[jase (Jason)]

Hi Ric,
I provided info on Ha filters in a previous thread - http://www.iceinspace.com.au/forum/s...ad.php?t=19749

There are a few things to consider when making a Ha filter purchase. All Ha filters to some effect will block unwanted light pollution regardless of the band pass size. Though I'd recommend you don't go over a 13nm band pass. Light pollution from sodium vapor lights typically transmits in the green/yellow wavelengths, while the moon transmits into the blue wavelengths. As Ha is centered on the 656.3nm you should not experience issues such pollutants. You can however still experience issues with gradients which flats and other processing can resolve.

The smaller the band pass of the filter, the more exposure time it takes to build an acceptable signal to noise ratio. If you have a reasonable fast scope, this isn't much of an issue. But be careful. If the scope/lens is really fast, and you're using a narrow band pass Ha filter, you can miss the band pass window all together due to the light cone. In such cases you need to increase the band pass window to 10nm or more.

More info;
Also consider the peak light transmission at the Ha centered bandpass of 656.3nm. The Astronomik 6nm has a transmission rating of ~85% and the 13nm is ~97%. These are incredibly high - a credit to the capabilities of the Astronomik filters. I can't see any stats on the Baader 7nm, but if its similar to their OIII (Doubly Ionised Oxygen) filter - it is listed as have a 93% peak light transmission. If this is indeed correct, there is not much difference between the Astronomik and Baader other than bandpass window size. I think you can't go wrong with either one. Though having stated this, what's senstivity of DSI II Pro around the ~650nm? If its poor, I'd go for a the larger bandpass window/higher light transmission filter.