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Light Pollution Filters for Luminance (broadband targets)

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#1 pbkoden

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Posted 10 April 2017 - 11:37 AM

Jon Rista posted a comment in another thread that I had to question (I hope he chimes in here). To capture faint broadband details, you are better off with a Lum filter than a Light Pollution Filter.

 

I have over the past year almost exclusively used a light pollution filter for my luminance frames when shooting LRGB. My filter is a Hutech IDAS LPS P2. My skies are are in the mid 18's on my SQM-L meter and this filter allows me to image 5 minute subs without blowing out the background. I only use the filter for galaxies and reflection nebula, I do not use the filter when imaging emission nebula where I instead switch to narrowband filters.

 

Am I handicapping myself on pulling out faint data? I know that I am filtering out some signal, but I would assume that I'm filtering out a much larger portion of light pollution and in the end would come out statistically ahead.

 

 

 

 



#2 Jon Rista

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Posted 10 April 2017 - 12:18 PM

For a galaxy, you are probably fine. The comment I made in the other thread was in regards to capturing much fainter details, IFN. There have been a number of discussions recently about whether an LP filter is actually better than an L filter. There are two potential issues with the LP filter.

 

First is softening. I've noticed this myself, and have been seeing examples from some other individuals recently, that IDAS LPS filters can actually introduce a lot of blur. I spent months trying to get sharp results with my AT8RC, and was using an LPS almost the whole time. The moment I removed the LPS, and just went with bare imaging, things improved. I didn't make the connection at first, however I did some testing and the LPS filter was definitely adding a non-trivial amount of blur. My FWHMs grew by around 70% or so. These results were recently corroborated by Ken S., and I've noticed some similar issues with some other people's data. For high resolution objects, because of this blurring, you might not want to use an LPS filter.

 

The other issue is whether the difference between the amount of light the LP filter blocks, and the total light the L filter acquires, is meaningful enough to hurt your SNR on faint details. While the LP filter may allow you to expose for longer, it is achieving that by blocking out some fairly significant notches in the bandpass. You may acquire more photons for some wavelengths, while others are blocked out entirely. If you did not block out those additional bands at all, what's the difference? Again, there seems to be some evidence that the differences are minor at best, if not even in favor of the L filter (probably depends on where you live and what you are imaging). For ultra faint details like IFN, the losses with the LP filter could mean you don't get much signal at all. Even if you stack, if you aren't picking up enough photons in each frame to eventually grow your signal larger than the noise, then the LP filter isn't going to help you. An L filter won't block anything...so it will definitely pick up those IFN photons. As long as you are picking them up, then it's ultimately a matter of stacking enough frames to build up that signal enough to overcome the noise. 

 

The brightness of your skies is still going to play a role in whether you can pick up something as faint as IFN at all, with either filter. If you are in a white zone, I wouldn't even bother trying. If you are in a red zone, it's probably still not worth trying. An orange zone or darker, and IFN could be within your reach...however, I would be very careful about filtering out any light. You need every IFN photon you can get, because it's a game of compounding that signal and making it stronger than the noise. Noise grows quadratically while signal grows linearly. As long as you aren't blocking those photons, it's just a numbers game. 


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#3 Nocturnal

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Posted 10 April 2017 - 12:20 PM

If you have LP in the frequencies that your filter rejects then you are doing it correctly. If however your LP is broadband itself (ie. caused by incandescent bulbs and LEDs, CFLs) then there isn't much you can do and an IDAS doesn't help much. Since galaxies often have emission nebulae you may still benefit a little because the IDAS does not knock down those frequencies. I've had an IDAS permanently mounted on my QHY8 for many years. I replaced the IR glass with it. Then again I have Sodium LP to contend with.

 

When it doubt, simply give it a try.



#4 Peter in Reno

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Posted 10 April 2017 - 12:22 PM

Personally, I dislike LP filters because they are harder to balance the colors than using Luminance filter. The stars are usually too bluish and unnatural looking with LP filters. You can use processing software like PixInsight to help control gradients with Luminance filter. I get better results with Luminance filter under fairly heavy LP where I live.

 

Peter


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#5 Nocturnal

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Posted 10 April 2017 - 12:47 PM

Anything you can do to keep unwanted light out of your image you should do. That includes using LP filters for color and the narrowest NB filters you can afford (mine aren't that narrow but I understand the drawbacks). If you have PixInsight you can easily correct color balance using the two dedicated tools for that. I do not understand why anyone would say filter X gives them trouble getting the color 'correct' (subjective anyway). QE for pixels is frequency dependent and if you use a color camera you have to contend with the better SNR of the rather useless G channel anyway. Your raw image is not going to look 'real' to start with. A little more or less of one color channel due to an LP filter is immaterial. You either know how to get the color right or you don't. Once you know how to do it it doesn't matter how skewed the balance is as long as you have enough signal in each channel. For the same reason I do not understand why some (many?) go through the trouble of finding color channel debayer ratios. A waste of time. You'll never get it right so you end up correcting afterwards again. Just debayer 1:1:1 and color correct in post. I had an article that showed how to use PI's color correction tools but alas my website was hacked. Have a look at the ColorCorrection tool Peter. Your stars won't be blue.


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#6 Jon Rista

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Posted 10 April 2017 - 02:03 PM

Anything you can do to keep unwanted light out of your image you should do. That includes using LP filters for color and the narrowest NB filters you can afford (mine aren't that narrow but I understand the drawbacks). If you have PixInsight you can easily correct color balance using the two dedicated tools for that. I do not understand why anyone would say filter X gives them trouble getting the color 'correct' (subjective anyway). QE for pixels is frequency dependent and if you use a color camera you have to contend with the better SNR of the rather useless G channel anyway. Your raw image is not going to look 'real' to start with. A little more or less of one color channel due to an LP filter is immaterial. You either know how to get the color right or you don't. Once you know how to do it it doesn't matter how skewed the balance is as long as you have enough signal in each channel. For the same reason I do not understand why some (many?) go through the trouble of finding color channel debayer ratios. A waste of time. You'll never get it right so you end up correcting afterwards again. Just debayer 1:1:1 and color correct in post. I had an article that showed how to use PI's color correction tools but alas my website was hacked. Have a look at the ColorCorrection tool Peter. Your stars won't be blue.

But what if unwanted light is at the same frequency as WANTED light? With broadband objects (and one could argue that's most of space ;P), blocking unwanted light comes with the consequence of blocking wanted light as well. With something as faint as IFN, that could be a real problem, when you get only a few photons a minute. It's unwise to filter when you aren't starting out with much in the first place.


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#7 Nocturnal

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Posted 10 April 2017 - 02:21 PM

You are only blocking a small percentage of the target light. Assume you are imaging a faint galaxy with only white stars. Now add HP Sodium LP which adds a lot of 589nm light. The flux from this LP can be larger than your galaxy which means the noise component is much larger too. Your image will mostly show LP all over with the galaxy hidden in the noise. That is not to say you can't see it but it is mixed in with the shot noise of the LP. Now consider what happens when you use an idea LP filter (IDAS is pretty close). You lose say 5% of the light from your galaxy but all of a sudden all this LP signal and noise is gone. Your galaxy target stands out clearly. You can shorten your imaging session even after loosing 5% (or more) of the signal of your target. Of course if crucial detail is hidden in *just* that blocked frequency then it is lost. But for white light targets this isn't the case.

 

All this depends on how much LP there is, how bright your target and all that of course. But the key is that LP ruins images. If you could simply 'subtract' LP then don't you think we'd all do that instead of buying expensive IDAS filters? Why do people travel to dark sites to observe and take pictures? Because the noise component of LP can drown out the signal of your target.

 

Using tools like DBE in PixInsight you can attempt to remove the *estimated* signal level of the background but it will leave the random component. I use DBE by necessity but it does not replace keeping out the 'bad' light in the first place. Once those randomly distributed photons start hitting your sensor you can't get rid of them anymore.


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#8 Jon Rista

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Posted 10 April 2017 - 02:32 PM

You are only blocking a small percentage of the target light. Assume you are imaging a faint galaxy with only white stars. Now add HP Sodium LP which adds a lot of 589nm light. The flux from this LP can be larger than your galaxy which means the noise component is much larger too. Your image will mostly show LP all over with the galaxy hidden in the noise. That is not to say you can't see it but it is mixed in with the shot noise of the LP. Now consider what happens when you use an idea LP filter (IDAS is pretty close). You lose say 5% of the light from your galaxy but all of a sudden all this LP signal and noise is gone. Your galaxy target stands out clearly. You can shorten your imaging session even after loosing 5% (or more) of the signal of your target. Of course if crucial detail is hidden in *just* that blocked frequency then it is lost. But for white light targets this isn't the case.

 

All this depends on how much LP there is, how bright your target and all that of course. But the key is that LP ruins images. If you could simply 'subtract' LP then don't you think we'd all do that instead of buying expensive IDAS filters? Why do people travel to dark sites to observe and take pictures? Because the noise component of LP can drown out the signal of your target.

 

Using tools like DBE in PixInsight you can attempt to remove the *estimated* signal level of the background but it will leave the random component. I use DBE by necessity but it does not replace keeping out the 'bad' light in the first place. Once those randomly distributed photons start hitting your sensor you can't get rid of them anymore.

But I'm not talking about a galaxy. I'm talking about something that could easily be 50x fainter than a galaxy. 


Edited by Jon Rista, 10 April 2017 - 02:32 PM.


#9 Nocturnal

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Posted 10 April 2017 - 02:54 PM

The dimmer the target the more important it is to filter out LP. I hoped that was clear from my explanation but I guess I wasn't clear, sorry. If you truly think that you can do better capturing extremely dim targets with LP then by all means shine a light in your scope while capturing. If you think that is a ridiculous suggestion (it is) then I don't understand why you would not block LP if you can. LP is bad. Blocking it is good. The only time we don't care much about LP is when the target brightness is orders of magnitude greater. This is true for planetary and lunar imaging for example. It is all about how the signal levels compare.


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#10 Jon Rista

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Posted 10 April 2017 - 03:03 PM

The dimmer the target the more important it is to filter out LP. I hoped that was clear from my explanation but I guess I wasn't clear, sorry. If you truly think that you can do better capturing extremely dim targets with LP then by all means shine a light in your scope while capturing. If you think that is a ridiculous suggestion (it is) then I don't understand why you would not block LP if you can. LP is bad. Blocking it is good. The only time we don't care much about LP is when the target brightness is orders of magnitude greater. This is true for planetary and lunar imaging for example. It is all about how the signal levels compare.

I understand what you are saying. I think it's that you don't quite understand my argument. What I am saying is, with a faint object, an LP filter will block many of the very RARE and IMPORTANT photons, thus DIMINISHING your signal, not improving it. Whereas an L filter will not...it'll acquire all those photons that are being blocked.

 

The simple fact of the matter is, these days, LP IS broadband. There are so many CFL and LED lights in play, both within neighborhoods as well as in cities and by municipalities, not to mention car headlights and all the other forms of broadband emission, and even for that matter high pressure sodium and mercury vapor lighting (FTR, an IDAS LPS-D1/P2 filter only blocks the primary narrow band emissions from LOW pressure sodium and mercury lighting...high pressure lamps emit significantly more light across a significantly greater number of emission bands, and most of them are not blocked). The difference between using an LP filter vs. an L filter these days is diminishing. When it comes to a DSO that may only have a few photons per minute emission level, blocking any of those photons is a bad idea. At least, that's my argument. I think there is some data to back it up, although nothing that was specifically done to officially test it. 


Edited by Jon Rista, 10 April 2017 - 03:13 PM.

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#11 Nocturnal

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Posted 10 April 2017 - 04:58 PM

"What I am saying is, with a faint object, an LP filter will block many of the very RARE and IMPORTANT photons, thus DIMINISHING your signal, not improving it. Whereas an L filter will not...it'll acquire all those photons that are being blocked."

 

And that is the core of your misunderstanding. A good LP filter only blocks a small percentage of the target light while blocking an important fraction of the LP. You can not image a target if it is drowned out by other photons. When you capture a number of photons for a target some percentage is LP, the rest is target. If the fraction leans heavily towards LP (such as when imaging a dim target like you are describing) then it is hard to see your target. Because an LP filter comparatively filters out more LP photons than target photons the fraction of the photons for the target is increased even if the total photon count is reduced. This makes your target easier to see. If you want to add well size into the mix then the attenuation of undesired LP allows you to image longer (at the same gain/offset) and capture more target photons. You'll say this is less important with short exposure CMOS imaging but the point is the same. No matter the camera type you have a certain target dynamic range that is ideal. Say you are looking for a max of 750 with a 1600. An LP filter will allow you to capture more target photons in the same 750 because the LP is blocked.

 

So yes, it is worth blocking a small percentage of your target photons if that goes together with blocking a lot of LP. Of course you'd never use a neutral density filter but an LP filter is not that. Its selectivity makes your argument void. If you have supporting math or data I'd be happy to take a look at it.

 

All that said not all LP filters are created equal. What makes the IDAS so unique is its several narrow gaps that block LP emission lines. And indeed, it is ineffective if LP is broadband. You are essentially screwed in that case. I have a bunch of broadband LP but also Na. One of the reasons I bought the mono 1600 with NB filters. Have a look at the transmission plot for the IDAS if you haven't seen it yet: http://www.sciencece...das/filtplt.htm


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#12 Jon Rista

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Posted 10 April 2017 - 05:04 PM

This is the IDAS LPS-D1 bandpass chart: http://starizona.com...lps_d1_plot.jpg

 

There is quite a lot of light blocked by that filter. Each notch is about 20-40 nanometers wide FWHM, resulting in about 150-160nm of the visible spectrum being BLOCKED. The visible spectrum spans from ~380nm to ~700nm. That means the full spectrum is only 320nm wide, which means even an IDAS LPS-D1 by FWHM of its notches, is still blocking half the spectrum. 

 

Personally, I think blocking half the spectrum is just a bit more than "a small percentage", and certainly not worth tossing if your chasing a few photons a minute. But, maybe that's just me. shrug.gif


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#13 pbkoden

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Posted 10 April 2017 - 06:43 PM

I appreciate the discussion, and didn't mean to bring up such a hot topic. I've posted this before, but here is an example of what the Hutech filter does for my skies:

 

Xjv7YyE.jpg

 

The mean ADU in the image drops from 7950 to 2950 at the same exposure setting. So it's cutting out about 63% of my skyglow. So my glow isn't pure low pressure sodium, but it still is well attenuated by the filter. This was trying to grab very faint details from NGC 2366 and it worked out okay I believe.



#14 Jon Rista

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Posted 10 April 2017 - 07:02 PM

Phil, now run dnaLinearFit on all of those files (register them to one as referece), then compare. The L filter is certainly going to be brighter, thats the skyfog offset. The real question is, for your system, how much is the LP filter benefiting you. Also note that for faint details, how well the bright core of a galaxy performs with the LP filter probably won't be as helpful as how well the fainter outer halo details show up (as IFN will usually be even fainter than that.)

 

I am also noting, again, that the LPS filter image is much softer. That could be just poor focus or bad seeing, but having seen this increase in blur with the LPS filter numerous times now, it's only solidifying the evidence that the IDAS LPS filters introduce some non-trivial blur.


Edited by Jon Rista, 10 April 2017 - 07:04 PM.


#15 pbkoden

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Posted 10 April 2017 - 07:14 PM

Jon, I purged those source files recently (only keeping the good calibrated subs), but I will try and do some test images when the moon and clouds calm down some. Is there any negative to mixing LPS and L frames in an integration? I can run a couple hours of each and integrate. That should provide a definitive answer at least in my case.

 

Another benefit to the LPS is the ability to integrate long periods of time without data overload. When breaking up 10+ hours of luminance, the difference between 5 minute subs and 2 minute subs on data set size is very noticeable. But.. if I will get better results on faint data with a straight luminance filter, I want to know.



#16 Jon Rista

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Posted 10 April 2017 - 10:10 PM

From a signal standpoint, I don't imagine there would be any harm from combining LPS and L data. However, from a resolution standpoint, I believe there would be some fairly significant differences, as demonstrated by your post above. Whether the loss in resolution and detail is something you are willing to accept is something only you could decide. Personally, I stopped using my LPS filters some time ago because of the blurring issue. Especially for galaxies, it's already tough enough to get decent resolution on them.

 

One question I do have for you is...are your 2 minute subs clipped? If they are, by how much? What is the difference between say a 2 minute and a 4 minute L sub? Or even a 5 minute sub? It may be that you could expose your L subs longer, so long as you are not clipping, or so long as any clipping is minor and acceptable to you. My ASI1600 only needs about 10-15 seconds to get read noise-swamping signal, however I usually expose for 60 seconds. I clip maybe 5-10 stars, lightly. I could probably go for 120 second subs, however I am really after as much resolution as I can get, so the 60 second subs strike a nice balance point (my issue is more wind than seeing though). Anyway...it's often a matter of balancing the factors that matter, and figuring out which matter most. 

 

There is also still that key factor to consider...the LPS is blocking about 50% of the visible spectrum. That's certainly better than the 75% or around there that a CLS or V4 would block, but it's still HALF the visible spectrum. Signal grows faster than noise. Your L filter will certainly pick up more LP than the LPS will, however the noise is growing slower and slower while the signal grows at the same rate. If you acquire a 100e- skyfog signal with the LPS, and 25e- object signal, your shot noise would be 11.2e-. However, if you acquired 200e- skyfog signal with the L filter and 50e- object signal (same exposure time, twice the spectrum), your shot noise would be 15.8e-. In terms of SNR (ignoring read noise for the moment), the LPS would be 2.23:1, while the L would be 3.16:1. Lets say your LP is even worse than that, and you end up with 300e- skyfog signal and 50e- object signal with the L filter. Your SNR with the L filter would still be better, at 2.67:1. What about a 400e- LP signal? Your SNR with the L filter would STILL be highe than with the LPS filter, at 2.36:1.

 

Note here that I have not changed the object signal...it's still 50e-. I'm only changing skyfog. So, this is assuming you expose for the same length of time with both filters. If your LP isn't that bad, then the alternative tradeoff is time. You could likely get away with shorter exposures, and still have the same SNR with the L filter, meaning you wouldn't necessarily need to stack that many more subs with the L filter than you were with the LP filter. I would strongly encourage you to do a proper linear fit of your test data and see how things look, and how things measure. You could very well find that the SNR with your L filter is at least as good, if not better, than the LPS filter. 

 

FTR, Things don't change much if you add in read noise, even 8e-, since a 400e- skyfog signal will totally swamp it. With 400e- skyfog, 50e- object signal and 8e- RMS read noise, your final L filter SNR would be 2.17:1. However the LPS filter would lose some SNR as well. wink.gif


Edited by Jon Rista, 10 April 2017 - 10:12 PM.

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#17 pbkoden

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Posted 11 April 2017 - 12:14 PM

Uggghhh.. Jon.. now that you pointed out the difference in sharpness, I will never be able to un-see it. Crap.. looks like I need to do some L filter imaging. I never would have thought that the "go-to" LPS filter would have that kind of effect on sharpness.

 

Even my LPS images clip on the brighter stars and as long as it's not too bad it's easy to work with in processing. My scope is f/4, so it doesn't take much. I assume I can mostly ignore the skyfog level and only worry about clipped stars? Or should I plan to keep my skyfog below a certain ADU value?



#18 Jon Rista

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Posted 11 April 2017 - 03:03 PM

Sorry. ;P But you image galaxies...so, resolution counts! I too was surprised when I finally learned that it was the filter blurring the crap out of my data. :( But, in the end, I think I've learned that an L filter is better anyway. I seem to get better SNR with the L filter DESPITE the LP, and I've explained above why...the underlying mechanics of SNR simply boil down to: Signal grows faster than noise! 

 

With your camera, I would figure out how long it takes to clip your stars. Once you know what exposure is required to do that, then check out your background signal. If you are swamping the read noise, then you are G2G. If not...then you'll need to figure out what you want to do. More exposures or longer exposures...those are your options. 



#19 jgraham

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Posted 11 April 2017 - 05:15 PM

I have done it both ways. I settled on using a Hutech IDAS LPS2 as a pre-filter in the nosepiece of my filter wheel. I liked the improved contrast and it toned down the red response beyond H-alpha that made color balancing my LRGB images much easier. The only time I removed it was when I was doing narrow band imaging (wasn't needed) or photometry (to keep from clipping my Johnson V filter.)


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#20 Jon Rista

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Posted 11 April 2017 - 07:20 PM

Xjv7YyE.jpg

I wanted to note something here. While I have not done an actual analysis of these frames so I have no concrete numbers...looking at the comparison of the two bottom versions, not only is the L filter much sharper...but it appears to have captured all the same signal as well. However, overall, it is obvious that the L image is much less noisy in the background. I know the lPS filter image looks more contrasty, and also appears to be "more deeply exposed"...however, I think that is an illusion. I think part of the reason the LPS image looks more deeply exposed is that the object is blurred so much, areas that appear fainter in the L image are just smeared about more. Additionally, the more contrasty noise of the LPS image means there is more noise!! 

 

At the very least, I think these images are comparable from a signal standpoint, and it appears to me as though the L image has better SNR. Actual measurements would certainly be needed to know for sure...however, what is AP but a visual art? Stack a few of these frames, and I think the L version would prove it's value. ;) 



#21 pbkoden

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Posted 11 April 2017 - 07:40 PM

Next decent night (looks like maybe 5-6 days out) I will run the night with Lum instead of LPS. I have a night of LPS on NGC 5906, I'll do a night of Lum and compare the two.

 

I'm also working on any light leaks and reflections, so hopefully my subs will overall be better looking. Thanks again for the help Jon.



#22 Jon Rista

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Posted 11 April 2017 - 08:13 PM

If you can acquire data with both filters from the same night, that would be most optimal. Bummer about comparing data across nights is you then have to guesstimate the impact of differing atmospheric/environmental factors, which can often be large enough to greatly skew the results. For an ideal comparison, interleaving small sets of L and LP filter data throughout a period of time would be ideal. That would distribute any changes in the atmosphere across both filters, so we could get more apples to apples comparisons of individual subs as well as the integrations. 



#23 freestar8n

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Posted 12 April 2017 - 12:38 AM

I have imaged from different locations around the world and they tend always to be light polluted - with pollution of different types. Right now I am about 15 meters from a bright sodium vapor light and across from it is a mercury vapor lamp. They are so bright you can read fine newspaper print - and the windows need heavy drapes on them. For that reason I recently ordered a 2" IDAS filter of the most recent kind - and I will be doing some tests with it.

I have a 1.25" IDAS filter of an earlier type and my impression was that it did help for hyperstar imaging - but it was too small.

I never noticed a difference in sharpness when using them - and I would be surprised if there is one. But I'll check to be sure.

I think that as long as there is no loss of sharpness - and as long as the dominant light pollution is in the narrow bands expected - there is good reason it should help with depth on both emission nebulae and continuous sources like galaxies.

But yes - if the light pollution really is more continuum from LED sources - there is reason to think they would not help - even if they don't reduce sharpness.

Frank
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#24 jhayes_tucson

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Posted 12 April 2017 - 09:26 AM

I used to use an IDAS filter with Hyperstar and for a lot of my other imaging with my C14 but I didn't like the results.  I found that it screwed up the color balance a little and that it decreased the signal a noticeable amount.  I didn't study it in any detail before abandoning the filter.   In my case, the light sky glow wasn't enough to justify the filter so my experience probably may not be relevant for those operating under a brighter sky.

 

John



#25 Jon Rista

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Posted 12 April 2017 - 03:47 PM

I have imaged from different locations around the world and they tend always to be light polluted - with pollution of different types. Right now I am about 15 meters from a bright sodium vapor light and across from it is a mercury vapor lamp. They are so bright you can read fine newspaper print - and the windows need heavy drapes on them. For that reason I recently ordered a 2" IDAS filter of the most recent kind - and I will be doing some tests with it.

I have a 1.25" IDAS filter of an earlier type and my impression was that it did help for hyperstar imaging - but it was too small.

I never noticed a difference in sharpness when using them - and I would be surprised if there is one. But I'll check to be sure.

I think that as long as there is no loss of sharpness - and as long as the dominant light pollution is in the narrow bands expected - there is good reason it should help with depth on both emission nebulae and continuous sources like galaxies.

But yes - if the light pollution really is more continuum from LED sources - there is reason to think they would not help - even if they don't reduce sharpness.

Frank

Frank, if you are using hyperstar, my guess is you probably won't run into much in the way of resolution issues. Your image scale would be pretty small, and would likely swallow the issue.

 

I have several IDAS LPS filters, and all of them blur. I've seen this blurring from Paul now, from a couple of people in the BII forum, and also in some data I've processed for people from PMs. I used to think people were just not focusing well, however I am starting to see mounting evidence, on top of my own experiences with these filters (BTW, I image at ~1.3"/px), that the blurring is not just misfocus...it seems to be inherent to the filters. 

 

You might be able to see the issue if you imaged with your SCT at f/10. It would be interesting to see if you could corroborate the issue. I've stopped using all my IDAS filters entirely as a result of it. I was already losing interest in them before, as much like John, I feel they block too much light (based on the bandpass charts for the IDAS LPS-D1 and -P2, these filters STILL block about 150-160nm of the 320nm wide visible spectrum, which is 50%!!), that loss of light definitely affects color (seems to make things too blue shifted, and calibrating that out is quite difficult), and the loss of light actually makes them perform worse than a similarly-exposed L filter. Even with stacking of two frames, the math and my own experiments indicate that the L is at least as good, since it acquires about twice as much or more signal per unit time as the LPS filters, so the added read noise or shot noise from two frames isn't enough to make it perform much worse. 




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