Astrodon 3nm versus 5nm Ha filter

[AlistairHowie]

I am about to take delivery of an Astrodon 3nm Ha filter, but having now read up some more on the subject I've just succeeded in getting myself confused!  All I want to do is take pretty pictures from an area of "average" LP - London 35 miles in one direction, and Luton 5 miles the other - both giving skyglow, at least in the lower elevations, but not overly bad nearer the zenith. I have an AZ-EQ6 mount and haven't taken my subs past 10 minutes yet, so taking 20 or 30 min subs will doubtless prove a challenge for me and my kit!

Given that I’m faced with average UK seeing (if that’s relevant), average mount and below average imaging skills, I’m now wondering whether I should swap this filter for the Astrodon 5nm one to make my life a bit easier.  I am concerned about the length of subs required as well as whether I’ll be missing signal that will, to put it bluntly, give me pretty pictures.

My first thought was no, stick with the 3nm filter, based on the below Q&A from the Astrodon website …

Do I Need a Longer Exposure with 3 nm Filters?

No, because the peak transmission is still above 90% for both 3 and 5 nm filters. If fact, it will take you longer to reach the sky noise limit with the narrower filter due to the lower background. I take 30 minute exposures with both filters.

Just wanting to make sure, today I traded some emails with Mister Astrodon himself, Don Goldman, and asked him ... "Is there any occasion with average seeing where a higher bandpass Ha filter would be better?"

He misunderstood my question a little, and replied ...

"Bandpass has nothing to do with effects of seeing. Of course, your stars will be smaller with 3 nm vs. 5 nm, but that's OK, since you'll tend to stretch your OIII more than your 5 nm H-a. I often image with a 5 nm H-a and 3 nm OIII. The narrower the better for light pollution, as you indicate, as you are reducing the background signal while still maintaining the same object signal at the emission line. I'd strongly urge you to stay with the 3 nm OIII. It is also the best choice for when the moon is up, which is like another form of light pollution."

I explained I was only starting out on LRGB imaging but had bought a 3nm Ha filter to dip my toes in the NB water and to supplement my LRGB images. So no OIII at the moment. He replied ...

"As you know from my FAQ page, I always recommend a 3 nm OIII to minimize moon issues, since we tend to do NB imaging when the moon is up. The H-a is more complicated due to the NII issue, balanced by light pollution concerns. You will likely give up some signal by losing NII in going to a 3 nm H-a, BUT if you have significant light pollution, e.g. London, you may have no choice. If you image in dark skies, then you can easily use the 5 nm H-a. So it is a balance of competing issues dependent upon your specific situation."

I have also subsequently found this good explanation of the issue on the Astrodon website ....

"This is a bit complicated. It is not well known that most H-a filters pass both H-a and NII. H-a emits at 656.3 nm and NII emits most strongly at 658.4 nm (and weakly at 653.8 nm). These are very close together spectrally. Thus, most H-a filters are wide enough (e.g. 4.5 nm bandwidth and wider) to pass both emission lines as shown for the older Astrodon 6 nm filter below. Our 3 nm H-a begins to separate both emission lines and reduces the NII contribution significantly, also shown below (blue curve). In this example the 3 nm filter only transmits 15% at the NII 658.4 nm wavelength, whereas the H-a remains unchanged. As mentioned earlier, some objects are enriched in NII, such as planetary nebula and Wolf-Rayet bubbles. The Dumbbell Nebula, M27, is a good example, as shown below, taken with 3 nm narrowband filters. The wispy clouds in the core of M27 are dominantly NII. A tricolor narrowband image is also shown below, mapping OIII to blue, H-a to green and NII to red to produce a beautiful color image. This information provides you with a choice based upon your light pollution, desire for more detail, or simply wanting all the photons you can get out of your H-a filter."

The problem is, I'm not quite sure what I want.  So …given my lack of skills, average mount, average LP and UK seeing and desire to create pretty pics … should I swap my 3nm Ha filter for a 5nm one?

All thoughts gratefully received.

[ChrisLX200]

I would stay with the 3nM personally, it should give you more contrasty images with lower background and better s/n of the Ha emission band that is of interest in NB imaging. You will need longer subs - but you need those anyway even with a basic 7 or 12nM filter. 15min or 20min subs is not unusual and will give a better result than trying to stack shorter (5, 10min) subs.

ChrisH

[johnrt]

I have the 3nm and love it. I would not swap my 3nm for a 5nm!!

My experience after upgrading from an Astronomik 12nm to Astrodon 3nm is that you do not require longer subs, you are still letting through all the Ha signal you need, just not the background rubbish you don't. I shoot a standard 10 minute sub frames, and perhaps 15 minutes on dimmer objects.

[AlistairHowie]

I just want to create pretty pics, so am I not better off with the 5nm which allows the NII to come through? With that goal in mind, should I be bothered about WHAT signal comes through? Isn't more signal better than less signal? I do have LP but I'm not in central London.

[Velvet]

I just want to create pretty pics, so am I not better off with the 5nm which allows the NII to come through? With that goal in mind, should I be bothered about WHAT signal comes through? Isn't more signal better than less signal? I do have LP but I'm not in central London.

I spent of lot of time googling and talking to folks about this and finally decided on the 5nm.  Most folks preferred set up was 5nm Ha and 3nm Oiii and Sii.

I decided that i did not want an extra Nii filter for some targets and that for me on balance of price/gain/benefit  i would stick with 5nm.

there are lots on conflicting information re sub length, seeing etc etc, in the end it is a personal choice.  Also speed of the scope should be taken into account.

My set up reduced is below F4 and there can be problems with the 3nm so I am told.

There is a guy who did tests on the difference between the filters on the web, will try and find the post

Velvet

[swag72]

Here's my thoughts Alistair, although they will not necessarily suit your criteria.

I moved from the 7nm Baader Ha to the 3nm Astrodons - I got the complete 3nm set instead of mixing them. I don't really know what my idea was behind it. Certainly I was recommended a 5nm Ha and 3nm OIII. But I decided to bite the bullet and just went 3nm all the way.

I've not found that I need longer subs (I always do 30 minutes as a minimum) but what I have found is the following,

• Undoubtedly I get better contrast than before
• The images are less noisy
• There is more signal

All in all I've not been disappointed with my decision, I LOVE the 3nm Ha filter and how it works. I think it gives you the very Ha data you can capture, the rest is up to you!! I can't help thinking that if I'd have got the 5nm I would have always wondered just how much better the 3nm would be!! But that's possibly just me and my own paranoia.

Have I missed the NII? I have no idea!! I'm developing my own processing style and will probably never know whether I'm missing out on NII or not. My understanding is that you can do some sort of 85/15 mix of Ha and OIII and pretty much replicate the NII in any case.

I have been using the 3nm's on my reduced Tak without any issues as far as I can tell.

For me it would be a no brainer, 3nm all the way, but that does rather conflict with people who know far more about these things than me, so take my ramblings as nothing more than that and probably best with a healthy pinch of salt as well

[Velvet]

http://eastcoastastro.com/boards/index.php?topic=3461.0

this was the link... as you can see it is a choice to be made personally and with regards to the setup/scope.

Velvet

[AlistairHowie]

http://eastcoastastro.com/boards/index.php?topic=3461.0

this was the link... as you can see it is a choice to be made personally and with regards to the setup/scope.

Velvet

Thanks for the link, I'd already seen that post.

I think what is clear is that moving from 7nm Ha to either 5 or 3 shows a marked difference.  That's what users are reporting on multiple threads on various forums, like Sara above.

What I think is far less clear is the move from 5 to 3nm ... and many would argue that in Ha it's not worth the extra money (my 3nm was the price of a 5nm here in the UK, so the price differential doesn't come into the equation for me).

So it seems the choice of 3 vs 5nm Ha is down to a) whether you want SII in your image, and the extent of light pollution.  I don't think is too bad for me - not great, but not central London -  and I have no clue about a).  For pretty pics I'm thinking I'm probably best just getting all the signal that I can get my hands on.

I feel a swap coming on ... unless anyone can talk about 3 versus 5nm Ha filters in a UK setting ...

[Velvet]

Thanks for the link, I'd already seen that post.

I think what is clear is that moving from 7nm Ha to either 5 or 3 shows a marked difference.  That's what users are reporting on multiple threads on various forums, like Sara above.

What I think is far less clear is the move from 5 to 3nm ... and many would argue that in Ha it's not worth the extra money (my 3nm was the price of a 5nm here in the UK, so the price differential doesn't come into the equation for me).

So it seems the choice of 3 vs 5nm Ha is down to a) whether you want SII in your image, and the extent of light pollution.  I don't think is too bad for me - not great, but not central London -  and I have no clue about a).  For pretty pics I'm thinking I'm probably best just getting all the signal that I can get my hands on.

I feel a swap coming on ... unless anyone can talk about 3 versus 5nm Ha filters in a UK setting ...

well there was not a lot of info i cold find about any UK camparisons... my main decision was the Nii and the speed of my scope reduced.  Hobsons choice really  best of luck with your decision.

Velvet

[davew]

I, like so many others, can only give an opinion on the merits of one but not compared to the other. I chose a 5nm Ha filter due to the speed I image at on occasions. At f2.8 a 3nm filter is going out of band to some degree although the background sky would be just as dark.

With regard to the graph provided on your first post ( I like graphs because they make things simple for a numpty like me ) I think it gives a slightly false impression. For one it compares to a 6nm filter and Astrodon stopped making them a while ago. Two is that the centre line is skewed towards the right ( Long ). Third, it gives the impression that there's as much NII to be had as Ha. This isn't necessarily so. In diffuse nebulae the NII content is way below Ha and OIII and with Planetary Nebulae it rapidly catches up to Ha but remains behind OIII ( Brazell.O. JBAA vol 104 ). How many PNs are you going after with an 85mm scope ?

There is little doubt the 3nm filter will be affected by LP less than 5nm, especially with the OIII.

[ The problem is, I'm not quite sure what I want. So …given my lack of skills, average mount, average LP and UK seeing and desire to create pretty pics … should I swap my 3nm Ha filter for a 5nm one? ]

Lack of skill ? Moving rapidly on.... The mount is good I believe.... LP won't be getting better any time soon unless you're very lucky.... Seeing doesn't come into it.

You DON'T have to image for longer with a 3nm filter over the 5nm but you can. I've just checked a 30 minute sub with mine and a very bright star in the frame got to 55,500 ADU. Still short of saturation.

If I was buying again I may still go for the 5nm filter due to my lenses. If 3nm was the same price as the 5nm I could possibly risk a 3nm. I would now buy a 3nm OIII because it would be less affected by LP and still give a good SNR.

Dave.

Re seeing slight improvements. I see you bought an FSQ85 on an AZ-EQ6. If you can't see a difference then it would have been cheaper to buy an ED80 and an HEQ5 ??     You'll murder 30 minute subs I'm sure.

[Stephen]

I think what is clear is that moving from 7nm Ha to either 5 or 3 shows a marked difference.  That's what users are reporting on multiple threads on various forums, like Sara above.

This is a fact having gone from 7 to 5 myself

[AlistairHowie]

Thanks all for the comments. I decided to save £75 and Don kindly swapped the 3 with the 5 just before the courier came to collect my parcel. So no harm done. Time will tell whether it's the right decision.