I own a 5 inch reflector. I am planning to buy a deep sky filter. Should I buy the UHC or O-iii. I own a small scope so I am afraid that oiii would darken the view.
Bear in mind, you will use these filters at low power (say, 10x/inch or lower). The eyepieces you use will determine whether to buy a 1.25" or 2" size.
Picture a gray square against a gray background. Hard to see.
Picture the same gray square against a black background. Easy to see.
That's how these filters work.
so long as the wavelengths emitted by the nebula are passed by the filter, the narrower the bandwidth, the blacker that background is.
Yes, the filters will dim the overall image of the entire field, but they won't dim the nebula. And it is the nebula you are seeking to see better, right?
So, the difference between those two filters is that the O-III filter passes the O-III wavelengths at 496nm and 501nm only, while the UHC filter passes
the O-III wavelengths AND the 486nm H-ß line. All nebulae emit H-α light in the deep red (a wavelength to which out eyes are very insensitive) and H-ß in the blue
(where our dark adapted eyes are very sensitive).
If you buy just one filter, I'd make it a "universal" nebula filter and get a UHC-type (narrowband) filter of 21nm to 28nm bandwidth that passes all 3 wavelengths at over 90%.
Some examples (not a complete list): TeleVue BandMate II Nebustar, Lumicon UHC, Astronomik UHC Visual, DGM NPB.
If your pocketbook is small and you cannot afford the above, there are many UHC-type filters of 36-49nm bandwidth. The background will be less dark, so the contrast will be less,
and if being used in a light-polluted area, these may not be ideal, but they are certainly better than no filter at all.
Some examples (not a complete list): Astronomik UHC-E, DGM VHT, Explore Scientific UHC, Optolong UHC, Starguy UHC.
Ultimately, with a scope of your size, you will want a narrower filter. I'd call these "mediumband" filters instead of "narrowband".
A few things to keep in mind:
--use them for nebulae only (bright nebulae, planetary nebulae, supernova remnants, Wolf-Rayet excitation nebulae, but NOT dark nebulae, reflection nebulae, star clusters, or galaxies)
--use at low power
--be completely dark adapted before using
--remember, the overall field, including stars, will be darker, but the nebula won't be. It is the contrast increase that makes the nebula appear brighter and easier to see.
Edited by Starman1, 22 October 2018 - 05:31 PM.