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7nm vs 12nm H-a filter

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#1 Aaron_tragle

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Posted 24 March 2018 - 05:31 AM

I am between the Baader 7nm H-a Filter and the Astronomik 12nm H-a filter. I am thinking the 7nm because of better bandpass and price, but how much longer am I going to need to expose to get the same signal with the 7nm compared to the 12nm. My (soon to be) new mount can do 10 mins guided by what I have seen. The 7nm is also 40 dollars cheaper, money is money. Which route would you guys take? 

 

 



#2 rigel123

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Posted 24 March 2018 - 05:36 AM

7nm



#3 vehnae

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Posted 24 March 2018 - 06:19 AM

how much longer am I going to need to expose to get the same signal with the 7nm compared to the 12nm

You will capture the same emission signal regardless of the filter. However, there will be less light pollution / sky glow with the 7nm filter which results in better contrast (darker background sky) given the same exposure time. If your exposures are not dominated by the sky glow the 7nm filter will also give you the option of exposing almost twice as long per frame compared to the 12nm. But that's not something which is required when you decide to go with narrower bandpass, you will get better performance regardless.



#4 Salty_snack

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Posted 24 March 2018 - 06:51 AM

7 nm at most. 5 nm would be better.

And if you have any significant amount of LP you’ll want the narrowest you can afford.

#5 mclewis1

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Posted 24 March 2018 - 07:02 AM

You will capture the same emission signal regardless of the filter. 

That actually is not true with most filters available today. The amount of emission signal is dependent on transmission of that area of the spectrum and for most of the Ha filters on the market today the 7nm models have a lower transmission. Yes of course the 7mn is also going to block more lp and unwanted light and with appropriate exposures will produce a better image but if you were simply measuring emission signal the 12nm will usually have a higher transmission of the Ha signal.



#6 Madratter

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Posted 24 March 2018 - 07:21 AM

The actual Ha signal itself is very very narrow. You don't need a wide filter to catch all of the signal.

 

However, as Mark just said, transmission values of filters do vary. That is part of what you are paying for with the very expensive filters.

 

If you believe the Astronomik website, their filter has transmission values of up to 99%. Assuming that peak is actually exactly at the Ha line, that is very good.

 

Looking at the graph on the Baader website, they run about 89% transmission.

 

However, transmission is only part of the story. The wider bandpass of the Astronomik is letting in non-signal. And that reduces your signal to noise ratio. In my opinion, unless you have very dark skies, the Baader is likely to produce an image with better signal to noise.

 

There are other factors as well. It is not unusual to get some haloing around the stars with these filters. I don't know which is better in that regards.


Edited by Madratter, 24 March 2018 - 08:27 AM.

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#7 otocycle

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Posted 24 March 2018 - 07:36 AM

I went with the Astronomik 12nm for easier focus, and it has worked out very well, BUT I am getting a lot more pollution closer to the horizon light dome that may be less with the 7nm.   For instance, right now I get less capture time for the Orion Cloud Complex as it sets in the west.

 

Here is the Angelfish Nebula @ 135mm with the 12nm from last week:

 

 

Angelfish_135mm_Ha_12nm.jpg

 

 



#8 TOMDEY

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Posted 24 March 2018 - 07:53 AM

Yes indeed, that is a trade among bandwidth, on-band T, light pollution, sensor [eye or cam] etc. Reasonable rule of thumb is that the ~information content~ of a signal is proportional to the product of the signal strength and the S/N level. That's what I'm using to design the chopper-blades for strobing the Crab Pulsar! But it applies to filters-selection, exposure times, coatings, baffling... all sorts of things! Here's the chopper... good Thor Labs one. I will be stacking two blades, for adjustable duty-cycle, so the convolution of the shuttering with (ostensible) pulsar intensity profile yields max S^2/N aka max probability of success. The 36 goes operational late April... hope. Will have to wait for Crab to roll around favorably.  Tom

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#9 bobzeq25

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Posted 24 March 2018 - 09:09 AM

7nm will block significantly more light pollution. Given roughly equal quality filters it will not block significantly more signal. So, you get better contrast, more detail.

It will _allow_ you to take longer subexposures. Generally a plus. You can get to the point where losing one is losing a lot of data.

I focus by using a bright star, and binning. Focus is relative, you just minimze FWHM, or use a Bahtinov.

Bottom line. I see no downside to narrower filters except cost (there's a rare exception for those with really high speed optics).

My personal cost/performance choice is 6nm astronomiks.

#10 johndias

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Posted 24 March 2018 - 05:42 PM

I can tell you I went with the 12nm and wish I had gone with the 6nm, if that helps.




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