Light pollution filters

[Oaky762]

Relatively new to this, I have pictured Saturn and recently Jupiter, but living in the city I obviously have issues with light pollution, apart from travelling away from the list, is it worth buying a light pollution filter? Thanks

[DaveS]

Probably not for planetary, though I've heard that a neodymium filter can bring out the detail on Jupiter.

LP filters, eg UHC are designed to transmit the emision lines of hydrogen and oxygen so emision nebulae stand out from the background.

[Ajohn]

There is still a lot of low pressure sodium lighting around where I live. I bought a sodium light interference filter years ago and it improves all view eg no sign of nebulae in orion but could be clearly seen with it. I went back to the same shop some time later to get a 2in version and was told that they couldn't obtain them any more. Actually I was looking around the web again today for one and found myself buying a TS Optical Contrast boost filter. Best bought direct as they are then cheaper than Amazon. How well that will work - pass.

Much depends on the lighting in your area really but it may well be worth trying one of the broader band moon glow / light pollution filters or one of the ones that state normal sources of pollution - mercury lighting. The mercury lighting aspect doesn't make much sense to me. I suspect it must be a problem in the USA or somewhere other than the UK.

John

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[scarp15]

I enjoy looking at the planets unfiltered personally, they are bright enough, and when positioned high enough, are not particularly effected by light pollution - and more subtle features will be discerned by using less glass in the optical path. However I always use a dew shield on my scope (C8) as this helps block out stray (street) light.

For anything else, unless you are aiming to locate certain nebulae, filters are not much use to combat light pollution, and I prefer using filters in darker locations anyhow. Best to enjoy what you can, to the best your skys will allow from home,concentrating on targets that are relatively high up, but nothing substitutes a drive out into the countryside for a darker sky.

For what it's worth I do from time to time use my Orion Ultra block filter, with low power eyepieces, to enhance (gain more contrast)the view of some planetary nebula from home though.

[Oaky762]

Many thanks

[JamesM]

I have the 2" Baader Neodymium filter and I would argue that you can see a difference when viewing deep sky objects as they appear to stand out more but not in proportion perhaps to its price (£80 ). Personally I use it more for planetary viewing where for me I believe it does help improve contrast.

If I'm honest, the best way to improve any view of the majority of objects in the night sky is to travel to a dark site to observe under dark skies - a bit of hassle to do sometimes but in my opinion certainly worth it. Of course planets, along with the moon and double stars, are free from the effects of light pollution and filters can assist certainly when viewing the first two but filters are not silver bullets and there are many who believe objects are better viewed without filters or as I would term 'raw'. I guess its down to personal preference and the size of your wallet!

James

[swamp thing]

Hi

As has already be stated LP isn't an issue when observing planets. In fact hazy skies often produce the nights of the steadiest seeing for planets. Planets require "steady" nights not necessarily the clearest most transparent ones, often the transparent clear nights have the worst seeing for planets producing terrible "boiling" views. It seems you cannot win huh? nights good for DSO,s, when its clear and transparent often means planetary views are awful, and vice versa.

LP filters work by blocking the wavelengths of light emitted by certain street lights (but unfortunately not all). If you have the "right" type of LP they can de effective, but. If you don't they can do next to nothing. Unfortunately most modern street lighting falls into the unaffected bracket.

Narrow band (UHC) filters work by blocking light from all wavelengths except the narrow bands in which nebulae emit light. By covering a few bands of light the UHC is perhaps the most versatile of the nebula filters, being effective on more targets than its line filter counterparts. UHC filters work on nearly all emission nebulae.

Line filters (O-III & H-Beta) block all light except from the lines of the spectrum after which they are named. This makes them much more aggressive than the UHC type. This also means they darken the sky background and dim stars in the FOV considerably. They can increase contrast much more than their UHC counterparts on certain targets.

No nebulae filters will help with globular, or open clusters. They can help pinpoint extra galactic nebulae within other Galaxies but not with the galaxy itself. They also will not work with reflection nebula unless said nebula is reflecting light from an emission line star.

No filter can replace a dark sky though. They can help with objects that are tough but, they still have most effect from a dark sky to start with. In other words a neb viewed from a LP sky even when a filter is used, will be nowhere near as good as the view with exactly the same set up from a dark location.

[dph1nm]

If you have the "right" type of LP they can de effective, but. If you don't they can do next to nothing

Actually they can make things worse!

NigelM

[scarp15]

Creating a screen, to block out street lighting, in your home environment will help to, there are various threads that have covered this. I think that this will also help your eyes to become /retain their dark adaption to, without being 'blinded' everytime you turn your head (something I have to take into account venturing into certain points from my own backyard).

If you use a reflector, then consider flocking at least the upper part of the tube, that is the section opposite the focuser and as I have mentioned a dew shield is effective at blocking out stray light to. Applying each of these will help gain a bit more contrast when observing in less than favorable sky circumstances.

[Ajohn]

I enjoy looking at the planets unfiltered personally, they are bright enough, and when positioned high enough, are not particularly effected by light pollution - and more subtle features will be discerned by using less glass in the optical path. However I always use a dew shield on my scope (C8) as this helps block out stray (street) light.

For anything else, unless you are aiming to locate certain nebulae, filters are not much use to combat light pollution, and I prefer using filters in darker locations anyhow. Best to enjoy what you can, to the best your skys will allow from home,concentrating on targets that are relatively high up, but nothing substitutes a drive out into the countryside for a darker sky.

For what it's worth I do from time to time use my Orion Ultra block filter, with low power eyepieces, to enhance (gain more contrast)the view of some planetary nebula from home though.

I would strongly advice people to disregard the comment "not much use" because if you happen to be able to find the right type of filter to suit your local street lighting they will more or less completely irradiate the problem. This is particularly true of low pressure sodium lighting as their light output spectrum is narrow and rather specific and easily removed with interference type filters along with little loss of light. It's a pity all street lighting isn't done this way but unfortunately more and more broad spectrum white light sources are being used despite that fact that they are a lot less efficient in terms of power needed for a particular light level.

Some older astro people I have met don't rate filters at all. An opinion based on types that basically use what in real terms are just coloured glass. Interference filters are entirely different. They are made in much the same way as the anti reflection coatings applied to optics but designed to reflect specific wave lengths of light and let the rest through.

Some info is even on the wiki http://en.wikipedia.org/wiki/Sodium-vapor_lamp Sadly a stretch of my local motor way is using the high pressure types now. Even discussed with the chairmen of a local astro club, No doubt he too thinks filters are useless - well in respect to the type of light used now they probably are.

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[LukeSkywatcher]

I have a SkyWatcher Light Pollution filter and also a SkyWatcher UHC filter and a SkyWatcher Oiii filter.

The Light pollution filter in itself works really well. Well worth investing in................

However the UHC and Oiii filters also block out the same orange glow LP from street lights while enhancing views of nebs.

I would say............kill 2 birds with 1 stone and get a UHC filter............

It will block most LP and allow you to see many nebulae in better contrast/detail.

The UHC or Oiii will not improve your views of planets though. Good views of planets really is down to local "seeing" conditions.

[Ajohn]

I'm one of the older astro people I mentioned - no hang ups though. I received a TS Optical UCF2 filter this morning. Initial impressions are disappointing but I can check this evening. All I have to do is look through a window at the front of the house with it. I may be wrong but I suspect that it will do little. The LP Sodium light filter makes a huge difference. Pitch black and all that can be seen is car head and tail lights. I've found that it increases the contrast of any objects rather noticeably but the broad spectrum street lighting is getting closer and closer. Strange really when the world is supposedly reducing power consumption as for watts in an light out it seems LP sodium is even better than LED or any other form of lighting.

It's easy to check that a filter is of the interference type. They reflect the light that they reject and usually their apparent colour changes when viewed at different angles. The UCF2 is just coloured glass and reflects white light rather strongly.

John

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[scarp15]

I would strongly advice people to disregard the comment "not much use" because if you happen to be able to find the right type of filter to suit your local street lighting they will more or less completely irradiate the problem.

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You have misunderstood my statement - 'unless you are aiming to locate certain nebulae, filters are not much use to combat light pollution'.

Implying of course, that light pollution filters such as UHC, only really work on detecting and somewhat improving contrast slightly on nebulae such as planetary nebula and do not enhance the view of any other DSO objects, such as galaxies and globulars.

[Ajohn]

How much contrast increase a filter gives depends on the level of light pollution. In my case the low pressure sodium lighting generates rather a lot of it. The level is for instance much greater than the nebulae in orion so without a filter I have little chance of seeing it. The same level of pollution exists where ever I look or what ever look at. ( Actually it's much worse looking north as I am directly south of B'ham city centre and only a few miles from it.) When it comes to stars and galaxies etc the same level of light pollution has a direct bearing on the local limiting magnitude. The situation in respect to planets differs but there is still an effect. Resolution is contrast. At Rayleigh limit for instance 100% contrasting detail only has about 7 1/2 % contrast when it has passed through the optics. Detail will generally have a lot less contrast than that and it will be reduced accordingly. The light pollution reduces contrast further.

Low pressure sodium light filters are ideal where this problem exists as it can be filtered out with very little light loss as only a very narrow range of wavelengths need to be rejected. UHC etc filters involve a lot of light loss due to the fact that they seriously limit the spectrum of light that passes through them. That will also limit the colours that can be seen. The TS Optic's UCF2 filter I have bought is an entirely different kettle of fish. It does reduce lp sodium light but doesn't get near completely removing it. Additionally the rejection notch is rather wide and the pass band outside of that has additional lossy areas. It even reflects a significant amount of white light. Despite this many people in the same situation as me are likely to see an improvement what ever they are looking at. All I have been able to do with it is my through a window in the front of the house at night test. Passing cars are still dimly visible through it. As I expected they are more visible than I would ideally like but there is a considerable improvement. Head and tail lamps are clear but there is a fair amount of light loss.

Having bought the TS Optics filter I suspect I have found something that may be better - The Skywatcher LPR filter. I found a spectral chart for it and it looks to be a decent compromise.

John

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