Filter effects on back focus

[Wonderweb]

Hi

I have added a filter drawer to my image train in-between the camera and the focal reducer/field flattener. I was wondering if the introduction of a layer of glass affects the back focus in the same way as it does the actual focus of the scope. 

I have used the field flattener with my dslr and a standard adaptor which gives the correct back focus and its worked fine. The filter has always been screwed into the end of the field flattener so has no affect on back focus.

I have recently got hold of a hypercam 269c and have carefully measured the back focus which i have set identical to the dslr (55mm) but I notice some star distortion at the edges of my image. Could the filter be the cause? 

Thanks

[Stefek]

When you have a filter between flattener and camera, you should add 1/3 of the glass thickness to the backfocus distance. 

[Wonderweb]

9 hours ago, Stefek said:

When you have a filter between flattener and camera, you should add 1/3 of the glass thickness to the backfocus distance. 

Thanks for the reply. 

I'll try it out. 

[newbie alert]

I'd add a third of the filter thickness, but the standard 55mm isn't a concrete figure.. machining tollerance comes into play... once you start to use Plate solving software you will find that  manufacturers native focal lengths are rarely right

[ONIKKINEN]

2 hours ago, newbie alert said:

I'd add a third of the filter thickness, but the standard 55mm isn't a concrete figure.. machining tollerance comes into play... once you start to use Plate solving software you will find that  manufacturers native focal lengths are rarely right

Also think this is the case.

Mine should be 855mm but it is consistently plate solved as 840mm. Also my backfocus should be at 55,5mm but i added 1mm more to reach a better result. So the correct amount of backfocus is probably not set in stone.