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Whippy

Light pollution filter review part 1

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If there's anything that causes more discussion on astro forums other than the lack of clear skies, it's light pollution. Britain is apparently the second most light polluted country in Europe. Only Holland is worse so light pollution filters are a popular accessory in most stargazers' toolkits. Question is which one? There are quite a few filters from different manufacturers on the market today and I've had the opportunity to review three different models to see what difference (if any) these can make to observing and imaging. I live in a town with a population of about 30,000 and my skies aren’t the worst but they’re certainly not the best either. My garden backs onto other gardens so whilst I don’t have any direct street lamps to contend with, I do on an average night have sky glow I can visually see up to roughly 40 degrees or so and on bad nights can be all over.

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The Baader Neodymium filter is by far the cheapest out of three and not only claims to cut down light pollution but can help on certain objects like the Great Red Spot on Jupiter and details on the Moon. Unfortunately I am unable to view Jupiter to clarify the GRB claim as it doesn't clear my house and it didn't seem to make any real difference to me on the Moon, not a great start! On DSO observing it did darken the sky somewhat but while objects like globular clusters such as M13 and M92 were almost resolvable to the core with no filter in a C8N, the Neodymium reduced this to just the outer core stars and the amount of stars outside of the core was lessened too. On open clusters such as NGC 457, the effect was the same. I managed to see approx. 50 stars without a filter but only 35-40 with the Neodymium. Other faint fuzzies such as galaxies and nebulae also suffered with noticeably less detail.

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The Astronomik CLS filter is part of the high quality Astronomik line of filters and has an excellent reputation, especially amongst imagers. Observing through the CLS I found to be a disappointing experience. Whilst it darkened the sky more than the Neodymium, there was considerably less detail coming through. Globs reduced to fuzzy blobs and open clusters reduced to a handful of the brightest stars. Forget galaxies but I did note slightly better performance on nebulae, especially emission (and planetary) nebs such as M27 and M51 where the drop in detail wasn’t as bad. I also noted that in smaller scopes (6” and 4”), the drop in detail is so great that it renders the CLS unusable.

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The Hutech IDAS LPS (Light Pollution Suppression) filter probably isn't quite as well known in the UK but in the US and Japan it would appear to the weapon of choice for imagers and observers alike. As an observing tool, I would pitch it slightly better than the CLS where it gives pretty much the same background darkness but slightly more detail on clusters and worse than the Neodymium because you still lose more detail. Again, using the IDAS on smaller scopes is pretty much a pointless exercise.

Conclusion.

If you’ve got this far then you’re probably thinking that I’m not impressed with any of the filters visually and you’d be right. I couldn’t really recommend any of them as a ‘magic’ filter that cuts out light pollution whilst allowing the faint detail through. This got me thinking is it actually possible to achieve this? Looking over the transmission graphs of each contender, both the Neodymium and IDAS have a more ‘notched’ graph where they have multiple frequency passes as opposed to the CLS which has two ‘blocks’ of light blocking. This would tally with what I found to be the CLS being the most ‘harsh’ of the three. I can only surmise that our streetlamps, ‘security’ lights and all the other nonsense that makes our hobby more difficult, their light transmission frequencies cut so far into the objects we love to observe that it makes manufacturer’s lives incredibly difficult, if not impossible. Nebulae do fare better probably because so much of their light is at certain frequencies (Hydrogen alpha for example) and are not affected as bad compared to clusters and galaxies. In which case, buy a UHC filter. All these do is let in light emitted by emission nebulae and cut everything else out and are very effective. Just don’t to use one on M13 and expect to be wowed.

This is first part of my review which deals with visual side; Part 2 will be the imaging side of things and will be completed once we get some clear moonless skies!

Tony..

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Nice one Tony. Its good to read a more down to earth descriptive review rather than a shedload of mind boggling statistics and wavelength graphs. Looking forward to your imaging review as I'm stumped as to which to go for at the mo.

Matt

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Cheers Matt. just looked at the metoffice website and it actually looks good for a couple of hours or so tonight so fingers crossed...

Tony..

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I agree with Matt, what you wrote is clear and concise and very helpful.

It looks like you used the newer neodymium - do you think the old one would be better or worse?

Mike

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Mike, I've got the old version and the only difference is that the newer model has IR blocking and the old one doesn't.

Tony..

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Trying to get your head round many of the requirements of this hobby is huge task, with filters just one example.

Thanks for the excellent review. Cleverly, your review leaves us to come to our own conclusion.

I was thinking of buying the Astronomik filter that fits into the Canon cameras.

John

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Mike, I've got the old version and the only difference is that the newer model has IR blocking and the old one doesn't.

Tony..

and only the 1.25" model

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Very helpful indeed.

I'm contemplating a light pollution filter in the near future and by the sound of it these aren't up to much.

More research is required I think.

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Very interesting Tony. I hear so many opinions about these filters and it's nice to read something so objective. Looking forward to the comparison for imaging.

I've used a CLS, IDAS and UHCS with both and ED120 and 12" skywatcher dob on DSOs. I found a benefit on M42 with all filters and with both scopes. The UHCS was particularly good, not surprisingly. On M51 I had the same experience as you with the ED120 but with the 12" newt the sky glow reduction made a big difference and the filters definitely improved the view by way of increasing contrast and revealing aspects of M51 that were previously obscurred. The CLS and IDAS were more effective than the UHCS (no surprise again) I guess fast, large aperture scopes are most likely to benefit from these filters.

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from an imaging perspective, I was impressed with my shrewd buy of a 2" Orion Broadband LPR filter, at Kielder 07. I have used it to image orion through the glow of Glasgow!

A snip at just £50

I have seen images through the IDAS filter, and they seem very similar

Hope this doesnt confuse the mix

Paul

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Thats a very interesting and useful set of notes Tony - thanks for posting them :D

As a visual observer, I've only used 4 types of this kind of filter, 2 broadband ones (Celestron LPR and Orion (USA) Skyglow) which I did not find very, if at all, effective and 2 narrowband ones (Baader UHC-S and Baader OIII) which had more impact although I found the OIII a little too "fierce" for my scopes as it filtered out so much light (as it is designed to of course :D ). So far the Baader UHC-S is the best of the bunch that I've tried but it's only really worth using on nebulae - it makes little difference to galaxies and clusters.

The best way to see more of DSO's is to find darker skies I reckon :D

John

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from an imaging perspective, I was impressed with my shrewd buy of a 2" Orion Broadband LPR filter, at Kielder 07. I have used it to image orion through the glow of Glasgow!

A snip at just £50

I have seen images through the IDAS filter, and they seem very similar

Hope this doesnt confuse the mix

Paul

Apparently the Orion and the IDAS have just about the same response curve, this obviously tallies with your experiences of it as well.

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didnt know that...the orion is broadband LPR, and I thought the IDAS was much more selective. I could be wrong. But i was impressed at the orion

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THANKYOU VERY INFORMATIVE

MY FEARS THAT A SMALL SCOPE WILL GET EFFECTED BY THE LIGHT LOSS

INCURRED BY THESE FILTERS LEADS ME TO AVOID THEM UNTIL ONE DAY MAYBE WHEN I GET A LARGER SCOPE......(MAK 127 AT PRESENT )

THE ADVICE OF FIND DARK SKIES SEEMS THE BEST I'VE HEARD

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THANKYOU VERY INFORMATIVE

MY FEARS THAT A SMALL SCOPE WILL GET EFFECTED BY THE LIGHT LOSS

INCURRED BY THESE FILTERS LEADS ME TO AVOID THEM UNTIL ONE DAY MAYBE WHEN I GET A LARGER SCOPE......(MAK 127 AT PRESENT )

THE ADVICE OF FIND DARK SKIES SEEMS THE BEST I'VE HEARD

Most of my viewing with the Baader UHC-S filter has been with scopes with less aperture than yours and it's still quite effective. I've managed to see parts of the Veil Nebula with an 80mm scope with it - the nebula could not be seen at all without the filter.

John

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Yeah I have a UHC-S and it's a good filter to have for viewing. Really brought out the fine filaments in Orions Neb

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Tony I thought you might be interested in this:

http://www.backyard-astro.com/equipment/filters/lumicon.html

The person reviewed Lumicons LPR Filters and found that the exit pupil of the EP you use under LP or dark skies, makes a difference to the result too.

If you were aware of this then just ignore me.

Good balanced review by the way.

Regards

Andrew

Andrew, it's an interesting read, thanks for that :D.

Tony..

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A really good read, thanks. I have the Baader OIII and Neo filters. I dont find much differance with the Neo filter and with my C8N the OIII seems to block to much light, but that is only my opinion.

I agree that to make the most of DSO's we really need dark skies.

Gary

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Great review Tony,

I've had a neo filter for a while.....You're right about the clusters and galaxies, couldn't make out cores etc.

However, I felt it did add a certain 'crispness' to the views of lunar crater rims.

I also couldn't see a lot of extra detail on Mars or Saturn when they were around. I also can't use it for Jupiter because it's behind my house at the mo.

Stef

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Yeah The Neo filter is a great planetary filter, really seems to bring out the detail.

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Tony, did you find the filters were of varying effectiveness according to the type of light pollution? My Neodymium works quite well looking at objects to the north but not the east. I've assumed that's because to my north most of the streetlights are the old Sodium ones whereas to the east there's a lot more 'modern' white lights.

James

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James, my surrounding area and most probably the whole town where I live have the same type of streetlamp. Those older (sodium?) ones that glow orange/pink as it starts to get dark. My LP is worse to the West as that's the direction of the high street and TBH I didn't notice any noticeable difference.

Tony..

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