Missing refractor baffles

[Ben the Ignorant]

My 80mm TS triplet has great optics but only one baffle. The rough black paint inside the main tube and drawtube is not enough to make for the missing baffles so I made the first one as soon as I could. It has to sit as close as possible to the objective, thus it has to be very thin and kinda hard to make without tearing the material. My brother's girlfriend is an architect so she gave me the rigid foamy sheet material she uses to build models. I crafted the ring with a compass fitted with a blade instead of the graphite tip. Installed it, saw how far behind it casts a shadow, and cut the second one accordingly.

Simply drawing the light cone at a 1:1 scale thanks to the short 480mm focal, and knowing where the second baffle goes, it's obvious how wide the baffle has to be. Its shadow reached down to the factory baffle, so the craft can stop there. Painted the baffles with a sharpie and got this:

 The serrated unit is the factory one, made of aluminum, well-cut and well-painted but mine were shiny. And strangely, the color from the black sharpie was a very dark purple, not black. But I came across some quality blackboard paint so I removed the friction-fitted rings and re-did the paint job. Good thing I had not glued them.

 

Fun fact: the 2mm thick hard foam rings are so light they don't trigger my kitchen scale, meaning they weigh less than two grams together.

 

No balance problem here. You could craft a dozen of those without bothering with mass spread across the tube. The layer of rough paint increased the friction, these will never move from their ideal spot. The four studs glued on each ring are a hair width outside the ring's diameter for better friction.

 

Nice uniform matte blackening, the suspicious homemade look is gone.

 

While I was at it, I removed the retainer rings behind the triplet glass: that critical place was shiny, never liked that (the lens edges were already blackened years ago).

Why two retainer rings? Well, turning a single ring could make the lens it touches spin, ruining rotation. The buffer ring prevents that, smart move from the scope designers.

 

After the blackboard job:

 

And both matte rings in the cell behind the glass:

Microfiber cloth, I'll never use cotton or any other fabric close to my optics, microfiber is non-abrasive unlike regular fabric, or paper, the worst offender in lens contact. Black sharpie on the lens edges, blackboard paint on the retainers, and the shiny thread will be hidden when it's screwed onto the main tube. No stray light from the cell can make it to the focal plane.

 

However the designers didn't get everything right, the rear ring was tapered to 77.3mm for some reason while the front one had the right 80mm diameter. This stopped down the objective a bit, causing a 7% loss of light. I have no idea why they did this, I have never seen a tapered retainer in the flesh or on photos or optical diagrams. So I resized the rear ring to 80.1mm for good measure. I had pics of the measurements on digital calipers but I lost them when I switched to a new computer, sorry.

Does this increase chromatic aberration, because the bulk of it always comes from the edge of the lenses? Well these scopes have a 23 or 30 microns chromatic spread, depending on which scope is tested, and which lab does it. A few more microns would be impossible to notice, since we are still below the super-apo 40 microns limit.

That's 20 microns on each side of median focus, about the tolerance of a micrometric focuser, doesn't seem like a sensor or an eye could ever pick the difference. However, if the switch from a 91% reflectivity diagonal to a 99% diagonal is worthwhile, the 7% gain is, too.

 

Back to the baffles, are they effective? Looking from the edge of the diagonal, check for reflections on the tube wall:

None, only the back of the baffles shows (and the flocking in the drawtube and the diagonal's nosepiece).

 

As a comparo, in my semiapo that hasn't received its extra baffles, some light bounces off the tube wall and reaches the diagonal:

That bright crescent shoud not be there.

 

But also, do the baffles eat up the light cone? That is verified from the front.

With the drawtube pulled at working distance (around 35mm to 40mm), the line between the edge of the objective and the edge of the drawtube is not interrupted. Thanks to the compactness of these apos, just two baffles sufficed.

 

But an antireflection job has to be complete, got to chase those stray lights like infectious viruses, leave not one alive, so the diagonal's nosepiece is garnished with the foam side of velcro, not glued directly to the tube, there is an intermediate plastic layer. Friction keeps the foam there, and it can be removed anytime with no effort.

The shiny metal front was black-sharpied but the job is sloppy, no problem, refinishing it is trouble-free. Remove stains with little rag and solvent, reapply carefully.

 

Same for the widest-field eyepiece for this apo, a 30mm Aero, except the velcro foam can't cover the last millimeters of the thread, or filters won't fit:

The drawtube was flocked the same, it can be seen on the seventh pic.

 

With this entire baffling, flocking, sharpie and blackboard job plus the 7% light gain from resizing the weird conical retaining ring, that little refractor has the blackest background and brightest image I could get from it. Next is my semi-apo.

[johninderby]

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[Ben the Ignorant]

No, never saw these discussions, I've been here for two years only. But stopping down such small and well-corrected optics is stupid, only the very large apos begin to show false color, and reducing the diameter by only a couple millimeters changes almost nothing. 5 or 10mm would make a visible change, but I won't start a debate, that just seems like an engineering, or worse, a marketing mistake ?. I'm glad I recovered my 7%.

That is so strange on the part of William Optics, they make clones of other brands' telescopes with one extra millimeter in frontal aperture, only to stop it down at the rear?

[Alien 13]

You should coat the internals in Vantablack its brilliant stuff, dont get it on anything else or you will never find it.

Alan

[johninderby]

Deleted

[Ben the Ignorant]

2 minutes ago, johninderby said:

It was so they could claim better ratings under testing rather than any real world difference. 

German optician Wolfgang Rohr tested a TS and an Astro-Professional version of the triplet at the same time. Both had polychromatic Strehl ratios between 0.95 and 0.99, measured in five colors, red, orange, yellow, green, blue! There was nothing to cheat about, just showing a 0.95 ratio in one color is enough to be deemed excellent! That's the Takahashi and LZOS norm!