Possible alternative to barlows/teleconverters (1 Viewer)

Quick test with the TN + Aperture control.

Without the Aperture adapter, the available diameter is 36mm. With the aperture fully opened, it is 25mm and it is 13mm half opened. There is a bit of vignetting at 13mm but it is easily corrected in LR, after that, vignetting kicks in big time.

First photo is with 13mm aperture, second one is 11mm.

3rd photo taken with an aperture opening of 13mm. Images with a 13mm aperture seem to have more contrast than when using the TN without the Aperture Adapter. I was surprised to see that there is no change in the amount of light reaching the camera. 36-25-13-11mm apertures all share the same settings: ISO 800, 1/4000 s.

4th photo is the SW80ED alone, no TN - ISO 800, 1/800 s.. All photos are untouched, except for resizing. TN assembly is 1.65X.

I'll try to test it on birds tomorrow but this quick test seems to indicate that results would be as good with a 13mm black cardboard diaphragm instead of the adapter. I can't see any benefits in being able to adjust the aperture.

Anything that close to the sensor will in effect cast a "shadow" on the sensor when it is stopped down. Further forward it will be beneficial to d.o.f., but that far back.... I doubt it. The proper place for a diaphragm is as close to the front nodal-point as possible, which will be inside the scope's lens group.
I experimented with a diaphragm mounted on the end of my extension tube (80mm), still a good ways behind the nodal point, but as far away from the sensor as I could get it. It did have some effect, but even there, perhaps 12-13cm from the sensor, it caused vignetting.

The Skywatcher I had before came with a plastic lens cap with a hole in the center with a second cap on it. I would use it when it was very bright out. It increased dof and there was no vignetting. Better there than bang in front of the sensor.
Only problem is adjusting it if it is inside the scope...

Thanks for the info Dan. I don't know enough about optics to comment but here is what I found building 3 TNs: adding a diaphragm at the camera end seems to increase contrast. I have not tested DOF but it is still very shallow.

This new TN seems to be the best of the 3. I won't keep it inside the aperture adapter. because it is heavy and I don't see any benefit in being able to adjust it. I'll put it inside a M43 extension set with the smallest diaphragm possible. I'll use the right extension length to make it about 1.65X, which will give me about 1000mm.

I'd like to read Paul on contrast and DOF. He has experimented a lot on this.

A few shots with the TN inside a M43 extension set using only 1 24mm tube. The TN is on the scope side and I installed a 15mm diaphragm on the camera side.

First shot at 100-125m.
Second shot at about 200m.
Third shot at 10m. - focus a bit off.

Temperature was sunny, cool and the air was crisp. Ideal for long range photography.

The 15mm diaphragm causes vignetting and I later replaced it with a 20mm one which does not. This Fujinon TN is definitely the best of my 3 TNs - the other 2 come from Minolta and Canon zooms. As installed, it gives me 1.71X, the SW80ED becomes 1000mm.