Baader Solar Continuum Filter

First of all, I would like to thank Kieron McGrath of SCS Astro for the long loan of the filter. This has allowed me to experiment with it over a long period of time and use it as part of my regular observations, instead of a quick look, couple of observations, then “back in the box”, as many magazine reviews are.
I was particularly interested in this filter, as I am a keen amateur solar observer and many astronomers, like me, are on limited budgets and always looking for low-cost options to improve their observing and imaging experience.

Safety Warning


You cannot use this filter on its own. You must use it with a full aperture solar filter or a Herschel wedge to block out 99.999% of the light from the Sun. Do not use filters that have been made for photographic use only for visual use. You should also use suitable filters for your finderscopes or cover them with their lens caps. Failure to observe this warning can lead to permanent loss of vision.

Technical Specification



The filter is designed to block all light except around a wavelength of 540 nanometres, where a nanometer is 0.000000001 of a metre. This is in the green part of the spectrum. The bandpass (the wavelength range that is allowed to pass) is 40 nanometres but only a small amount of light at the extremes of this range is let through. This gives a more realistic figure of 10 nanometres. By comparison, solar hydrogen alpha filters have a bandpass of 1/100th of this or less. It is available in the standard 1.25in and 2in fittings and cannot be used with some beginner telescopes with a .965in fitting. The 1.25in filter retails for around 60 GBP from most retail outlets and the 2in filter for about 100 GBP.

Principles


The white light emitted by the Sun actually consists of a mixture of light at various wavelengths and, hence, different colours. Much of this light is invisible to human eyes, although some (such as ultraviolet light) may be captured by cameras. By blocking out light from other wavelengths and concentrating on certain wavelengths, it is possible to see some solar features in more detail. With a bandpass of more than 1/10th of a nanometer, this filter is called a broadband filter, as it lets a relatively broad spectrum of light to pass through. It does not provide such dramatic views as narrowband hydrogen alpha and calcium K filters but definitely enhances the view over what you might achieve with a traditional white light filter on its own. It has been designed to make views of sunspots sharper and also to show a solar feature called granulation where the large scale cell structure of the solar surface can be seen.

And now for a bit of background



I am a self-confessed solar addict and view the Sun in normal light most days, using binoculars and full aperture filters. I make drawings of the sunspot sizes and positions and post them on the internet. I also view and photograph the Sun in hydrogen alpha light when conditions are clear enough and work and other commitments permit. When the Sun is especially active, I usually attempt at least a full disc photograph through a telescope. I have sometimes used a webcam for close-ups but I am mostly concerned with obtaining a full disc image. For those of you who have not tried it before, photographing the Sun in normal light is excellent at showing ever single grain of dust or hair and a session is normally preceded by furious optical cleaning! The need for squeaky clean optics is even more demanding than that for lunar photography. Owing to the preparation needed, I do not do much white light solar photography.

First impressions


The colour of the filter is s strong shade of green. Holding it up to the light showed that it seemed to be letting quite a small amount of light through and, holding it up to the Sun, the image seemed rather dim. This image persisted when I used it with my Skywatcher 127mm Maksutov, my usual instrument of choice for this type of viewing. Nevertheless, sunspot images definitely appeared sharper but I did not notice the granulation that I was expecting.

The Test


My initial results were quite poor! Not only was I having problems with dirty optics and cloud but I could not even get the focus right. A hasty review at this stage would have been negative. Fortunately, I was able to capture some great action in the 2nd week of July 2014. Not only had I managed to master the use of this filter but the Sun put on a good show for me. Although I had tried to use my DSLR at prime focus, the amount of light made it very difficult to find precise focus. Fortunately, I have a good Panasonic Lumix compact digital camera, with performance rivaling that of some bridge cameras. All of my observations and photographs were done using this camera. Some close-ups were taken using high power eyepieces and others using the excellent optical zoom of the camera. Note that I do not recommend the use of digital zoom for astrophotography.

The first picture shows the full solar disc. Limb darkening is obvious and it is possible to see umbral/penumbral shading in the largest sunspot and a hint of it in one of the others.

The second picture shows a close-up where the sunspot structure is more obvious. In the fainter regions near the edge of the frame there is a hint of granulation.

The third picture shows the largest sunspot in close detail. It shows how it is concentrated near the centre and the penumbral regions become more diffuse towards its outer edges.

The fourth and final picture shows some sunspots near the edge of the solar disc. Although the bottom right of the picture is somewhat over-exposed, most of the image shows the mottling effect of granulation. I have found it rather elusive to see and photograph.
If you encounter chromatic aberration, this filter would kill it stone dead! Fortunately, my Maksutov does not suffer from much, although I sometimes get it at high magnification. I thought it was worth trying it out on the Moon but I found that the light transmission led to a loss of detail, which is not what I expected! Nevertheless, if you own a big reflector you might like to try it for yourself.

Conclusion


There is little doubt that this filter improves the experience with white light solar viewing and photography but the question is how much? As it blocks quite a high percentage of light, it is unlikely to be of much use with small amateur telescopes and I suspect, it is best used with larger aperture telescopes than the ones I own. I was not able to see any faculae with the filter and, to be honest, I do not see them often without the filter. The granulation promised is there but rather elusive. Again, a larger aperture telescope would resolve it in more detail than I can.
Where the filter really delivers and more is the ability to resolve fine sunspot detail, at least the larger ones. I have even noticed a hint of the features that are normally only seen in professional instruments and this is not using some sort of advanced astro-imaging camera but a compact digital camera. On the other hand, if the solar disc is devoid of sunspots or only small ones are present, this filter will have little or no impact.
My recommendation is that if you have a 100mm or larger telescope, it is capable of delivering great sunspot views and photos. Do not expect too much in the way of granulation, though. Financially speaking, it is a much smaller investment than a narrowband solar filter and is capable of delivering solar views that are not possible without it. It is definitely a good buy for those who follow the Sun on a daily to weekly basis. However, it is unlikely to appeal to astronomers who are not frequent solar viewers in white light.

Footnote


I wrote the original review in August 2014. Since then, I have bought a better DSLR and have improved my processing techniques. I am convinced that if I had the filter today, my results would be better than those shown in the pictures.

Phil's Scribblings are 100% free and I make neither direct nor indirect money from them.

hits since counting started in March 4th 2008.

No responsibility is taken for anything, although attempts will be made to correct any errors or misunderstandings. There are not any naughty words on this site and every effort is made to make it suitable for use by young children. However, no responsibility is taken for the content of anything linked from here. No cute, cuddly creatures have been harmed in the creation or maintenance of this website.