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H2 alpha filters

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#1 PABresler

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Posted 17 July 2019 - 08:38 PM

How would the useage of a 7nm H2 alpha filter differ from that of a 12nm?

 

Peter

 

 



#2 Dan Crowson

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Posted 17 July 2019 - 08:50 PM

For Ha, the 7nm would just collect less other light (or light pollution) than the 12nm. Once you started to get narrower, you start cutting out some other lines and could run into issues with on quicker systems.

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#3 Kevin Ross

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Posted 17 July 2019 - 09:58 PM

As far as usage differences go, generally speaking, the narrower the band pass, the longer the exposures needed, since it lets less light through.



#4 PABresler

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Posted 17 July 2019 - 10:18 PM

Thanks. So the nm figure refers to the bandwidth, not the wavelenth?

 

Peter



#5 bobzeq25

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Posted 17 July 2019 - 10:21 PM

Thanks. So the nm figure refers to the bandwidth, not the wavelenth?

 

Peter

Yes.  Narrower is better; a rare exception is really high speed optics, and it costs more.

 

That's the simple tradeoff.  My personal compromise was a 6nm Astronomik.



#6 Kevin Ross

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Posted 17 July 2019 - 10:21 PM

Yes. The wavelength of Ha is 656nm. So a 7nm filter will pass light at 656nm +/- 3.5nm



#7 PABresler

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Posted 17 July 2019 - 10:23 PM

Got it!



#8 ks__observer

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Posted 18 July 2019 - 12:11 AM

Both a 12nm and 7nm let in the same exact amount of Ha signal.

12nm lets in more extraneous unwanted sky photons.

Many people set exposure time based on "swamping read noise."  This is just a ratio of sky noise to read noise -- when you reach a certain amount of sky noise then read noise becomes insignificant.

Because 7nm lets in less sky photons, it takes a longer exposure before enough sky photons are collected to make read noise insignificant.

But appreciate that even if you exposed a 7nm for the shorter 12nm exposure time, the 7nm will still yield higher snr than 12nm because less light pollution.  


Edited by ks__observer, 18 July 2019 - 09:54 AM.


#9 PABresler

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Posted 18 July 2019 - 12:15 AM

I own the 7nm filter, but bought one for EOS clip-in. I hope to be moving to a mono camera with a Hyperstar on my Celestron 8SE, so would need a 1.25 inch filter for that. I now have the EOS clip-in up for sale or trade.

 

Peter



#10 Jon Rista

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Posted 18 July 2019 - 12:15 AM

As far as usage differences go, generally speaking, the narrower the band pass, the longer the exposures needed, since it lets less light through.

This is not really true.

 

The narrower the pass, the less UNWANTED light gets through. However, the same amount of light on the desired band, such as Ha, gets through regardless of the bandpass. So technically speaking, you pick up the same number of Ha photons with a 3nm filter as with a 7nm or 12nm or 15nm or 30nm filter. So there is actually no NEED to expose longer with a narrower filter than with a broader one. There is the OPTION to expose longer, and so long as you are not clipping too many stars, then longer exposures could produce better results. However, the simple act of rejecting (filtering out) more of the unwanted off-band light will in and of itself also produce better results. The option to use longer exposures is more of a bonus benefit, rather than a necessity.


  • RazvanUnderStars, ks__observer, Salty_snack and 1 other like this

#11 Kevin Ross

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Posted 18 July 2019 - 12:48 AM

Thank you Jon for your excellent explanation. I knew someone would correct my gross (and slightly wrong) oversimplification. :)



#12 Jon Rista

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Posted 18 July 2019 - 10:40 AM

Thank you Jon for your excellent explanation. I knew someone would correct my gross (and slightly wrong) oversimplification. smile.gif

No worries. ;) I think it is the same mistake everyone makes. It's kind of the same as with LRGB or OSC imaging at a light polluted vs. dark site. Technically speaking, you could use the same exposures at a dark site as at a bright site. You get the same object signal in either case, same as with NB filters. The amount of light coming from space is the same...it is the pollutant light that changes. And even if those exposures were not long enough to be optimal at the dark site, the simple fact that you eliminated the pollutant signal, which increases contrast, means the short exposures at the dark site will produce better results.

 

The option to use longer exposures exists at the dark site, but there are similar limitations as with narrower NB filters. Once your stars start to clip, then longer exposures start to discard information. So while your dark site may be say 40x darker than your bright sky back yard, you usually cannot expose each sub 40x longer. You might be able to double, maybe triple exposure... Maybe. It is the TOTAL integration time that ultimately changes. You can, in fact, integrate about 40x LESS total data at the dark site to get good results. ;)




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