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Old 21-04-2008, 10:03 PM
Zuts
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Astronomic Ha/Astrodon Ha, which one?

Hi Guys,

I think I want to do some L RGB imaging. I plan on getting a Ha filter to use on the SBIG 2000 XCM and then use the camera unfiltered for the RGB.

Currenty I have Astronomic UHC and OIII visual filters and I find them very good quality. I was thinking of getting the Astronomic Ha 6nm 1.25 inch filter. However when I look at the following and I assume the 2nd filter is the 6nm Astronomic, while the 1st is the 13nm http://astrosurf.org/buil/filters/curves.htm#stronomik CCD Ha filter it appears that the Ha filter is shifted towards the blue.

This may be an old filter and the new ones may be far better. What do you guys think are Astronomic 6nm CCD filters good for imaging. My other option would be to spend a bit more and get the Astrodon http://www.optcorp.com/product.aspx?...-1157-861-4060

If the Astronomic is OK i would rather get it. Any help appreciated, what do you guys think.

Paul
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Old 22-04-2008, 11:42 AM
jase (Jason)
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Paul,
What do you mean that the Astronomik Ha filter appears to have a shift to the blue? All Ha filters are centered on the Ha emission line 656.3nm. The bandpass size simply implies how many nm on each side of this line i.e. a 6nm filter has a window from 653.3nm to 659.3nm. It hasn’t shifted the emission line centre. The 13nm simply has a larger window and as such lets in more continuum energy. The only exception to this are companion filters that are slightly offset from the narrowband emission line. Companion filters allow you to subtract the continuum energy from the narrowband data resulting in a pure emission line i.e no stars, zip, nothing; just Ha nebulosity (can’t get anything purer).

I haven’t personally used an Astronomik Ha filter, but can speak about the Custom Scientific or Astrodon filters. Given your light polluted environment, I’d go with a 6nm. You’ll need longer exposures than a 13nm (for obvious bandpass window reasons), but I suspect it would deliver a better result – certainly in the background ADU readings anyway.
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Old 22-04-2008, 12:16 PM
Zuts
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Hi Jase,

The link I gave is an analysis of the spectrum passed by the 6nm Astronomic filter. It appears from this that while the spectrum includes the Ha line it is not centered around it. The centre of the passwidth is right shifted towards the blue. I cant believe Astronomic would release a product with such an obvious flaw, but what do I know

Definately I am going with the 6nm but am a bit unsure if i should trust Astronomic in which case I would get it or the sites published spectrum in which case I would probably get the astrodon.

Paul
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Old 22-04-2008, 01:04 PM
jase (Jason)
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Looking at their site, it would reveal a typical Ha filter bandpass. First image is the 6nm and the second the 13nm. All appear to be around the 656nm. I wouldn't be concerned about it the "blue" shift. These graphs are misleading as there is a grey line that hovers over the visual spectrum. I originally thought that they were trying to explain that was what the ha filter bandpass was

The third image is the Custom Scientific 10nm 50mm Ha filter I use at present. They personally test each filter and provide you with a reading output result. Nice.
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Old 22-04-2008, 02:18 PM
Zuts
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Hi Jase,

I wasnt going from the Astronomic link, rather this link http://astrosurf.org/buil/filters/curves.htm#stronomik CCD Ha filter.

The 13nm seems nicely centered but the 6nm doesnt. I havnt heard anyone else complain about this so maybe the site is incorrect or made a mistake in the measurement.

If it was actually like this, how bad would that be, would it have much effect, it still lets through 90%?

EDIT, I uploaded the spectrum he obtained.

Paul
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Old 22-04-2008, 03:08 PM
jase (Jason)
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90% along the key emission line! Got to be happy with that. I'd seriously doubt you'd notice a difference. Keep in mind, there maybe some discrepancies in the spectral tests he performed or indeed, that is the filters true response.

While its nice to think that every filter is the same, they are not. This is the reason why the likes of Custom Scientific provide you with your individual filter's spectral analysis. Difference between them are typically minor. The shorter your bandpass, the more complex the filters are to make and subsequently cost more. You only need to look at the solar ha filters to get this into perspective. They have a very narrow bandpass.

Just keep in mind your overall filter goals. While you have a OSC now, you may go mono later. To save you the hassles you may want to select one manufacturer and stick with them. Manufactures don't all use the same thickness glass for filters thus if you use different manufactures it is almost guaranteed that they wont be parfocal. So if you think you'll eventually end up with heading the Astromomik, Astrodon, Custom Scientific, etc stick to one manufacturer.

There is of course a twist (just a warning). Even filters manufactured by the same company aren't always parfocal. For instance, my Custom Scientific LRGB filter set isn't parfocal with the Custom Scientific 10nm Ha filter. A focus change of approx 30 steps is required to hit the CFZ again. Some people are "hell-bent" on ensuring their filters are parfocal as it makes things a little simpler. But seriously, its not as big an issue as people make it out to be. It makes very little difference with automated focusing. You can also configure filter offset lists in MaximDL so that when a specific filter is requested, MaximDL tells the focuser to shift x steps to reach focus. Parfocal LRGB sets are probably more critical, not Ha - its just a nice to have imho.
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Old 22-04-2008, 05:00 PM
Zuts
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Hi Jase,

Thanks a lot for your help. I may go the Astronomic, as you say 90% is not bad and of course I have been happy with the quality of the filters to date.

Cheers
Paul
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