Which Nebulae filter

[Pete D]

I have a Skywatcher 200P and am looking to get either a SkyWatcher OIII Filter or the SkyWatcher Ultra High Contrast Filter and want advice on which would be the best for viewing Nebulae. I have read that the OIII Filter is better in scopes over 8" and the UHC filter is a better allround filter so not really sure which to go with. Also would either of these to be useful when imaging, thanks.

[swamp thing]

Not sure I for one agree with this "small scopes need this filter and big ones another" analogy myself.

I prefer the more aggressive O-III filters, others like the UHC, I think a lot depends more on the individual myself. Both work well in any scopes IMO.

I've used my O-III with just the unaided eye many times to search for the NA neb and it shows it fine at dark sky sites, you can't get a smaller aperture than that. I've also used it in an observing buddies 120mm scope and the view was very good indeed. At the other end of the scale it produced photographic views of the veil nebula in my old 16" from very dark skies.

Best to try both kinds yourself then make your decision. Try and get to a local society open night or hook up with some SGL social group, that way someone's bound to have one or two you can try yourself.

Filters are almost as personal preference ish as eyepieces and scopes

Clear skies

[Qualia]

Hiya Pete, I don't have much experience with your enquiry but I have made a sketch trying to show the relative difference between using a UHC-S filter and not which may be of some help to you:

On a personal note, I have found the UHS narrow-band filter a wonderful tool to bring out some of those nebulae. The only downside is that you really do get to the point where the stars are obviously dimmed and you may find that the star fields don’t look as pretty with the filter in place. For that reason, I generally swop between an EP loaded with the filter and one that isn't.

From what I understand, the UHS allows two electromagnetic 'bands' to be enhanced, namely, the hbeta region or red light band emitted by hydrogen and the OIII region or green light emitted by ionized oxygen. Both these regions are found in many nebulae. To this extent, they also work well as light pollution attenuaters because they reduce the light region of mercury vapour and other man-made light filth.

I have never used an OIII filter but I've heard they are also the business for bringing out nebulae. The only downside for me - an urban stargazer - is that I've heard they're pretty strong and will cut out a lot of the star field. For that reason, I guess, they're more suited to larger apertures, your 8" should in this case be fine. The other thing I've read about them is that they don't work on all nebulae. It was for this reason that my first purchase went for the general all-rounder that is the UHC-S.

[Spec-Chum]

Hi Pete, I was recommended and have actually ordered the castell uhc filter for my 200p, which comes in slightly cheaper than the skywatcher at £36 from 365astronomy, but I'm afraid I've not used either yet, so can't really comment on which is best.

[umadog]

What Steve is saying is true. The point is that these filters cut out light and so your useful magnification goes down (because higher powers mean dimmer images). So you can use an LPR filter in any telescope but you'll have to restrict yourself to lower powers. That will affect the objects you can go for. e.g. you probably won't be seeing detail small planetary nebulae with am OIII in an 8".

The image brightness depends on the eyepiece exit pupil. Exit pupil is eyepiece focal length divided by telescope focal ratio. This page gives you an outline of which filters work well with which exit pupil sizes: http://www.backyard-astro.com/equipment/filters/lumicon.html

[Moonshane]

it just occurred to me that you could use a UHC/Oiii 2" filter with naked eye as described by Steve above, by taking out your 2-1.25" adapter and assuming it is threaded for filters, screw the filter on and then look through the eyepiece end, holding the filter up to the sky. will be trying this next time I am out - seems better than the between finger and thumb approach I had in mind and will also block any extraneous lights for me too.

[Saleratus]

If you are only going to have one filter, I think the UHC is the one to get. I've seen wonderful images of Nebulae in a UHC. I have UHC, OII, and H-Alpha filters, and use the UHC 95% of the time.

[Pete D]

Thanks for all your replies. I think I am going to get the UHC filter but its a choice between the Skywatcher UHC, the Castell UHC Deepsky Filter, or the Baader UHC-S Filter. Anyone know which gets the best reviews.

[Spec-Chum]

My very same research suggested there was very little in the 3, so I went from the cheapest...

...Castell £36 from 365astronomy

[paul schofield]

Never tried the OIII filter. Use the UHC Filter a lot with my 200p and it works a treat. At first I found it a bit dark but the more I use it the better it seems. I may be wrong but I think the OIII Filter darkens the image even more I believe.