Working with the SV605MC monochrome, cooled, 14 bit CMOS camera.
The data were obtained while testing the plate-solving, object sync and assisted meridian flip developments in AstroDMx Capture.
The scope used was a William Optics Super Zenithstar 81mm ED Doublet APO refractor at f/5.5 with x0.8 flattener/reducer fitted with an Altair magnetic filter holder version 2. An SVBONY SV605MC camera was placed at the focus of the telescope which was mounted on a Celestron AVX EQ, GOTO mount. An SVBONY SV165 guide-scope fitted with a QHY-5II-M guide camera was mounted on the imaging scope. PHD2 autoguiding was done by a separate Linux laptop for convenience.
A simple stiff-felt cylinder was placed over the magnetic filter holder to prevent stray light ingress.
The felt cylinder can be easily slid out of the way to facilitate filter changing.
As usual, the mount was placed on marks on the ground which gives an acceptable polar alignment
The first experiments involved the Eagle nebula.
AstroDMx Capture, via the INDI server running on a Raspberry Pi computer, sent the scope/mount to the star HD168137 which is close to the ‘Pillars of Creation’ in the Eagle nebula. The field of view was plate solved and the star centred in the field of view.
AstroDMx Capture captured 20 x 2 minute FITS exposures of the Eagle nebula through each of H-alpha, SII and OIII narrowband filters plus matching dark frames.
Negative preview
Flatfields were captured using a green filter and an illuminated tracing panel as the diffused source of light at the end of the imaging session.
The separate groups of images were calibrated, stacked and part processed in Siril and stretched in the Gimp.
Monochrome images captured through each filter
H-alpha
OIII
SII
The colour channels were combined in the Gimp to different palettes. Star removal techniques were used with the Starnet++ plugin for Gimp and processed with Neat Image, Photoscape X Pro and the Gimp 2.10.
Star removal and replacement techniques are important because:
They allow stars to be largely desaturated if they have strange colours due to the palette being used.
They allow for the nebulosity to be stretched without also stretching and bloating the stars.
Pertinent steps in the processing of the Hubble Palette using the Gimp.
However, it should be understood that these procedures are illustrative rather than prescriptive, and that there are different ways of achieving an end result.
Register the three colour images so that the stars in the images exactly line up.
Combine the three monochrome images as component channels of an RGB image; mapping SII to Red, H-alpha to Green and OIII to blue.
SHO
It can be seen that most of the stars are a pinkish magenta. At this stage, or after they have been removed, the colour cast can be removed from them either generally or selectively.
The stars can be removed in the Gimp 2.10 with the Starnet++ plugin.
Stars generally desaturated
Stars selectively desaturated reducing mainly magenta colour
It must be said that the best way to produce stars for the image is to take exposures through red , green and blue filters and produce true RGB stars that can be added back into the starless images.
The starless image of the Eagle nebula
The Starless image can then be stretched and denoised to reveal more nebulosity
The stars can then be added back either using the addition mode or the screen mode for combining layers
Addition mode stars-back Hubble Palette
The Screen mode stars-back Hubble Palette
It can be seen that the Screen mode puts the stars back with more prominence than the Addition mode.
Producing hues more generally used for the Hubble palette, can be done by selective colour adjustment within specific colours. Various software can do this such a Photoshop, Affinity Photo and PhotoScape X Pro.
Here we have within the Yellows, selectively reduced the Cyans and increased the Magentas to produce the desired effect.
Selective colour processed Hubble Palette starless image
Selective colour processed Hubble Palette image with stars added back by Addition mode
Selective colour processed Hubble Palette image with stars added back by Screen mode
It can be seen that the person processing the images has considerable latitude with how the processing proceeds and on the appearance of the final image.
There are five other narrowband palettes where H-alpha, SII and OIII are mepped in various combinations to the red, green and blue channels of a false colour image that are less commonly seen than the hubble palette, but all equally valid. The palette of choice is ultimately a decision for the individual.
The other narrowband palettes
HOS
OHS
HSO
SOH
OSH
The SV605MC in conjunction with an Astronomik UHC-E filter
The UHC-E filter has two regions of transmission. One in the blue/green and the other in the red/far-red parts of the spectrum. It is an effective light-pollution filter and facilitates the study of emission regions.
Another imaging session involved using an Astronomik UHC-E filter in combination with the SV605MC camera for producing monochrome images. These sorts of images can be appreciated as they are or could be used as luminance data with narrowband imaging.
The Omega (Swan) nebula
AstroDMx Capture, via the INDI server in the Raspberry Pi computer, sent the scope/mount to the Omega nebula, plate solved and centred the object.
AstroDMx Capture captured 80 x 45s exposures with matching dark frames
Negative preview
The images were calibrated, stacked and partly processed in Siril
The Stars were removed in the Gimp with the Starnet++ plugin and denoised in Neat image.
The extracted stars
The starless image was stretched in the Gimp 2.10
Then the stars were added back and the image flipped horizontally to a more familiar orientation.
The Omega (Swan) nebula
Then AstroDMx Capture slewed the scope/mount to the Lagoon nebula, plate solved and centred the object in the field of view.
AstroDMx Capture captured 25 x 45s exposure before it became too light to capture worthwhile images.
The images were calibrated, stacked and partly processed in Siril
The Stars were removed in the Gimp with the Starnet++ plugin and denoised in Neat image.
The starless image was stretched in the Gimp 2.10
The stars were added back
The Lagoon nebula
The SV605MC camera in conjunction with the William Optics Super Zenithstar 81mm ED Doublet APO refractor at f/5.5 with x0.8 flattener/reducer proved to be a very capable imager with narrowband filters to produce false colour images in various palettes, and with an Astronomik UHC-E filter to produce monochrome images covering all of the wavelengths of the object or data to be used as luminance along with narrowband data.
Although not absolutely required, the star removal and replacement technique proved to be a satisfactory way of dealing with star colours and stretching the nebulosity the nebulosity without bloating or over emphasising the stars.
These data were, as explained earlier, obtained whilst testing the plate-solving, object sync and assisted meridian flip developments in AstroDMx Capture.
