Filters etc for light pollution

[StuartT]

I live in town where the sky is a bit rubbish (Bortle 6) and I don't have a car to take my scope out somewhere nice.

Any tips on any techniques to make the best of a bad situation? Maybe filters to get rid of the sodium lamps?

Thanks

Edited March 6, 2021 by StuartT

[Waddensky]

Not really. The light pollution filters available are not very effective against modern LED lighting, and they are not magically turning your skies to Bortle 1 anyway. It's sad, but to avoid light pollution you need to travel.

Another approach is to change your targets. Planets, the Moon, double stars, carbon stars and some open clusters for example withstand light pollution very well and they are lovely in their own right.

[RT65CB-SWL]

Hi @StuartT and welcome to SGL. 

As per @Waddensky says above.

One thing I have noticed, (and maybe others), is that since my local authority, [Wokingham], have changed to LED from sodium street lighting is that the night sky is a little darker and better higher up. You could ask your local authority, (i.e. Oxford City Council or Oxfordshire County Council), to shield the offending streetlights, if you think the light-spill is invading and disrupting your sleep pattern or privacy. You could or may need to submit some non-astro images to highlight your case.  

[Ricochet]

1 hour ago, StuartT said:

I live in town where the sky is a bit rubbish (Bortle 6) and I don't have a car to take my scope out somewhere nice.

Any tips on any techniques to make the best of a bad situation? Maybe filters to get rid of the sodium lamps?

Thanks

It depends on the target:

• Planets - these can be observed no matter the level of light pollution. In fact a bit of light pollution might even be an advantage here as is will help preserve your colour vision. A Baader Neodymium filter can help here, but this mostly to do with increasing contrast between areas on the planets (particularly Jupiter) although it does have a bit of a dip in the spectrum where sodium lights transmit.
• Point sources; double stars, open star clusters and globular clusters that can be resolved into individual stars (i.e. M13). - For point sources the brightness is pretty much only determined by the aperture of your telescope up until you can resolve the Airy disk, while the brightness of extended objects, such as the sky, is proportional to the exit pupil of your telescope/eyepiece combination. This means that you can decrease the brightness of the sky whilst keeping the brightness of the stars almost constant by increasing the magnification. Try to observe these objects with as high a magnification as possible while keeping the object within the field of view, up to the point where the eyepiece focal length equals the f-ratio of your telescope. This equates to a 1mm exit pupil, which is generated by a 10mm eyepiece in your f10 SCT. You may find it beneficial to upgrade to eyepieces with a wider apparent field of view so that you can fit slightly larger objects into the field of view at high magnifications. The ES82° range would work well with your SCT.
• Emission nebulae - The views of emission nebulae can be improved by the use of suitable UHC and OIII filters. These will not perform miracles and you will still find that some objects are invisible, but the right filter on the right object can make a worthwhile difference. As these filters work by blocking light, they dim the image and so you need to pay attention to the exit pupil when using them.As a guide I would suggest that a UHC filter should be used with exit pupils greater than 2mm and a OIII with exit pupils greater than 3mm. I have also found that it is a false economy to buy cheap filters. The worse your light pollution, the narrower you need the bandpass of your filter to be in order to block out more of the light pollution. You may even find that a OIII filter works better for you on some objects where the prevailing opinion is that a UHC filter is better. I would suggest you look at the Astronomik filters, and the new Televue filters, which are made by Astronomik.
• Galaxies - these are broadband, extended objects so there isn't really a lot you can do to counter light pollution. Anything you do to cut the light pollution will cut the emission from the galaxy as well. In the past I have seen a small improvement on some galaxies by using an Astronomik CLS filter, but the only real option is to go somewhere darker. As LEDs become more universal the effect of a CLS will be ever more reduced, so if you see a reasonably priced second hand Astronomik CLS you could give it a go, but I probably wouldn't buy a new one.

Generally speaking there are also a couple of things that you should try to do to counter light pollution on all DSOs. The first is to try to observe the object when it is highest in the sky, and/or in a direction that is less affected by light pollution. The lower the light pollution, the greater the contrast and even this small change could be the difference between seeing an object and not seeing it. Secondly, once you have found an object, you should sit and observe it for a long time whilst darkening your immediate surroundings so that your night vision can improve. I suggest that you get a patch for your non-observing eye, have a large hood or blanket over your head and use eyepiece where the eye cup is tall enough so that your head touches it and blocks outside light. If your chosen eyepieces don't meet this criteria you could make or purchase something to help block out glare. The Spot Bandit has recently come up in discussion on these boards as FLO have started selling the Bino Bandit and could well be worth purchasing.

[StuartT]

thanks SO much everyone for these super helpful answers. Really appreciate it.

[Pete Presland]

Hi welcome to SGL from Pete in Bedfordshire  🙂