Tag: rnc-color-stretch

Revisiting the Rho Ophiuchi cloud complex

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Several years ago I took a sequence of images of the Rho Ophiuchi cloud complex and posted the result on these pages. As noted at the time I used a Nikon D700 paired with a Nikon 85mm ƒ/1.8 lens all mounted on an iOptron Sky Tracker. Images were stacked using Deep Sky Stacker and post processing was done using Photoshop CS6 and Astronomy Tools v1.6.

Rho Ophiuchi cloud complex
Rho Ophiuchi cloud complex

Since then I have been experimenting with different tools for postprocessing astro photos. Along the way I discovered some interesting software called rnc-color-stretch from Clarkvision.com.

The rnc-color-stretch algorithm does 3 main things. 1) Analyze the image histogram to maintain a black point or use set low level color throughout the stretching process. The histogram is analyzed at multiple stages from beginning to end. 2) A power stretch while maintaining the black point. 3) Recover lost color after the stretching process. How far you can stretch an image depends on the signal-to-noise ratio.

I’ve been testing this software on both recent and older images. I thought it might be interesting to try it on the Rho Ophiuchi images taken in 2015. Once again, I used Deep Sky Stacker to register and align the images. Then I ran rnc-color-stretch. The result is the image shown above. I thnk it did a fine job of pulling out the details and the color.

Venus and Pleiades Conjunction—April 2020

During the first few days of April 2020 the planet Venus moved towards and then through the Pleiades star cluster. Venus and Pleiades have a conjunction every year but every eight years the conjunction is at its closest. This year, Venus moved right through the star cluster.

Conjunction of Venus and Pleiades
Conjunction of Venus and Pleiades

I shot images of Venus and Pleiades on three nights: 01 April, 03 April, and 05 April. Venus and Pleiades were closest on the night of 03 April. I then did a composite image of the three nights showing the progression of Venus past the star cluster. These were all shot at 8 seconds, f/4, 180mm, and ISO 800.

Additionally, I overlaid another image taken 13 February 2020. This is a stacked composite with 11 images shot at 120 seconds, f/4, 180mm, and ISO 1600. The images were stacked using Starry Sky Stacker. The stack was then post-processed using rnc_color_stretch. This composite image was used because it shows the nebulosity and color within the Pleiades better than the shorter exposures captured that show the motion of Venus.

Composite image of Venus and Pleiades.
Composite image of Venus and Pleiades.

The image above shows the composite from the three nights without the additional layer showing the nebulosity.

Photographing the North America Nebula

The North America Nebula has been on my To-Photograph list for a while. I had made one quick attempt previously to see whether I could actually resolve it with my Nikon 85mm f/1.8 lens. That was successful so I was ready to try again when the situation permitted.

I finally found the time, the right weather, and the right conditions. I shot a sequence of seven, 120-second exposures.

There are many star-stacking software packages available and I’ve often used DeepSkyStacker (DSS). More recently I’ve been testing StarrySkyStacker (a macOS-only app). The results have been pretty good.

The stacking complete, it was time to work on the histogram. Again, there are many histogram stretching packages. I’ve been evaluating rnc-color-stretch, available from Clarkvision.com. rnc-color-stretch is a set of scripts that calls the davinci application (not to be confused with the DaVinci Resolve video editing software).

North America Nebula (NGC 7000)
North America Nebula (NGC 7000)

The result is shown above. My next attempt at shooting this Deep-Sky Object (DSO) will be with my recently acquired Nikkor 180mm f/2.8 AIS manual focus lens.

Milky Way Closeup

Most of my Milky Way photographs are shot using a wide-angle (24 or 28 mm focal length), or ultra-wide-angle lens (16 mm focal length). These create an image that shows a large portion of the Milky Way. But sometimes it’s fun to zoom in a bit and focus (no pun intended) on a much smaller section of the sky.

Milky Way.
Milky Way.

A few days after the full Moon provided a great opportunity to do this. The Moon would not rise until about an hour after astronomical twilight ended and, more importantly, there were very clear skies.

I used a Nikon D750 body with a Nikon 85mm f/1.8 lens for this session. This is designed to be a portrait lens but I find it makes a pretty good astrophotography lens as well because of the excellent light-gathering f/1.8 aperture and the corner-to-corner sharpness resulting in nice round stars. At least, that is, when I get sharp focus and accurate tracking.

I shot 10 images of 120 seconds exposure time and used Long Exposure Noise Reduction (LENR). I was unable to use the 10th exposure because the sky was already getting brighter even though the Moon was still below the horizon. The result was 9×120 seconds or 18 minutes of light gathering.

I have several different applications (both Mac and Windows) for star stacking and alignment and chose to use Starry Sky Stacker this time with good results. Once I had the stack completed I used rnc-color-stretch for histogram stretching with final postprocessing done in Lightroom 6/Photoshop CS6.

Milky Way with annotations.
Milky Way with annotations.

This is the final result. I think the colors might be a bit too saturated—but I don’t dislike the result. Artistic license invoked here.