We are delighted to offer for sale this superb quality 1.25" Hydrogen
Beta 23.37nm FWHM deep sky and nebula filter.
All filters are not the same. You will find cheaper but just ask the sellers what the FWHM, the Maximum Transmission is and at what frequency and see if they can tell you - we can! If you can find one being sold by anyone else in the UK with such a narrow bandwidth for less then tell us - we will not just match the price but we will beat it!
Only Sky's the Limit:
- Test
every filter sold
- Include
an individual data graph for each filter (sent by email)
- Inform you of all the transmission and cut off frequencies before you buy
- Can provide a complete test result for your filter on request
It is an extremely useful filter for observing the very feint nebulae. It has a very limited transmission range of 484 - 490nm and 478 - 483 & 492- 497nm. The very sharp cut offs only allow the hydrogen beta light and the oxygen III that these feint objects emit to pass through. By cutting off almost all of the visual spectrum the filter effectively blackens the sky to make these objects show through.
This filter is best used on large aperture telescopes (8" +) under very good seeing conditions and is an essential item for those astronomers who enjoy tracking down the most elusive of deep space objects.
This is a filter for visual observational use under good to excellent seeing conditions and for astrophotography.
We test our filters independently in the UK and here is the transmission curve for the filter being sold - we do not rely upon manufacturer's average figures which most other suppliers do. A full nanometre by nanometre data sheet (xls file) is available upon request. Testing showed this filter to have an extremely narrow FWHM (Full Width Half Maximum) of 23.37nm.
The photo is of the curve of the actual filter for this listing:
| Nebula's light (nm) | Transmission T% |
| H-β 478nm | 88.33% |
| H-β 479nm | 92.71% |
| H-β 480nm | 92.76% |
| H-β 486.1nm | 95.62% |
| O-Ⅲ 495.9nm | 79.12% |
| O-Ⅲ 500.7nm | 23.68% |
| H-α 656.3nm | 0.07% |
| SII - 672.4nm | 0.06% |
| Artificial light (nm) | Transmission T% |
| Hg 435.8nm | 0.23% |
| Hg 546.1nm | 0.08% |
| Hg 577.0nm | 0.06% |
| Hg 578.0nm | 0.08% |
| NA 589.6nm | 0.03% |
| Na 615.4nm | 0.13% |
| Na 616.1nm | 0.14% |
| Max Tx%= | 97.03 |
| @ nm= | 491.77 |
We can guarantee our filters - can you?