Hi Don.

I read your post with interest. What set me down the path to finally get a definitive answer to how to calculate the Back Focus with Filters is that I just acquired an TV NP101is OTA, the little brother of the NP127is. Your setup is similar to mine. I have an ATIK 16200 with an EFW3 FW. I ordered the TV NP101is Field Flattener (LCL-1069), which per TV has a recommended Back Focus of 2.75" (69.85mm) for the ATIK 16200 34.6mm diagonal sensor. I also ordered TV's Set of 6 Accessory Tubes for 2.4" (TLS-2245). I plan use two of the spacers in the set to for the recommend 1.375" (34.9mm) spacing between the focuser and the FF. The remaining two spacers (0.500" and 0.250") will be placed after the FF. The remaining spacing will have to filled with a Precise Parts Adapter as TV does not make an adapter to go from their 2.4" accessories to the M54 thread I need to connect to the ATIK EFW3. I ordered the the Precise Parts adapter 1mm short and will use the shims included in the 6 piece spacer set to the required Back Focus. I could have just ordered a longer Precise Parts Adapter and skipped the two spacers after FF, but I wanted to keep at least two places where I could add shims as you can only use one shim for each threaded connection. Plus the shorter PP Adapter was cheaper... 

I have some questions about attaching this scope to the 16" DOVELM162 saddle that I have on my AP 1600 mount. I also need to figure out a guide scope solution. I have the 10 x 60 Vario Finder with Quick Release Guider Bracket Kit. (1060VGKIT) that I use with my TEC140, but the only way to use this scope with the NP101is is to commit the cardinal sin of mounting the guide scope to the rings. Although, this may be a less of a sin for a wide field scope. I may have to go OAG route, but I would like to see if I can make what I have work first. Do you mind if I PM you with a few questions about your setup?

Andrew