hanhait

I don't think you have the wrong gear. You mention the 16-50 2.8 which I would not call 'slow', but if you insist to have f 1.8 the choice is limited to the 50/55 mm that may be appropriate for ...

The iphone even has less CA. However bright sunshine is relatively easy for any camera.

Physically there is no reason to be different: light is light. It may be in the implementation, especially the color filter combined with the spectral sensitivity can give a different impression. ...

Tamron has the 10-24 mm f 3.5-4.5 in Pentax mount

You must consider the pixel size here; those of the K3III sensor might still be larger.

Well done, I meet similar limitations with the astrotracer. What camera ? ISO setting ? F2.8 I suppose ? What software ? Quite some cropping I presume. Looks like little light pollution.

Very impressive. What equatorial mount will you use ?

Use the distance scale on the lens.

35 mm 2.4 ; and nothing wrong with the 18-55, if not already attached.

The landscape looks very unpleasant. Maybe some astigmatism combined with the de-focus that is only partly compensated - or even amplified - by the small aperture ? If the new K5 helps: fine, but ...

On APS-C at 300 mm plus some 2x cropping, pentax K-5 Pentax K-5 + Sigma 70-300 at 300 mm, f 7.1 1/640 s ISO 200 2x crop ;