17-23mm Variable Spacer M48 Filter Thread

The M48 variable adapter is designed to help you fine tune the distance between your DSLR or CCD camera and your field flattener/coma corrector, for optimal results. By twisting the locking ring you can achieve spacing from 18 to 23mm in fractions of a millimetre.

 

Example use with a field flattener (or coma corrector) and Canon EOS DSLR: A Canon DSLR requires 44mm of backfocus from the bayonet mount flange to the CCD Chip or film plane. Let's assume your flattener requires a 75mm distance from the rear lens element to the imaging chip surface for best results. Therefore 75mm minus 44mm = 31mm, so you require 31mm more distance from the camera chip to the rear lens element of the field flattener or coma corrector. Now let's assume you're using a SkyWatcher M48 to Canon EOS adapter, which adds a about 10 mm to the imaging train. Subtract 10mm from 31mm, and you get 21mm remaining space required. Simply adjust the variable ring to 21mm and your field flattener or coma corrector now has the correct spacing. A Nikon DSLR requires 46.5mm of backfocus, therefore you need to adjust the variable extension tube inwards by an additional 2.5mm to achieve 28.5mm of spacing. In this way, you can experiment and optimise your unique setup, by varying the distance between the CCD and lens elements.

 

The 17-23mm M48 Variable Extension Tube has a male and female M48 filter type thread on each end.