Quote:
Originally Posted by ColHut
Great Article. Nice and concise. Do you have any comments about exit pupil and scope size?
regards
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Hi Col,
Most of the literature and specifications will tell you that you should aim for an exit pupil of between 2 and 4mm when using these filters. This makes sense with medium sized scopes and the general image brightness levels of most targets we view with filters. In practise I have used all of the Astronomiks and Lumicon filters with exit pupils from .5mm to 6mm and have found them to work fine within these parameters, however the scopes have been 14" or larger in aperture. Small scopes will certainly struggle with really small exit pupils, because the image will dim too much. There is a fine balance between reducing the exit pupil to increase contrast and then going too far and dimming the image.
In terms of filter types for different sized scopes, I recommend people always buy a narrowband filter as their first filter for all telescope apertures. My favourite is the DGM Optics NPB filter which is a very tight bandpass narrowband filter. The Lumicon and Astronomiks UHC and the Orion Ultra Block are also very good. People with scopes over 10" or 12" aperture will also get a lot of benefit from an OIII filter. This is because the OIII is a harsher filter than the UHC and generally needs more aperture than the UHC. For people who usually observe under semi light polluted or light polluted skies an OIII would be the best filter irrespective of aperture, as it will show the most improvement. However, observing DSO's under those type of conditions isn't how I like to spend my time. I believe if you have to endure those conditions, you should focus on targets not affected by light pollution like, double and variable stars, the moon and planets.
Cheers,
John B