Here’s a quick attempt to capture the galaxies featured in my previous three Astrobin posts and evaluate the field performance of an old Nikkor 50mm AI f/1.4 lens coupled with a ZWO ASI294MC camera.  Started imaging at 2:00 am, but was only able to capture twenty-four 240s light frames before clouds moved in at 3:30 am.

The image frame covers a 24° x 14° portion of the constellation Ursa Major that contains stars Alkaid, Mizar and Alkor.  To my delight and surprise, you can actually see some structural detail in galaxies M101, M51, and M63 — in spite of a very short integration time (1 hr 36 min), average seeing conditions, and a strong gradient from a waning crescent moon (33% illumination). 

The galaxies do indeed appear rather dull and insignificant in a sea of bright and colorful stars, but I like the “naked-eye” field of view that a 50mm lens provides.  I see around one-fourth this field of view when looking through 8° x 42° binoculars, which I like to do while the mount and camera are busy collecting data.   You can’t see these galaxies with binoculars, of course, but the stars are lovely and it’s fun to get your bearings in the night sky.  🧐

Images

A. Wide-field view of Ursa Major region with acquisition details provided at bottom of image.

B. Annotated version, with inset views of the M101, M51, and M63 galaxies captured using a Meade 8” LX200 “classic” SCT telescope and featured in my previous three Astrobin posts.

C. Wide-field image without acquisition details or annotations.

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 Light diffraction off the sharp edges of the iris blades in the Nikkor 50mm camera lens yields prominent 14-point diffraction spikes on stars of magnitude 7 or brighter (Figure 1).  I am not a big fan of the resulting star burst effect, but they don’t bother me in this image.  Probably because I subliminally associate many fond memories of 35mm film images I took with lens the wonderful Nikkormat F2 it came with when I bought from a camera shop in Reading, PA back in 1981.  It’s funny how our memories shape our perception of the present and vision for the future.



Figure 1. Iris blades on Nikkor 50mm AI f/1.4 lens, with aperture decreased three f-stops from 1.4 ➠ 2.0 ➠ 2.8 ➠ f/4.0 in order to more easily focus and achieve tighter stars.  Resulting starburst effect is shown for the three brightest stars in this image: Alcor (m=3.50), Mizar (m=2.20), and Alkaid (m=1.85).


Field performance

Image data were analyzed using PixInsight software to identify distortions and aberrations in the star field and evaluate the field performance of this Nikkor 50mm lens and ASI294MC camera combination for wide-field images.  The median full weight half mean (FWHM) was 1.917 pixels.  Contour maps of the FWHM distribution reveal there was a slight tilt in the optical train downward and toward the lower left corner (shown below).  FWHM values increase toward the outer edges of the frame, from around 1.6 to 1.8 in the center to greater than 2.8 in the corners of the frame.  Over much of the image, the star field is reasonably flat.  BlurXTerminator’s deconvolution algorithms decreased the median FWHM to 1.27 pixels and made a more uniform (e.g., “flat”) field of stars.




The pixel size of ASI294MC is 4.63 µm, so when paired with a camera lens with focal lengths less than 300mm, the final image will be under-sampled and resolution so low that small stars will not be round, but rectangular with only one or a few pixels.  So, while collecting the data, I configured the mount settings in the ASIAir Pro to ensure that every other frame was dithered.  The light frames were drizzled two times when registering and stacking them using PixInsight’s WBPP.   The 2X drizzled star field is much better, as you can see in the example shown in the lower right corner of the above figure.


The median eccentricity (Ec) of the stars is 0.4453, which I consider to be very good for a 42-year old “nifty-fifty” lens (Figure 2). Contour maps of the Ec distribution reveal little variation, with stars across the entire field of view appearing round.  Star shape becomes more distorted towards the lower left corner, which most likely is the result of the sensor tilt identified previously.  BlurXTerminator’s deconvolution algorithms do a terrific job reducing these distortions.



Rule of thumb: if Ec is above 0.7, the stars appear egg-shaped.  If the Ec is below 0.45, the stars will appear perfectly round.   Everything in between 0.45 and 0.70 is a matter of personal taste, but I am satisfied when the eccentricity of the star field is below 0.6.