Back Focus Calculation Atik 16200/EFW 3

[Eric Davis]

Hello,

I am new to the forum having just ordered an EFW 3 filter wheel, Atik LRGB filters and a 16200 camera. I am really looking forward to my first foray into mono cameras but I have bit of a problem.

I wish to connect my EFW 3 and 16200 to my TEC 140 scope. My scope has an Astro Physics Quad TCC field flattener with a quoted back focus of 80.8 +/- 1mm. I will need a purpose made adaptor to make the connection between the filter wheel and the field flattener.

My problem is understanding the back focus of the EFW 3/16200 combination. I intend to bolt the EFW 3 direct to the 16200 with the face plate removed. However, I am a bit confused as to the back focus as I have seen different figures quoted by Atik and on forums.

I have seen a figure of 19.5 +/- 0.5mm for the camera back focus. For the filter wheel I have seen both 34.5mm without the camera faceplate and also a figure of 21.8mm. Obviously they cannot both be right. I have also seen a figure on a dealer website offering a combination of the above with an Atik off axis guider which seems to conflict with any combination of these figures!

 Can someone advise me what backfocus I should use?

Another point I would like to clarify is does the 19.5mm allow for the thickness of the window of the camera? I know any filters or optical windows have to allowed for.

The purpose made adaptor will be quite expensive, so I want it to be right first time.

I would very grateful for any help.

Thanks in advance

Eric