Worth the cost?

[jetstream]

37 minutes ago, Andrew_B said:

Is it necessary to have a "visual" O-III filter or is one marketed as being for imaging just as good? I've got a Baader 8.5nm O-III it works well photographically but wondered if it's worth bothering with for visual observing.

Its best to get both OIII lines and imaging filters are typically (not always) lower in transmission.  Do you have the graph that came with it?

[jetstream]

[jetstream]

[globular]

@jetstream Have I got this right…?…..

When used visually / in real time the ccd filter will show a dimmer nebulae because it’s cutting 10% of the oiii light (rather than 1% with the visual filter) and because it’s losing all but the 501 band (while the visual one keeps the less intense but still important 497 band too).

When used photographically the lower transmission is not an issue because exposures are longer. And narrower bands keep the images tighter.

Edited August 1, 2021 by globular
typo

[Andrew_B]

1 hour ago, jetstream said:

Its best to get both OIII lines and imaging filters are typically (not always) lower in transmission.  Do you have the graph that came with it?

This is the graph and spectrum from Baader's website. There look to be a pair of lines in the second image but unfortunately the graph doesn't have an overlay with the relevant wavelengths.

Edit - looks like it's cutting off most of the 495.9nm line and prioritising the 500.7nm line.

Edited August 1, 2021 by Andrew_B

[Louis D]

I would guess you'd have to try them back to back to see which works better under your skies.  I could see the narrower band pass photographic filter pulling ahead in heavily light polluted skies where a 10% loss in transmission of one line and complete loss of the other line is more than offset by increased contrast against background skyglow.

[jetstream]

2 hours ago, globular said:

@jetstream Have I got this right…?…..

When used visually / in real time the ccd filter will show a dimmer nebulae because it’s cutting 10% of the oiii light (rather than 1% with the visual filter) and because it’s losing all but the 501 band (while the visual one keeps the less intense but still important 491 band too).

When used photographically the lower transmission is not an issue because exposures are longer. And narrower bands keep the images tighter.

Yes exactly. Also, from experience with filters it seems that smallish transmission differences can make big differences visually- such as a 5% difference. Many will say that the eye wont detect this...my eyes do. I have the old, wide Astronomik OIII, an excellent tight older Lumicon OIII and my best, the new TV OIII, made by Astronomik. In addition I have an excellent Lumicon UHC and the TV equivalent (by Astronomik). The new TV's transmission is higher than the Lumicons- and it shows.

IMHO a 10% difference is massive with these filters.

BTW, an astro colleague/vendor measured the bandwith and transmission of my new TV's and they were bang on.

[Don Pensack]

For visual use, and to not make the image too dark and cut out too much of the nebula, an O-III filter that passes both the 500.7nm spectral line (strength of 75) and the 495.9 spectral line (strength of 25)

is desirable.  Since high contrast is desirable, which means narrow bandwidth, the 493.1nm O-III line (strength of <1) is always excluded.

So, what is the optimum O-III filter?

To catch both of the strong visual lines, yet still have a narrow bandwidth for better contrast:

about 11.5-13nm bandwidth, >90% transmission at both spectral lines (w/94%+ being even better), and close to zero out-of-bandwidth (OOB) transmission in the 400-700nm range.

O-III targets: planetary nebulae (e.g NGC7293, NGC246), supernova remnants [some, not all] (e.g. The Veil nebula), Wolf-Rayet excitation nebulae (e.g.NGC6888, NGC2359)

 

My personal TeleVue BandMate II O-III filter has a tested 12nm bandwidth (most current model since 2018), 99.2% transmission at the 495.9 line, and 98.2% at 500.7nm, with OOB <1% anywhere else.

My personal Lumicon O-III Gen.3 O-III filter (most current model since 2018) was tested to have an 11.5nm bandwidth with 95.1% at 495.9nm and 94.7% at 500.7nm, with OOB <1.6% anywhere else.

The two are virtually indistinguishable in the field on O-III targets.

Another O-III filter of note is the current version of the Astronomik O-III (they make the TeleVue filter).

 

With such narrow filters, it is best to use exit pupils of 2.5mm or larger at the scope, and preferably >3mm.

Some small and bright O-III targets can be seen better without a filter with exit pupils <1mm, however.

 

NOTE: large hydrogen/helium gas clouds are usually best if the H-ß line at 486.1nm is also passed by the filter.  This includes nebulae like M8, M20, M17, M16, M42/43.

If the filter also passes the H-ß spectral line, it is referred to as a "narrowband" filter, and these vary from 22-27nm bandwidth.

Some good examples: Astronomik UHC, DGM NPB, TeleVue BandMate II Nebustar, Lumicon UHC.

It is important to realize that many filters in the market today are much wider than the aforementioned bandwidths.  These cheaper (usually) filters do provide some contrast enhancement,

but are much less effective at making the nebulae stand out because of lower contrast.