Price From 149,00€
Filters for astrophotography are constantly being improved to adapt them to modern imaging systems and changing environmental conditions. Our CMOS-optimized narrowband filters are the best example for this development and research process. There are other demands for those filters which are used for science instead of pretty pictures: they must deliver data that are comparable even with decades-old data sets. New developments must therefore only improve production and handling without changing the filter characteristics themselves.
The filters used today for scientific photometry with digital sensors go back to the filter sets introduced by Johnson and Cousins, which were developed in the 1950s-1970s and which were adapted in the 1990s by M. Bessel for CCD cameras - in each case using the techniques available at the time.
The original UBV filters consisted of coloured glass, partly even without protective coating. Therefore, the V filter, for example, worked wonderfully in dry observing sites. However, because its BGT39 material was exposed to the environment without protection and had hygroscopic properties, it aged if there was high huminity: so, under less favourable conditions, it quickly got hazy.
Another feature of the original filters was that the Cousins Ic bandpass filter, for example, was open at the red end, so the sensor or film - instead of the filter itself - limited the sensitivity at the red edge of the spectrum. At the blue end of the spectrum, on the other hand, it was the atmosphere (humidity, altitude of the observing site) rather than the filter that set a limit. Despite these disadvantages, thousands of observations were made with UBV filters and the filter systems based on them.
Whoever develops filters today has many more possibilities to influence the spectral transmission characteristics. While the original filters consisted of coloured glasses and protective glasses of different thickness and transmission, which on top of that were not resistant to ageing, we can now manufacture dielectric filters whose characteristics allow very well-defined transmission windows - but they are steeper than the original transmission windows, and depend on the focal ratio. On fast telescopes with f/2-f/4 they work somewhat differently than on slow telescopes with f/8, and since the angle of incidence of the light distributed over the field of view also changes, there are now some new problems on modern telescopes with large CCD sensors that were unknown on old, slow telescopes with film cameras. A modern photometric filter therefore does not have to provide perfectly delineated transmission windows with steep edges, but data that are comparable to the old data sets (even if only by using reliable conversion algorithms).
The UBVRI system was developed in the 1950s for the 0.9m McDonald Observatory telescope and initially consisted of a Johnson UBV filter set, later extended by Cousins to include the R and I filters for the red region of the spectrum. The filters of the extended UBVRI system according to Johnson and Cousins include:
According to the state of the art at that time, the filters had different thicknesses and transmittance values as well as no sharply defined edges (if any). Their spectrum therefore corresponds to a curve instead of a plateau.
In 1990, M. S. Bessel looked into the subject in order to find a filter combination for the new CCD cameras that were becoming increasingly widespread at the time. These Bessel-UBVRI filters are still standard today and are also the most widely used in amateur circles such as the AAVSO. The V-filter in particular has proved to be an inexpensive introduction to photometry: It corresponds approximately to the visual brightness; since only one measured value is used, this is optimal for getting to know the techniques.
Since 2010, Baader Planetarium has offered photometric UBVRI filters according to Bessel, which protected the coloured glasses from ageing with a dielectric coating and at the same time provided the desired transmission properties. This is not at all a matter of course: unfortunately, comparisons by the AAVSO have shown that not every modern "Bessel filter" actually shows the irregular flanks of genuine Bessel filters, but instead there are often plateaus with steep flanks. These steep slopes are easily achievable with dielectric filters, but the data obtained with them are simply not comparable, or only with difficulty, with the old data sets. Compare for yourself: You can find the transmission spectra in the product description.
We could not further improve the characteristics of our previous, Bessel-compatible UBVRI filters - but we could make the filters better. The new generation of Baader Planetarium UBVRI filters, introduced at the end of 2021, is a modern filter set that is both fully compatible with the characteristics of the original Bessel filters and at the same time meets modern demands for contemporary filters:
These terms and conditions are valid from November 7th, 2020.
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