Poor mans H alpha filter

[allan gould]

While chasing up several H alpha filters I came across this site (http://www.nexstarsite.com/Reviews/B...ingeKiller.htm) about filters and this very curious statement that does make sense.
"One last note on the uniqueness of this filter. Baader suggests that the Fringe-Killer combined with their Red filter is perhaps the best Hydrogen-Alpha filter commercially available. This combination has a 95% transmission rate at the H-Alpha wavelength and yet costs very little. H-Alpha filters are great for imaging emission nebula in great detail, though a smaller pass-band would produce better results."
Theoretically I and many other people bought an achro scope, then a violet suppression filter and have some coloured filters for other tasks. I thought that I would combine my sirius optics violet and red filters and see how this goes with my CCD camera. It may be a cheap way to broad band H alpha imaging?

[janoskiss (Steve H)]

I would find it very hard to believe that the combination of two broadband filters could replace a narrowband filter, i.e., it might have 95% transmission at H-alpha, but it will not have near-0% transmission in other parts of the spectrum. To really image in H-alpha you have to have a narrow band filter IMO.

[jase (Jason)]

Originally Posted by allan gould

It may be a cheap way to broad band H alpha imaging?

Broadband H-alpha imaging would simply be using a red filter. Traditionally H-alpha filters come in 4.5nm through to 13nm band sizes. There is nothing broad about these band sizes. They all transmit a very narrow window of light. Producing and testing narrowband filters isn't cheap, so I can't imagine a "so called" cheap alternative especially combining two broadband filters as a light cutoff mechanism.

[middy]

It is quite feasible. If the two broadband filters only have a small region of overlap you will get at narrower band of transmitted light when they are combined.

Looking at the site the Baader Fringe Killer transmits greater than 90% of the light between 475nm and 675 nm. A red filter would transmit everything above say 600nm. If you combine them you would get a transmitted band between 600 and 675 nm.

Does anybody here have a Baader Fringe Killer and a red filter and could do some experiments with it??

[jase (Jason)]

You're still passing a band of 70nm! You'd be better off with the dedicated 7nm H-alpha filter. Even the 35nm would result in a cleaner emission line (only slightly). http://www.baader-planetarium.de/zub...ter.htm#hapass
The effectiveness is also dependant upon where the two combined filters emission line is centered i.e where is the peak light transmission. If it’s not centered on the 656.3 emission line, I personally think you're wasting your time and money. Having stated that... go for it. I'd be interested in your results to see how they compare to a dedicated Ha filter.

Obviously, not all H-alpha filters are created equal. In fact, the identical filter produced by the same manufacturer is slightly different per filter. As luck would have it, a few days ago I received a H-alpha from Custom Scientific (CS). It’s a 10nm 50mm unmounted filter which I'll be installing into the STL11k when I get a chance. As can be seen (attachment), CS tests every filter and provides the customer with a transmission spectrum graph detailing that specific filter performance. I would recommend you go with a manufacturer that can provide this information. Sure its nice to read it off their website, but does the filter you've purchased meet the criteria they mention?

Selecting a narrowband filter isn't something I would recommend deciding on quickly. You need to consider the optimal bandpass for your optical system and environment. Indeed, the narrower the bandpass (say 6nm or less) will require greater exposure lengths compared to a 7nm and higher bandpass filter. A narrower bandpass can also be more effective a blocking unwanted light such as moon/sky glow, however narrower isn't always better when considering fast focal ratios in particular camera lenses.

"All dielectric interference filters shift predictably short with increasing angle. The combination of fast F/cone and wide field angles limits the use of narrow band filters to slower systems. Filters narrower than 13nm will be significantly degraded by the very fast F/cones where the sum of the F/cone and half field angle are greater than about 13°. Again, they are great for slower systems, where slow begins at F/4." Quoted from http://www.hbastro.com/Telescopes/Fa...derations.html
I would recommend reading the above link amongst many other narrowband /emission filter web info.

I would urge those considering doing narrowband work to carefully consider filter options. There are always compromises, but doing the appropriate research will ensure optimal bandpass for your astrophotography goals. There is no one size fits all.

[allan gould]

I really think some of you may have missed the point. The title said it all - a "poor" man's H alpha filter. Sure I've read all about the 7nm and 13nm narrow band H alpha filters but for a 1 1/2" its $200 and 2" $400+ for such a narrow band filter. Big bickies for some of us. If you took the time to look at the transmission characteristics of an MV1 from Sirius Optics and a red filter you would wind up with a 50nm H alpha filter which in light polluted skies of BrisVegas is better than nothing. Since most of us have both these filters, I thought it would be feasible and since Baader say in their literature (see above quote) its very good - then I thought it was worth mentioning. Ill give it a go with my ST7 for all the doubters and nitpickers.

[jase (Jason)]

I agree Allan, it has to be better than nothing. How effective this is can only be determined through testing. 50nm window is quite large. If you can effectively block the green and yellow wavelengths where the BrisVegas light pollution will reside, you may have a winning combination. As I said in my previous post "I'd be interested in your results to see how they compare to a dedicated Ha filter". Please keep us posted.