Narrowband Filters - 12nm or 6nm

roughy (Mark) · #1 26-05-2015, 09:54 AM

Apart from the difference in price (huge) what are the relative benefits of one over the other?

Joshua Bunn (Joshua) · #2 26-05-2015, 10:37 AM

My first thoughts are much more contrast in the 6nm, and the ability to use the 6nm in more light polluted skies. On the other hand, exposures will need to be longer with the 6nm filter, but you will be capturing the light you are after. In the case of a Ha filter, you will also be capturing NII, with either bandwidth.

Josh

gbeal · #3 26-05-2015, 10:46 AM

I had a 12Nm when I had a Canon with the clip-in system. It worked OK, but the difference between 12Nm and 6Nm (or in my case now, 7Nm) was quite marked.

Mind you, as has been suggested, exposure times will have to increase, but as Joshua says, better for polluted night skies.

One way to find out, try one or both, and decide from there.
Gary

rustigsmed (Russell) · #4 26-05-2015, 10:51 AM

how about going half way? Baader have theirs at Ha 7nm, Oiii 8.5 and S2 8?

gbeal · #5 26-05-2015, 11:08 AM

It's sort of what I did. Baader 7Nm Ha.
And "half-way" is a moot point, as there is 3Nm available too (I think?).
Gary

Slawomir (Suavi) · #6 26-05-2015, 11:57 AM

Hi Mark,

It largely depends on light pollution in your area.

I live in an area with heavy light pollution and have to image with 3nm filters- but they are also great when the moon is up; I would not go back to wider bandwidth when narrowband imaging. My set of 12nm filters have not been used since I got 3nm ones- the difference is that great.

Another factor to consider us that narrower filters may not be equally effective on fast telescopes.

Longer exposure for narrower filters is a myth, at least when considering mono CCDs, maybe except for 5nm vs 3nm Halpha filter.

I would go for narrower filters budget permitting.

clive milne · #7 26-05-2015, 01:46 PM

Quote:

Originally Posted by Slawomir

Longer exposure for narrower filters is a myth, at least when considering mono CCDs, maybe except for 5nm vs 3nm Halpha filter.

It is probably worth qualifying that statement.
The signal that you will record through a 5nm filter will often be higher (than the 3nm exposure) because the passband of the 5nm filter will be broad enough to include more of the Nii lines (654.8 & 658.3) which straddle the Ha line at 656.3.
However, when the frame is shot noise limited, the signal to noise advantage afforded by the 3nm filter should result in a reduction in exposure time for a pure Ha source to a ratio of 0.36 : 1
ie) The 5nm filter will need ~2.8 x more exposure to achieve the same result.

c

Octane (Humayun) · #8 26-05-2015, 03:01 PM

I live in OK-ish skies here in Canberra. Can see the Milky Way easily most nights when the Moon isn't out.

Question: I have an STL/FSQ combination with the Baader hydrogen alpha (7nm), sulphur II (8nm) and oxygen III (8.5nm) filters.

Will these be OK to image under moonlight, or, am I going to have to contend with gradients/background issues?

H

Peter.M · #9 26-05-2015, 04:14 PM

Quote:

Originally Posted by clive milne

It is probably worth qualifying that statement.
The signal that you will record through a 5nm filter will often be higher (than the 3nm exposure) because the passband of the 5nm filter will be broad enough to include more of the Nii lines (654.8 & 658.3) which straddle the Ha line at 656.3.
However, when the frame is shot noise limited, the signal to noise advantage afforded by the 3nm filter should result in a reduction in exposure time for a pure Ha source to a ratio of 0.36 : 1
ie) The 5nm filter will need ~2.8 x more exposure to achieve the same result.

c

And this is where the "you need more exposure for narrower bandpasses" statement comes from. If you are saying that the shot noise is reduced by a factor of 2.8 (signal surely isnt going up by changing a filter) then it stands to reason that to make a single sub shot noise limited, the 3nm filter will need each sub to be 2.8 times longer than the 5nm.

I think the thing that trips people up is that the 3nm filter image will be read noise limited, which will still be lower than the total noise in the 5nm image, even if they are exposed for the same duration. So the statement that it "needs" more exposure is not valid, the 3nm filter would benefit from more exposure, where the 5nm would not.

Amaranthus (Barry) · #10 26-05-2015, 04:26 PM

Quote:

...the 3nm filter would benefit from more exposure, where the 5nm would not

To further clarify, this is correct regarding the length of a single subexposure, but is different to the matter of total integration time (longer is always better, but returns diminish non-linearly).

Slawomir (Suavi) · #11 26-05-2015, 04:30 PM

Well explained Clive and Peter.

So going back to the original question and assuming a mono camera, the narrower filters will allow for capturing fainter details.

There are no miracles though, and even 3nm filters will perform better from a dark site and without the moon, but it is possible to capture reasonable data with 3nm filters with the moon up and from a light polluted area. Data captured through 6nm filters will be more strongly affected by the moon and by light pollution, and then 12 nm even more so.

Slawomir (Suavi) · #12 26-05-2015, 04:35 PM

Quote:

Originally Posted by Octane

I live in OK-ish skies here in Canberra. Can see the Milky Way easily most nights when the Moon isn't out.

Question: I have an STL/FSQ combination with the Baader hydrogen alpha (7nm), sulphur II (8nm) and oxygen III (8.5nm) filters.

Will these be OK to image under moonlight, or, am I going to have to contend with gradients/background issues?

H

I would recommend getting a narrower OIII filter for imaging under moonlight.

Octane (Humayun) · #13 26-05-2015, 06:55 PM

Cheers, Slawomir. More fights with the wife.

H

clive milne · #14 26-05-2015, 08:48 PM

Quote:

Originally Posted by Peter.M

And this is where the "you need more exposure for narrower bandpasses" statement comes from. If you are saying that the shot noise is reduced by a factor of 2.8 (signal surely isnt going up by changing a filter) then it stands to reason that to make a single sub shot noise limited, the 3nm filter will need each sub to be 2.8 times longer than the 5nm.

Ahhh... no.

The point I was trying to get across was that (assuming the signal was identical in each filter) and that the background noise in the 5nm filter is 5/3 greater. Then to achieve the same signal to noise ratio of faint features using the 5nm filter you would need to increase the exposure by the ratio of 5^2/3^2 = 25/9 = 2.7777

If you were to compare an exposure through a 3nm filter that had a duration 2.7777 times the length of one taken through the 5nm filter then it would have 2.7777^0.5 x (5/3)^2 = 4.63 times the signal to noise... for faint features. However, when the nebula is bright enough to dominate the background noise, the difference between 3 & 5nm filters completely evaporates.

Quote:

I think the thing that trips people up is that the 3nm filter image will be read noise limited, which will still be lower than the total noise in the 5nm image, even if they are exposed for the same duration.

The focal ratio, camera noise, sky background and signal intensity are variable to the extent that you cannot use this as an accurate premise.

Quote:

So the statement that it "needs" more exposure is not valid, the 3nm filter would benefit from more exposure, where the 5nm would not.

More exposure than what?
The fact of the matter is that there is an optimum exposure which can be derived from sky background, camera read noise, filter bandwidth, focal ratio and signal intensity. When the signal is bright, you can get away with a lot. For the faintest nebula, the formula is simple:
Narrow bandwidth
Fastest focal ratio (up to a point)
Darkest sky.
Optimum exposure.

When you align those ducks, the 3nm filter will record the faintest details 2.7777 times faster than through the 5nm filter.
- period -

pluto (Hugh) · #15 26-05-2015, 09:38 PM

A couple of comparisons I made when going from 7nm to 3nm Ha and 8.5nm to 3nm OIII:
http://www.iceinspace.com.au/forum/s...d.php?t=126305
http://www.iceinspace.com.au/forum/s...d.php?t=125922

clive milne · #16 27-05-2015, 05:37 AM

When comparing filters between manufacturers, there are a few more specs that would be helpful if known.

Optical density - how well does it block the unwanted (offband) wavelengths, and how far out in to the UV & IR is this rejection valid?

Centre wavelength tolerance - the narrower the filter and faster the focal ratio, the more this becomes a concern.

Transmission percentage - of the desired wavelength.

Transmission profile - does it look like a bell curve or is it mesa shaped (preferable)?

Alluxa is one company that does this. A pity though that they go to the trouble and expense of making a 1nm mesa profiled Ha filter but they have tuned the centre wavelength to the emission line instead of putting it on the shoulder (with the shorter wavelength).

Incidentally, Baader have a 3.5nm filter available now which is a fraction of the cost of the Astrodon (3nm)

Peter.M · #17 27-05-2015, 10:20 AM

You are assuming the images are shot noise dominated. This is what I am saying, if we assume that we expose the 5nm image to be just shot noise dominated, to get the 3nm image to also be shot noise dominated will take longer. We are both making the same point Clive. I understand that the 3nm will collect more signal in that time but it will require more exposure (than the 5nm) to be shot noise dominated.

Obviously every variable you mention that could be changed is kept constant (camera telescope ect ect) and I am assuming the transmission of each filter is similar. You work out 5nm image is shot noise dominates at 30mins changing to a 3nm filter The image will have less noise, because it is no longer shot noise dominated. The 3nm filter will benefit from longer exposure. The 5nm image will not.

roughy (Mark) · #18 27-05-2015, 11:38 AM

Ah.........wow. Thanks very much for the advice and the debate. Pretty clear that there is better perfomance to be gained with narrower bandwidth.

And, due to the debate, perhaps I should invest a bit of time learning more about the properties of light.

rustigsmed (Russell) · #19 27-05-2015, 01:24 PM

Quote:

Originally Posted by roughy

Ah.........wow. Thanks very much for the advice and the debate. Pretty clear that there is better perfomance to be gained with narrower bandwidth.

And, due to the debate, perhaps I should invest a bit of time learning more about the properties of light.

the other thing Mark is the price differential between the bandwidths ... you'll need to weight up if its worth the extra cost the narrower narrowband you go.

gregbradley · #20 29-05-2015, 12:14 PM

I went through this recently for the AP Riccardi Honders. In my case I already had a nice 5nm Ha and wondered if there was a gain going to 3nm.

I asked Don Goldman and he basically advised me to stick to the 5nm as it also picks up N11 emission whereas the 3nm does not. But if I were imaging in light pollution the 3nm was the one to use.

I get great results from the 5nm Astrodon. Good to know though there is a 3.5nm one from Baader now. Although I only see it in Ha not O111.

Greg.