Visual filters: UHC vs. OIII

[Andrew*]

As I understand it, a UHC filter is a narrowband filter that passes the specific wavelengths around Hb, OIII, and if you're lucky, Ha.

An OIII is also a narrowband filter, but it only passes around OIII.

What, then, would be the point in owning an OIII filter and a UHC filter? Surely the UHC does the same thing as an OIII, but allows useful Hb in too. Therefore it's more versatile, but still helps with the isolation of OIII in OIII rich targets.

Any thoughts?

Andrew

P.S. contemplating selling my Baader OIII for a Lumicon UHC.

[GazOC]

The UHC-S is a more versatile filter for the reasons you have mentioned. The justification for having both is that the OIII gives better defined views on OIII tatgets than the UHC-S. If you are only going to get one filter though, get the UHC-S.

[Whippy]

I'm just guessing here Andrew, but I reckon the OIII would give better contrast on certain objects if you had enough aperture to do it justice. As the UHC lets though more light, it's better for more objects and smaller apertures. I've got a UHC and never considered a OIII for visual.

Tony..

[Andrew*]

Gaz, I'm referring to the original UHC type, not the UHC-S, which is in between a LPR and a UHC.

I have a UHC-S, but I was wondering whether the UHC would offer me more than the OIII alongside the UHC-S

Cheers

Andrew

[narrowbandpaul]

the UHC actually blocks Ha, allowing a mix of Hb and O[iII] through. Perhaps blocking Ha is a silly thing to do, but the eye is very inefficient at Ha. It picks up Hb more effectively than Ha even though the Hb is 2.92 times weaker.

the rest wavelengths are separated by only 14nm (486 vs 500nm) so a wide FWHM on an O[iII] filter may pass Hb anyway. You cant constrict the bandpass too much or the flux from stars is massively reduced.

I used an Orion UHC witha 12" f4 dob, and easily saw the veil and north american

hope this helps

Paul

[ottUp]

I have both and would not want to part with either. If I could only have one, it would be the O-III.

From within the city and for most emission nebula objects, I find the O-III gives better results - my light pollution is quite bad, and the additional rejection helps. (For objects with O-III lines, obviously; fortunately that is most of the ones that my scope can see.) The Veils (NGC6960 and friends) and M42 are examples of objects that are better, for me, with the O-III.

From a good dark-sky site where light pollution isn't a problem, the UHC gives better results for some objects, as it allows more emission lines through, allowing you to see more detail and more structure. M27, for example, looks better with UHC in a dark site, but better with O-III from light pollution.

- Richard

[Andrew*]

Thanks Richard for that useful nugget of info.

I have also had excellent experiences with an OIII filter, but thought that the addition of Hb wouldn't do any harm, while improving its versatility.

Andrew

[Astronut]

This link's been posted before but it's a good read on filter comparisons for specific objects:

http://pages.sbcglobal.net/raycash/filters.htm

I've never tried an OIII filter so I can't comment on the difference. The Astronomik visual UHC I have lets through OIII, Hb and Ha. I'm not sure why Ha is passed though. Isn't Ha outside the visible spectrum? If so, how is it useful for visual use? Also, does street lighing emit Ha? If it does, then surely it would be better blocked :? I get the impression that Hb is emitted from street lighting because I've found the UHC doesn't perform nearly so well in my LP backyard when compared to its performance at a dark site. I'm a little puzzled on this.

[John]

I think the UHC-S is Baader's creation. It quickly gained a reputation for being slightly less "severe" than other brands of UHC filters and therefore more useable in smaller aperture scopes. I've owned a couple of Baader OIII filters and ended up selling them because I did not like the views they gave - virually all the background stars dissapear - but that's probably just my preferences. In contrast to the UHS-C I understand that Baaders OIII is more severe than other brands OIII's.

John

[bern]

the UHC actually blocks Ha, allowing a mix of Hb and O[iII] through. Perhaps blocking Ha is a silly thing to do, but the eye is very inefficient at Ha. It picks up Hb more effectively than Ha even though the Hb is 2.92 times weaker.

the rest wavelengths are separated by only 14nm (486 vs 500nm) so a wide FWHM on an O[iII] filter may pass Hb anyway. You cant constrict the bandpass too much or the flux from stars is massively reduced.

Depends on the brand Paul, the Astronomik UHC passes Ha and the OIII doesn't pass Hb:

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

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

bern

[Andrew*]

This link's been posted before but it's a good read on filter comparisons for specific objects:

http://pages.sbcglobal.net/raycash/filters.htm

Hi AN. Yes - that's a good link - it seems to suggest that a UHC does a good job on OIII targets about as well as an OIII filter.

I'm not sure why Ha is passed though. Isn't Ha outside the visible spectrum? If so, how is it useful for visual use? Also, does street lighing emit Ha? If it does, then surely it would be better blocked :?

I would like a UHC filter to pass Ha, so it can also be used as a general use narroband filter for imaging. You're right - The human eye is rather insensitive to Ha, but it's not totally outside the visible spectrum. Ha is not a light-pollutant. That is why imagers can get light purely from the target in bright moonlight and LP using very narrowband Ha filters.

I think the UHC-S is Baader's creation. It quickly gained a reputation for being slightly less "severe" than other brands of UHC filters and therefore more useable in smaller aperture scopes.

True. I also have a CLS and have compared them side by side on many occasions, and rarely experienced a real difference between them. Therefore I thought perhaps upgrading to a narrower bandpass UHC would benefit my DSO observing and save me having two very similar filters.

In contrast to the UHS-C I understand that Baaders OIII is more severe than other brands OIII's.

I had a chance to compare a Baader OIII versus an Astronomik one very briefly and not even at night. The Astronomik was noticably "brighter".

Cheers

[algol]

well i hope to be getting a UHC soon from aunt FLO, as i heard they are great and using the link posted above seem to be more versatile than the OIII