We highly advise against visual calcium observations of the sun - the 397nm of Calcium are already ultraviolet radiation, and UV-radiation can cause phototoxic reactions which can harm your eyes. In addition, even young people can't see these wavelengths very good, and sensitivity decreases with the age. Because of this, the eye doesn't feel if the intensity is much too high. If you now try to increase the amount of radiation (e.g. with another mirror) to achieve a brighter image, you will increase the risk for eye damage. According to the newest norms for eye safety and to ophtalmologist conclusions, the sun must not be observed visualy at 397nm.
We highly advise to only observe the sun in calcium photographically, then you also do not need a star diagonal. We can't understand why some manufacturers promote these calcium-filters for visual observations.
We also advise against placing a filter so close to the focus - any energy rejection filter belongs in front of the front lens, even if this sets high demands to the quality of such a large filter. Placing the filter close to the focus will save money, because you can use a much smaller filter, be we absolutely can't recommend it - because of our own conscience as well as because of EU-regulations. The filter would receive all the energy collected by the front lens. We can only recommend putting the energy rejection in front of the front lens, which is the ethically and technically correct method.
First Class H-alpha Solar Filters by SolarSpectrum
A SolarSpectrum filter transforms almost any telescope into a high-end H-α telescope with appropriate accessories. The filter design is optimized for observatories and amateurs who want to use the full aperture and resolution of large telescopes for solar observation and plan more than just an occasional view at H-alpha.
The filters have no central obstruction and are actively heated. This means that the filter does not need to be readjusted during the course of the day - ideal for long-term observations, time-lapse photography or public relations work.
Further information can be found on our detailed Blogpost:
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