Skywatcher UHC filter

[Matthew.Blake]

Yay, my UHC filter arrived today. Accordingly the weather prediction has changed from it being clear tonight to being cloudy within the space of the last 24 hours.

Does anyone know how these filters work? I had a peek at some street lamps through them and can make out a long line of repeated images of the street lamp suggesting some form of resonant cavity to me. Hopefully attached is a picture of what it looked like.

[John]

UHC filters are used to enhance the contrast of nebulae. The are most effective on planetary nebulae but also help a bit with other types of nebulae. They don't make any difference with galaxies I've found - dark skies are best for those !.

You may find that stars look a little distorted with the filter.

As far as I know they are not designed to filter out artifical light pollution.

John

[Astro_Baby]

What sort of scope was it in ? That looks like a tube current to me if its a reflector.

If its any use at all I did a comparison with the UHC versus the OIII filter.

OIII / UHC Filters

I was hoping by now to have had more time and targets to test with but the weather hasn't been kind to us astro fans.

[Matthew.Blake]

Sorry guys, you misunderstand me. I know what they do, I'm interested in how they do it i.e. do they just absorb light out of their pass band like a colour filter or are they set up as a resonant cavity filter? The multiple images of the lamp were included as my evidence of the second, but I am unsure.

[Astro_Baby]

As far as I know they are bandpass filters with a very narrow band - just the three nebula emission lines.

[John]

Sorry for the misunderstanding of the question. I'm not familliar with the term "resonant cavity filter" (sounds like something out of dentistry !) but I've found this useful info on how nebula filters, like UHC one, work:

Company Seven | Baader Planetarium UHC-S Filter Description Page

Hope that helps.

John

[roundycat]

they are interference filters made by the Fabry Perrot technique. Layer upon layer of dielectric material is deposited onto the glass substrate, the thickness of the deposition being relative to the wavelength of unwanted light. Thus most of the unwanted wavelengths are rejected by being effectively 'tuned out'. Your resonant cavity analogy is not far off the mark.

Dennis

[Matthew.Blake]

they are interference filters made by the Fabry Perrot technique. Layer upon layer of dielectric material is deposited onto the glass substrate, the thickness of the deposition being relative to the wavelength of unwanted light. Thus most of the unwanted wavelengths are rejected by being effectively 'tuned out'. Your resonant cavity analogy is not far off the mark.

Dennis

Excellent! So it's just a series of etalons then! Looks like I might get some clear sky tonight to try it out for the first time.