The first line of defense

[Ben the Ignorant]

Do these lens edges look black to you?

Uuh, yeah, sure!

 

But now?

Oooh!

 

That was the blackening applied to my new Astro-Professional doublet. Bear in mind that the light exiting at the front first had to pierce the layer of blackening on the other side, but it remained pretty intense, right? Good commendable will on the scope maker's part, and job done, but a second layer of felt marker paint was in order. Yes, paint, felt markers actually apply a kind of paint mixed with a solvent that evaporates very quick. Thus it's quite natural that a second layer is needed for opacity.

Stray light will take any path that's open, and bounce off the rough side of glass that has the ideal texture to spread it in every direction, so I also treat bevels. Look at the previous pics, they were not covered at all.

The real black distinguishes itself strongly from the mere factory gray. If you knew the lens' specs, and the angle of vision, you could reconstruct an image of that room with a specialized software, but that's another story.

 

While I had the lenses out of the cell, I made permanent rotation marks. A ghost of the factory inverted "V" is visible, I made scratches with a needle the first time I opened the cell, but during the first reassembly I suspect the lenses have rotated because a miniscule amount of paint at the wrong place made the rings sticky. Turning them made the lenses turn, I suppose, based on some loss of snap at the eyepiece, while the star test seemed similar. For future maintenance I dug two notches with the edge of a diamond-coated knife sharpener. Thanks to diamond hardness the job was done in seconds.

 

But what happened to those unblack edges, you ask?

That compact led flashlight is viciously powerful but not enough for two layers of black paint at entry and two at the exit. An optician wrote that paint (or ink?) hardens and shrinks enough to compress a lens out of ideal shape, that could be detectable in sensitive tests. Maybe, because the tolerances are only several dozens or hundreds of molecules; 500 nanometers in wavelength of light divided by a good lamba/20 wavefront would amount to only 25 nanometers, we're talking layers of molecules, here. But I still don't figure if the claim is far-fetched or not.

 

The two rear rings have been painted with ordinary blackboard paint. It can chip, I know, however unlike lacquer it dries fast enough to not interrupt the work session, and it's so easy to reapply. The black lens edges coupled to the matte rings produce this effect: the lenses seem to vanish. Direct light from two lamps, a white ring under the cell reflects light at the same angles as intrusive light during real-world observation, but only an extremely faint ghosting occurs. Do you see it?

Why two locking rings, by the way? Because a single one could make the lens it touches turn, and ruin the critical rotation alignment. The second buffer ring prevents that. A little not-yet-dry paint that bleeded between them may have caused the problem for a while.

 

Of course, at shallow angles the lenses reflect some light, but the rings behind them remain discreet. It took a white ceiling and a white column (plus a Strat) to make these reflections.

 

Some blackboard paint bled inside the cell scribing, which is not bright ink but an actual etching done into the aluminum with an interesting pointy tool, that's why it's shiny gray. Instead of spending time trying to scrape it, I filled all the letters and numbers; it created a nice, unique variant. Turns out, I always wanted a cell with dark engraving, and it's not lazyness talking.

The bevel and the inside of the outer cell is matted-painted, too. If you own an Hyperion eyepiece, the hard, dull black finish in the barrel is the ideal finish but I don't konw how to do it, unaware if it's a paint (doubt it) or some chemical process. I often stargaze from my city, lamps everywhere, so I don't want any shiny areas. Everything but the optical surfaces should be dull black. This will also be useful if I do astrophoto one day - uh, one night - because cameras are so sensitive, and exposures so long, any stray light could damage contrast.

 

But, Ben, but Ben! What are those white dots on the outside of the cell? I never liked the velours that barely puts the brakes on the dewshield's motion, and lets it wobble. The solution is those simple foamy rubber adhesive pads. Trial and error determined the right number was eight. A ring of pads at the very rear of the cell, and another at the very front suppress wobble completely, and they keep the dewshield in place even with a 700 gram tilt-lock (my invention so I get to name it) counterweight attached to it, with the scope pointing vertically.

A strip of tape takes marks at the screws' location. I remove the strip, measure the separation of the marks while it's on a flat surface, put it back on the cell, and voilà! Perfect centerline for the rubber pads.

 

Removing lenses means recentering them but their screws are not standard, so I recut a damaged mini screwdriver...

...and quickly glued pieces of any tubing I had to repair the handle and make it more ergonomic. The two notches in different shapes at the screwsdriver's collar help me keep track of turns and half-turns. This slot shape is not standard, a non-matching driver would damage the plastic:

The tolerance between cell and lenses was nicely tight, but thanks to the extra thickness of paint, they almost self-align. After inserting them, and driving the screws at the same depth, I did an artificial star-test, found only one screw needed 1/8th turn to get near-perfect centering. The lenses require a gentle push on the side with a toothpick in order to be seated in the cell. They just slide against the cell walls without rubbing, and exit the same without catching. If they were too close to the walls, centering screws would be disabled, and thermal pinching could occur.

It seems to have turned out pretty okay. The semi-apo quality shows in the very pale lime, almost white disk, and the purple ring. Intrafocal thru Explore 4.7mm eyepiece (120x). The Explore 4.7 increases chromatism in this refractor and in my achro, but the Myriad 9 reduces it, however it does not magnify enough for star-testing.

 

That rounds up my first line of defense against stray light. How does it perform? Well, with the dewshield retracted, and two lamps shining into the cell from less than two meters:

The glass edge and its retaining rings don't shine, and they cast a nice black shadow. The two baffles down the tube are not enough, though, I need to add a couple more, that will be for a later date.

[MarsG76]

looked black-ish.. 

a fellow tinkerer.. nice work

 

[25585]

Disturbing the seeming inadequate finishing on mass production scopes. 

[Ben the Ignorant]

Yes, but a marker job and cutting two baffles is easy. The optics are very fine, the cell is well designed and machined, nothing to complain about the focuser, it's more fluid than my triplet's after a little tweaking and honing. Makes me understand the triplet's focuser needs the same attention, strip it, polish the axle and the plate, rectify a few screws tips.

[David Smith]

Looks like a comprehensive job and you're braver than I dismantling your scope like that! Forgive my ignorance but with the lens encased in an opaque cell is the black paint really necessary? Does it help to minimise internal reflections within the lens itself?

Thanks

[Ben the Ignorant]

Yes, the rough finish on the lens edge scatters light very effectively if it's not painted black. All good eyepieces feature that, everything that's not an optical surface must be dark and dull.

[25585]

Be interested in your findings for fainter stars & objects when you next use it. What difference are you hoping for?

[John]

Will there be any benefit in return for the effort / risk I wonder ?:

https://www.cloudynights.com/topic/532314-blackening-lens-edges-of-cheap-refractors/

[Peter Drew]

My first "line of defence" is to leave well alone!  However, I'm happy to tinker with things if there is a proven benefit that is worth the risk.

[Ben the Ignorant]

8 hours ago, 25585 said:

Be interested in your findings for fainter stars & objects when you next use it. What difference are you hoping for?

I already know the difference, before this I practiced on unexpensive achromat scopes and finders. Finders are great for learning because you don't fear ruining them, and the job is brief. I painted the dewshield's inside, the doublet's sides, and either flocked or baffled the shorty tube. Then it was the rough sides of the Amici prism, and the miniature eyepiece tube took the same treatment as the finder's main tube.

Made a side-by-side of the blackened finder versus a plain one, aiming at the Hyades from my window with light pollution around. The darker finder showed faint stars more easily, I remember the image as a photograph in my mind. The difference is not enormous but the expense is not either: a few drops of paint.

[Ben the Ignorant]

9 hours ago, John said:

Will there be any benefit in return for the effort / risk I wonder ?:

https://www.cloudynights.com/topic/532314-blackening-lens-edges-of-cheap-refractors/

Is there any benefit when all eyepiece makers blacken their lenses and spacers? Is there any benefit when they advertise they've done it? Do we observers see improved contrast? Why is every optical housing in cameras, binoculars, microscopes and telescopes painted black?

As for the risk, I had to remove the lenses the first time to clean them, they were infected by mold. Not removing them was the risk. The second time I suspected rotation had changed, not removing them risked keeping the suspected defect permanent. Plus, the risk seems larger to those who have no experience but I acquired some with practice on cheap and simple instruments. I take so many precautions I annoyed a friend who wanted to take less. 

[Ben the Ignorant]

There's no nothing like an experiment to clarify things. Using this setup:

I shot a 5mW laser at these identical cemented doublets, aiming at 45° between the bevels to maximize scattering. One doublet is plain white, the second one is half-blackened. All three following pics done at 100 ISO under the exact same lighting, and from the same angle, no retouching, no processing.

 

Plain doublet:

No comment needed.

 

Half-black doublet shot at the white side:

An obvious improvement.

 

Half-black doublet illuminated at the black side is next. I had to leave the other half white or the - faint - light scattered by the black side wouldn't be visible. A full-circle blackening would be more effective!

 

My phone camera changed exposure times automatically; I can't set that parameter, that explains the slightly different backgrounds, but the images are consistent enough.