MLUnsold Digital Imaging

Creating Frequency Masks of Stars, Nebulas, & Galaxies

The Smooth Sharpen | Frequency Tool can be used to create masks of stars, galaxies, and nebulas. Stars and other small objects are expressed as high frequencies in the frequency domain. Larger objects like galaxies and nebulas are expressed as very low frequencies in the frequency domain. The low pass and band pass options of the Frequency tool are used to reject high frequencies from smaller objects like stars and keep low frequencies from large objects like galaxies and nebulas. The high pass and band stops options of the Frequency tool are used to keep high frequencies from smaller objects like stars and reject low frequencies from large objects like galaxies and nebulas.

For an introduction to using masks see the Process History - Basic Steps to Apply a Filter with Blend Mode, Opacity, and Image Mask and Combine Image & Mosaic - Basic Steps to Apply a Filter with Blend Mode, Opacity, and Image Mask tutorials.

Galaxy & Nebula Masks

Above is a narrow band image of M27 taken with a C14 Edge and QSI 583. Let's use this image to show how the Frequency Filter is used to make masks.

The Frequency Filter | Low Pass option is used to keep only the very low frequencies of M27 and reject higher frequencies of smaller objects like stars. So Cutoff is set to the far left at a value of 0.019 and Decay is set at 4 to produce a sharp frequency cutoff. Note that small detail is removed but large haloes around the brightest stars remain. Use the Scale Mask option on the Frequency Filter to adjust overall brightness.

The frequency cutoff is reduced from 0.019 to 0.010 with the same decay value of 4 as in the previous example. Now the haloes around the brightest stars are gone and only the largest objects with the lowest frequencies remain.

The Frequency Filter | Band Pass option can also be used to reject all frequencies outside of a range of frequencies. Cutoff is set to 0.000 and Decay is set to 11 for a very sharp cutoff. Width is set to 0.014 to create a very small frequency range about cutoff frequency 0.000.

The extreme low frequency image shown on the left can now be split into luminance, red, green, or blue masks.

A custom luminance mask could be create from the low frequency image using the Custom Luminance button on the left vertical toolbar.

A selective color luminance mask could be create from the low frequency image using the Selective Color Mask button on the left vertical toolbar.

Star Masks

The Frequency Filter | High Pass option is used to keep only the high frequency stars and reject the low frequencies of M27. So Cutoff is set at a value of 0.072 and Decay is set at 8 to produce a sharp frequency cutoff. Note that large objects like M27 are removed but smaller objects like stars remain. Use the Scale Mask option on the Frequency Filter to adjust overall brightness.

Special Functions | Pixel Math can be used to remove detail from the dark background that remains. On Pixel Math set the min, max, and feather range values to include only the dark background then use the multiply or subtract option to reduce brightness.

Haloes around brighter stars are removed by adjusting the min, max, and feather range sliders.

Above is a narrow band image of M27 taken with a C14 Edge and QSI 583. Let's use this image to show how the Frequency Filter is used to make masks.

The frequency cutoff is reduced from 0.019 to 0.010 with the same decay value of 4 as in the previous example. Now the haloes around the brightest stars are gone and only the largest objects with the lowest frequencies remain.

The Frequency Filter | Band Pass option can also be used to reject all frequencies outside of a range of frequencies. Cutoff is set to 0.000 and Decay is set to 11 for a very sharp cutoff. Width is set to 0.014 to create a very small frequency range about cutoff frequency 0.000.

The extreme low frequency image shown on the left can now be split into luminance, red, green, or blue masks.

A custom luminance mask could be create from the low frequency image using the Custom Luminance button on the left vertical toolbar.

A selective color luminance mask could be create from the low frequency image using the Selective Color Mask button on the left vertical toolbar.

Special Functions | Pixel Math can be used to remove detail from the dark background that remains. On Pixel Math set the min, max, and feather range values to include only the dark background then use the multiply or subtract option to reduce brightness.

Haloes around brighter stars are removed by adjusting the min, max, and feather range sliders.