IDAS LPS-P3 Light Pollution Suppression Filter

[Lee_P]

I recently saw FLO were selling a new light pollution filter: IDAS LPS-P3. I shoot OSC from Bortle 8, and figured this might help for broadband targets! (I use an L-eXtreme for narrowband, which works brilliantly). I already own an older-style IDAS D1, but don't routinely use it as I'm not sure of its effectiveness. Well, with these two filters in my possession, why not have a shootout and see the results?

For this test I used an Askar FRA400 f/5.6 72mm Quintuplet APO Astrograph and ZWO ASI 2600MC-PRO USB 3.0 Cooled Colour Camera. All shots were from Bristol (UK) city centre. 120-second exposures. The target was M81 / M82.

 

First up, I took a single sub using No Filter, D1, and P3. Each sub I edited slightly in the same way using PixInsight, just enough to make as fair a comparison as possible. Then I cropped very close into M82.

Hard to make out much difference! The D1 produces a colour cast that takes a bit of work to even out in post-processing. The P3 retains natural colours well, and perhaps gives slightly better definition than no filter. More testing needed... so next I gathered three hours of data for each filter:

 

Same again, to my eyes: all images very similar, but the D1 giving a slight colour cast.

Time for round 3. When imaging I aim for long integration times, so let's try that here. I'd need to cut one filter out just because there are only so many clear skies. So bye-bye D1! Now it's No Filter versus P3. 10 hours of integration time for each. Who will emerge the victor?

 

These are the stacks right out of PixInsight. The P3 has a bit of a colour cast, and a gradient -- to be expected actually, as the Moon was up more during the P3 test. Let's do some basic edits...

 

They look essentially the same to me. Some very slight differences in the background, but explained by my use of Pixinsight's DBE tool to remove the Moon's gradient. Let's have a tight crop on M81:

 

I really can't see any difference! These tests are leading me to conclude that the sources that make up Bristol's light pollution don't lend themselves to being suppressed by a filter. Maybe I've got lots of LED lights nearby.

 

It feels almost heretical to say, but is there any point in light pollution filters from a city centre? Sure, from towns or areas with particular street lights (e.g. sodium), but from the middle of a big city? It seems to me that long integration times are the only way to claw back some of that signal to noise ratio. This Iris Nebula photo was taken with the same kit as the above tests, and is 20.5 hours of integration time -- no filters at all. 

 

I'm probably going to return the L3, and put the D1 up for sale. Unless anyone can see something I'm missing?!

 

Edited April 26, 2021 by Lee_P

[Seelive]

LED lighting seems to have such a wide bandwidth that I believe anything other than narrow band filters will let most of it through. If you are surrounded by LED lighting I have the impression that any of todays so-called LP filters will just give a minor improvement in the background sky pollution level but at the expense of a reduction in the image colour range.  (Bring back LP sodium lighting, these days it's surely easy to filter out the two D lines whilst leaving the rest of the spectrum intact?)

[Lee_P]

14 hours ago, Seelive said:

LED lighting seems to have such a wide bandwidth that I believe anything other than narrow band filters will let most of it through. If you are surrounded by LED lighting I have the impression that any of todays so-called LP filters will just give a minor improvement in the background sky pollution level but at the expense of a reduction in the image colour range.  (Bring back LP sodium lighting, these days it's surely easy to filter out the two D lines whilst leaving the rest of the spectrum intact?)

My tests seem to support this. I am assuming that it's LED lights swamping my skies though -- I don't know for sure that they're widely used in Bristol, but I wouldn't be surprised.

[Dazzyt66]

17 minutes ago, Lee_P said:

My tests seem to support this. I am assuming that it's LED lights swamping my skies though -- I don't know for sure that they're widely used in Bristol, but I wouldn't be surprised.

I live in Bortle 8 (Doncaster) which is pretty much all LED now plus I live on a new estate which is definitely all LED so I’ve read this with interest - I was looking to maybe get a LP filter at some point, but now I’ve seen comparisons I don’t think they really make enough difference to justify. Great comparison! 😊

[Lee_P]

On 27/04/2021 at 07:28, Dazzyt66 said:

I live in Bortle 8 (Doncaster) which is pretty much all LED now plus I live on a new estate which is definitely all LED so I’ve read this with interest - I was looking to maybe get a LP filter at some point, but now I’ve seen comparisons I don’t think they really make enough difference to justify. Great comparison! 😊

I think we're in the same boat. Maybe there's a filter out there that would help us with broadband targets, but for now I'm happy to just brute-force it with long integration times. I took the 20 hours of data from my tests, added in another four for good luck, and produced this image. It's not great, and a 400mm focal length scope isn't ideal for these targets, but at least we can get something even with our skies.

 

[TerryMcK]

Interesting topic. I use the IDAS D2 which I believe is the predecessor to P3. I bought it primarily for use with my OSC camera as I am surrounded by LED lights in my area which is Bortle 7/8. I put up with the annoying green cast it gives but process the green out. I also have the Altair Astro TriBander which does much of the same blocking around LED wavelengths - that actually is a more of a narrower band filter letting Ha and OIII/Hb through. That produces a blueish cast.

Prior to installing the IDAS D2 filter I seem to recall that I couldn't get much longer than 30 second subs without the sky being washed out but I was using an astro modded DSLR at that point. Now I do 6 minute subs with no issues but I might try it out again without any filters in light of your tests.

 

Here's my M81/M82 mosaic which I have been doing over the last few days.

IDAS D2 in front of the ZWO ASI 183MC with William Optics ZS103 - just over 19 hours of 6 minute subs

Ha added from the mono camera ZWO ASI 183MM - 2 hours of Ha again 6 minute subs

 

 

It would be so much better if we had a bubble of Bortle 1/2 over our observatories

[Dazzyt66]

16 hours ago, Lee_P said:

I think we're in the same boat. Maybe there's a filter out there that would help us with broadband targets, but for now I'm happy to just brute-force it with long integration times. I took the 20 hours of data from my tests, added in another four for good luck, and produced this image. It's not great, and a 400mm focal length scope isn't ideal for these targets, but at least we can get something even with our skies.

 

It even looks like we get similar results - my attempts at same targets with around 4 hours of 30s subs using an unmodded DSLR and no filters...

 

[Magnum]

Hi Lee, Ive also been testing filters and on same target, but my goal was to see which one picks up the faint IFN in the background. So far I've used my standard UV/IR filter, my Idas P2, the Astronomik CLS ccd, though none of these are designed with LED lights in mind.

I see you went for the P3 but they also released a D3 at the same time specifically designed for LED LP. By the specs the new D3 would seam better as like the previous D2 it has a cut out for the brightest LED peak. Maybe you could ask to swap it for the D3? I would like to try one myself but they are too expensive to just buy on a Wim, I know FLO have the 30 day return policy but I only just retired another filter I was testing and its not really fair to them to keep buying things just to test.  Maybe as you've already bought the P3 it would make more sense to see if they would let you exchange for the D3 to add to your existing tests ?

https://www.firstlightoptics.com/idas-filters/idas-lps-d3-light-pollution-filter.html

Lee

P.S. im also a Lee P, P for Phillips what's your P for LOL

[Lee_P]

39 minutes ago, Magnum said:

Hi Lee, Ive also been testing filters and on same target, but my goal was to see which one picks up the faint IFN in the background. So far I've used my standard UV/IR filter, my Idas P2, the Astronomik CLS ccd, though none of these are designed with LED lights in mind.

I see you went for the P3 but they also released a D3 at the same time specifically designed for LED LP. By the specs the new D3 would seam better as like the previous D2 it has a cut out for the brightest LED peak. Maybe you could ask to swap it for the D3? I would like to try one myself but they are too expensive to just buy on a Wim, I know FLO have the 30 day return policy but I only just retired another filter I was testing and its not really fair to them to keep buying things just to test.  Maybe as you've already bought the P3 it would make more sense to see if they would let you exchange for the D3 to add to your existing tests ?

https://www.firstlightoptics.com/idas-filters/idas-lps-d3-light-pollution-filter.html

Lee

P.S. im also a Lee P, P for Phillips what's your P for LOL

Ooh interesting, I didn't know there was a D3. Thanks for letting me know. I'll give it a go and will report back -- might be a while though, given all the cloud that's forecast!

p.s. my P stands for "Pullen" 😁

 

 

[Lee_P]

Following @Magnum's suggestion, I bought an IDAS LPS-D3 to test as well. Obviously this purchase resulted in weeks of cloud (sorry), but over the last two nights I managed to obtain three hours of data to make a comparison with No Filter, IDAS LPS-D1, and IDAS LPS-P3. Drumroll...

 

 

 

 

 

Hmmmm, hard to see much difference. 

I ran the source subs for each stack through PixInsight to get a measure of seeing quality by calculating the average number of stars visible.
No filter: 876
IDAS LPS-D1: 1129
IDAS LPS-P3: 904
IDAS LPS-D3: 787

From that I infer that the D3 had an uphill struggle due to poor sky conditions. (The seeing was noticeably worse when I was out under the stars setting up for the D3 test). Also, I do think that the colours the D3 gives look like the best of the bunch, but it's admittedly hard to tell in the examples above. 

To be honest I'm not sure if the D3 is having a positive impact, or if I'm just making excuses to avoid returning another filter to FLO! What do you all think? 🤔 

[Dazzyt66]

6 minutes ago, Lee_P said:

Following @Magnum's suggestion, I bought an IDAS LPS-D3 to test as well. Obviously this purchase resulted in weeks of cloud (sorry), but over the last two nights I managed to obtain three hours of data to make a comparison with No Filter, IDAS LPS-D1, and IDAS LPS-P3. Drumroll...

 

 

 

 

 

Hmmmm, hard to see much difference. 

I ran the source subs for each stack through PixInsight to get a measure of seeing quality by calculating the average number of stars visible.
No filter: 876
IDAS LPS-D1: 1129
IDAS LPS-P3: 904
IDAS LPS-D3: 787

From that I infer that the D3 had an uphill struggle due to poor sky conditions. (The seeing was noticeably worse when I was out under the stars setting up for the D3 test). Also, I do think that the colours the D3 gives look like the best of the bunch, but it's admittedly hard to tell in the examples above. 

To be honest I'm not sure if the D3 is having a positive impact, or if I'm just making excuses to avoid returning another filter to FLO! What do you all think? 🤔 

Another great comparison test @Lee_PTo me, the difference isn’t worth the cost 🤔

[Magnum]

Wow thanks for being the ginny pig and doing this test Lee, they are so close aren't they! I think No filter looks the worse and Possibly D1 looks to be picking the Ha jets on M82 the best. but as you say the conditions weren't favourable to the D3.

Id be interested in seeing a single sub from each to see which has the least noise in the background like you did at the top of the thread but including the D3.

Maybe you could also upload a single raw fits file for each filter in dropbox or something, so I could try and compare in MaximDL.

Lee

 

[Lee_P]

11 hours ago, Magnum said:

Wow thanks for being the ginny pig and doing this test Lee, they are so close aren't they! I think No filter looks the worse and Possibly D1 looks to be picking the Ha jets on M82 the best. but as you say the conditions weren't favourable to the D3.

Id be interested in seeing a single sub from each to see which has the least noise in the background like you did at the top of the thread but including the D3.

Maybe you could also upload a single raw fits file for each filter in dropbox or something, so I could try and compare in MaximDL.

Lee

 

Yes, sounds good. My processing steps to produce these JPEGs could be smoothing out any differences, so it'd be good if you could run some tests too.

I've uploaded single subs and the three hour stacks here: https://drive.google.com/drive/folders/1sANwEglGFLrTOntqL5okqSLd32x82FBy?usp=sharing

Let us know how you get on!

[Magnum]

I've just had a look at the single subs in both APP and MaximDL. APP has an autostretch like Pix Insight and also a neutralise background checkbox, if I leave that unchecked then the colours vary hugely, but with it checked they all become similar except for the gradients and vignetting differences, the D1 has the least vignetting and gradients, next is the D3, then P3 and finally no filter is the worst. So id say with whatever makes up the majority of your local LP I think the D1 is performing best, however you said that the conditions were worse on the night you used the D3, so maybe you can try that again, I would take a single sub with each filter on the same night just to eliminate the conditions as a variable. I prefer to evaluate filters with just single subs rather than stacks of several hours purely because then they can be taken all within 10 mins and identical conditions.

Edited May 27, 2021 by Magnum

[Lee_P]

3 hours ago, Magnum said:

I've just had a look at the single subs in both APP and MaximDL. APP has an autostretch like Pix Insight and also a neutralise background checkbox, if I leave that unchecked then the colours vary hugely, but with it checked they all become similar except for the gradients and vignetting differences, the D1 has the least vignetting and gradients, next is the D3, then P3 and finally no filter is the worst. So id say with whatever makes up the majority of your local LP I think the D1 is performing best, however you said that the conditions were worse on the night you used the D3, so maybe you can try that again, I would take a single sub with each filter on the same night just to eliminate the conditions as a variable. I prefer to evaluate filters with just single subs rather than stacks of several hours purely because then they can be taken all within 10 mins and identical conditions.

Ah, no can do -- I sold the D1 and returned the P3!

Thinking about gradients, I just checked the Moon phase on the night of each test:

No filter    43%
D1             76%
P3             85%
D3            Not visible

Thanks for checking those subs. I think that the differences could likely be explained due to varying conditions on each night -- your approach of testing them all on the same night would be ideal, but alas isn't possible

Edited May 27, 2021 by Lee_P

[Magnum]

The difference between the D1 and D3 was minimal anyway, The D3 is the latest greatest one for LED pollution which is constantly replacing sodium, so probably the best one to have going forward.

 

Many thanks for doping the tests Lee

[Lee_P]

1 hour ago, Magnum said:

The difference between the D1 and D3 was minimal anyway, The D3 is the latest greatest one for LED pollution which is constantly replacing sodium, so probably the best one to have going forward.

 

Many thanks for doping the tests Lee

That's the question -- is the D3 worth having? I'm not sure it's benefiting the images appreciably enough over no filter to justify its £184 cost. *Maybe* its images look a little bit better if you zoom in a lot and squint 😅

Happy to have run these tests anyway! My main message to anyone else reading this thread wondering about light pollution filters would be that their effectiveness is very much dependent on your local sky conditions. So don't rely on reviews from other people, get hold of one and test it!

[Lee_P]

@Magnum Ok, two subs attached, both taken last night -- should negate any differences due to sky conditions. One IDAS D3-LPS, the other No Filter. Do you think there's an appreciable difference? @vlaiv, it would be great to get your opinion on these too, if you have time 😁

M81_NoFilter_120s.fit

M81_D3_120s.fit

Edited June 2, 2021 by Lee_P

[vlaiv]

4 hours ago, Lee_P said:

Do you think there's an appreciable difference? @vlaiv, it would be great to get your opinion on these too, if you have time 

Well, there is obvious difference between the two - one without filter has much higher average signal level in the sub:

I assume that these are taken successively and that conditions are pretty much the same?

Do you have calibration frames for them? At least darks? That way we can actually estimate SNR difference in some part of the image.

In the mean time I can estimate gradient (not very precise as there is vignetting) in each image and compare them

These are gradients "normalized" one against another - this shows that one without filter is much larger and in comparison, one with filter is almost no gradient at all - it just looks like gray background (but in reality, there is a bit of gradient as there as well)

So first iteration of gradient estimation for filtered version gives about 61ADU of difference between brightest and darkest part of gradient.

While no filter version has 137ADU gradient.

Btw - these are gradients of binned version of the image - that means R, G and B data combined / added together. If you like, I can do comparison of each channel for you, but I'd rather do that on calibrated subs as it makes more sense.

[Lee_P]

38 minutes ago, vlaiv said:

Well, there is obvious difference between the two - one without filter has much higher average signal level in the sub:

I assume that these are taken successively and that conditions are pretty much the same?

Do you have calibration frames for them? At least darks? That way we can actually estimate SNR difference in some part of the image.

In the mean time I can estimate gradient (not very precise as there is vignetting) in each image and compare them

These are gradients "normalized" one against another - this shows that one without filter is much larger and in comparison, one with filter is almost no gradient at all - it just looks like gray background (but in reality, there is a bit of gradient as there as well)

So first iteration of gradient estimation for filtered version gives about 61ADU of difference between brightest and darkest part of gradient.

While no filter version has 137ADU gradient.

Btw - these are gradients of binned version of the image - that means R, G and B data combined / added together. If you like, I can do comparison of each channel for you, but I'd rather do that on calibrated subs as it makes more sense.

Thanks vlaiv! Here are some dark files. To answer your questions: yes they were taken almost successively and with the same conditions; and RGB combined sounds just fine, especially if it's quicker for you.

Dark.rar

[vlaiv]

Ok, I have results for this D3 filter and interestingly enough - it's actually hurting you to use it.

Here are my findings. First background levels. Here filter helps quite a bit.

Without filter, ADU measurement of background values is as follows:

R: 2651.48
G1: 4433.10
G2: 4436.68
B: 2875.33

(I did not debayer - I just split bayer components and measured each - that is why there is G1 and G2 - two green components out of RGGB)

Background values without filter are:

R: 613.77
G1: 2222.72
G2: 2222.75
B: 1290.32

For R, reduction in background levels is x4.32, for green it is about x2 (almost exactly x2), and for blue channel - it is x2.23

There is definitive reduction in background, however, will target signal remain the same to justify filter use? These filter also eat into broad band targets as well.

Here are SNR measurements on sub without filter:

Red channel:

- signal : 173.99
- noise : 102.75

SNR: 1.6933

Green 1:

- signal : 216.14
- noise : 134.17

SNR: 1.6109

Green 2:

- signal : 216.58
- noise : 133.44

SNR: 1.6231

Blue channel:

- signal : 108.16
- noise : 108.24

SNR: 0.9993

I measured small square of 10x10 pixels in M81 between two stars (so I can repeat measurement between two subs) and background noise was measured on a patch 100x100px far away from galaxy.

Now measurement of sub with filtering (D3 filter):

Red channel:

- signal : 68.83
- noise : 49.26

SNR: 1.3973

Green 1:

- signal : 129.07
- noise : 96.04

SNR: 1.3439

Green 2:

- signal : 113.59
- noise : 94.5

SNR: 1.202

Blue:

- signal : 77.46
- noise : 72.29

SNR: 1.0715

There is more variation between G1 and G2 than I would like - maybe due to pixel shift a bit of star signal went into G1 or maybe it is just randomness of data.

In either case - both R and G suffered SNR penalty by using filter. Blue remained the same or improved slightly (it is hard to tell on just a single sub - there could be variance due to randomness of data - more data is really needed to form proper statistics).

In any case - target suffers reduction of about - x2.5 for Red channel, x1.9 for green channel and x1.4 for blue channel, versus x4.32, x2 and x2.23 for background. Not enough to result in SNR improvement - in fact - it results in SNR degradation for this particular target (or for that matter - any target fainter than LP).

Quite surprising for Bortle 8 skies. Maybe it is down to structure of LP.

 

[Alexis]

That's (given the filter) what I'd only expect if a large portion of the light pollution were mercury vapour (which the D3 purposefully ignores). But given the spectrum of M81 and M82, I'm baffled by the loss of the signal's amplitude too, though.

I'm not that suprised the results are so-so given the target: the arms of M81 are faint and yellowish, and even visually (I do use a D3 visually) it's one of the objects that doesn't respond well to any filter.

Edited June 2, 2021 by Alexis

[vlaiv]

4 minutes ago, Alexis said:

That's (given the filter) what I'd only expect if a large portion of the light pollution were mercury vapour (which the D3 purposefully ignores). But given the spectrum of M81 and M82, I'm baffled by the loss of the signal's amplitude too, though.

I'm not that suprised the results are so-so given the target: the arms of M81 are faint and yellowish, and even visually (I do use a D3 visually) it's one of the objects that doesn't respond well to any filter.

Welcome to SGL.

Another factor is that OSC sensor is used. OSC sensors don't have sharp cut offs for their components and that means that D3 gap 550nm and 640nm will affect both green and red colors (even blue to some extent).

Most of light from galaxies is in fact stellar in nature and as such has Planckian / Black body type spectrum that is more or less uniform in visible range (there is just "tilt" that determines if star is reddish, yellow or bluish - depending on temperature).

If we cut into spectrum with filter - we will just clip target as well.

[Alexis]

Thanks for the welcome -- but if you look at the filter specs (and I just looked at it through a pocket spectrometer as well -- that matches) you wouldn't expect a drop of signal of a factor 2.

All that being said, even visually there's a striking difference in how effective it is on different targets. NGC891 -- not much. M81 -- also not much. M101 -- a lot more. M51 -- even more. NGC 253 -- also quite a lot (but then that's down in the soup where I live). Globulars are quite a bit better in my really polluted backyard too.

 

Edited June 2, 2021 by Alexis

[Alexis]

Just now, Alexis said:

Thanks for the welcome -- but if you look at the filter specs (and I just looked at it through a pocket spectrometer as well -- that matches) you wouldn't expect a drop of signal of a factor 2. Even of M81 is indeed trickier than some other galaxies.