Binorum VariPol 1.25″ Variable Polarizing Filter
The Binorum VariPol variable polarizing filter with 1.25″ barrel diameter is a sophisticated solution for brightness control in both astronomical and terrestrial observation. This unique filter allows smooth adjustment of light transmission from just 1% to 40%, providing an exceptionally wide range of options for adapting brightness to the observed object and conditions.
How the Variable Polarizing Filter Works
The filter consists of two connected polarizing filters mounted in a rotating housing. Polarizing filters transmit only light vibrating in a specific plane. When both filters are oriented in the same direction, they transmit maximum light (up to 40%). By rotating one filter relative to the other, the polarization planes gradually rotate, reducing light transmission. At mutually perpendicular orientation of both filters, you achieve minimum transmission of just 1% - effective dimming of 99%.
This mechanism provides smooth, linear control over image brightness, which is far more versatile than a set of fixed moon filters with different densities. With one filter you cover the entire range of needed dimming levels.
Ideal for Larger Telescopes
The variable polarizing filter is especially useful for telescopes with objective or primary mirror diameter greater than 114mm. With these light-gathering instruments, the full Moon can be uncomfortably bright even with a standard moon filter (typically 13-25% transmission). VariPol allows you to reduce brightness down to 1%, providing comfortable observation even with large apertures at high magnification.
For owners of 150mm, 200mm and larger telescopes, this filter is almost essential accessory for lunar observation. Without adequate dimming, the intense light of the full Moon can cause eye discomfort and reduce the ability to perceive fine details due to pupil oversaturation.
Moon Observation with Maximum Comfort
The Moon is the brightest object in the night sky after the Sun. When observing through a telescope, its brightness can be literally blinding, especially during or near full Moon. The variable polarizing filter solves this problem elegantly:
Full Moon: Set maximum dimming (1-5% transmission) for comfortable observation of bright areas such as highlands or rays around fresh craters.
Terminator Region: The filter is particularly useful when observing craters near the terminator - the boundary between illuminated and unilluminated parts of the Moon. Here you can finely adjust brightness so that details are visible both in illuminated crater walls and in their shadowed portions. Too much dimming would remove details in shadows, while too little would oversaturate illuminated areas.
Old Moons and Crescents: When observing crescent phases or old Moons with low illumination phase, you can reduce dimming to 20-40% transmission, which preserves enough light for comfortable observation while maintaining contrast.
Planetary Observation
The variable filter finds application also in observing bright planets:
Venus: When observing Venus, especially when it's close to Earth showing a crescent phase, the planet can be uncomfortably bright. The polarizing filter allows reducing brightness while increasing perception of fine details in the planet's cloud formations.
Jupiter and Saturn: With large telescopes at high magnification, slight dimming can improve contrast of atmospheric belts and details.
Terrestrial Observation
The filter is very suitable for daytime terrestrial observations. Using variable dimming you can:
- Reduce glare when observing landscape in bright sunlight
- Improve contrast when observing distant objects through atmospheric haze
- Observe objects in bright light or reflections from water surfaces and snow
- Protect eyes from uncomfortably bright light at high magnification
Solar Observation (Only with Safe Solar Filters)
The variable polarizing filter can also be used for solar observation to monitor sunspots and eruptions. CRITICAL WARNING: The polarizing filter alone must NEVER be used for solar observation! It may only be used in conjunction with an appropriate certified solar filter (e.g., glass solar filter or solar film) placed on the front of the telescope.
In this configuration, the polarizing filter serves to fine-tune the brightness of the solar image after primary light reduction by the front solar filter. This can be useful for optimizing sunspot contrast or compensating for brightness differences in various atmospheric conditions.
Technical Specifications
Light Transmission: Continuously adjustable from 1% to 40%
Dimming Range: Effectively from 2.5× to 100× brightness reduction
Barrel Diameter: 1.25″ (31.75mm) - threads onto eyepieces like a standard filter
Construction: Dual polarizing system with rotating mechanism
Optical Quality: High transmission in set range, without color shift
Material: Metal housing, glass polarizing filters
Advantages of Variable Filter Over Fixed Filters
Versatility: One filter replaces an entire set of fixed moon or ND filters with different densities. You save money on purchasing multiple filters and save space in accessories.
Precise Adjustment: Continuous regulation allows finding exactly the right brightness level for specific conditions, object, and personal preferences. You're not limited to several fixed transmission values.
Quick Adaptation: You can immediately adjust brightness by rotating the filter without needing to change filters during observation. This is particularly valuable when observing different Moon areas with varying albedo or in changing atmospheric conditions.
Economic Solution: Instead of purchasing three or four moon filters with different densities, one variable filter suffices, covering the entire range.
Use and Operation
Using the filter is simple: thread it onto the bottom of the eyepiece (most eyepieces have standard M28.5×0.6 filter threading). Then insert the eyepiece with filter into the telescope focuser as usual. During observation, rotate the filter's outer ring to change dimming until you find the optimal brightness level.
Start with higher dimming and gradually reduce it until you find a comfortable level. Remember that eyes gradually adapt to darkness, so you may want to slightly adjust dimming after several minutes of observation.
Who Is This Filter For
✓ Owners of telescopes with aperture greater than 114mm
✓ Observers who frequently watch full Moon or phases near full
✓ Astronomers working with high magnification on lunar details
✓ Users seeking a universal solution instead of a set of fixed filters
✓ Observers using telescopes for terrestrial observation as well
✓ Owners of telescopes with fast focal ratios (f/4 to f/6)
