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Omicron (ο) Cephei, Σ 2883 (STF 2883), and Σ 2923 (STF 2923)

August 23rd, 2010, just after midnight

Fall is in the air again tonight – already the temperature is a crisp fifty-one degrees.  Adding to the autumn ambience is a very bright moon in the southeastern sky which is just a few hours away from being full, so this will be a night for turning the scopes to the north to do some exploring in Cepheus.

Sears 80mm on the left, Mizar-Carton 80mm on the right.

And I’m going to do something I’ve been wanting to do for a while, which is to mount a pair of 80mm f/12 refractors side by side on a Giro III mount.  The beauty of having two scopes of the same aperture and focal length on a side-by-side mount is that I can use different magnifications in each, and then jump between them to compare views.

And THAT means – this is going to be SMALL eyepiece night!  Both scopes will only hold a 1 1/4″ diagonal, so I’ll restrict myself to using Plössls and Orthos tonight – none of those large 1 1/4″ Radians, and definitely no monster two inch eyepieces!  So prepare to do some squinting into some small lenses.

From my observation site Cepheus is framed on either side by some trees, and oriented so that Gamma (γ) Cephei — the pointed end of the constellation — is pointing down, as shown in the map below.  That puts Omicron (ο) Cephei just to the left of a few tall firs.  In the bright moonlight, I can barely see it, but I’m able to line it up with a protruding tree limb very close to it.  So, resorting to a seldom used high tech approach, I center the tree limb in the 6×30 finder of the scope on the right and move to the left – and we have success!

Stellarium screen shot with labels added. Click on image for a larger view.

Omicron (ο) Cephei  (Σ 3001)           HIP: 115088    SAO: 20554
RA: 23h 18.6m   Dec: +68° 07′
Magnitudes: 5.0, 7.3
Separation:  3.35″
Position Angle: 222°  (WDS 2012)
Distance: 211 Light Years
Spectral Classification: G8
Status:  Physical, orbital chart can be seen here.

The Omicron pair is a tight 3.4″ apart.  I find that with a 20mm Plössl (60x), the secondary is just budding from the edge of the primary.  I drop a 15mm Plössl (80x) in the other scope and have a definite split, but replacing that eyepiece with an 11mm Plössl (109x) and then a 9mm Plössl (133x), I can see a respectable amount of black sky between the two stars.  I decide to try a 7mm Ortho (171x), but that dims the view just enough that the secondary becomes hard to see.  The best view is with the 9mm Plössl.  The primary is a very definite yellow in both scopes and in each of the eyepieces.  Observations in the Haas book include a yellow-green, which I don’t see at all.  That book also describes the secondary as blue, but in my scopes it’s too faint to determine a color – pale white is the best I can do.

Σ 2883  (H N 121)           HIP: 109474    SAO: 19922
RA: 22h 10.6m   Dec: +70° 08′
Magnitudes: 5.6, 8.6
Separation:  14.5″
Position Angle: 252°  (WDS 2007)
Distance: 106.5 Light Years
Spectral Classification: F2

Moving into the space between Beta (β) and Iota (ι) Cephei will get us near Σ 2883 (STF 2883).  Again, the moonlight is making it difficult to make visual contact, so I point the scopes to the  southwest of Beta (β) and then peer into one of those little 6×30 finders.  There is one advantage to the bright moonlit sky — I can clearly see the cross hairs in both finders.  I find what looks like the ideal candidate, center it, peer into the scope on the right which now has a 25mm Plössl (48x) in it because I need the larger field of view it offers — and we have contact!  These two are farther apart, but each component is a bit fainter.  The primary is a bright white, and the secondary is a just a pinpoint of faint light, clearly separated.  On a dark night, this would be a beautiful pair, but against the background of the bright sky, the 8.6 magnitude of the secondary is tending toward the weak side of faint.  The best view of this pair is with a 15mm Plössl (80x) — in the 11mm Plössl (109x), the secondary is difficult to see in the glare of the primary.  Moisture in the air seem to be doing it’s part to enhance the glare.

And, I see, there is quite a bit of dew collecting on the dew hoods of both scopes, and on the deck, and on the tops of my shoes!  Time to get up and take a short break — tea and cookies for me, a dog bone and water for Astro Klaus, my trusty side kick — stretch my legs, and dry things off  just a bit.

Image from Stellarium with labels added.

Σ 2923  (STF 2923)         HIP: 111325    SAO: 10418
RA: 22h 33.2m   Dec: +70° 22′
Magnitudes: 6.3, 9.2
Separation:  9.7″
Position Angle: 47°  (WDS 2007)
Distance: 400 Light Years
Spectral Classification: A0

Last, but not least, and not easy,  is Σ 2923 (STF 2923) — not easy because I can’t see it at all.  By now, Cepheus has rotated enough that Σ 2883 and Σ 2923 are just about horizontal, which should make navigation easy on the alt-az mount.  I can see by my chart that if I move to my right about 1.5 degrees, and then down slightly, I should pick up a distinctive group of three stars — the one degree field of view of the 25mm Plössl makes the distance easy to determine.  Finding the stars is not a problem, but visual confirmation of the double is another story.  It’s clear from the more detailed chart that the star in the middle of the configuration of three is Σ 2923, but it takes a good fifteen minutes and several repeated searches before I can confirm it.

small EYEPIECES! (Click image for larger view.)

My plan here was to start with the brightest of these three Cephean doubles and end up with the faintest, mainly because I needed the other two to help locate it.  So we’ve gone from magnitudes of 5.0 & 7.3, to 5.6 & 8.6, and now to 6.3 & 9.2 — a declining pattern of brightness that I hadn’t thought too much about when I started this.  But as the moon gets higher, the sky is becoming brighter, and the moisture in the atmosphere is following in step.  Leaving the 25mm in the scope on the right in case I need it for reference, I put a 20mm Plössl (60x) in the one on the left and finally am able to see the secondary, very faint and very close at 9.5″ from the white primary.  A 15mm Plössl (80x) brings it out well, but again, any more magnification than that just increases the glare and makes the secondary more difficult to see.

So, a damp night, damp clothes, dew-covered scopes, and all in all, a rather successful night.  I think I’ll come back again, though, when the sky is darker.  These stars are not at their best in this bright moonlight, and I suspect that Σ 2883, especially, will be a very rewarding sight in a 60mm scope.  Don’t wander off to far — I’ll be back!

The moon caught in the act of casting its reflected photons at a pair of 80mm f/15 refractors. The one on the left is a Sears model, with a Towa lens; the one on the right is an old Mizar, with a Carton lens installed in it.

2 Responses

  1. While imaging Wolfs Cave (vdb 152) it so happens that STF2883 is pretty much on the edge of my image and since I use a RC for imaging, I keep watching this region to check the focus.
    “Interesting double”, I thought. Now that I’m going through the second night of imaging, I decided to look it up and stumbled across your page. So thank you for all the info about this truly beautiful couple.
    Greetings from northern Germany.

  2. Vielen Dank, Sebastian. Glad it was of some help to you.


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