Attaining the magic 55mm

[jimboozle]

Hello again troops, and sorry to have to invoke the power of your collective intellect once more. Should be the final time for a while, and will bring Jaffa Cakes to the star party for anyone who can assist  

I'm finally about to push the button on the camera.  I'm not quite decided between the 460ex and 490ex, but for the purposes of this question, it is immaterial.

I image through my SW120ED DS-PRO.  I have the dedicated 120ED field flattener, which has an M48 thread on the camera end.  Optimum sensor distance is 55mm give or take a millimetre. 

So I obviously need a filter wheel.  The SX standard filter wheel with 7 spots looks good, I use 36mm unmounted LRGB/NB, so I can fit them all in and avoid dust bunnies (and pigs might fly).  I think this is 25mm thick.  I then need to add an OAG, the SX one is 13mm.  Total so far is 38mm.

Atik cameras of this ilk have a back focus of 13.5mm, so lets add that in.  This gives me 51.5mm.  With me so far? Is my maths correct?  For this very reason Martin has dissuaded me from the SX814 as it's back focus is much bigger.

Questions:  How do I attach this assembly to the M48 thread on the focal reducer - any ideas?  Will it fit, or do I need an adaptor - anyone else done it?  Once I get this sorted, I should be able to pop a delrin shim or two into the mix and tune the last couple of MM, dependent on the adaptor thickness (if needed).

Do I have to account for the optical properties of the filters? do they add/remove physical distance to sensor?  Or am I invoking too much Physics into this...  For the record, they are all the Baader 36mm unmounted, and should be, ahem, parfocal with each other (or thereabouts)...  I use a moonlite autofocuser, so not an issue, but just wondered if it affects the spacing of the sensor - focal reducer thingy.

I really hope someone understands all this better than I do - I'm getting rather confused by the whole malarkey

Cheers,

Jim

 

[DaveS]

SX say the thickness of the FW is 29mm, but when I put a dial calliper on mine it came in at 30mm, so a total of 56.5, 55.5 if yours does come in at 29mm. Add about 1/3 the thickness of the filter to the light path.