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Old 17-01-2025, 09:19 AM
Bluebeard (Giles)
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Light Pollution filters for Sydney

Hi,
Are there any light pollution filters someone can recommend for inner-city Sydney. These would be used for planetary viewing through a 127mm SCT. Would something like a SkyGlow or specific colour filters be best? Any advice will be appreciated.

Thanks
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Old 18-01-2025, 08:54 AM
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mental4astro (Alexander)
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Light pollution filters are not for planetary viewing. These filters are made to transmit very specific wavelengths of light at which certain nebulae types glow at but absorb all other frequencies of light so to increase contrast with said nebulae.

The thing about the planets and the Moon is you don't need to be under a dark sky to view them as they are sufficiently bright. Also, because they glow across the entire spectrum, light pollution filters are not going to help.

What CAN help are colour filters to show up certain filters with different planets. But these are still a double edge sword as the features they can help bring out won't necessarily make these features jump out to be plainly obvious. Some planetary features are vey low contrast and a colour filter will only help to show them up just a little bit easier.

The chart below shows what the different colour filters are best suited to show according to the different planets and Moon. Note that while this Orion filter set is no longer available, the information in the chart is still current as these colour filters are all made to the Kodak Wratten standard.

NOTE!!! 1
Many places sell colour filter sets with only deep colours. DO NOT BUY THESE! Such sets are designed to sell to chumps who have no clue about what colour filters are for or what they are supposed to show. The majority of filters that are useful are actually subtle in their colour density with those deep colour filters having very limited applications and in reality have little practical use for what people actually want. Look for places/sites that offer individual colour filters and have a wider range of colours than just four as this way you will be able to select and buy what will actually best work for you.

I do happen to have the Orion Planetary Filter Set noted in the chart below. It really is an extraordinary bit of kit but it really is VERY specialised. Many of these filter get little use with there being just a few that you will use a lot. You don't need all of these filters but maybe two or four. Once you get your experience up you might like to get a few more filters.

NOTE 2,
Aperture is King with astro. Not only will this show fainter stuff but resolution also increases. This is particularly important with the Moon and planets. The larger the aperture the finer the detail you will be able to see and better these filters will work.

But to get you started, I would suggest the 80A for the planets and the 82A for the Moon. These are by far the filters I most use with the planets and Moon. With your 5" scope, these filters will serve you very nicely. With the 80A I was able to see subtle clouds on Mars and helped lift the Great Red Spot which is very soft at the moment.
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Old 20-01-2025, 03:25 PM
Bluebeard (Giles)
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Thanks for that advice, Alexander. Saved me from pointless expenditure! I have a Moon filter already, so I'll try and source an #80A.
regards,

Giles
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Old 20-01-2025, 09:19 PM
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mental4astro (Alexander)
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Giles,

It wouldn't be a pointless exercise. "Light pollution filters" are actually poorly named. They are specialised filters that will do a specific job, but not in the way that the name suggests. Bright objects like the Moon and planets don't suffer from light pollution. But these Light Pollution Filters also won't help you with galaxies, open clusters or globular clusters as these all glow across the whole spectrum, unlike emission nebulae.

Nebulae such as M42 and M8 and planetary nebulae are what will benefit from these filters. There are three main types of Light Pollution filters:

1, General purpose - these are what are normally known as Light Pollution Filters. While they do help, not as effective as the next two.

2, Oxygen iii (Oiii) - these are designed specifically for planetary nebulae.

3, Ultra High Contrast (UHC type) - these are best for emission nebulae like M42 and M8, what we would see as red nebulae.

There is a forth type:

4, Hydrogen Beta - these are even more tight in their transmission and are used for things such as the Horsehead Neb. But an UHC type filter will also show the Horsehead, but not with quite the same level of contrast*

* HOWEVER, the Horsehead Nebula requires MORE than just a filter to see. Its reputation for being a difficult object to see is ILL DESERVED. It can be seen with as small an aperture as 4", but there are people who have never seen it with a 12" scope. Very faint objects, such as the Horsehead require first and foremost a TRANSPARENT sky. Anything less and it will be very difficult to impossible to see no matter how large your aperture is.

My own experience with the Horsehead will demonstrate this. Using the same scope and from the same location, I have had nights when the Horsehead was an easy target without needing a filter. I have also had nights when it took all my experience, knowing where to look, with a filter and a lot of patience to tease out a hint of it. And I have also had nights when it remained totally invisible no matter what I did. The ONLY difference is entirely the quality of transparency of the night.

Just because a site is "dark" does not mean it is actually good for astro. And part of your learning is to be able to recognise a transparent sky vs everything else. And this comes with experience in just looking up and actually noticing what can be seen naked eye from one night to the next. And this goes not only from a dark location but also very much applicable from under urban skies! With time and experience, you will begin to recognise conditions that will be more favourable to chase really faint stuff vs setting up yourself for a night of frustration. Taking this a little further, it comes down to site selection, which is beyond the scope of this thread.

Below is my sketch of the Horsehead and Flame Nebulae done on a night of very good transparency from a Bortle 4 location (not the darkest, but a site with outstanding transparency which will outdo a Bortle 1 sky with poor transparency) using a 17.5" dob. And yes, I sketch at the eyepiece directly on to black paper.

Alex.
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Old 21-01-2025, 10:15 AM
Bluebeard (Giles)
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Hi Alexander,
Thanks for the further clarification. being as close to the city as I am, I really am limited to planets and the moon. Even though my 127mm SCt is reasonable portable - fits in a backpack - no car really limits my exploration of darker sites to parks that I can walk to. Definitely Bortle 8 territory!

These additional filters would be useful if I was in a place to see Nebulae and the like, but at the moment, I think they'd be an indulgence. I will refer to this post as time and my experience increases.

Oh and you really are an excellent artist!

regards,

Giles
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Old 23-01-2025, 01:59 PM
glend (Glen)
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The best advice I can give you is to take your scope as far from Sydney as you can. It's worth the drive to the western side of the Blue Mountains, or spending a weekend camping at Bretti Reserve near Gloucester NSW.
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