A little advice on a Olll Filter

[Taff]

Hi' Can anyone give me some advice on a Olll Filter.What i like to know can you use the filter for capturing the night sky,or is the filter  used just for viewing Thanks.Mark

[Davey-T]

Two different things, the imaging one is a very narrow wavelength as little as 3nm.

Visually the preferred ATM is the Astronomic which is useful for some deep sky objects such as the Veil Nebula.

Dave

[Taff]

Thanks Dave' For your help.Mark.

[ronin]

Much will depend on what you want to accomplish. Classically they allow through only a small section of the spectrum, so I guess that "capturing night sky" may be nonapplicable.

Oftan used to remove (block) a lot of the general background that surround objects that emit at the OIII band and so this allows the object to stand out better. However then you have reduced the amount of light that gets to the eye or the sensor. So bigger scope or longer exposures.

Least costly I know of are the ones at Skys the Limit, I will eventually get one of Alans to try them out.

[Tim]

It is a great question.

Oiii filters for visual use are slightly different than those for imaging, and the imaging ones are usually available in a selection of wavelengths, with the smaller number versions blocking more non Oiii wavelength light.

In use they help the eye or the camera pick out Ozone emission nebulosity, increasing its contrast against the background. Really good targets for Oiii include, NGC 6960, NGC 281, NGC 6888, NGC 7635, M27, M57, M76, M97, M42/M43, and pretty much every planetary nebula. Although the fainter planetaries might be easier to locate with a filter, I prefer their natural colour in the eyepiece, as quite a few will exhibit a ghostly blue.

For imaging, they work best on a mono camera, but even with a DSLR or OSC camera they can be used effectively, as 3/4 of each pixel will respond to the oiii wavelength to some degree (blue and green elements, blocked by red).

Are they interchangeable or are two filters necessary? I have both versions, visual and imaging, and under average to poor skies it is very difficult to notice any tangible differences. An Oiii filter for imaging also provides reasonable views when used visually.

Cost wise there is a lot of variation. The process of creating a decent filter with its many layers of coatings is expensive, and as a general rule, you get what you pay for. Cheap filters? No thanks!

Hope that helps a bit.

Tim 

[Davey-T]

Also they block a lot of " normal " light so the more you capture to start with ie: more aperture the more you will see and a dark sky helps as well otherwise by the time you've blocked out everything but a bit of O111 you may see nothing at all 

Dave

[Taff]

Thanks to everyone for your help and advice on the filter.Mark