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The NextGEN series telecompressors have a design back-focus distance of 2.49 inches or 63.1mm as measured from the flange or camera mount seat. The on-axis back-focus distance varies somewhat between the three NextGEN models, but Optec designed the NextGEN camera mounts to be interchangeable between the NextGEN MAXfield (NGM 0.33X), the NextGEN WideField (NGW 0.5X - now discontinued), and the NextGEN Ultra WideField (NGUW 0.7X for apochromats - also discontinued).

Each of the camera mounting plates is designed to maintain the optimal design back-focus for the best overall image quality. Especially with the fast NGM 0.33X, varying the back-focus as little as 1mm will have an adverse effect on image quality.

Each camera mount consists of a 2.1" ID tube of various lengths and a mechanical interface such as a T-thread, C-mount, or STL thread. This 2.1" ID tube fits over the camera side of the NextGEN telecompressor which features the OPTEC-2100 male circular dovetail mount. The camera mount is held securely to the Lepus dovetail using three oval point setscrews located 120 degrees apart. The dovetail and setscrew arrangement allows rotational adjustment for image composition.

Most of the mechanical camera interfaces require an additional 1/10th inch or 2.5mm of back-focus, though each mount interface is fixed to the 2.1" tube and is not removable. The camera back-focus distance, BF, should be known to be within about 1mm for best performance and should include the thickness of any filter wheel, off-axis guider, or other adapters between the Lepus and camera sensor. Use the table below to find the appropriate mounting plate for many commercially available cameras and configurations. If your camera is not listed below and you know the optical back-focus, BF, and mechanical interface (i.e. T-thread) you can determine the stock number as with the following calculation:

TUBE LENGTH = 2.49" - FLANGE THICKNESS - CAMERA BACK-FOCUS

Mechanical Interface	Flange Thickness	Stock No. Prefix
T-thread (M42x0.75mm)	0.1 inch	#17392
M48x0.75mm thread	0.1 inch	#17394
SCT thread (2" x 24tpi)	0.1 inch	#17396
STL 2.156" thread	0.1 inch	#17398
C-mount	0.1 inch	#17399

 

Next use the TUBE LENGTH value calculation above for the suffix. For example, nearly all C-mount cameras share a 17.5mm back-focus distance or 0.69-inches. The C-mount stock number prefix is #19399 and the TUBE LENGTH would be equal to 2.49" - 0.1" - 0.69" = 1.70-inches. The Optec stock number for a NextGEN C-mount camera would therefore be #17399-1.70 as shown in the table below.

Similarly, a camera having a 17.5mm back-focus distance but with a T-thread interface would require stock #17392-1.70.