Multiple multiple-star systems

The motivation for this imaging session was to see if I could successfully record the companion star to Polaris (answer: yes).  This multiple star is a moderately-challenging  visual double for small telescopes. The equipment used was a Sky-Watcher Skymax 102 MCT (Maksutov-Cassegrain Telescope) amplified with a Tele Vue 2.5x Powermate.  The nominal base focal length for this scope is 1300 mm (L/D = 12.7).

Prior to setting up the camera, I first did some visual tests with several different eyepieces without the Powermate amplifier.  The eyepieces used were: Tele Vue 24mm Panoptic, Takahashi TPL 12.5 mm, Masuyama MOP 10mm, and a Brandon 12mm.  

The 24mm Panoptic (54x) was used as a "finder" eyepiece.  With some concentration I was able to see the companion star with this eyepiece, in spite of the relatively low magnification.  The star was visible with only occasional lapses with the Takahashi TPL (112x).  I was never certain about seeing it with the Masuyama (130x).  The best and steadiest view was with the Brandon (108x).  The Takahashi and Masuyama eyepieces are recent offerings in the market and consequently the subject of much comparison and discussion on astronomy forums.  Brandon eyepieces have been around since the 1950s and are revered by some and dismissed by others.  On this night, the Brandon came out ahead.

Polaris, Alpha Ursae Minoris, the North Star or Pole Star, is a triple-star system.  The magnitude-2 primary star Polaris Aa has a close companion Polaris Ab separated by less than 0.2 arcsec.  This companion is 420 times fainter than the primary and has been imaged by the Hubble Space Telescope. The  more easily-visible companion is Polaris B.  It has magnitude 9.1 and it separated from Polaris A by 18.6 arcsec.  Polaris B is seen below and to the right of Polaris A in the image below.

Polaris A+B.  E-M5iii + SW102 + 2.5x Powermate.  ISO 800, 5 s.

The nominal focal length of the SW102 + 2.5x Powermate is 1300 × 2.5 = 3250 mm.  However, that applies only to a specific focal-plane position.  Moving-mirror telescopes such as this one are focused by shifting the focal plane rather than the eyepiece or image sensor.  That changes the effective focal length.  I determined the effective focal length of the present setup by measuring the separation between the double-star pair Mizar and Alcor in Ursa Major.

Mizar and Alcor.  ISO 800, 4 s, 4x binning.

The angular separation of the two stars is 0.1968°.  In the E-M5iii image they are separated by 2895 pixels.  This works out to L = (0.0033 mm/pixel)*(2895 pixels)/tan(0.1968) = 2781 mm for the true effective focal length and an image scale of about 0.245 arcsec/pixel.

Mizar is an easy visual double star in small scopes, and each of the two components is itself a double, as is Alcor.  This is therefore a six-star system.  It is the second-closest sextuple star system, about 86 ly distant.

Mizar A + B.  Each star is also a double.  ISO 800, 1 s + 0.5 s average.

The closest sextuple star system is Castor, Alpha Geminorum, at a distance of about 51 ly.  The two brightest components, each of which is a spectroscopic double, are currently separated by just over 6 arcsec.

Castor A+B.  ISO 800, 1/4 s.  Stack of 4 images.

The third component, Castor C, also known as YY Geminorum, is separated by 73 arcsec.  This magnitude-8.8 companion requires a longer exposure to be visible.  Castor C is an eclipsing binary system of two red dwarfs.

Castor C (YY Gem) is the reddish blob to the left and up from the overexposed primary. ISO 800, 4 s.  North is to the right.

 Although Castor is designated Alpha Geminorum, it is slightly fainter (mag 1.6) than its twin Pollux, Beta Geminorum (mag 1.2).
 

Castor (top R) and Pollux (bot L).  E-M5 + Rokinon 135mm f/2 + softon filter.  ISO 1600, 30 s.

The "Trapezium", also known at Theta 1 Orionis, is an asterism of four stars at the heart of the great Orion Nebula, M42.  The component stars are part of a tight open cluster associated with this nebula and are too closely packed to be in a gravitationally stable arrangement.  They will eventually disperse.  The brightest one will  go supernova in a few million years.

Theta 1 Orionis, the "Trapezium".  ISO 800, 1 s.