Experimenting with narrowband palettes with the Rosette nebula

Starting with balanced monochrome channel images, I used our Rosette nebula Ha, OIII and SII data to produce the following palettes.

The balanced mono channels

Ha: 

OIII:

SII:

For comparison purposes we shall first show the Hubble palette and the Canada, France, Hawaii Telescope palettes.

The Hubble palette: SHO

The Canada, France, Hawaii Telescope palette: HOS

These two palettes use whole channel combinations as RGB images.

All of the palettes shown here also have gentle, selective colour processing to enhance to hues in the final palettes. Also all of the processing was done on starless images, where the stars were removed by the Gimp 2.10 plugin Starnet++.

There are however, palettes that use static factors for blending narrowband data

The 'Natural' palette

The ‘Natural’  palette is a narrowband color palette that uses a combination of Ha, OIII and SII filters to create a pseudo-natural color image. It was developed by Robert Gendler, an astrophotographer and physician.

The Natural palette uses the following formula for each color channel:

Red: 0.75 SII + 0.25 Ha

Green: OIII

Blue: 0.1 Ha + 0.9 OIII

The balanced monochrome narrowband images were blended in these proportions to produce the 'Natural' RGB channels

Natural palette image of the Rosette nebula

The ‘Gendler’ palette 

The ‘Gendler’ palette is a narrowband color palette named for Robert Gendler, who developed it as a variation of his Natural palette. 

The Gendler palette uses the following formula for each color channel:

Red: 0.8 SII + 0.2 Ha

Green: 0.6 OIII + 0.4 Ha

Blue: 0.4 OIII + 0.6 SII

Gendler palette image of the Rosette nebula

I think that this is a pleasing palette that compares favourably with the Hubble palette.

There is a bicolour palette that is commonly used that makes use of the full Ha and OIII narrowband images. This is the HOO palette.

HOO palette image of the Rosette nebula

I produced a palette inspired by the HOO palette but using blended data from the Ha & SII channels and OIII balanced monochrome narrowband images. I call this the 'SHOO' palette.

The SHOO palette uses the following formula for each color channel:

Red: 0.5 SII + 0.5 Ha

Green: OIII

Blue: OIII

SHOO palette image of the Rosette nebula

It is clear that there is scope for the experimental development of palettes and that the final appearance of each palette depends on a number of factors such as the initial balancing of the channels and the degree, if any, of selective colour enhancement.

Although this article is about static factor channel blending, it is also possible to blend channels using dynamic narrowband combinations. An example of this methodology is the FORAXX palette developed by a software engineer and astrophotographer who goes by the Internet identity of Paulyman Astro who wrote a script for PixInsight that automates the process of creating narrowband images in the Foraxx palette. The Foraxx Palette uses dynamic factors for each pixel, instead of static factors for the whole image. This kind of dynamic pixel math can be done in other software such as Siril or Image Magick and the same results achieved.

Narrowband imaging of the Pacman and Rosette nebulae with AstroDMx Capture

The Pacman and Rosette nebulae were imaged with AstroDMx Capture, a William Optics Super Zenithstar 81mm ED Doublet APO refractor at f/5.5 with x 0.8 reducer/flattener, and an Altair 2" magnetic filter holder v2 fitted with  a 7nm H-alpha, or a 6.5nm OIII filter or a 7nm SII filter. An SVBONY SV605MC monochrome, cooled CMOS camera was placed at the focus.

The equipment used

Part way through the imaging session we found that the ZWO AEF had failed likely due to the cold temperatures. A cursory internet search revealed that this is common and that the EAF incorporates domestic components rather than industrial components; it is not guaranteed to work at temperatures below minus 5 degrees C. This was a disturbing discovery. Whilst the 5 volt focuser that receives its power via the USB (in our case via an active, powered cable) is more convenient than the more powerful 12 volt version that it replaced, which required a separate 12 volt power supply, (as does the Pegasus FocusCube 2 that we use on a different telescope); the convenience is completely negated if it is necessary to do what we did to solve the problem, to fit a powered lens-warming, anti-dew strip to keep the focuser warm:

The lens-heater strip fitted to the AEF

As usual, the mount was placed on marks on the concrete base which give a fairly good polar alignment. AstroDMx Capture passed the time, altitude and location coordinates to the hand controller via the AstroDMx Capture integrated INDIGO client, and server which was running on the imaging computer. The hand controller which now contained all of the correct information was set to its previous alignment and was unparked by AstroDMx Capture.

AstroDMx Capture was used to send the scope/mount to a Altair to check focus with a Bahtinov mask. 

AstroDMx Capture plate-solved and sent the scope/mount to the Pacman nebula and captured 6 x 5-minute exposures through each of the H-alpha, OIII and SII narrowband filters, giving a total exposure time of 90 minutes.

AstroDMx Capture capturing Data on the Pacman nebula through the H-alpha filter

AstroDMx Capture capturing Data on the Pacman nebula through the OIII filter

AstroDMx Capture capturing Data on the Pacman nebula through the SII filter

The data were registered and stacked in Deep Sky Stacker. The OIII and SII light frames were registered with the best H-alpha light frame so that the three channels would be perfectly registered. The data were processed in Gimp 2.10, the Starnet++ Gimp plugin, Neat image and Photoshop CS2. The Channels were balanced in the Gimp 2.10, combined into false colour image palettes and gently selective colour processed to achieve the required colours of the Palettes used.

The Pacman nebula SHO, Hubble Palette

The Pacman nebula HOS, Canada, France, Hawaii Telescope Palette

AstroDMx Capture plate-solved and sent the scope/mount to the Rosette nebula and captured 6 x 5-minute exposures through each of the H-alpha, OIII and SII narrowband filters, giving a total exposure time of 90 minutes.

AstroDMx Capture capturing Data on the Rosette nebula through the H-alpha filter

AstroDMx Capture capturing Data on the Rosette nebula through the OIII filter

AstroDMx Capture capturing Data on the Rosette nebula through the SII filter

The data were registered and stacked in Deep Sky Stacker. The OIII and SII light frames were registered with the best H-alpha light frame so that the three channels would be perfectly registered. The data were processed in Gimp 2.10, the Starnet++ Gimp plugin, Neat image and Photoshop CS2. The Channels were balanced in the Gimp 2.10, combined into false colour image palettes and gently selective colour processed to achieve the required colours of the Palettes used.

The Rosette nebula SHO, Hubble Palette

The Rosette nebula HOS, Canada, France, Hawaii Telescope Palette

All of the procedures produced the required results and the integrated INDIGO client in AstroDMx Capture worked well for mount and focuser control. It was disappointing to discover that the ZWO AEF was so susceptible to the cold, but the heated lens anti dew strip solved the problem. In fact on the highest heat setting, the temperature of the AEF reached double Celsius figures despite the cold environmental temperature with enhanced radiative heat loss due to the clear sky.

Imaging the Cave nebula (Caldwell 9) and the Ghost of Cassiopeia nebula (IC 63 and IC 59) with INDIGO mount and focuser control in AstroDMx Capture.

Exposures were captured with AstroDMx Capture through an Altair Starwave 60 ED refractor with an 0.8 reducer/flattener, a Pegasus FocusCube 2 Motorised Electronic Focuser, an Altair Quadband filter and an SV605CC 14 bit OSC CMOS camera. The scope was mounted on a Celestron AVX mount.

The equipment used

The mount was placed on marks on the ground which quickly gives quite a good polar alignment if care is taken with the placement of the tripod feet.

AstroDMx Capture running on the imaging computer indoors sent the scope/mount, using the integrated INDIGO client, via an INDIGO server running on the same computer, to a bright star and the scope was focused on the star using a Bahtinov mask using the Pegasus focuser controlled by INDIGO.

An SVBONY SV165 guide scope with a QHY-5II-M guide camera was used for PHD2 multistar pulse auto-guiding via the INDIGO server. The auto-guiding was controlled by a separate Linux laptop indoors.

AstroDMx Capture plate-solved and sent the scope/mount to the Cave nebula and captured 18 x 5 minute exposures.

AstroDMx Capture capturing data on the Cave nebula

With a negative preview

The data were stacked in Deep Sky Stacker and processed in Gimp 2.10, the Starnet++ Gimp plugin, Neat image and Photoshop CS2.

The data were processed with unmodified data and also using a channel balancing plus selective colours technique to reveal more of the colours.

Normally processed image of the Cave nebula

Balanced channels processed image of the Cave nebula

AstroDMx Capture plate-solved and sent the scope/mount to the Ghost of Cassiopeia nebula and captured 13 x 5 minute exposures.

AstroDMx Capture capturing data on the Ghost of Cassiopeia nebula

With a negative preview

The data were stacked in Deep Sky Stacker and processed in Gimp 2.10, the Starnet++ Gimp plugin, Neat image and Photoshop CS2. The channel balancing technique was used to render the final image.

The Ghost of Cassiopeia nebula

Annotated image

AstroDMx Capture worked well with the INDIGO mount and focuser control. It was interesting to balance the channels and reveal more of the colour present in the nebula that would normallyh be overwhelmed by the red from H-alpha and SII, both of which are passed by the Quadband filter as are H-Beta and OIII spectral bands.

More work for the old, cheap Meade RB-70 refractor

Encouraged by the good lunar results on the first testing of the f/10 Meade RB-70 air-spaced doublet refractor I removed the scope from the rings within which it had been mounted when it was used as a guide scope some 13 years ago. A small dovetail bar was fitted so that the scope would be parallel to the declination axis of the mount rather than in the arbitrary alignment determined by the guide mounting rings. This is essential for GOTO to be able to send the scope reasonably accurately to an object.

The AVX mount was placed on marks on the ground which quickly gives quite a good polar alignment if care is taken with the placement of the tripod feet.

An SV705C OSC uncooled CMOS camera fitted with a UV/IR cut filter was placed at the focus of the scope. The mount was controlled by AstroDMx Capture via an INDIGO client and server running on the imaging computer indoors.

Click on any image to get a closer view

The Meade RB-70 mounted on the AVX mount by means of a dovetail bar

AstroDMx Capture sent the scope/mount to Vega and the star was brought to focus using a Bahtinov mask.

The scope showing the Bahtinov mask

The Bahtinov mask focused star

AstroDMx Capture plate-solved the field of view and the scope was sent to M42.

AstroDMx Capture capturing data on M42

With a negative preview

Twenty five images of each of 2s, 5s, 15s, 20s, 25s duration were captured and stacked in Deep Sky Stacker. The resulting image was post processed in Fitswork 4 and the Gimp 2.10.

M45 with a Meade RB-70

 The scope was then sent to the 92.1 % waning Moon

AstroDMx Capture captured a 1500-frame SER file. 

The best 80% of frames in the SER file were stacked in Autostakkert! 

The resulting image was wavelet processed in waveSharp

The image was post processed in Gimp 2.10

92.1% waning, gibbous Moon

The Meade RB-70 was significantly improved by fitting the dovetail bar and the scope proved its worth for lunar imaging and even a bright deep sky image, the Orion nebula.

The INDIGO client in AstroDMx Capture worked without issue.

Continued testing of the Indigo client with AstroDMx Capture

Four nebulae were Captured with AstroDMx Capture, controlling the mount via a newly implemented INDIGO client. The scope was an Altair Starwave 60 ED refractor with an 0.8 reducer/flattener, an Altair Quadband filter and an SVBONY SV605CC 14 bit OSC CMOS camera. We were testing Nicola's implementation of the INDIGO client. PHD2 pulse auto-guiding was also done via INDIGO. The mount was placed on marks on the ground which quickly gives quite a good polar alignment if care is taken with the placement of the tripod feet.

AstroDMx Capture running on the imaging computer indoors sent the scope/mount, via an INDIGO server running on the same computer, to Vega and the scope was focused on the star using a Bahtinov mask and the Pegasus focuser.

An SVBONY SV165 guide scope with a QHY-5II-M guide camera was used for PHD2 multistar pulse auto-guiding. The auto-guiding was controlled by a separate Linux laptop indoors via the INDIGO server

The exposures for each nebula were:

Heart 18 x 5 min = 90 minutes

Soul 14 x 5 min = 70 minutes

Pacman 17 x 5 min = 85 minutes

Helix 30 x 2.5 min = 75 minutes

The images were calibrated and stacked in Deep Sky Stacker, background corrected in GraXpert and post processed in the Gimp 2.10, the Starnet++ Gimp plugin, Neat image and CS2. Channel decomposition, combining and composition, selective colours processing and Channel mixing were used to produce an HOO type palette rendering of the Heart and Soul nebulae.

The Heart nebula

AstroDMx Capture capturing data on the Heart nebula

Negative preview

The Heart nebula

HOO type rendering

The Soul nebula

AstroDMx Capture capturing data on the Soul nebula

Negative preview

The Soul nebula

HOO type rendering

The Pacman nebula

AstroDMx Capture capturing data on the Pacman nebula

Negative preview

The Pacman nebula

The Helix nebula

AstroDMx Capture capturing data on the Helix nebula

Negative preview

The Helix nebula

The session was a success. The few anomalies that were detected were noted and have been resolved.

INDIGO support is getting closer to release.