CLS filter "experiment"

[jambouk]

I've been meaning for some time to do a little experiment to investigate the impact that the CLS clip filter has on widefield DSLR imaging in terms of limiting magnitude detection with and without the filter. The prompt to do this was partly the interesting post I saw yesterday about faintest start detection at differing elevations above the horizon.

As Orion was putting on a nice show at 4am today I ran a quick experiment from my light-polluted back garden looking south-ish, away from the light pollution of the city which lies to my north/northeast.

Canon 6D on a static tripod. 5 second exposures as this is often what I use for star trails under a light polluted sky. Various ISO settings. Two settings with the CLS clip filter in, one with the automatic white balance (AWB) and one with a custom white balance I'd set up recently (see below); then another shot without the clip filter and using the AWB setting.

From this very simplistic experiment, it looks to me that the filter results in a detection difference of about 1-2 magnitudes. At ISO 200, when I zoom in to Orion's belt, the faintest star I can easily spot with the filter in situ is 5.35, whereas without the filter I can see a magnitude 8; at ISO 800 the difference becomes less, with a magnitude 7.85 being seen with the filter but a magnitude 8.7 without. As to the relevance, I just wanted to see for myself the impact the filter had with my setup in my garden on limiting magnitude detection.

An interesting and incidental finding, however, was that the custom white balance really does allow hydrogen alpha rich regions to be clearly detected from light polluted sky, and easily seen on the LCD of the camera at higher ISOs - useful to know for framing when using a DSLR.

That's it really. Just thought I'd share my findings. Going to stick to using the custom white balance now when imaging with the CLS filter in.

James

EOS CLS filter clip experiment.pdf

[jambouk]

Custom white balance:

I made a grey sheet of paper by printing 9% grey with my laser printer.

I look an image of this under natural sun light with the filter in situ.

Then it is just a case of selecting custom white balance and selecting that image and hey presto

I think it is important to do this under unobstructed sunlight as clouds may interfere with the process.

There are various guides online about doing this.

I makes everything look quite red under artificial light, but under natural light the image looks much more normal than the blue/cyan hue the filter usually casts on images.

James

[Ruud]

There's a clear difference. The no-clips capture more light and much sooner.

Thanks for sharing these results.

[michael.h.f.wilkinson]

Very nice experiment. I think I shall be using my CLS clip filter based on these findings, rather than using none

[brendant]

If you are shooting in Raw mode White Balance has no effect on the Raw data from the camera. White balance is only relevant to Jpeg modes. If you open the Raw file in PS or DPP your can assign any colour balance you like regardless of shooting mode. Jpeg files lock in the white balance set on the camera.

[jambouk]

Have the custom white balance does allow the LCD on the camera to display the impact of the custom white balance, whether it affects the RAW data or not, which is useful for framing. Opening the RAW files on my desktop with Faststone also causes the custom white balance to be applied - all the images in the pdf above were captured from Faststone by opening the RAW files.

James