which filter?

[ian_j_kirk]

Hi everyone,

I have a skywatcher 200p dob, and use it under reasonable skies in my back garden but also have a nice dark location a short drive away. I am looking at getting either a uhc or an O111 filter to get more detail out of nebulas, and was wondering which one people recommend, as in which one provides the best views and which one has more of the types of nebula to view ( as i understand, both filters can only see one type of nebula)

Thanks in advance

Ian

[faulksy]

hi ian, your right in what you say about 1 filter sees 1 thing and another filter sees another, but if your only buying 1 filter there is more objects dso,s to see with a uhc filter, the uhc is actualy good on jupiter as well

[DOBBY]

The UHC is probably the better bet with an 8inch

[Qualia]

I have found the UHS a useful tool to bring out nebulae. The only downside is that in use, light is cut, so you reduce useful magnification simply because as you up magnification the image gets darker anyway. Effectively, then, stars are obviously dimmed and the surrounding sparklers don’t look as pretty with the filter in place. If you do buy this kind of filter, you'll probably end up restricting yourself to lower end magnifications.

Here's a useful number-guide which shows what kind of thing is going on regarding exit pupil (image brightness at EP for want of a better term) when using this kind of filter.

I have never used an OIII filter but I've heard they are also the business for bringing out nebulae. The only downside is that they are a little more aggressive than your UHC filter.

I use the Baader UHC-S and have no complaints.

[michaelmorris]

All filters will make your image darker, that's what they do! The point of almost all filters (not neutral density filters) is that they selectively darken some wavelengths of light, therefore increasing the contrast between the wavelengths of light that the filter lets through when compared to the wavelengths the filter blocks/suppresses. An OIII filter will preferentially let through light from by certain types of emission nebula (e.g. planetary nebulae), whilst reducing transmission from other wavelengths. A UHC filter will have high transmission rates in OIII and hydrogen alpha wavelengths and hence be useable for increasing contrast over a wide range of emission nebulae. An OIII filter will have high transmission rates in OIII wavelengths and hence be useable for increasing contrast of a range of nebulae that emit strongly in OIII wavelengths. The relative amounts of light let through/rejected at different wavelengths of light is known as a transmission graph and this tells you how the filter will behave. See examples here http://www.astrosurf.com/buil/filters/curves.htm

If you look at the transmission graphs for a range of filters (see link above) you'll see that selectivity and maximum transmission rates in your target wavelengths varies a lot between filters of the same type. 'Aggressive' (i.e. expensive) filters have high transmission in a narrow band of wavelengths and low transmission in other wavelengths. If you have a relatively small aperture scope, aggressive filters may darken the image so much as to make the filter practically unusable. On larger aperture scopes you have more light to play with, so you can get away with more aggressive filters. Therefore it's important to know about someone's observing equipment and the targets they have tested a filter on when evaluating whether their experience of a filter is likely to be relevant to you.

Here is a bit of useful reading on the subject http://www.cloudynights.com/item.php?item_id=1520

I hope this helps.

[LukeSkywatcher]

The UHC filter will be of more use over a wider range of nebulae. Most people who have both (myself included), prefer the UHC over the OIII.

[JamesM]

Ian, if you have the time you might want to read this report which was compiled by someone with a 10" newtonian using a variety of different filters. It's fairly long though not too technical and is mostly comprised of performance scores being allocated to a list of targets of which many I am sure you are familiar with.

James