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Old 28-09-2018, 01:25 PM
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mental4astro (Alexander)

mental4astro is offline
Join Date: Jun 2008
Location: sydney, australia
Posts: 4,815
Hi Brendon,

Colour that you are seeing is no mistake. Two factors at play here, one is your eyes and their capacity to make out colours at low levels of light, and two is the actual colour/colours that these filters transmit. The light they transmit is not white. It is in the red and greenish range.

If you see an intense red red around individual stars this is an artifact of the way filters are produced transmitting internal reflections. Most obvious with brighter stars. A clue to their filter origin is this colour is too deep and intense, where actual red stars are more orange in appearance.

With respect to LED lights, what "light pollution filters" do is center their wavelength absorption around those of mercury vapour and sodium lights - older technology street lights. These broadband light pollution filters will still work, however their effectiveness is reduced because these LED lights transmit across the whole spectrum, as you said. Narrow band filters, OIII & UHC are less affected as they are more aggressive, but their effectiveness is still reduced in the Big Smoke.

It then becomes a balancing act between the light pollution vs aperture and filter you are using. While aperture is King, in the Big Smoke smaller apertures are the Knight in Shining Armour! Telescopes collect light, and hence light pollution too. Me, from my home in Sydney I now never set up my 17.5", instead limit my aperture to no more than 8".

There's also another issue with Bigger Apertures - they are also more sensitive to the prevailing seeing conditions of the night. Particularly if you are maxing out the magnification. Dropping the aperture sometimes means the difference between a productive session or a night of frustration...

My last dark sky session, seeing was abysmal. Two of my mates had their 18" dobs, but I only took my 8" f/4 dobbie. They had a crap of a time as even their lowest magnification was too much for the conditions. Yet I was only doing low power wide field observing and the image I had was sensational!

For those of you not aware of this, ripping the maximum amount of magnification out of your scope is not the best use of any scope. There's a lot of factors at play, seeing (atmospheric stability), transparency (clarity of the sky, humidity, fog, smoke, etc), aperture size, object size andx brilliance. And it takes a bit of time to understand ho w they all come together to play nice or give you grief!


Last edited by mental4astro; 28-09-2018 at 04:26 PM. Reason: Typo
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