Filters: Narrowband vs. Broadband...?

[akilgore]

Hi all,

I bought a telescope (repaired Meade ETX125) in Dec '05 and I'm ashamed to say that, so far, it's had pretty limited use. This is due to a number of reasons, most significant is living quite close to the city centre, but I intend to resurrect it in the coming months.

Before I do, I want to get a suitable filter for it as my viewing location (back of my house, Ravenhill area) suffers quite badly from light pollution. Initially, I'd be interested in looking at nebulae (especially M42) and possibly some galaxies (if conditions permit).

So:

• What's the difference between narrowband and broadband filters and which do I need?
  
• Which makes do people recommend for a medium-ish budget (£50-£75)?
  
• Where's the best place to buy?
  

Many thanks,
Andy.

[Administrator]

Hi Andy,

I'm not really qualified to answer this question since I don't own any filters but as I understand it a narrowband filter will block out all light apart from a specific region of the spectrum. This will give high contrast views of certain objects depending on the object and the type of narrowband filter. For instance a common type of narrowband filter is the OIII filter which only lets the light of glowing oxygen ions through the filter. This type of filter is said to be particuarly useful for planetary nebulae.

A broadband filter will attempt to let most wavelengths of light through except those wavelengths commonly associated with light pollution. E.g sodium (yellow) and magnesium. Unfortunately the old yellow sodium streetlights are easier to filter out than the new street lights, so broadband filters are becoming less useful.

Lumicon supposedly have the best filters but they are expensive. I think the Orion UHC filter is a good one, but again, I'm not qualified to fully answer this question!

Best place to buy is www.northdowntelescopes.co.uk/ , as Andy McCrea who runs it is a member of the IAA and knows what he's talking about!

[bigeye155]

Hi Andy, welcome to the IAA forum.
Hopefully with some decent clear nights (before it gets too cold), you'll get some observing done with your etx125. There is quite a wide range of filters available for astronomy so it's easy to get confused between different types. The difference between broadband and narrowband is essentially to do with the bandpass characteristics of each filter type. Broadband is used to suppress light pollution (commonly streetlighting etc) which permits a broader spectrum of light to pass thru than the narrowband, which is used specifically for emission and planetary nebula.
I'd recommend the Baader Planetarium range of filters (made in Germany)as they are quite reasonable and are optically excellent, they also are pioneers in the use of the latest coating technologies. I'd suggest the Moon and skyglow filter from Baader for the broadband, and the UHC-S filter for nebulae(both 1.25"). Check David Hinds or SCS Astro for more details. The best place to buy is from in N.I. is our own Andy Mc Crea (North Down Telescopes)........ he won't even charge P+P that other dealers do from England!
There is plenty of info in books or online so read as much as you can!!

Regards
David