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Ric
17-05-2007, 03:05 PM
Hi all, I am nearly in a position to purchase a Ha filter for my DSI II Pro, I am very impressed with the level of detail they show up as well as their ability to cut out moon glow and light pollution.
I have found two at the moment for sale, one is a 13nm Astronomik and the other is a 7nm Baader.
I am wondering which of the two would be the better to purchase, there's only a $10 price difference between the two. Is it correct to assume that the 7nm would be suited more for the suburbs but as I will be moving to a dark sky site to live within twelve months perhaps the 13nm would be a better choice.
Also with a Ha filter is the IR/UV blocking filter still used or does it need to be removed.

Cheers all :)

jase
17-05-2007, 04:19 PM
Hi Ric,
I provided info on Ha filters in a previous thread - http://www.iceinspace.com.au/forum/showthread.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.

Ric
17-05-2007, 11:58 PM
Hi Jase, thanks for the info and the link to the other other thread, it was very helpful info and has given me a lot better understanding of these filters.

Cheers

jase
20-05-2007, 12:25 PM
One last thing to consider with narrow band pass Ha filters (say 3nm or 4.5nm) is that they transmit less continuum radiation (starlight, skyglow, etc). From a Ha purist point of view, this is ideal as all they want to capture is nebulosity. However from an aesthetics perspective, this will also suppress many stars. You will find that many astrophotographers actually blend a narrow band Ha image with a luminance (unfiltered or IR/UV blocked) image. Thus, you can maintain strong nebulosity while keeping as many stars in the field.

The trick is the blending process is to ensure the background of luminance image is slightly brighter than the Ha image. In addition the highlights of the Ha image should be brighter than the luminance. When the two images are combined in PS, the brightest components of both images will be shown, thus maintaining nebulosity, subtle details and star formations.

Ric
20-05-2007, 01:22 PM
Thanks Jase for your help.

Cheers