Recently Don Goldman of Astrodon posted this interesting article on his website regarding the Baader Hydrogen Alpha filters.

Baader H-a Comparison

This highlights the differences between the expensive astrodons and inexpensive baaders. Don is claiming some IR leakage, and I have also heard reports of some leakage in the green. This have provoked some stern response from various quarters.

I've been following the jolly debate in the States for the last few days and I think I've made my mind up, at least for now.

What this comes down to is a small difference in contrast when dealing with fainter regions (or corresponingly high background lp). This also depends on your QE curve. Astrodon maintain they are providing the best possible solution.

By all reports, Thomas Baader was not best pleased at Don's article, and we await further measurements from Baader.

Richard Crisp, always a man with an opinion has published the following on his website...

Filter scans on Richard Crisp's website

...which doesn't really prove much about anything in my view

Until futher information is forthcoming, my recommendation is: I think the Baader filter is extremely good value for money, and will give the best bang-for-buck for most astro-imagers - especially those having to pay for 2inch plus filters.

If you do want the best available, then it appears, with current published data that the astrodons will have a slight edge, but at a price premium.

As for wider filters - I think products like the 13nm astronomik have now had their time. Fantastic products in their day, but I think the market has moved forward now. Astronomik are a ground breaking company offering some of the best filters available.

Astronomik sell a 13nm filter. Given the narrower offerings available at competative prices, I don't recommend a 13nm filter any more. Some might say the stars get too attenuated in a narrow filter, but I think this is part of the point of using an Ha filter.

Astronomik also sell a narrower 6nm (heard rumour of 5nm?) filter. This is available competatively at around 85-88% (roughly) transmission and also at a higher transmission for a price premium. To be honest, the difference between 90% and 85% transmission isn't that serious. Some poeple harp on about peak tranmission, but I don't think this should be treated as the "single most important" measurement on a filter. I think other aspects are perhaps even more key, such as out of band blocking and halo production. Peak transmission is also a function of focal ratio, due to the bandpass shift on faster systems.

I imagine this is a great filter, but I don't have any data to support it - despite asking. A graph like this:
does nothing expect explain the basic function of a Hydrogren Alpha filter.

I'm not trying to attack Astronomik, and I have no doubt that the 6nm filter is very good, and I know it is recommended by some leading astronomy suppliers in the uk. My point was that the 13nm is just a bit passed it now as a marketable product.

This interpretation is based on my own views on narrowband imaging. The point of narrowband imaging is to capture light of a particular frequency, and discard all other light. Most of the light from stars falls into this "bad light" category. Others view it differently.

To be frank, until filter manufactures publish proper logarthmic based plots of their trasmission curves, which have been independantly verified, all filter purchasers must be guided by here-say and marketing instead of cold hard facts. The single fact that most filter suppliers fail to describe their products clearly is a disgrace imho.