Antlia H-Alpha filter 2.5nm ULTRA, model 50.8mm

All you need to know about SLOAN Antlia filters

The Sloan Digital Sky Survery System (SDSS) photometric system is the most widely used filter set in professional observatories today. This universal system makes it easy to compare data... every user (professional or amateur) uses the same transmission curves.

The SLOAN Antlia filters comprise 4 filters (g'/r'/i'/z') covering the visible spectrum and beyond: from 400nm to the sensitivity limit of silicon CCD cameras (about 1100nm). The technology used (ion gun assisted deposition on the substrate polished on both sides) makes Antlia filters references in terms of durability and performance.

Filter transmission reaches 98% for the r', i' and z' filters and 95% for the g' filter. The optical density outside the transmission zone is OD5 (0.001%), which offers an excellent signal-to-noise ratio and a clear cut-off of the other lines. The clear cut-off of the signal outside the transmission zone is an important parameter because it determines the quality of the filtering. The steeper the cut-off slope, the higher the contrast between the received signal and the perfectly black background sky. In addition, a sharp cut-off minimizes halos around bright stars.

Note: Due to the very low sensitivity of cameras below 400 nm, Antlia has decided not to develop a u' filter (300/400 nm).

Transmission curve of Antlia SLOAN filters. The wavelength in nanometers is on the abscissa axis. The transmission percentage is on the ordinate.

During manufacturing, Antlia performs an extremely strict quality control. Each filter is tested and measured to ensure that it meets the high performance requirements of the specifications. Small tolerances require perfectly homogeneous substrates and uniform thicknesses; deviations from specifications should not exceed 0.05 mm.

Technical specifications

• Mounting diameter: 50mm circular unmounted
• Lens type Schott optical glass
• Surface quality: 60/40 (per MIL-O-13830)
• Focal Ratio compatible from f/1.8 to f/15
• Substrate Thickness 3mm +/- 0.05mm
• Peak Transmission 98% minimum
• Spectral Range: Above 812 nm
• Coatings Polished filter on 2 sides with an accuracy of λ/4
• Multilayer and anti-reflective coatings on 2 sides