Quote:
Originally Posted by jase
Steve,
I've provided plenty of info on narrowband filters, specifically Ha on these forums before. Here's one - http://www.iceinspace.com.au/forum/s...ad.php?t=31029
To cut to the chase, a larger bandpass will let in more energy, thus will not require such a long exposure. According to some (Don Goldman and others), filter categories are as follows - ultra narrow band uses a bandpass window of < 4nm, narrow band is < 10nm, intermediate band is 11 > 60 nm. Quite a broad range. If you've got dark skies and not trying to cut through light pollution, an intermediate band can work well. You'll find the background ADU will be lower with a narrower bandpass. If your intention is to use a red continuum filter to subtract the stars from the Ha data, i.e. so you can simply just add the nebulosity to the RGB data, then a narrow band filter is best (less than 10nm, preferrably 6nm or 3nm). You're probably thinking why would I do this...good question...match the stars from Ha data with RGB can introduce headaches, therefore sometimes its easier to get rid of them from the Ha data entirely. The red continuum filter can also be used on SII data.
Hardcore narrowband enthusiasts will most likely go for the narrowest bandpass. A 3nm filter will also attempt exclude the NII (Nitrogen) emission lines 653.8 and 658.4 which are on either sides of the 656.3 of Ha for the purest of data. Personally, I don't see the much value in this. I'd want the highest S/N regardless if it came from Ha and NII.
What Merlin mentions is correct about fast light cones and narrowband filters. There is a general fear that fast optics will shift the bandpass off the emission line. However, its not as drastic as once believed. Its more prevelant in OIII filters than Ha. At 6nm, the shift is barely percievable, at 3nm the shift results in a slight drop in transmission efficiency i.e. 90% to 75%, thus on a worst case scenario, you would need to increase your exposure time...however with fast optics, this would not be a huge disadvantage.
You can't go wrong with most bandpasses. The trick with narrowband is to use long subs. I know a few guys with reasonably fast optics, F/5, 6nm filter and an ABG chip that use 30min subs everytime. The results speak for themselves. A little experimentation can go a long way in determining what will work for you. |
Thanks for the information Jase ... much appreciated !
So would this be a good filter to consider ?
http://www.optcorp.com/product.aspx?...-1157-861-6965
