Open Star Cluster M34: Colorful Stars without a Blue Filter
Posted: Sun Nov 15, 2020 2:29 pm
No doubt, you will read this and ask yourself: Why?
To answer that question, I must explain that I have an Achromat refractor, not an APO. The William Optics ZenithStar 71 was marketed as "APO-like", and the price point was good! Well, I discovered only later that a lot of people are unhappy with this scope. It does not focus blue light well at all. Please look at the photo at the following link to see the difference. The left-hand photo uses a luminance filter which includes blue light. The right-hand photo uses a Wratten #12 filter which passes only red and green. The difference is dramatic:
viewtopic.php?f=10&t=3367
I was actually very happy with that monochrome image of M34 with the Wratten #12. In fact when I got my start in astrophotography 50 years ago, nearly every amateur used monochrome emulsions. There was color, but the results were disappointing. Today, everyone's attitudes have changed, so you MUST have color or you are a "nobody". My preference is to use a monochrome camera with LRGB filters, and because of that I have more degrees of freedom.
So I replaced my luminance filter with Wratten #12. (From now on, I will refer to it as "W12".) W12 is a "minus blue" type of filter. That's great, but in order to create color I need R+G+B, or so I thought! How can I make a color image only out of R+G? The answer is to borrow a technique from narrowband astrophotography, specifically bi-color narrowband, like Hydrogen Alpha and Oxygen III but no Sulfur II. There is a particular palette that I like: HOO. Normally, people assign Hydrogen Alpha to the red channel, and split Oxygen III equally between green and blue. I am not too fond of those proportions. Hydrogen Alpha is very strong, so you end up with a screaming red image. I like to split Hydrogen Alpha between red and green, so that I get a pleasing shade of reddish-orange. I used that technique for the following image of M34:
Color assignments:
67% of the Red stack to the Red channel,
33% of the Red stack to the Green channel.
33% of the Green stack to the Green channel,
67% of the Green stack to the Blue channel.
There is another reason why this works. From my work in Photometry, I knew that stars emit light over all wavelength but some bands are stronger than others. For example, a blue star strongly emits in blue, a little less in green, and even less in red. Likewise, a red star strongly emits in red, a little less in green, and even less in blue. So you see, it is not all or nothing. So, my technique takes advantage of that natural distribution of light, but the window of wavelengths it uses is from 500nm to 700nm, instead of 400nm to 700nm.
One last note about the image. It looks muted. The reason is that the weather conditions were variable last night, and I had tracking issues so I had to discard a lot of frames. For star clusters I like to achieve a minimum signal-to-noise ratio (SNR) of 100, preferably 200. Last night I was only able to capture enough data to achieve SNR 60.
Brian
To answer that question, I must explain that I have an Achromat refractor, not an APO. The William Optics ZenithStar 71 was marketed as "APO-like", and the price point was good! Well, I discovered only later that a lot of people are unhappy with this scope. It does not focus blue light well at all. Please look at the photo at the following link to see the difference. The left-hand photo uses a luminance filter which includes blue light. The right-hand photo uses a Wratten #12 filter which passes only red and green. The difference is dramatic:
viewtopic.php?f=10&t=3367
I was actually very happy with that monochrome image of M34 with the Wratten #12. In fact when I got my start in astrophotography 50 years ago, nearly every amateur used monochrome emulsions. There was color, but the results were disappointing. Today, everyone's attitudes have changed, so you MUST have color or you are a "nobody". My preference is to use a monochrome camera with LRGB filters, and because of that I have more degrees of freedom.
So I replaced my luminance filter with Wratten #12. (From now on, I will refer to it as "W12".) W12 is a "minus blue" type of filter. That's great, but in order to create color I need R+G+B, or so I thought! How can I make a color image only out of R+G? The answer is to borrow a technique from narrowband astrophotography, specifically bi-color narrowband, like Hydrogen Alpha and Oxygen III but no Sulfur II. There is a particular palette that I like: HOO. Normally, people assign Hydrogen Alpha to the red channel, and split Oxygen III equally between green and blue. I am not too fond of those proportions. Hydrogen Alpha is very strong, so you end up with a screaming red image. I like to split Hydrogen Alpha between red and green, so that I get a pleasing shade of reddish-orange. I used that technique for the following image of M34:
Color assignments:
67% of the Red stack to the Red channel,
33% of the Red stack to the Green channel.
33% of the Green stack to the Green channel,
67% of the Green stack to the Blue channel.
There is another reason why this works. From my work in Photometry, I knew that stars emit light over all wavelength but some bands are stronger than others. For example, a blue star strongly emits in blue, a little less in green, and even less in red. Likewise, a red star strongly emits in red, a little less in green, and even less in blue. So you see, it is not all or nothing. So, my technique takes advantage of that natural distribution of light, but the window of wavelengths it uses is from 500nm to 700nm, instead of 400nm to 700nm.
One last note about the image. It looks muted. The reason is that the weather conditions were variable last night, and I had tracking issues so I had to discard a lot of frames. For star clusters I like to achieve a minimum signal-to-noise ratio (SNR) of 100, preferably 200. Last night I was only able to capture enough data to achieve SNR 60.
Brian