Is this good enough

[alacant]

22 hours ago, shropshire lad said:

If I lose the mask

You'll see coloured halos with or without the mask.

The mask spikes register the most in focus -red and green- light, leaving the blue out of focus. 

Compromise focusing is what happens after the mask is removed. Focus the blue, which at the same time will defocus the red and green; a compromise.

HTH

Edited August 19, 2023 by alacant

[alacant]

On 16/08/2023 at 09:47, shropshire lad said:

do I need to get it spot on

We can't tell because we don't know what images taken through the lens look like.

My guess is that with that pattern, the red and green will be in focus leaving blue rings around bright stars.

Perhaps post an image focused with the mask of, say Deneb? We should then be able to offer more precise help.

HTH

[ollypenrice]

Just going back to your original B-mask image, I'd call it a significant way out. The central spike is clearly on the high side in the orientation below. When using a B-mask I want it to be strictly central. In reality I usually feel with mine that one central spike is slightly offset relative to the other so I compromise with one a tad high and the other a tad low. I never know whether this is an illusion but, when I do it this way, I find I have the best FWHM as measured in camera software.

When I split the channels in your image above and pasted the red onto the blue in Layers, I could erase a section of the top layer's central spike to see how it aligned with the bottom. Red and blue were almost identical with an error only a fraction of the visible error in focus (ie the high central spike.) I certainly think that being more precise with the mask will make a difference.

Olly