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I finally got out to a dark sky site yesterday evening. I had to drive about 40 miles south of Glasgow. I set up my LX90 at 7.30pm and roved around for an hour (Albireo, Mizar, Castor, M57, M33, M15 & M2). I then "tweaked Bob's knobs" to get perfect collimation, (using Capella). By now it was 8.30pm and moonshine was starting to seriously effect seeing (which had not been that great anyway) - so I decided to go to a bright object - M42. I was seriously disappointed, moonlight was ruining the view I had come to expect.

I decided to add the Baader OIII filter I had bought some time ago, but never really used, to my 16mm T6 Nagler. WOW - what a difference. The trapezium and other stars were much dimmer, but the nebula just jumped out at me. Lots of real detail in the swirls of nebulosity - massive sweeps of "Gulls wings" of gas away from the centre, a really obvious concentration (in a linear formation) and twisted streamers of gas in all directions. Why had I never used the OIII filter on M42 before?

SO - the OIII filter will be going out with me every time from now on.

I was wondering - if an OIII works really well on nemulae, and a broadband filter for skyglow etc. What niche does a narrowband filter fit in?

Tom

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Well the OIII IS a narrowband filter. The narrow band filters work on emission nebulae by filtering out everything except the narrow band. The emission line light has got to be there in the first place, obviously, which is why they work on stuff like the veil, NA Neb. M42 etc. These emit light because something is happening to emit the light. The Ha emission line is so called because the light comes from hydrogen atoms changing their ionisation level, ditto the OIII with oxygen atoms.

Captain Chaos

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Thanks CC - I understand that the OIII is a type of narrowband filter. I see adverts for filters labelled "narrowband" - I suspect they are "narrow" in a different part of the spectrum to OIII - so are they for more general use, or do they have specific targets in mind for use?

Tom

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I use narrowband filters for photometry-the precise measurement of specific radiation from the stars. I use a Johnson "V" band and Johnson "B" band and plan to add "R" and "I". Precisely measuring the output of stars helps us understand their internal processes as well as their temperature and mass. I kinda doubt you're refering to these type filters, though, since they're not advertised much for observing. You're probably talking about the OIII and HA and such that are narrower than colored glass but wider than a solar H alpha. CC and Steve have answered your question, I think.

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Be aware that it will darken the view a LOT. You might need to get more aperture to go with it.

The OIII filter is to be used from a aperture of more than 6" (15cm) on. At smaller instruments there is not enough light available for a sensible use. Many experienced Deep- Sky- observers rather use the OIII filter at instruments with more than 10" (25cm) despite the smaller field of application than the more versatile UHC filter.

Snipped from

http://www.astronomik.com/english/eng_oiii.html

Captain Chaos

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For the amount of use you'll get out of the filters, a rule of thumb (for visual) is: If you're going to buy one filter get the UHC, if you are going to buy two filters get the UHC and OIII, if its three filters get the Ha as well.

I've got a UHC and a OIII, the UHC gets used a lot more than the OIII.

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The Baader UHC-S is a popular filter with a reputation for being brighter than most regular UHC filters so is particularly well suited to smaller aperture scopes.

cheers Santa erm....i mean Steve. So would you not recommend this one for the Newt ?

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Yes,

If you don't agree that it is worth every penny - return it for a refund 8)

Have made that offer several times but so far, nobody has ever returned one :)

(I have had a couple of OIII returned by those with small aperture refractors)

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