spikes and a detective story

Here's an image of open cluster M 25 taken with a small refractor.

Open Cluster M 25

^{An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring (wikipedia).  Most of the stars have the same appearance, though there can be variation.  Many open clusters are visually large objects in the sky, best seen with a small telescope or binoculars.}  

diffraction spikes are those pretty lines that you see radiating from bright stars in photographs. interestingly, the spikes are not inherent to the starlight, but rather an artifact of light interacting with the optical system used to capture the image.

reflecting telescopes typically have the most prominent diffraction spikes, large cross shaped lines emanating from bright stars.  they are caused by cross bars in the light path used to hold a secondary mirror in place.

the classic vertical and horizontal line are created by "spider vanes' 
which hold a secondary mirror in place in many types of reflecting telescopes
wikipedia diffraction spikes

refracting telescopes (lenses only, no mirrors) have no such obstacles in the light path, they produce the "cleanest" stars without prominent diffraction spikes.

a few use curved spiders in their reflectors to minimize the effect, but they can have issues:

destinycomp.com

as you can imagine amateur telescope makers have tried all kinds of different curved shapes (modeling with fourier-transforms) to minimize the effect.  while curved shapes can mitigate the effect, they scatter a small amount of light throughout the field.  so many prefer straight spiders with spikes and a cleaner surrounding field.

^{I know what you're thinking now: wait a minute.  the brighter stars in your image have diffraction spikes which are caused by cross-bars suspending the secondary mirror of a reflecting telescope, but you said this was taken with a small refractor.  What is this, some sort of misguided...}

April fools, I Photoshopped  the image, adding the spikes in to accentuate the stars in the cluster. 

some find the spikes aesthetically pleasing and will actually add them into their image by stringing fishing line across the front of the telescope, or using photoshop as I did above.  

A Detective Story:
Having an image selected for the astronomy picture of the day (APOD) is the pinnacle of amateur astronomy.  If you look at the caption for the APOD for 1/12/17, you'll see the following cryptic message:
"Editor's Note: The NGC 891 image used in today's APOD posting has been replaced and the credit corrected to indicate the author of the original work."        
Apparently, an amateur astronomer from Italy who had been boasting that his images taken with modest equipment were comparable to those of the major observatories (thanks to his excellent technique) posted the image that was originally selected.  It turned out, the image was plagiarized.  The astronomer had taken an older image from the Mount Lemmon Observatory and photoshopped it into his image.  

How did they catch him?  take another look at my December image below.  This was just a star test, an image of a bright star (Capella) slightly out of focus used to to diagnose a problem in my optical train (telescope, filters, filter wheel, off-axis guider, camera...).  looked pretty cool so i figured i'd share it:

Capella star test

The telescope used to take this shot doesn't have crossbars, so you don't see a single set of large diffraction spikes, but if you brighten it up like this, you see hundreds of tiny spikes radiating out from the star.  if you look closely, you can see a slightly more prominent set of lines going vertically and horizontally.  the vertical and horizontal line are caused by the rectangular architecture of the camera while the myriad surrounding spikes result from subtle irregularities such as dust on the lenses.  

Here's an image of a bright set of stars taken with a refractor.  The brightest set of stars have pairs of lines 120 degrees apart.  These are caused by 3 clips holding the lenses in place:

Omicron-1 Cygni, the Patriotic Triple 6/09

Thus, the pattern of lines around a star reveal the instrument used, just as ballistics on a bullet can be linked to a gun.  

The "author" of the original APOD image, claimed to have used a telescope that does not have crossbars, but the diffraction pattern around the stars in the plagiarized image exactly matched the diffraction pattern in the observatory image with large diffraction spikes.  someone thought the image looked familiar and compared the two...

why someone would do this is beyond me, but apparently it's not that uncommon. 

more sordid details here:


the smoking gun image comparing the two:

lunar libration

After the recent lunar eclipse, I attempted to line up a before and after image, superimposing the full moon from the night before with the eclipsed moon just before sunrise, and found something odd: they don't match.  it wasn't just a matter of lining things up--which i can correct, or size--which i expected to be minor, but the surface of the moon itself appeared to have rotated:

lunar libration, full moon at sunset to eclipsed moon at sun rise 1/31/18

In a moment of doubt, i feared that a processing bug was creating horrific artifacts, since i know that the moon is tidally locked to the earth (it doesn't rotate, the same side always faces us).  then i recalled the term lunar libration, referring to minor rotation in the appearance of the moon relative to the earth.  a number of factors contribute to this, but the easiest to grasp is that as the earth rotates, the observer's location shifts.  in this case, viewing the moon at sunset and then sunrise, i had moved by the diameter of the planet earth, explaining why the moon appeared to rotate slightly.  

Here's a really cool animation of lunar libration throughout a lunar month:

wikipedia lunar libration animation

as well as a more in depth explanation of the causes of lunar libration:

wikipedia

imaging details:

close up:

ZWO ASI120MC, Takahashi FS-60C at native F/5.9, 355 mm.

skywatcher star adventurer tracking mount  

full moon .325 ms exposure 12 seconds captured at ~17 fps

eclipsed moon 152.6 ms exposure 68 second capture at 6 fps

13:24 UTS

1/31/2018

Eastbluff, CA

abell 33, the diamond ring nebula

Here's a spherical planetary nebula with a bright foreground star conveniently superimposed on the edge, providing the "diamond" for the ring:

Abell 33, the diamond ring nebula OIII RGB
click on image for full size

The small blue star in the center is the white dwarf creating the nebula.  A close up of the RGB version shows a hint of a second star to the lower right.  This is likely another superimposed star or a "visual double", as a double central star should (in theory with rare exception) create a bipolar, rather than a spherical nebula.  

central star of Abell 33
RGB, upsampled 2x

apparently an open issue in astronomy is whether the percentage of planetary nebulae double central stars matches the percentage of doubles in the general stellar population.  there are several recent surveys which address this, and may confirm my supposition, but i don't have access to them.  

The binary fraction of planetary nebula central stars - II. A larger sample and improved technique for the infrared excess search

The binary fraction of planetary nebula central stars: the promise of VPHAS+



The RGB image shows two other things:
1. how faint the nebula is, as it was barely detectable on the RGB image, requiring narrow band filters to bring it out.
2. how bright the "diamond" star is as it created a "bloom" (bright white artifact to the right of the brightest star).  
should have shot the RGB unbinned to better resolve the central star and minimize blooming.  

I did take a few trial Ha shots, but got only extremely faint signal in a circle, matching the OIII, but without detail.  

Image details:
8" LX200R, SX Trius 694 binned x2 to 0.8"/px,
astrodon 3nm OIII, RGB E SERIES GEN-II
ASA DDM60
R 18x4 minutes, G 16x4 minutes, B 8x4 minutes
OIII 24x20 minutes.
4/14/17-5/1/17, bortle white skies
eastbluff, CA