Thanks to a generous loan, I have the opportunity to test Baader's entire current range of preshifted and "normal", non-preshifted H-alpha filters. In the new CMOS-optimised series which has superceded the former CCD versions, the range comprises:
6.5nm without preshift
6.5nm preshifted f/2
3.5nm without preshift
3.5nm preshifted f/3
3.5nm preshifted f/2
Baader developed the preshifted versions because of the bandshift issues that arise when a filter is positioned between a very fast objective lens and the camera sensor. This is explained briefly here https://www.baader-p...r-cmos-filters/ and is presented in depth, together with all the transmission curves, in Baader's 2022 White Paper on the use of narrowband filters on astronomical telescopes: https://www.baader-p..._telescopes.pdf
In night vision astronomy, the use case for which these filters were made arises when a very fast lens is used and the filter is placed between lens and night vision device (NVD).
However, there is also a second use case when the filter is placed in front of the smaller lens of a handheld NVD. It is then no longer the focal ratio of the lens that determines bandshift. Instead, it is the angle between the central axis of the field of view and the position of a celestial object in the field - the further out from the centre, the more bandshift. In other words, the larger the field of view (FOV) becomes, the more H-alpha signal will be lost, to the point of total loss at a certain angular distance from the centre of the FOV.
The FOV of a handheld NVD depends upon the focal length of the lens. Lenses around 25mm focal length show fields in the sky that are around 40° in diameter. With a "normal" 7nm or 6.5nm H-alpha filter, two thirds of that field are compromised due to bandshift! This is where preshifted filters come in. To determine precisely what they can do, and what any downsides there may be, I used a clear night around the last New Moon to carry out systematic tests in Orion and Monoceros.
This is the hardware used:
Bushnell Wolf 3S night vision device with Gen 2+ tube; 25mm and 50mm Cosmicar C-mount lenses, both f/1.4; Baader 3.5nm f/3 in 2-inch filter cell is shown on the 25mm lens, Baader 3.5nm without preshift is shown on the 50mm lens.
Observations with 25mm lens giving approx. 1x (estimated) and 41° FOV (measured in the sky), filters front mounted in 2-inch filter cells. Suburban site, NELM ca. 5m0, good transparency of sky.
"Good field", "deterioration", "loss" refer to galactic nebulae. Stars are generally less affected by bandshift and loss towards edge of field. M42, a very bright and compact nebula at 1x, proves useful to determine the point of complete nebula loss. The Rosette Nebula is useful to determine where loss sets in, as it is more vulnerable to deterioration than M42.
Unfiltered: Stars that are at limit of perception at centre of field remain visible until 90% out towards edge of field. They then fall prey to gradual softening plus edge darkening that begins 95% out. Good field of unfiltered lens is hence 37°.
6.5nm without preshift: I notice a central brightening affecting 10% of the field (4°). I put the filter on the 50mm lens to see whether the problem arises there; no, it doesn't. Then I check with an Astronomik CCD 6nm H-alpha filter on the 25mm lens; in this configuration the central brightening affects 25% (10°). So it seems to be a systemic issue and the Baader 6.5nm does a better job of suppressing it.
Field is otherwise good over 35% (14°). It then deteriorates gradually until major but not complete loss at 75% (31°).
6.5nm preshifted f/2: The preshifted filter is free of the central brightening seen with with the non-preshifted 6.5nm filter. Field is good to 65% (27°). Then loss of galactic nebulae sets in, becoming complete at 75% (31°).
3.5nm without preshift: No central brightening issues. Nebulae are good to 35% out (14°) then deteriorate gradually until complete loss at 60% (24°).
The good central 14° of the field is enough to frame the whole of Barnard's Loop. All objects in the good part of the field are seen much more clearly with the 3.5nm filter than with both 6.5nm filters in their good parts of the field: While with the 6.5nm filters Barnard's Loop peters out shortly after Saiph (Orion's right foot), with the 3.5nm filter the Loop continues a good part of the way towards Rigel (Orion's left foot). Similarly, the Cone Nebula complex is seen larger and clearer.
3.5nm preshifted f/3: There is a central patch out to 15% (6°) in which there is loss (but not complete loss) of nebulae. From then on nebulae are good to 65% (27°). Then loss sets in, becoming complete at 75% (31°).
All objects in the good zone are seen much better than with the 6.5nm f/2 filter. The overall visual effect is very pleasing. I only need to remember that the central 6° are diminished and avoid putting an object of interest at the centre of field. I notice during the observing session that this quickly becomes automatic behaviour that I don't find annoying at all. Putting the centre of field roughly 2° due south of Betelgeuse, I can frame Barnard's Loop, the Angelfish Nebula Sh 2-264 at Orion's head and the Rosette Nebula in the good part of field, all these objects seen distinctly. This is the most memorable sight of the night.
It is so pleasing that I start looking for other groupings. Two are quickly found: The Rosette Nebula in Monoceros plus the Seagull Nebula at the Monoceros-CMa border, almost 20° apart, are framed well together, both seen clearly and much better than with the 6.5nm filters. So are the California Nebula in Perseus plus the Flaming Star / Tadpole Nebula complex in Auriga.
3.5nm preshifted f/2: With this filter, the central patch with (not complete) loss of galactic nebulae covers the central 50% (20°) of the field. From then on nebulae are good to 70% (29°). Then loss of nebulae sets in again, deteriorating quickly to complete loss at 75% (31°). In other words, the good zone for nebulae is an annular ring of 9° angular width (20° to 29°).
At 1x (25mm lens) all celestial objects suffer loss of visibility and/or detail compared to 2x (50mm lens). So the point of going to 1x is to frame within one FOV objects that are distant from each other on the celestial plane. How do the five filters examined here fare in this regard?
Within the 6.5nm pair of filters, the preshifted f/2 variant is the big winner. At 1x it delivers a good field for galactic nebulae from the centre of FOV all the way out to 27°. If I were under a pristine mountain sky, I might develop a preference for the 6.5nm f/2.
However, under my suburban NELM 5 sky the 3.5nm f/3 also delivers a good field to 27°, with the caveat that the central 6° are diminished (yet not lost entirely). At 1x with an f/1.4 lens, I see all nebulae in the 27° zone, with the mild exception of the 6° central zone, much better than with the 6.5nm filters.
So under the home conditions in which I spend 99% of my observing time, the 3.5nm f/3 is a great gain and absolutely a keeper at 1x. I only need to remember that the central 6° are diminished. I found during further observing sessions that it quickly became second nature to frame several objects - the whole purpose of 1x viewing in a 41° field! - automatically in such a way that none of them are at the centre of field.
The question now is: how do these filters shape out on a 50mm f/1.4 lens giving 2x and 21° FOV? I'll be reporting my findings here shortly - stay tuned!
Edited by C.Hay, 11 March 2023 - 05:41 PM.