[avandonk]

Actually Alex I have two apertures inside the lens hood. These apertures are baffles from an achro refractor. The one nearest the lens is 56mm diameter and the one at the entrance to the lens hood 84mm diameter.

This effectively makes the lens f/5.2 and there is no diffraction spikes from the len's iris. This way all light passes through the 'allmost perfect' center of the len's optics. Another benefit is no light not contributing to the image even enters the lens to be scattered. No light even hits the lens's internal baffles. This would lower contrast.

As usual it is all about signal to noise not brute force. It took me a while to figure this out. I can also use the lens the same way at f/3.5 but the f/5.2 configuration has stars almost perfect to the corners and vignetting is less of a problem. This is a real plus for mosaics.

The proof is in the image. If you look closely you can see dark dust starting to appear.

If I used the lens wide open at 1600ISO the aberrated and scattered light just swamps the image. This has the effect of losing all dim stars and bright haloes around all the brighter stars.

Bert