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LP filter for a small scope under urban skies


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Hello all

I was recently in Bortle 5 skies with a small 70mm Celestron Travelscope, and managed to pick out the Ring nebula with averted vision at 17x which I was pretty pleased with. Fast forward to me being back home in the city under Bortle 7/8 skies, and even with a 100mm scope with far superior optics and a variety of high quality EPs to choose between, the job was much harder. I also did a "direct" comparison by choosing an EP that gave me 17x and about the same TFOV, and the background sky brightness was terrible. 

Having a direct comparison within just a few days of each other highlights (ha ha) just how bad my LP is and I'm a bit tired of it. 

Moving house is not currently an option, so I'm seriously looking again at high quality LP filters. What are my options? 

 

Criteria are:

1. Visual only

2. Primarily for small/medium fracs (73mm F6, 102mm F7), 2" fitting

3. Primarily used under urban skies, so LP is pretty bad

4. Should be useful for general observing, all types of targets

 

I should mention I do have Astronomik and Oiii and UHC filters and I have read some conflicting information that suggests the UHC can be helpful for all targets, not just nebulae, but I've only used it for nebulae and in limited tests found it blocks light from stars, clusters and galaxies which is kind of what I expected unless I am getting confused. 

Astronomik seems to do two LP type filters, the CLS and CLS-CCD both shown in this graph - I presume the CCD one is literally designed for CCDs and not human eyes. 

Astronomik-Filters-2-CLS-CCD-filter.jpg.87f2e1fef637cc84f330709e9d5dd05d.jpg

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Not sure that light pollution filters work well with new led lighting that most cities now seem to have. They were primarily made for the older yellow lighted (sodium) street lamp. I tend to use a Baader contrast booster or semi apo filter on my ‘fracs which helps pull out detail a little more on things.

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I'll have to look into what the street lights are but I imagine that there are still a lot of sodium and Mercury lights in Berlin. Interesting side note, Chris Hadfield took a photo from ISS about ten years ago which actually showed the division of east/west Berlin, because the light technology was different in each part. They may of course have been replaced by now. 

"Daniela Augenstine, of the city's street furniture department, says: "In the eastern part there are sodium-vapour lamps with a yellower colour. And in the western parts there are fluorescent lamps – mercury arc lamps and gas lamps – which all produce a whiter colour." The western Federal Republic of Germany long favoured non-sodium lamps on the grounds of cost, maintenance and carbon emissions, she says." 

image.thumb.png.c7daed1fb1fbd5c70ef2e9da4d634ffe.png

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56 minutes ago, Knighty2112 said:

Not sure that light pollution filters work well with new led lighting that most cities now seem to have. They were primarily made for the older yellow lighted (sodium) street lamp. I tend to use a Baader contrast booster or semi apo filter on my ‘fracs which helps pull out detail a little more on things.

Sorry, didnt look at where you where from. My bad! 😉

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You could try an IDAS D3 filter, which is supposedly designed to reduce LED as well as Na emission. How well it actually works I have no idea as I don't need a LP filter here (Bortle 3)

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I have recently upgraded to an Optolong LPro filter for visual use in fairly bad suburban skies and I would say the jury is still out on it. It darkens and therefore improves the sky background, but it also dims the image generally. Where the target is bright enough to be visible with the filter and I am at large exit pupils that's where it seems most likely to have a benefit, but where a target is right on the margins of detectability (which it usually is) or I'm at smaller exit pupils it's less often of benefit. I'm hoping it may turn out to be useful on a selection of particular types of target just like the UHC etc, I don't think it's going to be used as a general purpose filter that gets left in all the time.

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10 hours ago, badhex said:

Astronomik seems to do two LP type filters, the CLS and CLS-CCD both shown in this graph - I presume the CCD one is literally designed for CCDs and not human eyes. 

Astronomik-Filters-2-CLS-CCD-filter.jpg.87f2e1fef637cc84f330709e9d5dd05d.jpg

It looks like the CLS-CCD block IR where the CLS does not.  Since CCD can see IR and humans cannot, the CLS should be fine for visual.  Also, it looks like the CLS will pass the C2 lines at 511nm and 514nm where the CLS-CCD might not pass them quite as well.  This is important for comet hunting.

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16 hours ago, DaveS said:

You could try an IDAS D3 filter, which is supposedly designed to reduce LED as well as Na emission. How well it actually works I have no idea as I don't need a LP filter here (Bortle 3)

Thanks Dave. Very jealous of your Bortle 3!

I'll have a dig about to see if I can find any first hand experience of this filter. 

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16 hours ago, Paz said:

I have recently upgraded to an Optolong LPro filter for visual use in fairly bad suburban skies and I would say the jury is still out on it. It darkens and therefore improves the sky background, but it also dims the image generally. Where the target is bright enough to be visible with the filter and I am at large exit pupils that's where it seems most likely to have a benefit, but where a target is right on the margins of detectability (which it usually is) or I'm at smaller exit pupils it's less often of benefit. I'm hoping it may turn out to be useful on a selection of particular types of target just like the UHC etc, I don't think it's going to be used as a general purpose filter that gets left in all the time.

Thanks Paz. I have seen a bit of stuff on the Optolong L Pro, such as Astrobackyard's video, but I had (perhaps incorrectly) understood it was primarily designed for AP. This is sort of the problem I'm finding with researching filters in general. Almost all videos or reviews tend to be for AP.

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Posted (edited)
10 hours ago, Louis D said:

It looks like the CLS-CCD block IR where the CLS does not.  Since CCD can see IR and humans cannot, the CLS should be fine for visual.  Also, it looks like the CLS will pass the C2 lines at 511nm and 514nm where the CLS-CCD might not pass them quite as well.  This is important for comet hunting.

Thanks Louis. The CLS-CCD blocking IR was my understanding as well from some of the literature, but I'm not super familiar with all ideal/desired wavelengths for visual, so info like this is very useful. Are you able to determine from the graph whether this filter is likely to cause significant image darkening in visual use, or block any other important visual wavelengths? I wonder if there is a full list somewhere of specifically which wavelengths are important for visual. 

Edited by badhex
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This was the bit I found confusing in the description for the CLS:

- Visual observation (dark skies): Good, to reduce light pollution by mercury-vapour lamps (streetlight)
- Visual observation (urban skies): Good, an UHC-E or UHC filter is more suitable

Surely this is the wrong way round? If you're in dark skies why would you need to reduce light pollution from street lights? 

Also, as already discussed doesn't the UHC cut out some wavelengths desirable to general observing? 

Very confusing. What we really need is someone who has both filters who can advise in the real world performance. Not sure if I want to spend €175 to be that person though 😂

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I think because light pollution filters apply such a light touch, that they're actually more effective at increasing contrast in areas with little light pollution.  Rural skies mostly deal with street and yard lights that are more limited in the spectrum they pollute.

In heavily light polluted skies, light pollution filters don't appreciably increase contrast as compared to the more heavy handed UHC filters in my experience.  I think it's because there's so much light pollution across the entire spectrum in urban skies thanks to myriad light sources.

Simply put, you're unlikely to ever see low surface brightness, face on galaxies in Bortle 8/9 skies with any filtration.  However, they might stand out better in Bortle 3/4 skies with a light pollution filter.

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Posted (edited)

Observing from Amsterdam, next to Schiphol Airport, I have found that various filters have not helped and instead made the stars unnatural colors. In theory an OIII filter should help on the right targets, but I prefer the unfiltered views of things I can see.

On the subject of M57, this is quite a bright nebula that should punch through light pollution. It is hard to spot at low powers because of its size, and with a large exit pupil the sky becomes white (at my site) so the nebula becomes unspottable. Observing with an exit pupil of 1-2 mm makes fuzzy objects much more visible for me - the larger scale helps and the sky background is at least a dark grey, which seems to help with contrast with the DSO. And of course the darker sky background makes the view more beautiful. In a city I never use exit pupils over 3 mm.

Obviously magnification makes the DSO fainter too, but the eye seems to apply an S-shaped stretch to the view, and you want the DSO in the middle of your neurological histogram!

Edited by Ags
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Posted (edited)
4 hours ago, Louis D said:

I think because light pollution filters apply such a light touch, that they're actually more effective at increasing contrast in areas with little light pollution.  Rural skies mostly deal with street and yard lights that are more limited in the spectrum they pollute.

In heavily light polluted skies, light pollution filters don't appreciably increase contrast as compared to the more heavy handed UHC filters in my experience.  I think it's because there's so much light pollution across the entire spectrum in urban skies thanks to myriad light sources.

Simply put, you're unlikely to ever see low surface brightness, face on galaxies in Bortle 8/9 skies with any filtration.  However, they might stand out better in Bortle 3/4 skies with a light pollution filter.

Thanks Louis. I guess I'm just trying to find some small improvement wherever I can as most of my observing will be from this location for the foreseeable, but in my heart of hearts I know it might be a fool's errand - obviously the best improvement will always be darker skies. 

Perhaps I'll persevere with the UHC for now and see how I get on. Most people seem to say that Orion see great improvement with a UHC, however I personally found the Oiii to work better in that particular case. 

I could do with a list really of which filter of the two will work best in which targets. 

Edited by badhex
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4 hours ago, Ags said:

Observing from Amsterdam, next to Schiphol Airport, I have found that various filters have not helped and instead made the stars unnatural colors. In theory an OIII filter should help on the right targets, but I prefer the unfiltered views of things I can see.

On the subject of M57, this is quite a bright nebula that should punch through light pollution. It is hard to spot at low powers because of its size, and with a large exit pupil the sky becomes white (at my site) so the nebula becomes unspottable. Observing with an exit pupil of 1-2 mm makes fuzzy objects much more visible for me - the larger scale helps and the sky background is at least a dark grey, which seems to help with contrast with the DSO. And of course the darker sky background makes the view more beautiful. In a city I never use exit pupils over 3 mm.

Obviously magnification makes the DSO fainter too, but the eye seems to apply an S-shaped stretch to the view, and you want the DSO in the middle of your neurological histogram!

Cheers Ags, good tip on exit pupil. I'll have to double check next time I'm out but in general I've found something similar from my location, although it does vary from scope to scope. I can tolerate a bigger exit pupil with my ZS73 presumably due to the lower light gathering power. 

I'm not sure if I've actually ever had two observing sessions within a few days from a dark(er) site then back to urban skies which I think is what has frustrated me, especially because I was using a new EP with a rather cheap scope in the dark sky vs bigger, better quality kit at home

I also suspect that my surprise at Lyra containing so many stars was that I can't see half the damn things from here! 

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15 hours ago, badhex said:

Thanks Louis. I guess I'm just trying to find some small improvement wherever I can as most of my observing will be from this location for the foreseeable, but in my heart of hearts I know it might be a fool's errand - obviously the best improvement will always be darker skies. 

Perhaps I'll persevere with the UHC for now and see how I get on. Most people seem to say that Orion see great improvement with a UHC, however I personally found the Oiii to work better in that particular case. 

I could do with a list really of which filter of the two will work best in which targets. 

David Knisely of the Prairie Astronomy Club has written some nice web pages about nebula and LP filters that you might find useful:

Useful Filters For Viewing Deep-Sky Objects

Filter Performance Comparisons For Some Common Nebulae

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On 11/05/2022 at 10:49, badhex said:

Thanks Paz. I have seen a bit of stuff on the Optolong L Pro, such as Astrobackyard's video, but I had (perhaps incorrectly) understood it was primarily designed for AP. This is sort of the problem I'm finding with researching filters in general. Almost all videos or reviews tend to be for AP.

I agree, many filters are designed for or marketed at photographers. I recently got a 2" Baader IR/UV filter and it seems to have been rebranded for astrophotography which left me wondering for a while if it was ok for visual (I think it is).

When looking at LP filters in the end I was guided more by looking at the spectrums they pass than by whether they were marketed at observers or photographers.

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26 minutes ago, Paz said:

I agree, many filters are designed for or marketed at photographers. I recently got a 2" Baader IR/UV filter and it seems to have been rebranded for astrophotography which left me wondering for a while if it was ok for visual (I think it is).

When looking at LP filters in the end I was guided more by looking at the spectrums they pass than by whether they were marketed at observers or photographers.

Yeah, I guess that's probably where the money is at the moment given the popularity of AP, especially for small rigs in urban locations. 

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Quick update on this. I did quick test last night on the Ring comparing naked eye, UHC and OIII. 

The OIII definitely brought out more detail in this case, at the cost of making everything green and dimming the other stars, as expected. The UHC was also helpful but not quite as much contrast, although it doesn't muck about with the colour and brightness quite so much. 

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I never had much luck with filters but hoods or clothes than can block stray light when at the eyepiece and allow your eyes to get better dark adapted have always improved things for me. So an over sized hoody or just an old t-shirt can work wonders not to mention its a cheap fix. 

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On 19/05/2022 at 21:01, StarryEyed said:

I never had much luck with filters but hoods or clothes than can block stray light when at the eyepiece and allow your eyes to get better dark adapted have always improved things for me. So an over sized hoody or just an old t-shirt can work wonders not to mention its a cheap fix. 

Cheers, yes I do the same whenever possible and it definitely helps for local LP and faster dark adaptation, although sadly it's thebglow from the city causing most of the issues! 

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On 19/05/2022 at 14:01, StarryEyed said:

I never had much luck with filters but hoods or clothes than can block stray light when at the eyepiece and allow your eyes to get better dark adapted have always improved things for me. So an over sized hoody or just an old t-shirt can work wonders not to mention its a cheap fix. 

Only an option for me in the short winter season here.  Putting any covering over my head from April to October leads to profuse sweating even at night for me here.  From June to September, I run a box fan across me to help keep me from sweating at night.  That, and it keeps the mosquitos at bay.

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I didnt realise your in Texas. You probably need a cold room then! I lived in the tropics for a few years and developed a loathing for mosquitoes that persists to this day. More so than sand flies because they live on the beach and I was a couple of miles in land. Ive even brought my kids up who are now young men to kill on sight. Brings me great satisfaction having three men in the house who will even team up to mercilessly hunt one mosquito down. 

Back to Filters, I have considered two opposites a bigger scope with a filter a smaller scope for travel for better skies. But opted for a camera lens and a second hand planetary camera which are quite sensitive and I can keave it out all night throw a BBQ over it during the day. Thats been a lot of fun. But the most fun is hunting things down visually in the London light pollution and whilst at times it appears a bit disappointing I have learnt to appreciate the hopping and glimpses of what I am looking at rather than trying to see more so to speak.

But my favourite astronomical filter is a solar filter! 

 

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7 hours ago, StarryEyed said:

But my favourite astronomical filter is a solar filter! 

When I lived in New York state in the late 80s/early 90s, the haze was so bad most days that I couldn't see the sun even with cloudless skies.  Astronomy of any sort was often a no-go there.  It wasn't until I moved to Texas that I took up astronomy thanks to the clear, dark skies I enjoyed at that time.  Nearing retirement, I'm looking to move further out to more rural surroundings.

I've found that if I can get my Dob on target, I can resolve M22 at high power despite it being completely invisible in the murk at low power.  It's amazing how high power works well together with stellar objects even in bright skies.

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