cls moonlight m101

[alacant]

Hi everyone. Moon in Virgo, just got a CLS filter, impatience and was expecting miracles... This was as far from the moon I could get to try something to compare with previous efforts. Alas, the CLS does nothing -or very little- about the moonlight; the longest snap I could get was 150s, about the same as my trusty moon and skyglow. Anyone else using CLS? Tips, comments, what to expect etc. most welcome. Thanks for looking.

[LightBucket]

The CLS filter is an excellent LP filter, for the older street lamps with the sodium orange glow, and its not really to be able to image in full moonlight, for that you need to image in narrowband such as Ha, then the moon with be beaten into submission....  

[michael.h.f.wilkinson]

But then narrowband filters are best for narrowband targets, and only the H-II regions in galaxies qualify as narrowband targets. Planetary nebulae more generally diffuse emission nebulae would be better targets

 

Still a very nice image, given conditions

[alacant]

1 hour ago, LightBucket said:

CLS filter is an excellent LP filter

Thanks.

A few other bit's I'm not sure of... Mine's a cheap Chinese version and looks like a coloured mirror. You can only see through it if you're in a direct line of sight. Is that correct?

Also the rbg histogram peaks are closer, making colour balance  (I was about to say easier- it's not!) different. But maybe that was the effect of the moon?

Oh, and the length of time I need to takes flat frames has doubled; presumably it's blocking more of the light panel frequencies than my moon and skyglow.

TIA for any pointers.

[Filroden]

The filter works by not passing a large chunk of light in the green/yellow part of the spectrum so your colour balance will be very different to unfiltered images. You can experiment during the day by taking a filtered and unfiltered image that includes all three main colours (red, green and blue). Check the difference in colour a see what you need to do to adjust the balance in the filtered version to match the unfiltered. That should be roughly the same adjustments for Astro images.

You will need longer exposures (lights and flats) as it filters a lot of light.

[Nigel G]

3 hours ago, alacant said:

Thanks.

A few other bit's I'm not sure of... Mine's a cheap Chinese version and looks like a coloured mirror. You can only see through it if you're in a direct line of sight. Is that correct?

Also the rbg histogram peaks are closer, making colour balance  (I was about to say easier- it's not!) different. But maybe that was the effect of the moon?

Oh, and the length of time I need to takes flat frames has doubled; presumably it's blocking more of the light panel frequencies than my moon and skyglow.

TIA for any pointers.

Yes correct, here's a picture of my astronomik cls at an angle and square on. I find  colour balance much better  and take my flats the same way, on auto AV.

Great addition to my kit. Worth every penny. 

Nige.

 

[jambouk]

Yes to all of the above.

Below is graph showing the wavelengths the CLS filter allows through, and cuts out - whilst this isn't necessarily the filter you have, the principle is similar.

Moon light is a reflection of sunlight, so includes all wavelengths in the visible spectrum (and some outside that). As such, there is no good “moon filter” when it comes to imaging as moon light contains all the wavelengths your targets will also contain. As said above, the best way to image is to image in narrow band, as the relative amount of light from the moon in these narrow bands is very small; the very narrow filters can be used to image targets on nights with a full moon, and people report being able to image just 10 degrees or so away from the full moon! But you need targets which emit sufficient photons in the wavelength of the chosen filter.

It is far easier to just use a one shot colour camera or RGB filters and a mono camera on a moonless night.

Yes, the CLS filters look like a coloured mirror.

Yes, they also mess up the colour balance, and often make the image look excessively cyan. Depending on your camera, you may be able to do a custom white balance in the daylight with a grey test card, this will help give the image a more natural colour balance, but you can achieve similar results without by just adjusting the colour channels at the processing stage.

Yes, because fewer overall photons get through the filter when looking at a diffuse white light, you will need to alter your exposure settings / duration to get a similar shaped histogram.

James

 

[alacant]

21 hours ago, jambouk said:

CLS filter allows through

Thanks for the excellent explanation. So it looks like 90% of what we want and nothing of what we don't? Is there anything in the 0% areas we need?

[jambouk]

If I've understood your question correctly, then yes. I've attempted to overlay the graphic above onto the visual part of the electromagnetic spectrum. The bits in grey are areas of high transmission so stuff the sensor will be exposed to and register as "light". The bits between the grey are photons which will be rejected by the filter and not get to the sensor so will not contribute to the data collected. So most of the UV end and deeper blues are rejected, most of the yellows are rejected, a good proportion of the orange is rejected, and red is allowed through. It depends on your target. If you target is predominantly emitting red light then you are fine; if it emits mostly yellow light or blue light, then only a small amount of this will get through the filter so it will take a lot longer to get sufficient data to make a good image.

James

 

 

[alacant]

Hi James and thanks again. That's even clearer now. I can think of loadsa red emitting targets like the Ms in Scorpio and ngcs in cygnus.. Blue, M45 maybe? Yellow; can't think of any... Do you think I'm missing blue or yellow in the galaxy above? TIA.

[jambouk]

Hopefully this will work; an animated gif comparing your M101 with an image of M101 taken by the HST. I think yours has less blue. Also, if you zoom right into your image many of the stars have that odd cyan hue which the CLS filters often give. As I said before, a custom white balance helps to remove this.

James

 

[alacant]

Yep, I can see it fine. Thanks so much for the effort. I'm gonna have a go at the custom white balance thing although I must admit that of all processing, it's the colour I find the most difficult.

[jambouk]

What camera is it? Looking at the gif above, it might be that you are missing out on some of the Ha data, and replacing the IR cut filter in the camera (if it is a DSLR) may help with that.

The custom white balance is easy to do, and will make the colour part of the processing a bit easier, maybe a lot easier.

James

 

[alacant]

17 minutes ago, jambouk said:

What camera is it?

Hi. It's a 700d which is modified but I don't know what filters remain; it shows up red stuff much better than my daytime camera if that's any clue...

[jambouk]

Who modified it? They will likely know what filter is now in it.

It sounds like it has something useful if the red is more prominant. It's modified, so I wouldn't worry about that any further.

James