A startlingly bright sundog, and worthy of submiting to OPOD - (Atmospheric) Optics picture of the day!

Your one was clearly much brighter than the one I saw on the way home the other night (Which faded further shortly before I could safely park to photograph it.)

For those that are interested OPOD discusses Sundogs here, here & in many other entries.

It seems if Wikipedia is correct, that the Battle of Britian one is currently the only flying example. The Canadian one will not be flying until 'returned to flying condition'
Though how it can be 'Airworthy' whilst not in 'Flying condition' confuses me!

Welcome to Pentax User, Klaus!
Yes your certainly well known on the MFlenses, your UV work is impressive enough that I recognized the name even though I'm a fairly rare visitor to MFlenses. (I'm dConvert over there)

I have managed to get UV images that show luminosity differences to the visible equivalents, but still nothing worth sharing...

Hi Tony
None that I've seen yet
If you're not too far south perhaps we could change that though??
(I'm right in the NE corner of Essex)

I've just got a K7 (it arrived a week ago), so I have little experience in having 2 DSLRs. However I have no plans on selling my old camera (K100d) for several reasons:

The K100 has good Infra Red response, and I like dabbling in such things.

Resale value of the K100 is ~£150 - not enough to be worth the hassle.

My wife is interested enough in the possibility of using the K100 that Photography expenses might be considered more favorably.

I've already got one, so can't justify helping. It's an excellent lens, but not so rare that I feel the need to have a spare.

Rather than another lens, what I want/need is a better way of taking my lenses & accessories with me, (and a newer Body!).

My normal kit (in a large backpack) covers nearly everything (I've missed the 17-18mm range) from 10mm(fisheye)to 300mm in four overlapping zooms, together with a fast 50, a 100mm macro and a macro teleconverter (theoretically doubling the whole lot). There's also spare batteries/cards, flash & a few filters. On some occasions I add a second bag (generally for the 600 reflex, or a prime telephoto) and maybe a tripod.
Even with all that lot it's not that uncommon to want a bit of kit I've left at home to photograph something right.

Today (taking the kids through the country park to the swings) I just took the K100d, F50 & an IR filter, and my back seems to approve of the change!

Does anyone know where I can get a cheap Sherpa? He'd have to sleep in the garden, but I'm sure we could feed him.

The Optomax 200mm f4 sounds like a T2 rather than M42.
T2 mounts are m42*0.75 rather than M42*1. ie a finer pitch screw thread.

Adapters for T2 are readily available for just about all mounts - I've got several for mirror lenses & digiscoping.

Useful links

I've found ICE can be quite forgiving of parallax errors, It stitched together a row of some 15-20 telephoto shots taken (in quick succession) from a moving ship with only one visible problem point (looking up a river so the foreground/middle distance were less constant).
Given the width /height ratio I was left with it probably works better as 2 panoramas anyway.

To answer the other points:
Magnification for a simple lens system is total extension divided by focal length.
(Total extension is the added extension from tubes/bellows + the focusing extension of the lens itself)
The more extension you add the more magnification you get. It makes no difference to the image if the extension is from tubes, bellows or a (suitable lined/lighttight) pringles can.
I have used a section of steering gaiter but this introduces extra effects as the lens is not parallel/in line with the sensor.

DOF is a function of magnification & aperture. The greater magnification you get the less the DOF. I've not checked but DOF should be the same for both extension & close-up lenses when giving the same magnification.
If you use a tripod & focus slide (some bellows have one built in) you can take multiple shots with the camera at slightly different distances (ie. another part of the subject in focus) which can then be combined using software 'Combine ZM' is free & work well enough for me, 'Serene Stacker' is apparently better when you have large numbers of images. (Note refocusing the camera instead of moving it changes the magnification so doesn't work for this.)

My Digital microscope (not Pentax so I won't post images) really suffers from the shallow DOF needing ~10 shots to get the width of a fly's leg all focused at 100x.

For hand held macro, Extension tubes are generally better (with electrical links if you can afford it) otherwise even £5 Chinese ones work but it's worth paying a bit more for some second hand one that have bayonets at each join, include the aperture linkage and are likely to be more robust.

One other possibility you may not be aware of, is reversing a shorter focal length prime in-front of your K105 (a std 50mm should work well). The reversed lens works as a highly corrected Diopter (Close up lens). Magnification is the ratio of focal lengths and focus has to be done by moving the camera. You can try it out by just holding the lens in place, if it suits your purposes a coupling ring to screw the two together by filter threads can be had for £5-20. This is the technique I use most for macro, though having the camera control the aperture of the forward facing lens ('A' setting required) simplifies matters.