I am still puzzled by why adding a 2mm filter will make the problem worae that having no filter, since the 2mm glass should lengthen the path. But the actual measurements did show that the EAF delta was larger with the 2mm than with the EAF delta for the empty filter holder!
There is on possibility that I had not thought of before, and that the minimum EAF delta is in between the empty filter holder case and the 2mm case, i.e., instead of extrapolating the two EAF deltas to -3mm, it actually looks more like this:
If so, adding 0.7mm worth of shimming can make the star bloat out more than not shimming it at all.
I will try tonight to measure with a 1.1mm thick filter, viz. the Astronomik L1, to see if the EAF delta is smaller than both the empty filter and the 2mm filter cases.
The Rikonon lens has so many elements that the image plane might not be a simple low order curve, but one wth multiple hills and valleys.
All that said, I think I have found a way to get a much more relaxed EOS adapter arrangement, instead of the EOS filter drawer, where there is no way to shorten the backfocus.
The solution is this adapter, the ZWO EOS adapter with no filter drawer:
https://www.zwoastro.com/product/zwo-zwo-eos-lens-adapter-for-2-efw-filter-wheel/
Notice that the M54 male thread at the camera end is internally threaded for a 2" (M48 thread) filter. So, indeed it is possible to include a filter in the light path, even though it is not as convenient to change filters as a real filter drawer.
That ZWO EOS adapter is meant to screw into a 20mm thick EFW that is bolted to the camera without the tilt plate (i.e., the "flange" focal distance of the camera has to be 12.5mm, instead of 17.5mm with the tilt plate. 12.5mm + 20mm + 11.5mm for the EOS adapter comes to 44mm, which is the flange focal distance of an EF lens, if there is no extra filter glass in between. With a "normal" image plane, you would need to add some shims to make the distance 44.66mm instead of 44mm if you want to use a 2mm filter.
The removal of the tilt plate requires any attachment to be bolted on, since the M54 threaded hole is on the tilt plate. This leaks light like crazy, causing ZWO to manufacture what they call "darkening rings" (which now come with their EFW that permits bolting. And I still don't trust that will prevent light leaks sufficiently.
I also do not want to use an EFW with my "MC" ASI2600 and ASI6200, while ZWO's filter drawers don't permit bolting.
However, since there is a way to screw in a 2" filter, I do not need even need a drawer. If I now leve the tilt plate as is, we get (without filter) 44mm (EF flange) = 17.5 (ZWO camera flange) + 11.5mm (EOS lens adapter) + extra M54 spacer. This makes the extra M54 spacer be 15mm when there is no filter, and 15.66mm with an internal 2mm thick filter. It is easy to make up 15.66mm with a 15mm Blue Fireball spacer that I already have on hand, together with 0.66mm worth of shims. (https://agenaastro.com/blue-fireball-m54x0-75-spacer-ring-with-15mm-extension.html)
This arrangement is very relaxed, there is 15mm worth of backfocus space to play with. It is just a small pain when changing filters (unscrewing the M54 spacer to expose the internal of the M54 thread on the EOS adapter). But I really only need to add a fixed light pollution filter like the IDAS NGS-1 for my Bortle 5/6 skies, and don't really need to swap the filter often, if at all.
But, I will check out the current EOS drawer first, repeating last night's measurements, but with the 1.1mm thick filter.
Now, if the image plane of this lens is so complex, it makes adjusting backfocus the normal way (trial and error) a real pain. You need many spacer swaps (perhaps adding 0.2mm at a time) to discover one that produces the best corner stars (and it is so subjective too). Stepping 1mm at a time may make you miss the best backfocus completely.
The Rikonon 135 has such sweet optics that I am not about to give up on it yet :-).
Chen
