Tag: ZWO ASI482MC

I just recently got around to finishing up processing a batch of images of Pluto that I captured on August 3rd.  At the time of the imaging session, Pluto was just two weeks past opposition, with opposition having occurred on July 20th.

On August 3rd, Pluto was moving slowly against the starry background at 3.5 arcseconds per hour. Pluto’s apparent movement on that night was slow in comparison to other minor bodies in the solar system, but pretty fast in comparison to itself. For example, earlier in this month on October 2nd, Pluto was only moving at 0.5 arcseconds per hour. At this speed it would have taken two successive nights of imaging to show the same amount of movement I captured in just two hours on August 3rd.

Pluto animated GIF.
This animation shows the dwarf planet Pluto’s movement across the night sky during a two-hour period on August 3, 2022. At the time, Pluto was 33.6 astronomical units (3.12 billion miles/ 5.02 billion kilometers) from Earth. The field of view of the animation is approximately 17×12 arcminutes, cropped from a wider field of view image sequence. North is up. East is left. Image details below.

Capturing the images for this animation was a bit of a challenge. On August 3rd, Pluto’s V magnitude was predicted to be 15.0 by the Minor Planet Center and Lowell Observatory’s Ephemeris Service (NASA’s Horizons service was an outlier, predicting a somewhat brighter magnitude 14.34).

A magnitude 15 object is a pretty faint target for my 8-inch telescope, especially since I was imaging from a bright, Bortle 7, backyard, looking straight into the washed out Oklahoma City light dome to the south. In any event, I was pleasantly surprised to see that after slewing the telescope to Pluto’s coordinates, the dwarf planet popped up in the field of view, albeit faintly, right where the Lowell Observatory’s Asteroid Finder chart showed it would be. I haven’t done any photometric measurements on these images, but by my eyeball estimation, Pluto looks more like magnitude 15 than 14. 

Processing the image sequence into an animation also proved a bit of a challenge. But, ultimately, the challenge taught me a new processing technique. I normally process animations by opening the time-sequenced group of images as a set of layers in the image processing program GIMP. I then manually align the background stars in each layer with a base layer (usually the first in the time sequence) and export the entire aligned sequence as an animated GIF.  

In this case, however, for some reason, I could not manually align the layers. There was always a stubborn half a pixel difference between layers that I could not wring out. This tiny misalignment of background stars between layers caused the stars in the animation to appear to bounce around. After posting the animation on the Cloudy Nights Forum, I received some helpful comments and suggestions that led me to a solution, a solution that made the animation process easier and produced a much better alignment of the background stars.

The solution was using a program called DeepSkyStacker (DSS). I already had DSS, and had some familiarity with using it. The trick was to load the time-sequenced images into DSS and let it produce a sequence of registered (star-aligned) images, but not have it take the final step of stacking the images into a single image. Instead of having DSS create a single stacked image, I opened the registered sequence of images it produced as layers in GIMP, added some stretch and red circle highlights to each layer, and then exported the layered image as an animated GIF. The result: near rock-steady background stars with the only discernable motion being Pluto itself and the natural twinkling appearance of the background stars.

But, there’s more. DSS also calibrated each image so the background, star brightness, and color levels of each image were more uniform. I usually do this manually in GIMP, but for some reason, this was also a vexing problem with this group of images. Thanks to DSS, when I moved the sequence to GIMP, I only had to apply a simple stretch to each layer to bring the background and foreground levels to an eye-pleasing state.  And finally, there was another bonus, DSS is freeware.

This project challenged my image capture and processing skills. But, it showed that my modest 8-inch telescope and camera could capture a magnitude 15 target even under heavily light polluted conditions. And, it also taught me a faster, easier method for producing animations of the smaller members of our solar system. Thanks to this project, I’m looking forward to finding and capturing smaller and fainter objects.

Image Details:

August 3, 2022 04:31:02-06:31:13 UT
Two hour time-lapse animation.
North is up. East is left.
FOV: 16.9×12.1 arcmin   Original scale: 1.49 arcsec/pixel
Seeing: Good
12 image animation sequence. Each image is stack of 10 frames at 30 seconds, gain 200.
Captured with SharpCap
Processed with DeepSkyStacker & GIMP
Telescope: Celestron C8 (203mm SCT f/10) + Celestron 0.63 focal reducer/flattener (f/6.3)
Camera: ZWO ASI482MC
Mount: Celestron CGEM
Vmag (MPC): 15.0
Motion (MPC): 3.5″/hr  toward 256 degrees

In late May, I took my Orion ST-80 out of storage and mounted it on my Celestron CGEM mount. I hadn’t used this little telescope much for several years and wanted to test its suitability for capturing images of the brighter asteroids with an eye toward mounting it on an old Meade LXD-75 mount for use as a lighter weight travel scope. For this test, I paired the ST-80 with a ZWO ASI482MC camera.  The two animations below are generated from images acquired during this test. A discussion of the set up and results follows the animations.

 

Asteroid 10 Hygiea on May 28, 2022, taken with an Orion ST-80 telescope and ZWO ASI482MC camera.  Hygiea is the moving object. As discussed in the main text, distorted star images caused by field curvature are very apparent moving outward and away from the center of this full frame animated image. The distortion is especially pronounced near the right and left edges. This image covers a field of view of 1.06 x 0.6 degrees. North is up. East is left.  The animation spans a period of approximately one hour. [1]

 

This animated image is cropped from the full frame image above and slightly enlarged. The image was cropped to remove the outermost areas of the field showing the most distorted stars. The resulting field of view is approximately 20×15 arcminutes (0.33°x0.25°). Again, the moving object is asteroid 10 Hygiea. North is up. East is Left.[2]
The Target

The target for this test was asteroid 10 Hygiea. On the evening of May 28th, Hygiea was positioned high in the southern sky near the celestial meridian in the constellation Virgo. At magnitude 9.9, it was a target that could be easily captured by this telescope-camera combination with relatively short 15-second exposures. According to the Minor Planet Center’s Ephemeris Service, Hygiea would be moving at 0.31 arcseconds per minute (18.4 arcseconds per hour) towards the northwest (303°). This amount of movement would be easily detectable over a one-hour period. Asteroid 10 Hygeia is a main belt asteroid with a diameter of approximately 450 km. On the evening of this test, Hygiea was approximately 1.89 astronomical units, or 282,000,000 km/ 176,000,000 miles, from Earth.

The Set-Up

From past experience, I knew that images taken with this scope exhibited significant chromatic aberration that showed up as purple-blue halos around the stars. This is expected because the inexpensive ST-80 is a simple achromat whose design does not include the lenses made of the specialized glass necessary to correct this optical effect.

To tamp down on the chromatic aberration, I added a Baader Fringe Killer filter to the camera’s 1.25-inch nosepiece (I’d acquired the filter several years ago for another purpose). Because the ST-80’s objective lens is not made from Extra-low Dispersion glass, the lens does not focus all the red, green, and blue light at exactly the same point. The resulting defocused wavelengths create the purplish-blue fringe or halo around the stars. The Fringe Killer blocks much (but not all) of the defocused blue and red light that cause the halos (the Fringe Killer’s light transmission specs are here).

Another concern with this setup was image scale. Pairing the ST-80 with the ZWO ASI482MC camera produces an image scale of 2.99 arcseconds per pixel with the scope operating at its native focal length of 400mm (f/5). This scale produces an undersampled image. So, I added a 2x Barlow lens and a 0.5 inch spacer to the camera’s nosepiece. The spacer reduced the Barlow’s performance to 1.5x and thereby reduced the image scale to 1.99 arcseconds per pixel. Still undersampled, but better.

Placing the Barlow lens in the optical train had an additional benefit of adding some extra focal length to the system. The additional focal length in turn helped reduce chromatic aberration a bit more. In the end, this combination of telescope, camera, and Barlow lens produced a system operating at an effective focal length of 600 mm with a focal-ratio of f/7.5. The field of view of this configuration was 1.06 x 0.6 degrees.

 

 

 

This is the disassembled sensor configuration used for this test. This combination of camera, optical components and spacer, paired with the Orion ST-80 telescope resulted in a 1.06°x 0.6° field of view with an image scale of 1.99 arcseconds per pixel.

 

This is the assembled sensor configuration. The camera nosepiece consisting of the 0.5-inch spacer, Barlow lens, and Fringe Killer filter was inserted and locked into the Orion extension tube, which was then inserted and locked into the ST-80’s focus tube.

 

 

The complete rig used for this test. The Orion ST-80 is the white scope in the middle. The ZWO ASI482MC camera is attached to the ST-80’s focus tube. To the left of the ST-80 is 9×50 mm finder scope. This scope is used to visually calibrate the Celestron StarSense camera (egg-shaped object to the right of the ST-80). The StarSense camera automatically aligns the mount to the sky for accurate go-to operations. On the far right is a SVBONY SV106 (50mm f/4) guide scope with ZWO ASI120MC camera. The guide scope and camera send commands to the mount to keep the target object centered in the main imaging camera’s field of view. The scopes and cameras sit atop a Celestron CGEM mount. A small mini PC mounted on the front of the eyepiece tray just below the counterweight (not shown well here), connects to the home wireless network and allows remote operation of the mount and cameras from the back porch or inside the house.

Results

The results of this test were somewhat surprising. I was pleasantly surprised to find that chromatic aberration seemed well controlled by the Fringe Killer filter and extra focal length. There was little or no purple-blue fringe around the brighter stars, or Hygiea. The tradeoff for this, however, was an unnatural yellow color to the stars, correctable to some extent in post-processing.

I was also pleasantly surprised to find that the wide field of view of this telescope-camera configuration detected enough stars, even though many were severely distorted, to use plate-solving for accurate slewing to the desired target. In fact, plate-solving worked remarkably well.

I was not so pleasantly surprised, however, to find that while the chromatric aberration was seemingly brought under control, another defect of short focal length refractors, field curvature, was much worse than expected. Field curvature distorts the shapes of stars on the outer edges of the field of view. In this case, the distorted shapes started not far from the center of the field of view. But, there seems to be a workaround for this.

After separating the stacked color image of Hygiea into its Red, Green,  and Blue components, I found that the Red channel layer was much less affected by field curvature than the Blue and Green channel layers. By cropping away a portion of the outer edges of the one-degree by half-degree image, the Red layer produced a monochrome image with a workable field of view of a little less than approximately a half-degree by half-degree. Not as wide a field as I had hoped, but useable.

So, the results of this experiment are mixed. It seems that chromatic aberration is well controlled by the Fringe Killer filter and extra focal length provided by the Barlow lens. However, because field curvature is so pronounced, this setup will only allow production of monochrome images from the red channel, severely cropped to an area of approximately one-half degree square around the center of the field of view. This probably rules out photometry, but at least will allow capturing hour-long sequences of the brighter asteroids to produce animations of their movement across the sky.

Update: This post was updated on March 8, 2023, to show the correct focal ratio for the Orion ST-80 telescope as f/5.

Notes:

1. May 28, 2022 04:32:00-05:31:42 UT. 15 images (16 x 15 sec, gain 400).  Monochrome images from red channel only.  No noise reduction.  Telescope: Orion ST-80 (80 mm f/5 refractor). Camera: ZWO ASI482MC with Orion Shorty 2.0x Barlow, 0.5″ spacer, Baader Fringe Killer Filter. Captured with SharpCap. Processed with GIMP. 

2. Same as above. Cropped and enlarged.