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#1
01-09-2013, 08:18 AM
 PRejto (Peter) Registered User Join Date: Jan 2011 Location: Sydney, Australia Posts: 1,290
Test Comparing 2 Light Pollution Filters

I've been contemplating a new setup where I use a LP filter in place of the usual luminance filter (rather than a situation where I use a LP filter in front of all filters (LRGB). I decided to compare the Hutech IDAS to the Astronomik CLS using my Moravian G2-8300.

First I tried comparing the filters just using a light box that I use for flats. I collected the following information:

IDAS: 8 seconds = 28,510 adu
16 sec = 55,960

CLS 8 seconds = 17,270 adu
16 sec = 33,880

Comparing these numbers to a situation without any filter (no luminance filter) I calculated that the IDAS would require increasing exposure by 2X and the CLS by 3.4X.

I then selected a dim target galaxy low to the Southwest where I see considerable light pollution. I attempted to equalize adu (density) at the core of the galaxy by adjusting the exposures. The results are shown below. The photos showed little without stretching. What you see are auto stretches in CCDStack (without DDP).

Image File CLS-6 min
centroid {X=836.4806, Y=626.423}
FWHM 2.5
background 7,212.35
maximum 19,096
flux within halfMax 30,961

Image File IDAS 210 sec
centroid {X=833.3436, Y=627.2254}
FWHM 2.3
background 8,403.20
maximum 19,259
flux within halfMax 23,692

Clearly the IDAS transmits more light but the CLS does a better job of controlling the background and gradient at the expense of rather longer exposure.

However, comparing both photos further in terms of S/N it seems the IDAS may have some advantage:

Image File IDAS
rectangle {X=769,Y=561,Width=162,Height=155}
diagonal 224.21
# pixels 25,110
# rejected 0 (0.00%)
mean 8,175.46
STD 1,175.27
S/N 6.96
median 8,087.00
int mode 8,019
min 7,640.00
max 65,535.00
R:G:B ratio 1.00 : 1.00 : 1.00

Image File CLS-6 min
rectangle {X=769,Y=561,Width=162,Height=155}
diagonal 224.21
# pixels 25,110
# rejected 0 (0.00%)
mean 6,856.59
STD 1,388.67
S/N 4.94
median 6,738.00
int mode 6,697
min 6,306.00
max 65,535.00
R:G:B ratio 1.00 : 1.00 : 1.00

Perhaps I don't understand how S/N is calculated. Subtracting the background from the signal should give the usable signal over the background. For the CLS this is about 12,000 adu, for the IDAS about 11,000. It would again appear to me that the CLS is doing a better job if the price of time collecting data is to be paid.

Peter
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Last edited by PRejto; 01-09-2013 at 01:54 PM.
#2
01-09-2013, 09:14 AM
 gregbradley Registered User Join Date: Feb 2006 Location: Sydney Posts: 15,408
That's a considerable improvement in contrast. It would be good to see 2 equal exposure shots as well to see if the contrast is still there.

This sort of post is very informative and helpful.

Greg.
#3
01-09-2013, 03:10 PM
 PRejto (Peter) Registered User Join Date: Jan 2011 Location: Sydney, Australia Posts: 1,290
Hi Greg,

Thanks for the feedback. I can post a 6 min IDAS exposure when home tomorrow. Of course, because the IDAS passes more light both the signal and the background are greater. To my eyes it made the CLS look even better, but what it does bring into clarity is how much slower imaging through the CLS will be. I'm starting to think I might be best served by both filters. The IDAS for mild LP and the CLS for massive LP.

Peter
#4
01-09-2013, 09:24 PM
 gregbradley Registered User Join Date: Feb 2006 Location: Sydney Posts: 15,408
Yes that sounds like a useful conclusion from that test.

The question often comes up which light pollution filter is best so its a valuable test.

Greg.
#5
02-09-2013, 07:22 AM
 Shiraz (Ray) Registered User Join Date: Apr 2010 Location: ardrossan south australia Posts: 4,787
Agree with Greg, nice test Peter. CLS seems to work well in your location.

If you want to do your own approximate SNR calculation, use the same exposure for each filter, remove any bias level and then calculate:

0.4*(signal-background)/sqrt(background*0.4)

I suppose that SNR is only part of the problem though - the fact that the CLS appears to have removed that nasty background gradient is also important - it might well be worth putting up with lower SNR if you can get that sort of result. Distortion of colour balance must also be considered?

regards ray

Last edited by Shiraz; 02-09-2013 at 08:18 AM.
#6
02-09-2013, 10:29 AM
 PRejto (Peter) Registered User Join Date: Jan 2011 Location: Sydney, Australia Posts: 1,290
Hello Ray,

The more I ponder this the more I think that I should continue experimenting. Perhaps the conditions that I tested the two filters was too severe as I would never try to actually image so low towards the horizon. And, yes, I do think that the large frequency of light blocked by the CLS could prove to be a problem for some targets, particularly those containing a lot of blue. The frequencies cut by the CLS are quite broad as can be seen in a comparison of the spectrum graphs.

I'm not sure I have done the Hutech IDAS filter any favour in the first photo attached in my first post as I just let CCDStack stretch each in an automatic fashion. I suppose that isn't entirely fair. So I also attach below the unstretched raw frames as displayed in Photoshop.

Next clear night I will repeat this experiment but close to the meridian where I more typically image. There is much less LP and I expect that I might see a better result from the Hutech IDAS filter. At least I hope so because I'm not sure I want to pay the price for very long exposures in luminance using the CLS vs the IDAS, nor do I want to miss collecting faint blue stuff. Looking at the transmission in blue of Astrodon filters frequencies from about 380 nm up are transmitted. The IDAS will capture some of those and the CLS will not.

Peter
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Last edited by PRejto; 02-09-2013 at 11:11 AM.
#7
02-09-2013, 01:16 PM
 DJT (David) Registered User Join Date: Nov 2011 Location: Sydney Posts: 1,352
Interesting your comment on the blue.

Grab a trifid with the CLS then compare to the Hutch. I use CLS from Sydney and didnt seem to have any issues with the blues in the image in the link below (the rest of the processing is another story..) .
M20 Trifid Nebula in context (C)
#8
02-09-2013, 04:07 PM
 skysurfer Dark sky rules ! Join Date: Dec 2011 Location: 52N 6E (EU) Posts: 1,043
Indeed that is my experience too. I have had the Astronomik and it filters too harsh. The phohos have a strong greenish blue tinge which is hard to remove at the expense of reddish stars.
The Hutech is much smoother and gives a more natural color and despite this, it filters out the yellow sodium light very well.
I compared pictures of the Banana (East Veil) nebula NGC 6992, and the colors with the Hutech were more natural after processing.
But with *strong* light pollution as close to Sydney an Astronomik CLS might benefit a bit more.
The CLS costs \$130 and the Hutech about twice as much (\$250) but it is well worth the double price.
#9
02-09-2013, 07:28 PM
 PRejto (Peter) Registered User Join Date: Jan 2011 Location: Sydney, Australia Posts: 1,290

Firstly, yes, that is a nice blue in the Trifid shot so obviously the CLS works fine for you with your DSLR!

My aim is to use the filter in place of luminance and not in front of any colour filters, so I want to capture as much light as possible in all three colours (but represented as grey scale of course). The CLS takes a rather large bite out of blue and green, and some of red. It certainly deals with severe LP better than the Hutech IDAS. I am anxious to test these both again in more ordinary conditions.

Peter
#10
04-09-2013, 09:45 AM
 PRejto (Peter) Registered User Join Date: Jan 2011 Location: Sydney, Australia Posts: 1,290
I have continued testing the two filters in an area of the sky with less light pollution. The results are somewhat different and I am finding it difficult to decide between the two filters. Please share your opinion!

I photograped NGC6384. I chose a dim object so that I could use longer exposures and compare core ADU. I was able to match core ADU with exposures of 330 sec with the CLS filter and 210 sec for the IDAS, a 2 minute difference in exposure.

I also took equal exposures of 6 minutes with both filters.

Comparing photos with equal core ADU it seems there is little difference between the two filters, except that the exposure for the CLS is 2 minutes longer. Subtracting the background from the core adu is 7000 for the CLS and 6,800 for the IDAS, however CCDStack measures rather a large difference in S/N. The IDAS = 2.07, the CLS = 4.81

Comparing the 2 6 minute exposures the difference between the background and core ADU is CLS = 8,200 and IDAS 12,700. CCDStack measures the S/N CLS = 4.83, IDAS = 6.86.

It appears that the IDAS is ca 30% faster, but pays a price in S/N when exposures are equalized in terms of core ADU. The difference in background brightness is striking between the two filters, especially in heavy light pollution, and less striking (though there) in lower light pollution.

In conclusion I cannot say exactly which filter would do a better job. I think it depends on the severity of light pollution. The IDAS is certainly faster so would minimize guiding errors through the ability to collect light faster and still do a reasonable job with LP gradients. Nontheless, there is something vicerally attractive about seeing such beautifully dark backgrounds in raw frames that make it hard to reject the CLS.

Peter
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#11
05-09-2013, 12:39 PM
 g__day (Matthew) Tech Guru Join Date: Dec 2005 Location: Sydney Posts: 2,564
Doesn't the Hutech IDAS do the best at preserving colour fidelity for non mono camera users?

I use IDAS filters with my modded Canon DSLR's - as after research I concluded it allow a combination of near best pass through of signal and best ratio of all colour channels allowed equally. Other products may skew your colour bass through in hard to process out ways.
#12
05-09-2013, 01:45 PM
 PRejto (Peter) Registered User Join Date: Jan 2011 Location: Sydney, Australia Posts: 1,290
Quote:
 Originally Posted by g__day Doesn't the Hutech IDAS do the best at preserving colour fidelity for non mono camera users? I use IDAS filters with my modded Canon DSLR's - as after research I concluded it allow a combination of near best pass through of signal and best ratio of all colour channels allowed equally. Other products may skew your colour bass through in hard to process out ways.
Hi Matthew,

I believe you are correct, and that has been my experience. If I were going to continue to use a LP filter in front of all my filters (LRGB) I would certainly pick the IDAS. However, I now intend to just replace my luminance filter with a LP filter and deal with LP gradients in RGB as best I can. Picking the best filter to replace luminance isn't entirely clear. I believe the effect of LP would be greatest in luminance.

Peter

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