It is possible to use the merging of multiple shots to get an effect very similar to an ND filter, I think some Olympus bodies have this feature.

That pretty much sums it up from me too. Well done Spad!

On a side note, I think all my astro shots have been failures. It's very rare I even think to give it a go now - when special astronomical events are brought to my attention you can guarantee extensive cloud cover.

I'm sure it was, fortunately I've played with it enough to remember diopters are additive.
Simply change the lenses focal length into diopters add the close up lenses diopter (+8 for the DCR250, or +4.8 for tha DCR150) then convert back to focal length.
As mentioned upthread diopter is 1/focal length but the focal length must be in meters

A 250mm lens has a diopter of +4, add the DCR250 & the combination is +12, which gives 0.083333m or 83mm
at 100mm the lenses diopter is +10, giving 45.5mm combined with the DCR250

It's also useful to realise the furthest you can focus with a close up filter is determined by it's diopter (calculate it's focal length & that's your answer) This is one of the reasons I prefer the weaker DCR150. It typically gives quite enough magnification on a 300mm lens & the added working distance is always helpful.

I don't see how extension can change a lenses focal length as I believe focal length is defined as the point where infinity is brought to a focus.

Close up lenses have to decrease the focal length to work. The distance a lens has to be from the sensor is a function of focal length, to focus closer the distance is increased. (for thin lenses it's calculated using 1/f=1/i+1/s where f is focal length, i is image distance & s is subject distance)
The lens is set to hold the optics at the right distance to focus at distant objects, to focus closer either the lens has to move further out or the focal length must be reduced,

I'm also more than a bit partial to IR.
One, three & five are my favorites, while I love the sky in the sixth, I see too many loan trees.

It certainly looks like OM, while the buttons are not usually round I have at least one third party lens that does have round buttons.
To confirm the greater diameter of the OM mount (across bayonet tabs) is ~46.7mm and without the tabs the diameter is ~44.2mm.
OM has a horizontal locking pin near the end of one of the bayonet tabs.

Perhaps useful is the note I have that the lens can be used on PK bodies upside down if it is held against the cameras mounting flange.

I hope so, rather than just challenging!
The only images I've recorded through this so far have been on my Pentax digitals. Through the viewfinder shots solves shutter issues, but I've not managed one yet with the image looking focused.

I've no plans to try wet plates on my 5x4 cameras, but a few other alternative processes may creep in.
His images all seem to have considerable artifacts from the processing - much more than I've seen in other wet plate stuff.

The big lens I finally got mounted on a lens board over christmas might actually work reasonably well for cyanotypes, it's fast (5" wide & 150mm focal length makes ~f/1.2) yet still transmits UV better than most - managing 50% of it's peak transmission at 375nm.. In camera cyanotypes might just be practical

I'm certainly keen to try it, getting my huge range of Pentax lenses autofocusing on an IR converted body will be nice. Somewhat typical that i hear about it just 2 days after recieving a DA-nex adapter that will have no use once this new one is in my hands.

As Lenny says cropping can make a big difference to images. But in the first two examples, I find his crop too tight making the images look cluttered which the original didn't. The second two I find the cropped versions an improvement.

He's definitely right about these things being personal opinion too!

I must admit I'm always a sucker for well taken blue hour shots. Hard to say why but I think no 2 is my overall favourite.