Rik's sandpaper - a modification

[NikonUser]

Rik showed a method with lots of details re. using scored sandpaper as a lens test subject.

http://www.photomacrography.net/forum/v ... 0843d4ec45

I found the sandpaper to be too 'blurry' (for want of a better word); looked to me like an amorphous mass.

The idea, of course, was excellent.

Copying from him I sprinkled 320 grit Carborundum powder (Silicon Carbide with a median diam. of 40.5µ) on to sticky paper.

Lots of nice sharp edges.

Full frame (23.6 x 15.8 mm) with Nikon 20x ELWD M Plan Achromat, NA 0.4

NU1210

[g4lab]

Brilliant. I LOVE this forun!

[rjlittlefield]

Indeed, "brilliant" in every sense of the word. I'm fascinated by the variety of appearance in these crystals. Checking with Wikipedia, I find that:

The material formed in the Acheson furnace varies in purity, according to its distance from the graphite resistor heat source. Colorless, pale yellow and green crystals have the highest purity and are found closest to the resistor. The color changes to blue and black at greater distance from the resistor, and these darker crystals are less pure.

I found the sandpaper to be too 'blurry' (for want of a better word); looked to me like an amorphous mass

I have the same problem with un-scored sandpaper. I suspect a lot of it is due to that resin bed that Betty mentioned -- the one that holds the particles down to the sandpaper in the first place. Several years ago, the first sandpaper that I tried was awful, but then when ChrisR prompted me to look again, I got hold of some stuff that worked a lot better (HERE). The scoring was something I thought might improve an easily available material. Loose abrasive did not occur to me, but it looks great!

--Rik

[ray_parkhurst]

That's just beautiful as well as a fantastic subject for lens evaluation. I'm going onto amazon or wherever right now and getting some...Ray

[ChrisR]

amazon or wherever right now and getting some

I'd got as far as buying a new sheet of 600 "wet and dry" for a comparison test, to see how similar it looked to Rik's sheet. I just tried rubbing two new pieces together and collecting what came off - looks like a nice wide range of particle sizes.

The particles "stuck" to a business card like grated pencil lead does, but not so much to a shiny-surfaced train ticket. Less, presumably, to a glass slide.
It didn't look as though much was there until I looked under a loupe ( 20x ELWD Mplan). It wouldn't take long to accumulate enough for a target for a 4x test.
Perhaps a coarser grade would provide more ammo. If there are particles down to very fine, I don't think the size of the largest matters too much. If that's all true then it's an easier source to get hold of than pots of powder, I think.

What's your grey "glue", NU? It looks a lot smoother than sticky tape. Would nail varnish do the job?
I don't have Canada Balsam but nail varnish came from the "£1" store.
I think we just need something that dries fairly quickly in a smooth thin layer.

[ChrisR]

The research budget has now extended to manage a can of grey "plastic" spray paint ( for car bodies), a pot of two grades of valve grinding paste, and a sheet of 120 grit waterproof paper.

Grinding powder wasn't on the shelf, though I thought I had some somewhere. The paste is carborundum grit in grease. I tried to wash the particles out, with WD40, and isopropanol, but without much success.

The 120 grit paper does, as I hoped, produce a lot more material to work with. Rubbing two surfaces together rapidly produces a little heap. If you leave it in on the paper, or tip it back on, the particles get smaller as you rub in circles.

Grey primer works just fine as glue. The thicker the layer you spray, the longer it takes to dry, but times around 10 seconds to a minute are easily achievable. Dropping the grit onto the "primed" surface while the surface is tacky, makes it stay there. It needs to be fairly wet, or the larger particles stick precariously by one point.

I'm attracted to the possibillities of transparent "paint". There are many of course. Apart from nail varnish, I have a few, including gold size, French polish, polyurethane varnish and so on. It would be convenient to spray onto a surface which had a millimetre scale on it, such as a steel rule, though the reflectivity of the metal could be undesirable. Using a coverslip, a scale placed underneath may be useful.
(I've always wondered why rules are not made of white-anodised stainless, engraved and filled in black )

A tester for your stacking software lighting and lens behaviour when dealing with specular highlights, could be a nail varnish or paint with metallic sparkly bits in it - there are many of those.

A microscope slide is the obvious substrate, though I used packing plastic initially. Using the inside of a small mineral specimen pot, which has a lid, would make a target with an easy protective cover.

Sorry I''m rambling, but I remember now how us students used to flatten metallographic specimens under the miscroscope so they were easier to photograph ( 5 x 4 inch films....) The specimen was buried in a bakelite button, sitting on a blob of Plasticene on a slide. We had special presses to make the surface parallel to the slide, but we soon learned that the rim of the objective was much better. So yes, we routinely rammed the objective into the specimen. I can only assume that we were using "BD" style objectives, which wouldn't get wrecked by the procedure.

Don't do this at home.
Crunch:

[NikonUser]

Thanks guys for the feedback.

Chris R wrote "What's your grey "glue", NU? It looks a lot smoother than sticky tape. Would nail varnish do the job?"
It's simply 3M double-sided mounting tape, very slightly grey, stuck to a slide and the slide held way above a grey background paper; vertical set-up.
I suspect all the 'thicker' bits of glue are holding the carborundum particles leaving very smooth spaces between the particles.

Nail varnish tends to shrink on drying, may work; tacky grey paint would also work. But why would you want to use anything other than the 3M tape??

Image shows quick and dirty point-and-shoot photo of actual specimen.

NU12011

[ChrisR]

That's the sort of tape I used for my butterfly wing, though no particular make.
It does pick up fluff over a period, and at higher magnfication is anything but smooth. Is your tape OOF?

[NikonUser]

There is an image of the tape:
http://www.photomacrography.net/forum/v ... hp?t=15887

appears to be little ridges (glue) with smooth areas between. Perhaps the carborundum sticks to the ridges leaving the smooth areas as background.

Sticky labels seem to be a smoother overall but then you would have a white background.

I have some clear sticky labels that are designed to fit over the classification codes they place on the spines of library books. These could be mounted, sticky side up, on a card of any colour.

When not in use keep the specimen on its sticky tape in a closed box.

'No name' brands are OK for some things, but Scotch Brand (3M) stuff is often of much higher quality.

3M also makes a double-sided sticky 'tape' that is pure glue, Not particularly easy to work with and I believe used by professional at galleries for mounting.
May not be available in stores. I have not checked its smoothness as I am happy with their regular tape and labels.

[NikonUser]

Chris R: how about this? Chat up your local hospital, university, microscopy club technician/member and get am embedding-wax block cut into a 1 micron thick ribbon (microtomes can cut 1 micron sections routinely). Mount ribbon on glass slide, sprinkle on whatever, heat gently to melt the wax and then cool. Lay on a grey card and photograph. Result should be a very smooth non-sticky subject that can be gently rinsed with water if it gathers dust.

[ChrisR]

Sounds neat. I think a painted-on layer of shellac (French pollish) would do it too. Once dried (by evaporation of alcohols) it can be remelted (restorers use solid sticks - "beaumontage") - or redissolved, come to that. Surface tension would pull the liquified film flat (?).

I was taken by the colour of the tape in your slide - grey being more useful for standardising between exposures than clear or black.
Unless someone scrapes the car I have a lifetime's supply of grey primer now.

[Chris S.]

Great idea and demonstration, NU. I think you're onto something. Thanks!

--Chris