back focus spacing question

[iwols]

hi just going to fit a reducer to my scope with a filter wheel and atik 414ex ccd,the reducer says 60mm working distance from thread, and my atik 414 has a back focus of 13.5mm,so does that mean from the reducer face to the ccd face i need to be 46.5mm,including the filter wheel,and can the filter wheel go anywhere inbetween the reducer and ccd as long as i maintain the 46.5mm face to face (reducer/ccd)thanks

[symmetal]

Yes, your 46.5mm distance from reducer to camera face is correct although you will need to add 1/3 the thickness of your filters to this figure, add 0.7mm for 2mm thick filters. The thickness of the filter wheel is included as part of the spacing distance so the extra spacing you need is 46.5 - (filter wheel thickness) + (filter thickness / 3) mm.

The filter wheel can go anywhere in the spacing gap as far as spacing goes, though they are usually fitted as close to the camera as possible to avoid possible vignetting when you use large sensors with 1.25" filters. The 414 sensor is a 'medium' size sensor so the filter distance to the sensor is not so critical. There have been some reported issues with noticeable reflections when the filters are fitted close to the sensor but many people with close filters have no such problems.

Alan