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  #1  
Old 21-08-2012, 06:37 PM
Garbz (Chris)
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Anything to fight light pollution?

Hey all. I just experienced my first dark sky the other day, and WOW that made a difference to my typical photography session. I know I won't ever reach that kind of quality from the burbs but I'm wondering if there's anything I can do to help against light pollution from the city?

I live unfortunately directly west of the Port of Brisbane and my only decent view is towards the east, which means many interesting objects are obscured by a glow of sodium vapour lamps. I've seen recommendations to shoot narrowband however I'm working with a DSLR here and one thing I've noticed is an epic lack of Ha response in the sensor so doing that won't work too well. Likewise the UHC may be out of the question for the same reason as it seems to pass only OIII and Hb within the spectral sensitivity of my camera.

Does anyone out there have any ideas? Any examples of how badly a UHC would affect an unmodded DSLR camera's performance? I've seen ranges from "barely" to "don't bother".

Or maybe it's time to move to the country.
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Old 21-08-2012, 07:11 PM
Dennis
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I use the Astronomik CLS (EOS-Clip) filter and I am very impressed with its performance in my Canon 40D. It is simple to clip it in although it could be a bit tedious if you had to insert/remove it for night time then day time shots on a regular basis, I don’t know if was designed or intended for repeated insertions/removals.

Here are 3 shots:
  • Raw straight out of camera, without filter, 60mm lens, 60 secs.
  • Raw straight out of camera, WITH FILTER (17-40mm lens at 17mm), 60 secs.
  • Same image as above but processed to look pretty.
Well worth the investment in my opinion.

Cheers

Dennis
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  #3  
Old 21-08-2012, 07:39 PM
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Screwdriverone (Chris)
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Hi Chris,

I agree with Dennis, the best thing I have bought for my unmodded Canon 1000D is the EOS CLS clip in filter. It's quite easy to remove and re-insert, so dont worry about this at all, however, you CANT use it with EFS lenses as they are too long and bash into it inside the camera body and scratch it to hell (speaking from experience here....)

As you can see in Dennis' test shots, it introduces a blue cast into the image if the white balance is set to auto (or anything else) but this can be fixed by taking a photo with it in of a white piece of paper in broad daylight (no shadows) and then setting this to the CUSTOM white balance in the camera. When you then take night shots, the colours are more realistic (although slight skewed towards red).

I have attached a pic of my latest (and best so far) M20 Trifid using 54 mins of 3 min subs at ISO 800, taken from my light polluted backyard in Kellyville Ridge (Sydney) with the EOS clip in place. In my opinion, it reveals MUCH more nebulosity than before as it cuts down so much of the grey "wash" that is up there due to the light pollution.

It also therefore makes it easier to process the resultant stacks of subs.

Well worth it if you have a Canon camera. If not, I think you can get LP filters for other cameras too, just not as convenient as the EOS clip in one.

Cheers

Chris
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Old 21-08-2012, 09:42 PM
Garbz (Chris)
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Very nice. The Clip in issue won't affect me since Nikons don't have that facility, however getting a 2" threaded one would be the answer.

I was looking at the CLS one after I posted the question. The other one that comes recommended is an LPS-P2 filter which has a very similar spectral response as CLS with one additional peak passing.

Do you have any experience with shooting reflection nebula with the filter in place? Apparently these filters do have quite a bad effect on them, that said the blue part of the triffid is a reflection nebula and seems to have come out quite well in your picture...
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  #5  
Old 21-08-2012, 10:01 PM
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Hi Chris,

No, I dont think the blue reflection nebula is an issue with the CLS or the LPS-P2. I was going to get the Hutech IDAS instead of the EOS clip simply so I could use it on other cams and doesnt have the blue tinge issue, however, I couldnt be bothered waiting for one from OS, so I snapped up a EOS clip at Bintel when I bought some other goodies.

Chris
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Old 22-08-2012, 08:50 PM
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The blue cast from the cls clip is a none issue once you've calibrated your stacked image.

I have both the clip and the 2" version, and they are great.
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  #7  
Old 23-08-2012, 11:37 AM
Garbz (Chris)
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A cast is relatively easy to eliminate for me coming from a long background in digital photography.

The question is, is the resulting cast dimmer than the background lighting that I would get? I would hope the answer is yes otherwise what's the point of the filter right?
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  #8  
Old 23-08-2012, 02:32 PM
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I looked at this issue in the past and my conclusion was the filter doesn't help, as it actually reduces the final S/N.
The most of LP background can be removed by simple processing in DPP.
Have a look at this thread:
http://www.iceinspace.com.au/forum/showthread.php?t=48651&page=2&#37
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Old 24-08-2012, 08:34 PM
Garbz (Chris)
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Simple processing requires a higher signal noise ratio. I.e. more data. That's the problem I have come to realise. My eagle nebula looks much better with only 1 hour of data compared to the 3 hour shot I took in the city.

In theory the filtering should work quite well depending on the lighting source. I'm battling with the port of Brisbane which is lit predominantly by sodium vapour lamps. I understand the filters do nothing for light pollution from stadiums (metal halide floodlights), cars (halogen or vapour discharge), or homes all of which produce mainly broadband light pollution.

So in theory the light pollution filter may actually increase SNR during certain parts of the day (like after 10pm when the people go to bed and the cars disappear).

Can you relate these scenarios to your experience with the filter making not much difference?
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Old 24-08-2012, 09:15 PM
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My conclusion was based on comparison between shots taken (by others) with and without filter. When tweaking images to achieve the same (or similar) colour balance, the noise with filter appeared to be actually higher - so I simply stopped thinking about it as a solution.
Also, sodium lamps have quite a wide spectrum (they are mostly high-pressure models) so filter is not really effective because it cuts off too much of wanted light from celestial objects.
The only real and significant improvement in light polluted areas is possible with narrow-band filters - but I haven't done this (yet).
Other way is simply to do what's possible to do - stellar photometry, for example - with high F-numbers, LP is not such an issue.
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Old 25-08-2012, 07:28 AM
Garbz (Chris)
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bojan does that account for all stellar objects? I understand reflection nebula, stars, etc given they are wideband and as part of that they would be cut in brightness along with the light pollution.

But thinking about it emission nebula should survive the filtering without any darkening at all given they are almost very exlusively in the Ha Hb and OIII bands.

Sorry if I sound like a little kid arguing about something he's never tried, but I'm trying to rationalise the theory with the results you were seeing.
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Old 25-08-2012, 10:54 AM
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All depends on what you want to achieve - pretty pictures, or useful and accurate measurement data.
The ultimate way of eliminating LP is moonless, dark site away from city lights.
However.. this is ideal situation, sometimes impossible to arrange

Stellar objects are very small, almost point like - so spreading their light across wider surface using objective longer focal length (as long as the size of the star image is comparable or smaller than the size of the pixel) will not significantly reduce surface brightness (number of photons per pixel) of the star-like object image.
So, to reach faint stars in LP affected areas, the best strategy is just a bigger telescope. If you compare two OTA's featuring the same F-number, the bigger one will show stars as brighter spots on a similarly LP-illuminated background. And, the additional benefit of this approach is the reduction of exposure times and higher resolution.

Nebulae, being spread across wider area, are affected by F number and aperture just like the LP.
But if their spectral contents are different from those of LP, for them to stand out, theoretically the right filter could be used to remove unwanted spectral content, introduced by LP.
I think there are two issues here:
- filtering affects reflective nebulae the same way it affects LP.
- Emission nebulae, being narrow-banded in their emissions, should survive filtering nicely (and they do), so aggressive narrow band (for example H-alpha) filtering definitely make (hydrogen emission nebulae it this example) stand out nicely, even in full Moonlight.

But, wide-band LP filtering for colour images must be different.. it is very complex in terms of spectral response [to remove most common LP spectrum, which in practice varies from place to place - some cities use high pressure sodium lamps, some other use mercury.. or combination of anything available on totally uncontrolled (in terms of LP) market], and there is also some attenuation present in pass band..
Now, with results I have seen - they were not good (or, better to say, they were not up to my expectations) probably because of the following reasons:
- removing the parts of the spectrum from reaching the sensor caused colour imbalance
- restoring the colour balance to the similar level by stretching the histogram in particular colour channels introduced some additional quantisation noise.
On the other hand, apparently (at least according to my experience) removing the background (LP) on images in DPP had similar effect as using LP filter+restoring colour balance, but at no $cost for filters.
Longer exposures and bigger number of them for stacking will help here to increase the S/N - so histogram stretching and removal of the background produces more eye-pleasing result.
And my standards for image appearance are probably much less demanding.. as long as I can extract some useful photometric data from my images, I am happy.

Last edited by bojan; 25-08-2012 at 02:26 PM.
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  #13  
Old 25-08-2012, 05:11 PM
Garbz (Chris)
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Ok it's starting to all make sense now.

The assumption here is that for the most part I've enjoyed shooting emission nebula which is why I was at odds with your initial comment of having no benefit.

The colour balance thing may actually remain an issue. I was thinking still emission nebula and solve any background problems with a histogram stretch, but the nebula are never in the photos on their own. The stars will have the same colour balance issue so you can't confine the colour correction to only darker background. Hence my guess the noise.

Hmm this will require pondering.
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Old 26-08-2012, 10:50 AM
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I use an unmodded DSLR and find I get far better pics using a light pollution filter. The camera also seems to "wake up" in Ha.

There's a Plethora of filters out there with multi channel band pass for single shot colour, from something like the wide band Baader Moon and skyglow, to the relativity narrow band Astronomic UHC and everything in between like the CLS and Lumicon Deep Sky. The narrower multi-band filters tend to make the colour go a bit funky, but the contrast boost is often worth it.

In my experience no amount of stretching will equal the contrast gained with the correct filter for the job.

Bintel have very useful spectrum graphs along with their Astronomic filters that show what they pass and what they don't. This is also a good page for comparisons. http://www.astrosurf.com/buil/filters/curves.htm

Keep in mind the more narrow filters require longer exposure.

I shoot just about everything now with filters, except comets. Some comets have spectral emissions in the light pollution bands which a filter would attenuate.

M8 and M20 with a Lumicon Deep Sky filter. M42 with some subs using Lumicon Deep Sky and a different stack of subs using a Baader Semi-Apo filter, same for the Horsehead.

M42, because it's so bright shoots pretty well without any filter, but IC434 really benefits.
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  #15  
Old 27-08-2012, 09:58 PM
Garbz (Chris)
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Those are very nice results Kevin. What filter did you use? It looks like your reflection nebulas are keeping up quite well with the emission nebulas. I've seen some results of the triffid taken through LPR filters and the blue was almost non-existent.

How hard was it to colour correct your results as well?
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  #16  
Old 28-08-2012, 07:45 AM
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Quote:
Originally Posted by cometcatcher View Post
I use an unmodded DSLR and find I get far better pics using a light pollution filter. The camera also seems to "wake up" in Ha.
I definitely agree with Kevin, my 1000D shows up HEAPS more Ha details with the CLS EOS Clip in place than without it.

Cheers

Chris
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Old 28-08-2012, 11:00 AM
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Quote:
Originally Posted by Garbz View Post
What filter did you use? .... I've seen some results of the triffid taken through LPR filters and the blue was almost non-existent.... How hard was it to colour correct your results as well?
For the Lagoon and Triffid I used a Lumicon Deep Sky filter. It's a little more narrow band than the Astronomik CLS. The horsehead was mainly with the Lumicon also but M42 was a 50/50 sub mix of Lumicon Deep Sky and Baader Semi-Apo filter. The Semi-Apo is a combined Moon and Skyglow + Fringe Killer. It's a wide band filter designed to reduce CA in refractors and I think passes blues a little better than the Lumicon but doesn't reduce light pollution as effectively. It's so wide it can be left on for most objects, including star clusters and galaxies.

The colour shift of the Baader Semi-Apo is minimal, but I still use manual colour balance in the camera.

The colour balance shift with the Lumicon Deep Sky is stronger so I use camera and photoshop adjustments. In level adjustment it's just a single click with the middle eyedropper to get a neutral background. Well sometimes more than 1 click but it's not too bad.

If you want blues, do take note of the filter's spectral graph cutoff, especially at the shorter wavelengths around 440nm.

The Astronomik CLS should pass a little more blue than the Lumicon Deep Sky, but the UHC and UHC-E will clip blue more strongly.
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Old 29-08-2012, 11:14 AM
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Watching this thread closely, I'm also looking at options for my canon 600d (unmodded) I notice there is an astronomik cls and ' cls ccd' both in clip format, anyone know the difference? I'm guessing the ccd version is more narrow... I was also thinking of getting the Oiii filter and by passing the Ha.

Contemplating modding, but I still use during the day...
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Old 29-08-2012, 11:55 AM
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From memory I think the CLS CCD has an integrated UV and IR blocker where the plain CLS doesn't. For an unmodified DSLR use the plain CLS as the camera's stock filter already blocks UV/IR.
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Old 29-08-2012, 12:29 PM
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Quote:
Originally Posted by cometcatcher View Post
From memory I think the CLS CCD has an integrated UV and IR blocker where the plain CLS doesn't. For an unmodified DSLR use the plain CLS as the camera's stock filter already blocks UV/IR.
Thanks Kevin.
Is there any significant advantage to the clip in system as opposed to the t-thread filter?
Cheers
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