The above schematic shows the layout required for using the Alan Gee II for CCD imaging at f/5.9. This sets the camera at the distance required for the nominal design f/5.9 focal ratio. The layout above is for an SBIG ST7/8/9/10 camera, which has a back focus distance of 0.920" (23.4mm) as measured from the imaging chip to the T-threaded front mounting surface. For other cameras, adjust the distance accordingly by selecting a different T-2 Extension Tube combination.
Note: the thin plastic spacer ring is used to fully center and clamp the Alan Gee into place. If you are using a larger SCT (Celestron 11/14 and Meade 10/12), you should mount the telecompressor and short T-adapter to the SCT reducer plate with the plate off of the telescope and held out horizontally. This will help you to see when the spacer ring and telecompressor are properly centered and seated (if installed with the reducer on the scope and the scope horizontal, it may be possible to get the telecompressor to be mis-located and clamped off center).
The following parts are recommended as shown, in addition to those supplied with the Alan Gee Mark II Telecompressor. This configuration lends itself to precise and convenient removal/replacement of the camera with other accessories, due to the use of the T-2 Quick Change System. If you desire the simplest configuration without the use of the Quick Change System, the Quick Changer can simply be replaced by an additional 15mm Extension Tube (the effective optical path length of the quick changer is 15mm).
- Short T-Adapter (order # T2-21)
- T-2 Quick Change System (order # T2-06,07)
- T-2 Extension Tube 15mm (order # T2-25A)
Another neat way to make use of the capabilities of the Quick Change System, is to use it to permit exchanging a parfocal eyepiece with the CCD camera. This allows you to visually center an object in the field, which is particularly handy given the small chip sizes in today's expensive CCD cameras. If a high powered eyepiece is used, it can easily be adjusted to be parfocal with the CCD camera and used to visually focus on a bright star making focusing more precise and faster. Addition of an optional Focusing Collar (not shown) enables you to set a reference collar on your eyepiece's barrel, enabling you to remove an eyepiece and return it to the a repeatable focus distance. The following parts are used to build the eyepiece adapter:
- T-2 Quick Change Ring (order # T2-07)
- FR-4 Focusing Collar FR-4 (order # T2-30) (optional)
last updated 4/06/02
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