Narrowband Filter Selection 5nm vs 3nm

[alaskacam]

Hello. I'm exploring getting into narrowband photography. In looking at the various Astrodon filters available there seems to be 3nm and 5nm filters to choose from. I'm assuming that this is the width of the wavelength band that gets passed through. Any pros or cons between the two would be appreciated. Thanks.. Rob

[Whirlwind]

Briefly, the narrower 3nm will be limit the amount of light almost to only these wavelengths whereas the 5nm will let just a bit more non narrowband light through. Hence in theory you should get a sharper image of just the wavelength you are looking at. Also in the halpha emission area you can then image NII and Halpha separately which you can't with a 5nm halpha filter as this filter will include both emission lines. The narrower 3nm should also reduce more the impacts of light pollution and the moon. But because you so narrow you are going to need to increase your exposure times to compensate for reduced light coming through. Finally if you are using the intergrated guide chip behind the filter you may find it more difficult to find a guide star.

Hope this helps

[alaskacam]

Yes, thanks Whirlwind. I would get a remote guider to use on a ED80 that I can attach to my TOA-150B. I posted the question on a yahoo forum and the response also mentioned that it would be a challenge with the 3nm filters to shoot with a scope mush slower than f5, especially in Ha. My current setup gives me f7.3.

[MG1]

Are you using the remote guiding head?

I'd probably consider an off axis guider using either the remote head or a lodestar rather than going side by side.

I know you are within the capabilities of your mount but weight reduction is never a bad thing and you certainly want to be guiding in front of the filters.

With the TAC and lodestar you'll not have any problems picking a guidestar...and I'm guessing the guide head is even better!

[Whirlwind]

I'd agree an OAG may be a better choice in the long term as you avoid having all the issues arising from using a different scope and guding with that. It will also be lighter as well. I have a QSI wsg and I find it easier to co-ordinate compared to having a separate scope.

Not sure I agree with the comment about being more of a challenge with anything slower than f5. I have imaged successfully at f6.3 (WO FLT98) and f9 (VC200L) with these filters. By having a narrower filter the only thing you are excluding is more background light that is less near the relevant wavelength. If both filters have 90% transmission they both let through the same amount of narrowband light (excluding any doppler shift issues) from the same source - you just get less non narrowband source light in the 3nm. But you should get more detail as your image is being less swamped by non narrowband sources. In theory to get the same narrowband signal your exposures can be the same.

In practicalities however as you have a more restricted view of the universe your images will be dimmer but more wavelength specific hence longer exposures will show you better detail in the wavelength that would have otherwise been swamped by non narrowband sources. However given your equipment - very nice by the way I don't think that should be an issue. A faster scope will still be helpful, but that goes for all imaging...

Ian

[MG1]

I'm guessing that the yahoo response was suggesting that it's more challenging because you will still get less photos per sec of exposure and for the overall image this will result in longer subs being needed for the same EV.

But that's if you're looking to end up with a composite image (hubble pallette for example) in which you're wanting to combine various channels with luminence.

The result should be the same EV for the pure Ha regions and less to zero exposure for N3, but unless you're looking to really focus down on creating the purest Ha images possible, you may loose a little brightness, but gain more contrast for more general targets.

...wish I had this dilema. For me it's more Astronomik or Baader! Both don't get close to the Astrodons from what I hear, but then your settup is much more worthy of investment in the dons than mine. Can't wait to see some of these images!

Best wishes and clear skies

Michael

[davew]

Hi Rob,

I’ve seen all the replies you’ve got and agree with most.

Just over 12 months ago I was in the same situation as you in that I was looking for good narrowband filters. Regardless of price I wanted to buy just once. I very carefully checked out as many images on the net as I could. I also asked a very good UK based imager and retailer about his experiences with various makes and band widths. I bought Astrodon 5nm.

From what I see, Astrodon produce less halos than some others ( Cue argument )

I don’t know the specifics of your camera but most seem to have a QE of around 50% in Ha for example. Don Goldman’s graphs indicate above 95% transmission for 5nm and around 90% for 3nm. With the gear you’ve got I don’t see any problem for 30 minute subs so are you worried ?

3nm should give the best contrast but with the moon about the difference, I think, would be slight. For instance, try imaging the moon through an Ha filter. What will you get ? A very nice moon image. If the moon’s up and close by you will get gradients in a damp sky even with narrow band ( Cue further arguments )

Most of my imaging is done at f4 or faster. I don’t see all the problems people talk about. Goldman claims his 3nm filters will work down to f3. Custom Scientific will make you filters for these speeds I believe.

With guiding try the built in camera first. It may well work binned 2x2 or 3x3 at your focal length. If it doesn’t you’ve lost nothing and can the buy a separate camera.

Any specific questions just ask. I may or may not be able to help.

Dave.

[alaskacam]

Thanks guys for all the responses. I had my Tak mount in a dome and it is well aligned. A 30 minute guided sub is crisp, at least with the internal guider. That may change a bit with a separate scope/guider. I'm leaning towards the 5nm Astrodons, but still looking around. I have a Baader Ha that came with the filter set and can see the challenges with using the internal auto guider with anything other than very bright stars in the field. In decent seeing conditions, I prefer most of my subs are unbinned. With my scope / ccd combination, I can take advantage of the unbinned resolution, most of the time. If I'm adding to Lum subs then 2x2 is usually ok. Thanks again....

[drdavekaplan]

IMHO, Fastjimmy from the Mallincam Yahoo user groups is the Boss on filters

From what I read. I have never seen any one else who so comprehensively compares all kinds of filters

Check it out

Dr Dave

South Africa

Sent from my GT-I9100 using Tapatalk

[MG1]

Topjimmy2010...aka Jim Thompson?

[drdavekaplan]

That is the man. A Fantastic knowledge of a complex subject.

Sent from my Samsung Galaxy S2..... The coolest gadget I own. Tapatalk..The coolest App.