from incoming light. Increased demands for the optics is the reason we did not introduce the C5-OAG together with C5 cameras. But users asked for it, so we introduce it now.

The C5-OAG (left) and C5-OAG on the C5 camera with XL External Filter Wheel

Off-axis guider adapters provide many advantages compared to standalone guiding telescopes, especially the fact that the guiding camera uses the same optics like the main camera. This means the guiding corrects also for many subtle effects like optical tube and focuser flexure etc.

But as mentioned earlier, OAG adapter in general increase the demand to the field of view. The used optics must cover the reflecting mirror with good-enough image to be functional.

Comparison of the C4-OAG (left) and C5-OAG (right)

Comparison of the C4-OAG (left) and C5-OAG (right)

Standalone guiding telescope typically flexes differently than the main OTA and also the junction between both telescopes introduces some angular differences as the telescope tracks the object. So, guiding using standalone telescope typically leads to slight shifts of the main telescope field of view through observing time.

On the other side, guider telescope can have shorter focal length and thus greater field of view compared to main OTA, which makes it easier to find bright enough pointing star. But the necessity to attach it to the main telescope increases the complexity, weight, and of course cost of the observing setup.

Location of the mirror in the C5-OAG

Location of the mirror in the C5-OAG

The C5 OAG mirror is located 31 mm from the optical axis. The 10 × 10 mm mirror, tilted 45°, thus reaches up to 38 mm from the optical axis. The required diameter of the telescope field of view is 76 mm, so the OAG mirror is well covered.

 
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