Field Flattener Positioning & custom spacers

[MG01]

I have a TMB field flattener for the WO FLT132 and am trying to work out how on earth I achieve the illustrated lens to chip distance of 113.13mm.

In the past I was told that the distance isn't that critical, but having read a number of other posts I'm starting to think that was wrong.  I know it's easy to chase pixel accuracy, but as I'm returning after quite a long break from AP I thought I'd check and see if there's anyone making custom Aluminium or Copper spacers/adaptors that might not cost me a leg!  May as well try to get close at least, right?

I've attached a drawing from the William Optics website....any suggestions?

[Richard_]

Looking at the diagram, 58.14mm is already taken up by the "camera adapter" which you don't need to insert spacers for. Taking 58.14mm from 113.13mm gives you 54.99mm which is pretty much the standard 55mm back focus a lot of flatteners/reducers work to. 

For a DSLR, all you'll need is a T-mount to fit your selected camera. No spacers required! 

For an astro camera (like ZWO or QHY), the camera usually comes with sufficient spacers to achieve the 55mm back focus distance. 

[MG01]

21 hours ago, Richard_ said:

Looking at the diagram, 58.14mm is already taken up by the "camera adapter" which you don't need to insert spacers for. Taking 58.14mm from 113.13mm gives you 54.99mm which is pretty much the standard 55mm back focus a lot of flatteners/reducers work to. 

Trouble is I have a SX filter wheel and LP Filter and would like to use that with the Camera Adaptor (B) which I don't have a drawing for...I'll try and sketch something later...

21 hours ago, Richard_ said:

For an astro camera (like ZWO or QHY), the camera usually comes with sufficient spacers to achieve the 55mm back focus distance. 

I think the Adaptor (B) is supposed to be used with CCDs...but again, I'll have to draw out the image train to figure out how close to the 113.13 (or 113.14 maybe?) I can get.

How important is it to hit this number?  Is it just a case of the centre will be in focus but the periphery will show some aberration?  Given that most images will be smaller than my sensor I guess I can get away with a certain amount of error?

[Richard_]

OK so the picture shows Camera Adapter (A) but you want to use Camera Adapter (B)? What is the difference between the two adapters? T-ring threads are M48 so you'll get loads of adapters which will attach to this. 

Which camera are you using? 

Here's a link from ZWO where they talk about achieving 55mm back focus with their cameras. Seeing as you're using a filter wheel and a light pollution filter, I'll assume you're using a colour CMOS/CCD camera instead of monochrome camera or DSLR. 

https://astronomy-imaging-camera.com/tutorials/best-back-focus-length-solutions-55mm.html

Scroll down to the heading "2) With 1.25”/36mm/31mm EFW:" where they illustrate which spacers to use to achieve 55mm and indicate whether the telescope side terminates in M42 or M48 threads. Follow this logic for your setup (back focus of sensor within camera body, thickness of filter wheel, what threads you have between each device and Camera Adapter) and work out if you hit 55mm or not. The general rule of thumb with filters is to add two thirds the thickness of the filter to back focus, so most people round this up to 56mm.

How important is back focus? Depends on how flat you want the stars to be 🙂 if you can get it to within 1mm then that may be acceptable. 

[MG01]

Sorry for the delay @Richard_ I've been having a mild panic about the CCD I just purchased, bit that's another thread! 

I've managed to etch out something and I think what I'm looking for is either a 35mm (ish) SCT extension tube, or a similar length extension for the position of the green arrow?

image train.pdf

I was concerned that the SCT tube might y affect the light cone, but without drawing the whole tube it's difficult to tell.

 

[MG01]

I'm pretty certain I've sorted this now and so thought I'd share the final combinations just in case anyone else finds themselves in the same situation.

I'm still slightly concerned that I'm going to get vignetting and/or OAG shadows, but won't know that until I can get it all assembled.

Option A uses the TMB flattener with the M42 back shown in the William Optics drawing, but with the filterwheel and OAG the backfocus is about 4mm longer than the 113.31mm I'm aiming for from the WO spec, and there's not really anywhere to lose it.

Option B uses the Flattener with the optional SCT back.  I've used a this Astro Essentials Low Profile SCT / 2" to M48 Adapter with TS M48 Variable Adaptor with M63 which allows me to get a pretty bang on spacing for the field flattener.  Again I'm a little worried that the light cone is getting truncated here but it seems the best option for attaching the Atik in this train.

Option C is how I'd plan to attach my DSLR.  Surprisingly it's only a fraction off the Flattener backfocus so should give a pretty good result. This uses the following ultra short EF adaptor  with an M48 extension tube.

So if anyone has any comments, particularly on how you might trace light paths to work out if there will be any problems before I get all the bits, I'd love to hear them.  Cheers