NGC 4236, Extremely Faint Galaxy In Draco – May 2018 – Observer’s Challenge Report #110

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MAY 2018 OBSERVERS CHALLENGE – NGC-4236

Rogers NGC-4236 Inverted b

10-inch reflector, and spending four hours in my attempt to see galaxy NGC 4236, of which the first three were unsuccessful.  However, during the forth hour, at well past 1:00 AM EDT, could glimpse an extremely faint, elongated NNW-SSE oriented blur of light as shown in my sketch.  The galaxy appeared featureless due to the extreme low surface brightness, and visible only intermittently with averted vision. 

Sky conditions were poor with a NELM of 4.9, which is about normal for springtime in the foothills of North Carolina.  There is a distinctive tilted half-circle of five stars, NNE of the galaxy which works well to assist in determining the exact location of this very faint galaxy, or better said, extremely faint galaxy. 

On the previous night, under the same conditions, using a 6-inch reflector, the galaxy was invisible, despite spending two hours in my search. 

Roger Ivester

 

Image by Dr. James Dire from Hawaii

NGC4236

NGC 4236 By Dr. James R. Dire

NGC 4236 is a faint, nearly edge-on barred spiral galaxy in the constellation Draco.  The constellation is often overlooked due to its low surface brightness.  The galaxy is relatively large in our sky, spanning 24 x 6.8 arcminutes. Its integrated magnitude is 9.63. NGC4236 is located 11.7 million light years away and is part of the M81 galaxy group.  It rivals M81 in size. However, M81 is 3 magnitudes brighter!

NGC 4236 is located “above” the open cup of the Big Dipper Asterism. Its distance above the cup is approximately the same as the distance between the two starts forming the top of the cup – Dubhe and Megrez.  NGC4236 lies just over a degree west of a linear trio of stars: 4, 5 and 6 Draconis.  Both 4 and 6 are 5th magnitude and are orange and yellow, respectively, while 5 is 4th magnitude and blue in color.

Despite its brightness and location near naked-eye stars, NGC 4236 can be very difficult to find, even with a GOTO mount.  The Image was taken with a 70mm f/6 apochromatic refractor using a 0.8x focal reducer/field flattener.  The exposure was a whopping 3 hours.  In the image, north is up and east is to the left.

In the image, the bright blue star on the left edge is 5 Draconis.  The orange star is 4 Draconic. Those two stars are separated by 40 arcminutes.  The smudge just above the center is NGC4236.  The exposure captured about half the actual length of the galaxy, essentially the galaxy’s central bar and the inner, brighter spiral arms.

To help identify the galaxy in the eyepiece, note the arc of three stars on the northeast side of the galaxy.  The top star is magnitude 8.3, while the lower two are magnitude 10.5.  There is also a magnitude 9.8 star on the south end of the galaxy’s major axis.  Seeing these four stars in the eyepiece allows one to center the galaxy in the eyepiece and then using averted vision to see the galaxy.

I viewed NGC 4236 this month in three telescopes.  The first was a 190mm f/5.3 Maksutov-Newtonian reflector using a 14mm eyepiece (71.4x).  I centered the galaxy using the four stars around it as described above.  Direct vision did not produce the galaxy. However, with averted vision I was barely able to make out the elongated shape of the very diffuse galaxy.

I next viewed the galaxy using an Orion 12-inch, f/4.9 Dob using a 20mm Televue Nagler eyepiece (75x).  The galaxy was just as difficult to see in this scope as the Mak-Newt.  This was probably because the Dob’s optics are nowhere as good as the Mak-Newt.  Plus, I later discover that the primary mirror on this astronomy club telescope was extremely dirty (I have since professionally cleaned it and recollimated it).

Finally, I viewed NGC4236 with a friends Orion 14-inch, f/4.6 Dob with clean optics and a good collimation. Through this telescope, the galaxy could be seen directly.  I could see about the same visual detail as in my image!   JD 

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