UHC Filters - How should they be used

[Guest]

I understand the basic idea of a UHC filter but...  They are supposed to improve contrast when looking at certain types of object but they also make the image a lot dimmer since obviously they are filtering out some of the light. For visual use they don't seem to help very much since you may be looking at a high contrast image but it's one which is really really dim. Therefore are they better used for astrophotography and you can compensate for the dimness by using longer exposures.

 

[John]

I find UHC and O-II filters very useful for observing nebulae, especially planetary nebulae and super nova remnants. They don't provide any benefits for viewing galaxies and star clusters - the opposite really, the diminish the view of those.

I tend to use them at low to medium magnifications but sometimes at high magnifications on smaller targets such as the little planetary nebulae.

Smaller aperture scopes (less than 100mm) probably get better results with a UHC filter because those have a wider light band pass width than the O-III filters.

Astro photographers find these filters very useful as well but there are filters that are optimised for imaging and those optimised for observing. The latter will work for the former purpose OK but the former type are limited in their usefulness for observing.

Dark skies will help get the best from them, as with all deep sky observing.

You are correct that these filters do only permit certain wavelengths of light to pass through. Pick a target that emits strongly in those wavelengths and you get the results. Stars do get dimmed and can take on a different tint but it's the nebulosity that you are using the filter to see.

 

 

[Ricochet]

1 hour ago, woodblock said:

For visual use they don't seem to help very much since you may be looking at a high contrast image but it's one which is really really dim.

For visual you increase the brightness by observing with a larger exit pupil (= lower magnification) than without the filter. For a UHC you probably want to be looking at an exit pupil of around 3mm or more, depending on framing, whereas you would be aiming for an exit pupil around 2mm for an unfiltered view.