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.
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.
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.