Certainly an excellent set.

The 150-500 is great for motorsport but I found out at Brands that a wider option is worth having with you too! Your 70-300 might suffice, but even the 17-50 could prove useful.

I find it hard to find anything to critise on these, I like the Saturn V (especially the chimp behind it) though the green thing is a little distracting. I do find the Dunlop sign peaking out at the back of Giovanardi's car a minor annoyance (it's barely noticable in the Ginetta shot).

The 3/4 shots are very impressive I've never managed to pan sucessfully with something like that.

I suspect the biggest difference will be in the quality of materials. The cheap ones will probably bend in use, but might well be sufficent for occational use. The more expensive models might also have interchangeable tips so that a tight fit for the recesses can be used.
A DIY solid one would be possible but I think I'd try the cheap ones first. If they aren't man enough make your own from something tougher, but the time involved is likely to be worth more than £20.

If it's your only M42 lens then IMO it's best left on the lens. I modified my earliest adapters to keep them om the lens & speed up lens changing.

FWIW T2 threads (42mm X0.75) are sufficently different that the adapter will only go on about one turn maximum. It seems there is also a 42mm x 0.5 size thats been used for some filter threads - I've never seen one of these but came across stepping rings for them, as well as a few listed as 'm42' without specifying the thread which turned out to be T2.

IIRC the much longer wavelength would mean there'd be very low resolution. In addition I think only the rock would reflect radio waves to any significant extent.

The few radio telescope images I've seen have looked very unimpressive, so I don't think it would be worth investigating even if the hardware was readily available.

Pentax doesn't make it's own sensors, so has to choose from those made by others & available to them. If they use a very old sensor with lower MP they'll lose out on low light capabilities...
Previous examples suggest to me that they're not going for extra MP at the expense of IQ.
If sensor manufacturers made a new generation of 12MP sensors, I suspect the low light & dynamic range would beat the latest high MP sensors. Unfortunately many still see high MP as the most important factor of a sensor, so it's not likely to happen.

IMHO 'of interest to and usable by Pentax users' would include a handful of photos of:

A potential location/event,
A unusual or advanced technique if suitable for use with Pentax cameras,
Comparisons between Pentax & other systems,
Alternative lighting/storage etc. options...

Even if the photos are taken with a third party camera.

I've always thought the moderation here has pretty much ignored this part of the rules. Certainly in any of those situations more than a handfull of non Pentax images would not be approriate.

Nothing that I'd expect to reflect quite like that.
It could be the sky reflecting off the flared wheel arch could make the helmet (with the safety barrier giving the trim).
There was a big yellow (Dunlop?) sign in the area but none o the features look right for that.
I suppose the road surface could reflect so as to look white, but I don't think there were any ripples in the door skin...

Quite a bit of internal mail still goes by air. So the distinction is not very relevant.

My more recent batteries have come with dedicated plastic clip on covers. Prior to that they just came in plastic bags inside a cardboard box, but even that should prevent shorting.

I still use the original boxes for my day to day storage, putting the battery in a bag if its charged. This means I can tell at a glance which ones need charging when I get home.

This morning I had a quick look at this image I took over the weekend at Brands Hatch, and saw the reflection just above the Autocar sticker looks very much like a white faced man in an open faced crash helmet.
I did a little investigation into fatalities at the circuit and found amoungst the 50 names that came up was one that may be a relative!

Norman Glanville Parker died at the circuit in 1960.

Glanville isn't a very common surname (more common than Kanssen) and my Family tree links to their's about 5 generations back.

I usually photograph the whole coin, and haven't yet done anything for your challenge (need to find the right coin)
I have got a shot of a 20p coin though;


20p detail by Analyst 1, on Flickr

The W was under 1mm accross, and the set up might loose me more points - not a macro lens, but a microscope (not on high power!) with the camera mounted via a self modified adapter.

I'll try & do this properly later