Skywatcher Light pollution Filter

[Fathead]

Hi I have been looking for a used skywatcher light pollution filter (LPR) does anyone know a cheap place to get one looked on the usual places I did find one for a £5 but got no response from the seller.

 

Many Thanks

[Fathead]

Forgot to add I need a 1.25" one

[sophiecentaur]

I see that Fathead wants a 'sky pollution Filter'. How does he 'know'?

I think that pollution is not too bad where I live. There is no local street lighting, for instance but, taking the Andromeda galaxy as an example, I can see it very clearly - but as a fuzzy circle with hardly a hint of ellipse about it.

Are there any cheap and cheerful ways of finding what it is about the local sky that's limiting visibility? If it's just too many scatterers up there? If so,  there is presumably not much I can do about it but, short of using a spectrometer on a dark patch of sky, how can I find what filter would be best? I spent a few quid on an unbranded 'Skyglow' filter and that seems to have no effect at all. Should I just buy a whole assortment of filters at great cost, and pick the best or can I attack this in a more targeted way? 

My eyepieces are all 2". Very nice but it would't make me happy to buy an expensive filter and find it's useless for my problem.

[Cosmic Geoff]

You'd need to know what spectrum any offending lights are emitting. IIRC some light pollution filters assume sodium (yellow) street lighting and block out the sodium lines, but if your council is changing to LEDs, that filter would be of no use.

[sophiecentaur]

On 10/5/2016 at 16:18, Cosmic Geoff said:

You'd need to know what spectrum any offending lights are emitting. IIRC some light pollution filters assume sodium (yellow) street lighting and block out the sodium lines, but if your council is changing to LEDs, that filter would be of no use.

Yes, in principle but easier said than done. A while ago, on a whim, I bought a small spectroscope (about 30GBP) and that is good for medium / high levels of light. Looking through my 8" Dobs, I can see nothing through the spectroscope. The cost of getting a measurement of the spectrum would be greater than the cost of a bucket full of filters and suck it and see.

The thing is, though, that I am not sure whether it's man made light pollution or just a problem with atmospheric absorption. There's only a limited answer to that with filters - except to deal with the 'slope' due to rayleigh scattering. Perhaps a reddish filter could help?

Interestingly, I have found that the spectroscope gives a convincing continuous ROYGBIV band when looking at the Sun and a Halogen lamp but, looking at a ' pretty' setting sun, I see distinct absorption bands in the Yellow / Orange region. This seems to be significant effect and I would like to know what mechanism is causing it. This is purely qualitative, of course, but I have repeated the observation several nights running. I would like to get some pictures of it but it's very fiddly with my iPhone because the exit pupil is so tiny.

 

[ronin]

I see that Fathead wants a 'sky pollution Filter'. How does he 'know'? 

May have tried one and so knows how it performs for where he lives, or if the LPR filter is "old" style and he has only low pressure Na lights it should be reasonably effective.

With the evolution of lights it is getting less and less probably that a LPR filter will do a great deal of use. So much is now a "white" light that in effect no filter will help to any extent.

Still trying to think out the sun set spectrum, would expect a reduced blue+green, which there may be but not mentioned.

[sophiecentaur]

I looked at stuff on absorption by pollutants but could find nothing to account for those definite bands. Weird.

[Geof James]

On 10/3/2016 at 23:35, Fathead said:

Hi I have been looking for a used skywatcher light pollution filter (LPR) does anyone know a cheap place to get one looked on the usual places I did find one for a £5 but got no response from the seller.

 

Many Thanks

 

[Geof James]

Hello...........This is Geof James "speaking", from Queensland, Australia. Regarding LP filters. I have a 1.25 inch "Sky Watcher" telescope (120 mm Objective lens, with a 600 mm focal length), and over the years I have found that the majority of filters for different applications, made by the astronomy equipment manufacturers, are good. I have use such LP filters, from time-to-time. Be that as it may, but I am not wholly convinced that they eliminate all the background light, especially in built-up areas. However, my night-skies here in Queensland, may have less light-pollution than yours in suburban UK.

My LP Filter is manufactured by "Orion". I have had a this a number of years. I have other "coloured" filters for specific uses, which are good when used on the appropriate object. I guess that I seriously look after all my astronomical equipment, which keeps everything in excellent condition.

My opinion is that an astronomer's real enemy is dust, and I do take all the required precautions to eliminate this, and especially from all my "optics".

Personally, I would not buy any second-hand optical equipment, unless I knew the person selling such, kept their equipment in first class condition. The price that you have mentioned, being  £5  (the equivalent of Australia's $10), does to me, sound very cheap. In fact too cheap, but that is just my opinion.

Over the years, I have seen other second-hand filters for sale, and these were somewhat scratched which I suspected that they had been wrongly cleaned and /or dropped. Consequently, such second-hand filters as these would not 'deliver' exactly what they were supposed to; leaving the purchaser rather disappointed and also out-of-pocket; buying something that was to be the proverbial "lemon".

I honestly believe that it would be in your best interests to purchase an new LP filter (or any other astronomical equipment), from a reputable "optics" business. It really is worth the "saving" and "waiting", to obtain something worthwhile; which if properly cared for after purchase, would last you a lifetime. Furthermore, optical manufactures do give a guarantee on their sold equipment; which maybe a second-hand dealer would not do. 

I trust that my comments have been of some assistance to you. Sincerely........Geof

[Cosmic Geoff]

I thought I'd try some practical science, so found a diffraction grating and checked the local street lights with it. I just have to hold it up and see an image of the lamp spread out into a low-res spectrum. 

White lights give a continuous spectrum. Pale yellow lights give a continuous spectrum with a bright orange line. Orange lights give an orange line only.

Indoors, a white LED lamp gives a continuous spectrum. So do fluorescent tubes.  Energy-saving bulbs give a discontinuous spectrum of around 6  emission lines spread across the spectrum.

Of all these, only the orange (sodium) lights look like they could be screened out by any sort of filter.

[Dave S]

58 minutes ago, Cosmic Geoff said:

I thought I'd try some practical science, so found a diffraction grating and checked the local street lights with it. I just have to hold it up and see an image of the lamp spread out into a low-res spectrum. 

White lights give a continuous spectrum. Pale yellow lights give a continuous spectrum with a bright orange line. Orange lights give an orange line only.

Indoors, a white LED lamp gives a continuous spectrum. So do fluorescent tubes.  Energy-saving bulbs give a discontinuous spectrum of around 6  emission lines spread across the spectrum.

Of all these, only the orange (sodium) lights look like they could be screened out by any sort of filter.

Sadly Geoff is right, as the councils replace streetlighting with other types than sodium then LP filters lose their effectiveness as they can only really deal with sodium and sometimes mercury lighting.

Dave...