Advice on filters to enhance visual observing, please.

[stephenb (Stephen)]

I need some advice on a filter (or filters) to assist with visual observing of DSO's.

I see that the blurb for the Astronomik CLS filter...

"...gains contrast for most objects under light-polluted suburban skies. The Astronomik CLS blocks the emission of Hg and Na streetlights as well as all Skyglow-lines . All major emission lines from deep-sky objects are passed without surpression".

And the Astronomik UHC filter...

"... is the best filter for visual Deep Sky observation of nebulae. Almost 100% of the radiation from both O-III lines and the H-Beta line is transmitted. A second bandpass for the H-Alpha line ensures that this radiation reaches the observer's eye. Scattered light from other, unwanted wavelengths is filtered out. The sky background is darkened allowing much more detail in gas nebulas and planetary nebulas to be seen. This filter may be used successfully in telescopes 125mm and above (The UHC-E is a better choice in telescopes of smaller diameter)".

blah, blah, blah... Reading between the sales pitch, both appear to be handy.

I do not need them for imaging, just to enhance visual observing only, particularly in partial-suburban skies.

Are either one or both of these type of filters worthwhile? What do other members use? Your opinions greatly appreciated.

[wavelandscott (Scott)]

Here is my take for visual...

A good UHC filter is a helpful tool to help pull out some types of faint fuzzies.

A good OIII filter is also a useful tool to pull out some different Deep Sky Objects.

While a CLS (or similar light pollution filter) will work as advertised (with appropriate expectations set) it to my mind is not as helpful as either the UHC and OII.

I have all three and the CLS does not get any use anymore. I've written at length on my advice and experiences in other threads here so I won't repeat it all here...in summary, instead of a CLS invest the money in petrol and get to a darker sky.

The Astronomics filters are very good in my opinion...expensive but good. I am also exceptionally fond of the DGM Optics™ NPB Nebula Astronomy Filter

From their website:
The NPB (Narrow Pass Band) filter is an Ultra High Contrast type of filter that is very effective for small and fainter emission and planetary nebula and a variety of bright nebula, especially through larger scopes. Another nice feature of the NPB filter is the very natural coloration yielded by this UHC type filter. Star images retain a much more natural color, in addition to pinpoint star images. High transmission at the 656.3nm Hydrogen Alpha emission line is a bonus for this design.

http://www.omegafiltersebuyer.com/se...ontrast/Detail

I have both their (DGM Optics) NPB and OII filters as well as an Astronomics CLS...I used to have an Astronomics UHC. When I compared the DGM to the Astronomic I found I preferred the DGM (very slightly) and at the time it was much less expensive than the Astronomics...I kept the DGM and sold the Astronomics for a reasonable return. The Astronomics is a very good filter too and I know others that preferred it to the DGM.

[erick (Eric)]

Interesting that you should bring this up, Stephen. I bought the DGM Optics NPB filter (cosmetic second) last time the exchange rate was favourable and I am very happy with it. It does a great job on the Carina Nebula in particular. Lots more things I haven't tried it on as yet. I now want to buy the DGM Optics OIII filter (2").

Anyone interested in joining my order to share the postage cost? Took two weeks to deliver last time I ordered.

ps. DGM Optics say "Star images retain a much more natural color..." Eta Carina turns violet! The rest are fine.

[wavelandscott (Scott)]

Originally Posted by erick

ps. Scott, you say "Star images retain a much more natural color..." Eta Carina turns violet! The rest are fine.

Read carefully, I was quoting from their website on the above (I should have used quotes or italics to be more clear).

What emission lines is Eta Carina giving off?

[erick (Eric)]

My apologies Scott. I'll edit my post to correct that attribution.

[erick (Eric)]

I'd better stop "shooting from the hip". In 2007 I said:-

"It had an interesting effect on Eta Carina - it went deep red. For fun, I turned the scope on Antares which was also a deep red colour."

I think that might make more sense than violet?

Time to have another careful look. Of course, apart from using a star to refocus with the filter in place, one is too busy looking at the enhanced nebula to pay careful attention to the stars!

Edit: Spotted some more on CN:-

"Yes, the NPB does yield small secondary red flarings on bright objects, but it does so at a level which is less than that seen with my Lumicon UHCs." David W. Knisely