Backfocus woes

[raadoo]

I believe I've landed in stump-land with a new imaging setup I'm putting together.

In a nutshell: dialling in proper backfocus for my reducer causes the focuser to lose inward travel.

In depth: I used the classic "look at the stars in the corners and add shims until they no longer radiate" and I'm very close to getting it perfect (<1mm). Trouble is, as I've increased the distance between sensor and reducer, the focuser (situated between scope and reducer) has had to move ever more inward to achieve focus. This to the point that I now have no more inward travel available and cannot achieve proper focus.

I'd massively appreciate any ideas, thoughts, opinions or guidance.

_

Image train is:

FS-60CB ⇢ CAA (TKA21200) ⇢ Reducer (TKA20580B) ⇢ Adapter to M48 ⇢ OAG ⇢ Filter Slider ⇢ Shims ⇢ Camera

Edited March 19, 2023 by raadoo
Misspelling.

[Which one is Polaris]

I had this problem.

I moved the CAA to downstream of  the Reducer.  Now the Reducer is hard up to the backend of the focuser.

This released a lot of inward focus travel (the length of the CAA).  Also, the CAA is now used as a Spacer, and in my case was M63, so helps delay the necking down to M48,.  This improved vignetting.

 

Cheers

[raadoo]

9 hours ago, Which one is Polaris said:

I had this problem.

I moved the CAA to downstream of  the Reducer.  Now the Reducer is hard up to the backend of the focuser.

This released a lot of inward focus travel (the length of the CAA).  Also, the CAA is now used as a Spacer, and in my case was M63, so helps delay the necking down to M48,.  This improved vignetting.

 

Cheers

Brilliant solution and one that certainly makes sense!

My CAA is a measured 20mm thick so I'm slightly doubtful that moving it after the reducer will not overshoot the backfocus requirement.

What does your image train consist of, if I may? It'd be massively helpful. Thanks!

[Which one is Polaris]

OK, great.  I will send the details later today.  I have to go out right now.

Cheers

[Which one is Polaris]

I have a TS 80mm Photoline Triplet and TSFlat25del Flattener with its red spacer tube, I added a William Optics CAA, then to a Nikon Z6ii camera.  No OAG, Filter Wheel or EAF.  Guiding is a separate Guide scope and camera.  I only do One Shot Colour, as in the UK a clear patch of sky is infrequent.  Also,  I have to drive to a site, setup / breakdown every time.  It isn't possible to image from home so I try to keep everything as simple as possible.

This is my original configuration when I had added the CAA (63mm both ends).  Note I ended up with only 2mm inward travel left.  I still needed approximately 2mm more backfocus to get sharp corners.  So I ended up in your predicament.

From the back of the red spacer tube flange I had the following:

2mm shim -> 30mm spacer tube  -> 10mm spacer tube -> 1mm shim -> 0.5mm shim -> M48 / T2 adapter for Nikon Z mount (9mm) -> Z6ii Flange to Sensor (16mm) = 68.5mm

 

This is the final configuration with the CAA downstream of the Flattener.  This gave me back the 20.6mm inward focus travel taken up by the CAA.  It also delays the necking down M63 to M48 to the camera, which seems to have helped reduce Vignetting.  I needed to buy different spacer tubes so I bought a TS Variable M48 Adapter 17 to 23mm.  Now set approx 21.5mm.  I have not finished precisely tuning the backfocus (clouds) but am very close, in the 0.2 to 0.4mm adjustment range, so it will all work.  The Variable tube is brilliant.

The Flattener comes with an empty red spacer tube and I use it.  Removing it would give around 20mm space for additional equipment.  FLO has all these parts.

From the back of the red spacer tube flange I now have the following:

CAA (20.6mm) -> 2mm shim -> Variable spacer set at ~21.5mm -> 1mm shim -> M48 / T2 adapter for Nikon Z mount (9mm) -> Z6ii Flange to Sensor (16mm) = 70.1

Hope that helps.

Cheers

Tim

[raadoo]

Thanks so much for taking the time to reply, Tim!

Thankfully, I won’t need to move the CAA’s position as, upon close examination, it turned out that there was a 30mm extension sandwiched between focuser tube and CAA. It came attached to the scope so I didn’t give it a second thought, believing it to be part of the focuser tube itself.

Having regained about 25-30mm of focus travel, I’m positively jubilating, not to mention grateful for your help!

Edited March 20, 2023 by raadoo

[Which one is Polaris]

Yup, some of these parts look very much like they have to be there.  Sounds like you have it all sorted out.  That is good news !

Cheers

Tim