S-FPL51 : degradation and corrosion ("rotting") ?!

S-FPL51 is a premium, low-dispersion optical glass manufactured by Ohara.

While generally machined as bulk substrate for telescope lenses Ohara provides separate "Low Tg" i-Line and molded glass variations.

This allows S-FPL51 (and its analogs like FCD1) to be precision-pressed into high-volume aspheric shapes without traditional grinding.

example photo

Unlike the higher tier S-FPL53 or FPL55, S-FPL51 features thermal characteristics that make it more challenging to mold into complex geometries while avoiding thermal stress.

However, its base cost is lower than S-FPL53 making it a popular choice for budget ED (Extra low Dispersion) telescopes and commercial optics.

An acidic atmosphere can accelerate the degradation and corrosion ("rotting") of FPL-51 optical glass (manufactured by Ohara), a fluorophosphate material used in telescopes.

This glass is chemically delicate and susceptible to environmental stress degradation, particularly in the presence of moisture and acidic agents.

1. Why it deteriorates :

Low-dispersion glasses like FPL-51 contain elements (such as fluorine) that can react with airborne acids, causing the glass's lattice structure to break down, resulting in clouding, the formation of surface micro-cracks, and corrosion.

In the optics industry, this type of degradation is known as "glass rot" (breakdown of surface properties and loss of transparency).

2. How to protect it :

To prevent this phenomenon in acidic or highly polluted environments several measures can be taken. 

Use of filters :

Install UV/IR or other protective filters in front of the lens.

Airtight storage :

Store the optics in sealed environments and use silica gel to keep relative humidity very low (< 30–40%).

Temperature control :

Avoid sudden temperature changes that lead to condensation forming on the lens.

Condensation combined with acidic gases creates an extremely corrosive environment.

Astro KMan