Lunt BF1800 2"Blocking Filter H-alpha Solar Systems

The Lunt BF1800 2 inch blocking filter is an essential component of a Lunt H-alpha solar observing system. It is not simply an accessory used to define the available field of view: its housing contains additional filtering elements required for both the safety and optical performance of the complete solar system.

Lunt specifies that a blocking filter is required on all of its solar systems using an H-alpha etalon. The manufacturer only warrants the safety and performance of the system when a Lunt blocking filter is paired with a compatible Lunt H-alpha filter or solar telescope.

The BF1800 provides an 18 mm clear aperture. This is significantly larger than the apertures of the B600 and B1200 and provides a wider usable field, particularly with longer focal length instruments, imaging configurations and cameras requiring a larger light cone. Lunt confirms that the blocking filter designation corresponds to the clear aperture, with the B1800 providing 18 mm.

For visual observing, Lunt recommends the BF1800 for systems with focal lengths up to 1800 mm. Its larger aperture provides sufficient field for viewing the full solar disc with additional margin as telescope focal length increases.

Solar imaging places different demands on the optical system. Cameras, adapters and other accessories generally require additional backfocus, moving the blocking filter farther up the converging light cone. At this position, the light cone has a larger diameter. Lunt therefore recommends selecting a larger blocking filter for imaging than would normally be required for visual observing alone.

For imaging applications, the BF1800 is recommended by Lunt for telescope focal lengths up to 1200 mm. This provides sufficient aperture for the larger light cone encountered when a camera is positioned farther back in the optical train.

The 18 mm clear aperture is particularly valuable for solar imaging with larger camera sensors. Lunt currently uses the B1800 in configurations intended for wide-field imaging and specifies full-frame sensor capability on selected systems equipped with this blocking filter.

Selecting the appropriate blocking filter therefore depends primarily on telescope focal length and intended application. A larger blocking filter does not fundamentally alter the H-alpha system bandpass. Its main advantage is a larger unvignetted field. Lunt states that its H-alpha blocking filters share the same optical quality and bandpass specifications, with clear aperture being the primary difference between sizes.

The BF1800 can consequently be a useful choice when a system originally intended for visual observing will also be used for imaging, or when additional field is desired for a larger sensor.

Lunt offers the BF1800 in different mechanical housings, including a 2 inch diagonal and a 2 inch straight-through tube. T2 connections are available within these housing options for integration into visual and imaging configurations.

The BF1800 blocking filter must always be used as part of a compatible Lunt H-alpha solar system. It is not a standalone solar filter and does not replace the etalon or the other safety elements of the complete system.

Technical specifications

• Manufacturer: Lunt Solar Systems
• Model: BF1800
• Type: Blocking filter for H-alpha solar system
• Application: H-alpha solar observing and astrophotography
• Clear aperture: 18 mm
• Mechanical format: 2 inch
• Recommended visual focal length: Up to 1800 mm
• Recommended imaging focal length: Up to 1200 mm
• Function: Additional blocking within the Lunt H-alpha system
• Additional safety filtering elements incorporated: Yes
• Lunt proprietary filtering system: Yes
• Required with Lunt H-alpha solar systems: Yes
• Optical quality between Lunt blocking filter sizes: Identical according to Lunt
• Primary difference between models: Clear aperture diameter
• 2 inch diagonal version: Available
• 2 inch straight-through version: Available
• T2 connection: Available depending on housing
• Suitable for configurations requiring additional backfocus: Yes
• Suitable for larger imaging sensors: Yes
• Standalone solar filter use: No
• Required combination: Compatible Lunt H-alpha etalon or solar telescope