Bring Out Details in Light Polluted Skys

Back on October 11 I did a post on looking for a solution for light pollution which backyard astronomers face more and more these days. I was looking at performing some research on the skyglow filters which are available to resolve that very problem. Well I did that research ……and the problem was…. I was seeing a lot of mixed reviews on how good these filters were. There were some bad reviews which pretty much killed the notion of my spending hard earned money on a filter that might not do what I wanted.

None of the reviews, good or bad, provided any pictures to back up their reviews to show an image before and after using these skyglow filters. And that makes it diffacult to judge the filter because you are relying on word alone….. and as they say…… a picture tells a thousand words.

Astronomik CLS Comparision (Click to Enlarge)

Astronomik CLS Comparision (Click to Enlarge)

Last night I was checking into a filter called an Astronomik CLS Filter which is marketed as an inexpensive solution for visual deep sky observation and CCD imaging. This filter is said to gain contrast for most objects under a light polluted sky. Well the words inexpensive solution grabbed my attention and the fact this filter was optimised for telescopes with a focal ratio from f4 up to f15.

Sounds good but would it work and give me the results I want.

So I focused my research on this one specific filter to find some visual evidence from users in the field to show if this filter was up to scratch or not. Then last night I discovered the picture shown above which was taken by an astronomer in Victoria using the Astronomik CLS filter for CCD imaging. The picture above was taken under a near full moon with the image on the left showing the view without a filter and the image on the right showing the same shot but with the Astronomik CLS Filter.

Skyglow filters are generally shot down by critics because while they reduce sky glow and they are also reported to decrease the brightness of other objects such as stars, clusters and galaxies. But by how much and is there a huge reduction in sky glow and a minimal drop in star brightness?

So I first look at the before and after picture above to see if the image with the Astronomik CLS filter has a reduced brightness in stars. Looking at some of the brightest stars in the image on the right, there is possibly a very small drop in brightness when compared to the left….. but then it looks very minor to me and could just be a trick of the eye.

So my next step is to process these before and after images as I would do with my images to see what the final look would be. In this test as shown in the picture on the left, I simply used paint shop pro to adjust the brightness and contrast of the first picture above. A fairly simple step and not requiring any use of unsharp masks or modifying curves.

Astronomik CLS Processed Shot (Click to Enlarge)

Astronomik CLS Processed Shot (Click to Enlarge)

If you look at the picture on the left, this is the result I get by simply fine tuning the first picture shown above. I adjusted brightness with a setting of 13 and contrast with a setting of 45…… and i think it shows off the true comparision of using the Astronomik CLS Filter.

Keep in mind that the picture was taken with the glow of an almost full moon. Even after adjusting brightness and contrast, the image on the left in the second picture looks better than the original taken without the filter….. but still looks washed out. Stars are brighter but thats because of the modifications by Paintshop Pro.

But looking at the images with the  filter attached….. I would have to say the completed shot with the filter on in the second picture is incredible considering the effect the moon had on the original picture. And if you look at the right hand side of both pictures, you will see that using paintshop you can actually get brighter stars than even the original picture taken without a filter.

So my view on the Astronomik CLS Filter is this:

The pictures do not really show a significant drop in star brightness using the filter. I am keeping in mind the fact the pictures above were CCD images and does not reflect what one might see through a scope. But I suspect that even through the scope there would be only a minimal drop in star brightness. Of course you will not see the image as shown in the right hand picture of the second picture as your scope does not have a built in paintshop or photoshop.

Because of the light pollution from the moon, you will still get a washed out image…..though much improved with the filter added. The best trick is to avoid observing when there is a full moon around…no filter will solve that problem. But for taking images of the objects and then processing them in paintshop or photoshop….. then I think the Astronomik CLS Filter looks like a must.

Saying that I need to plan on purchasing the filter so I can personally test it out. Bintel sells the Astronomik CLS Filter for $139 for the 1.25″ so in about 3 weeks I’ll purchase one of them and try it out on my scope.

~ by sydneystargazers on October 27, 2009.

3 Responses to “Bring Out Details in Light Polluted Skys”

  1. Hey Roy,

    Ash here mate (NSAS member also) – interesting read, I too have been in the market for a “decent and reliable” light-pol filter. Im looking forward to the follow up on this post.


  2. I wonder how much effect this has on the image color curves. I know the HUTECH filters are renowned for maintaining white balance and the Astronomik require aggressive hysteresis adjustments. Having said that I ordered the Astronomik CLS Clip filter for my Canon from OptCorp the main reason being the price difference between the two filters.

  3. Hi,

    Nice article.

    I am curious to know whether you bought the Filter and what was the experience seeing through the eyepiece?

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