FSQ 106ed metal back measuring

[libraryman]

Hi, all, I have my reducer and adapter ready to fit to my fsq106ed, I'm not entirely sure how to measure the back focus or metal back distance as I believe it referred to..

I use Astrodon narrow band filters in my qsi583 but I also have Astronomik lrgb filters and the L filter is also currently fitted.

Anyone know how I should measure it and if the mix of filters will complicate this?

Ray

[libraryman]

O.k. looks like I can answer this myself, it might be useful info for someone else.

It seems that the metal back distance for the new Q or fsq106ed is 72.2mm

Using a qsi583 wsg with STD T adapter the back focus is 50.17 mm for this model.

Taking from this the thickness of the filter (possibly termed refractive index) 1mm for my Astrodon filters

Leaves a figure of 49.17mm

Subtract this from the 72.2mm gives a required spacer of 23.03 mm

My particular spacer is 22mm. Therefore in an ideal world I need a 1 mm shim to ensure perfect back focus!!

How tolerant the Tak is with this figure I don't know.

Ray

[strutsinaction]

Hi Ray

This might help you, or confuse you 

http://stargazerslounge.com/topic/236625-do-filters-add-to-back-focus/

Regards

John

[libraryman]

Thanks for that John, I really wish I'd seen it before, its a very good read with some very comical posts!

It does seem confusing but it has answered my question completely, I was on holiday for a month so I guess that's why it sailed past me??

Cheers

Ray