There are a lot of narrowband astrophotography astronomy filters on the market with a wide variety of prices. Several manufacturers boast seemingly identical products, but with a massive price difference. The most common astronomy filter products are Hydrogen Alpha (Ha), Oxygen III (OIII) and Sulphur II (SII). Although exotic filters can be obtained such as helium and nitrogen II, for most normal purposes the trio of Ha, OIII and SII are all we are shopping for. So which narrowband astrophotography product is best?

Customers based in the UK are largely limited to Baader and Astronomik products unless we feel brave enough to import products from the states from companies like Astrodon, Finger Lakes and customer scientific. Is it worth it? People tend to exhibit strong herd behaviour when it comes to narrowband filters. A few years ago people looked at you a bit odd if you bought anything except an Astronomik 13nm Ha filter, not realising that that product was out of date yet still selling in high numbers.

We spend a mind-numbing amount of money on equatorial mounts, reflecting and refracting telescopes and our cooled astronomical CCD cameras. Yet people seem to think it is fine to chuck a sub £100 filter on their astrophotography rig without really thinking it through!

There are a number of key factors that go into making a narrowband filters. First the easy one Ė band-pass full-width-half-maximum (FWHM). This tells us how narrow the narrowband filter is. In theory the narrower the better. The widest narrowband filters in the market for cooled CCD cameras start at about 12nm and go down from there. The narrower the better, and the narrower the more expensive. Narrower filters let through less unwanted light, and therefore increase contrast . They also reduce the size of stars and help in poor conditions (e.g. moonlight).

Next, peak transmission. We want our filters to have a very narrow band-pass around the target wavelength, but we also want them to pass as much light as possible at this wavelength. Filter technology has come on a lot in recent years and high transmission (90%+) in very narrow filters is becoming the norm.

Out of band blocking is the third critical parameter of a good narrowband astronomy filter. Whilst we want a great deal of light to come through at the desired imaging wavelength, it is critically important that all other wavelengths of light are suppressed. CCD cameras are sensitive from the ultraviolet through to the infra-red, the filter must block a wide range of wavelength. This is one area where those little transmission graphs the filter manufacturers put on their websites are totally useless. A poor filter might have 99.9% blocking, whilst a good filter will have 99.99% blocking, and no leaks in the infra-red! You simply canít tell that from a linear graph. The blocking is critical to getting good contrast in poor conditions and good suppression of stars. There is no point having a very narrow filter with poor out of band blocking.

The final issue is optical quality and substrate. Decent filters have a ľ wave surface, good parallelism between the surfaces and most critically, made from a single substrate. It is this final issue that stops halos forming around bright stars. This is an area where Iíve seen some dreadful images from Astronomik filters. They have pages on their website claiming that they have resolved the halo issues, but I have seen several images posted by users of newer filters from Astronomik that still exhibit halos.

Conclusion: I would not advise buying Astronomik. From the images I have seen, I think they still give halos. For this reason alone I discount them for serious imaging. What about the baader filters?. Neither Astronomik or Baader will publish a decent logarithmic transmission graph or state accurate out of band blocking.. By failing to provide proper information, these manufacturers invalidate their products in my eyes.

I do not think the standard European filter suppliers make narrowband filters of a quality high enough to do justice to the expensive kit owned by the average amateur astronomer. Go stateside. Various US companies make narrowband filters, look for Finger Lakes Instruments, Customer Scientific and Astrodon.

Astrodon in the US will sell you a 5nm high transmission filter with guaranteed minimum 99.99% blocking which are properly designed and do not give halos. The cost? Each filter in the 5nm range will be starting at £180 plus shipping and duties, but I believe buying a cheaper filter is a false economy. Astrodon also have a premium 3nm product which is probably well worth the money!

If we look instead at Customer Scientific we find offerings around the same price point, with options for even narrower FWHM.

My advice: It is better to have one good filter rather than 3 lesser filters.

If anybody would like to lend me some Ha filters so I can do some side by side comparisons, I would be very grateful , as such reports are thin on the ground on the internet.