Nothing has changed in terms of handling: The film, which is coated on both sides, is easy to cut and install in your own filter cell (for my first attempt, I simply taped it in front of the objective with masking tape, which already worked excellently). The material is robust – to destroy it, you really have to make an effort and use a sharp object. Nevertheless, you should not touch it, in order to avoid leaving fingerprints that would reduce contrast.

On my telescope (Celestron ED80/600), there is no difference at first glance between the old and the new film: In both, the image appears pure white and high in contrast. Only when switching between the two films does the difference in brightness (and possibly better suppression of stray light) become noticeable: With the old OD5.0 film, the sky is brighter (using a Hyperion 36 at 17x magnification and a 4.7 mm exit pupil), and the veils of cloud next to the Sun are clearly visible. With the new film, the sky is darker, while the Sun itself appears hardly any darker. Even at high magnification, the new film shows no weaknesses compared with its predecessor and delivers a high-contrast image of the Sun with clearly visible granulation.

Photography

In recent years, there has repeatedly been some confusion caused by the fact that, in addition to the visual OD5.0 film, there is also a photographic OD3.8 film. The photographic film is far too bright for visual use and also does not provide sufficient protection against UV/IR radiation. But it is also too bright for use with camera lenses: With modern, fast cameras and lenses, the image is too bright. Its field of application is high magnifications with correspondingly low light intensity, or use together with narrowband filters such as the CaK filter Calcium GEN-II Filter 1¼" (incl. AstroSolar Photo Film OD 3.8, A4) (#2961590 , € 285,- ) .

The visual OD5.1 film, on the other hand, is just as suitable as its predecessor for “standard” photography of the complete solar disc through a telescope or telephoto lens. This is likely to be the most common photographic application of the visual film. visual film. On a typical system camera, a focal length of at least 1.5 meters is required if the sun shall fill the frame. The native focal ratio (without extending the focal length) of camera lenses is still fast enough that you do not need the photographic film. With a camera which cannot expose faster than a few thousandths of a second, additional neutral density filters were already necessary with the previous ND5.0 film, depending on the speed of the optics, in order to expose the image correctly. On my f/4 guide scope, which I use with a small, sensitive planetary camery, I still have to add additional neutral density filters to avoid over-exposure, as it can't be stopped down to ~f/8 like a camera lens. Only if you are extending the focal length with a Barlow or a telecentric system for chasing details on the solar disc at high magnification, the focal ratio gets so slow that you need the photographic film.

Especially for solar eclipse photography, you have to remember to stop down the camera lens if necessary. With my Panasonic G71 (minimum ISO 200 and 1/5000s exposure time), for example, it was not possible to use the lens wide open, which causes problems especially when using the camera’s automatic exposure mode. Fortunately, a camera lens can be stopped down.

In general, it must be said: Use manual mode for solar photography; automatic exposure usually can not handle a small, bright Sun against a black background. Autofocus also does not always deliver exact results, so focus manually whenever possible. For the test, I used a Nikon D7100 (APS-C) with a Sigma DG 120-400mm telephoto lens; the shortest possible exposure time of this camera is 1/8000s and the lowest ISO setting is 100. I carried out further tests with a Panasonic G71 (MFT sensor) with a 45-175mm lens. Since the autofocus of the Panasonic was unable to cope with the still quite small Sun during my test, I could only use the images from the Nikon at 400mm.

Aperture and exposure times

At f/5.6 and ISO 200, I was able to go down to the camera’s shortest possible exposure time of 1/8000s and still had a nicely exposed Sun. Even at f/16, the exposure times are not excessively long. At f/8, which is the range in which most telephoto lenses deliver their sharpest images, only about 1/1000 second is required at ISO 200. This makes it no problem to expose briefly enough to “freeze” atmospheric turbulence in the individual images.

The exposure times are only reference values; the exposure time depends not only on the focal ratio, but also on the transparency of the sky (cloud cover, haze, dust) and the altitude of the Sun above the horizon.

For maximum image sharpness, multiple images must be stacked and averaged; solar photographers often use many hundreds of images to extract all details from the data. This single image was only adjusted in brightness and contrast and slightly sharpened in Affinity Photo. The size is unchanged; the image was only cropped.

The first “proper” photo

So far, the conditions have unfortunately not been good enough for extensive tests on the telescope, but the expected exposure times can already be estimated – after all, they depend only on the focal ratio and the sensitivity of the camera.

A first test on the telescope with the Baader 7.5nm Solar Continuum Filter (540nm) (various versions available) and a monochrome camera already shows the potential: Even though the conditions were far from ideal and only a small portion of the images could be used for stacking, the first test photo is already quite respectable!