Pegasus Astro 2inch mounted photographic filter - Ha 7nm

H-alpha Imaging at 7nm: Over 80% Transmission at 656nm

The core of this filter is its ability to isolate the 656nm H-alpha line, the dominant wavelength emitted by star-forming regions like the Orion Nebula (M42) and North American Nebula (NGC 7000). By restricting the incoming light to a narrow 7nm window, it dramatically increases contrast by rejecting moonlight and light pollution from sodium and mercury vapor streetlights. The greater than 80% average transmission within this bandpass ensures that faint nebulosity is captured efficiently, reducing required exposure times compared to filters with lower transmission rates.

1/4 Wavefront Polish for Sharp, Parfocal Imaging

Optical quality is maintained by a 1/4 wavefront polish across the filter's surface, preventing degradation of the image formed by your telescope. The filter is also specified as parfocal, meaning it is designed to maintain the same focal point as other filters of the same series. This minimizes the need to refocus when switching between LRGB and other narrowband filters, streamlining your imaging sequence and ensuring all data channels are equally sharp.

• Multilayer Anti-Reflection Coatings: Applied to the 2mm thick glass, these coatings suppress halos and internal reflections that can occur around bright stars, preserving contrast and detail in the surrounding nebulosity.
• Strict Optical Tolerances: The 1/4 wave specification ensures that the filter will not introduce astigmatism or other aberrations, preserving the performance of your optical tube assembly (OTA).

2-inch Mounted M48x0.75 Format with 2mm Glass

This filter is housed in a standard 2-inch cell, which uses M48x0.75 threads for direct installation into any 2-inch filter wheel, filter drawer, or camera nosepiece. The total thickness of the mounted filter is 5mm, with the thread itself accounting for 2.5mm of that height. This low-profile design ensures compatibility with most imaging setups without introducing spacing or clearance issues. The glass substrate itself is 2mm thick, providing a rigid and stable optical element.

7nm H-alpha vs. Broader and Narrower Bandpasses

Choosing a bandpass is a critical trade-off in astrophotography. A 7nm filter strikes a balance between performance and versatility, making it a common choice for a wide range of equipment and skies.

• vs. 12nm Filters: A 7nm filter provides significantly better contrast and light pollution rejection than a broader 12nm filter. This makes the 7nm a superior choice for imaging from suburban or moonlit skies. The trade-off is slightly dimmer stars, which can be beneficial for making the nebula the hero of the image.
• vs. 3nm or 5nm Filters: Ultra-narrow filters (3nm/5nm) offer the absolute highest contrast, ideal for heavily light-polluted areas or for separating fine details in bright nebulae. However, they are more demanding on your mount's guiding accuracy and can introduce halos on some optical systems. The 7nm bandpass is more forgiving and often a better starting point for those new to narrowband imaging.