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How I Pried a Piece off of Propus (Eta [η] Geminorum)

Ah, the various vexing vagaries of a Star Splitter’s career can be so — well, very vexing.  😉

But sometimes, just occasionally, and almost always when you’re about to fold up the tent and call it a night, those vexations can unfold into a surprisingly successful night of splitting.

The Clear Sky Chart hinted that tonight (March 3rd) would have some periods of clearing around 9 PM, so in view of the fact that I hadn’t seen the least hint of a star for a week, and according to the forecast, probably wouldn’t for another week, I figured I would be wise not to take a chance on missing a chance.   And since the sky was about 75% clear, at least it looked like a promising start.

So after setting up my 102mm f/10 Celestron refractor, I went inside to get some warmer clothes while the scope cooled down, fished out a few eyepieces and put them in bolt cases, put those in my coat pockets, and went back out into the outer world.  And the sky was down to about 10% clear.  Which translates to 90% cloudy.

Hmmm, I muttered to myself — well maybe not that, but it was something close.   Hoping to arouse the sympathy of the sky gods,  I raised a few eyepieces in their direction in salute to their inscrutable, all-knowing, always frustrating ways, and the 10% of the sky that was clear disappeared — completely.  Uh oh, they must have heard my hmmm and other related words.

But, I waited, and eventually a few of the clouds parted, and I began that hilariously funny exercise called sucker-hole hopping.  Which is fantastically funny if you’re watching it, but frequently frustrating when you’re the one doing it.  After about 30 minutes of that, I latched on to M35 and NGC2158, both located at the southwest corner of Gemini, which would be the right foot of the twin on the right if you were looking at him as he was reclining right on the meridian.  I think I got that right.  🙂   You can check the chart below, though.

Gemini, with Propus shown down at the lower right corner. Click on the image for a larger version. (Stellarium screen image with labels added)

On the other hand, if you’re in the southern hemisphere, the twins would almost be standing on their heads as they transit the meridian, which would put the right foot of the right twin on the left, like this:

The Gemini Twins as seen from down under when on the meridian, click to enlarge. (Stellarium screen image with Propus label added)

At any rate, the sky was jet black — no moon — and when the clouds weren’t meandering past and through the Celestron lens, the transparency up there was pretty darn good.  The nebulosity that was stretched across some of the strands of stars in M35 was particularly captivating.  And the small nebulous glow that is NGC 2158 kept trying to explode into individual stars — and never did quite make it.

But the clouds kept pestering me, so I was about to call it a night, when Propus popped into my star clustered conscience and rolled off my tongue.  PROPUS!

Yes, Propus — that pretty little pesky red-orange glow that has defeated my Star Splitting implements for the past few months.  I had almost forgotten about Propus — on purpose.

Propus, properly positioned in relation to its neighbors, M35 and NGC 2158. Click on the image for an even larger version. (Stellarium screen image with labels added).

Propus (Eta [η] Geminorum)  (Bu 1008)        HIP: 29655    SAO: 78135
RA: 06h 14.9m   Dec: +22° 30′
Magnitudes: 3.5, 6.2
Separation:   1.607″
Position Angle: 252.5°   (WDS Ephemerides 2016)
Distance: 349 Light Years
Stellar Classification: M3.5
Status: Physical, orbital chart and data can be seen here.

Well, anyone who stands outside chasing madly from one sucker hole to the next with a telescope on a wet deck in a thirty-seven degree temperature and with dampness dripping from his clothes isn’t likely to know when to quit.  So I slid the scope two degrees to the southeast and found the familiar reddish-orange glow of this alleged double star staring back at me.

Now I had a 15mm TV Plössl (67x) in the scope at that point, and I didn’t expect to split it with THAT — believe me, I knew better, I’ve met failure here too many times to get reckless or ridiculous — but I could see that the image was very stable, so I moved up to a new eyepiece I had barely begun to become acquainted with, a 12mm Brandon (83x).

Chanting the position angle to myself — 257 degrees, 257 degrees — I looked hard.  Boy, did I look hard.  I thought I saw a slight flaring at about the right spot, but the image wasn’t quite as stable at this magnification, so I kept tweaking the fine focus knob just a very slight bit until I could get the flaring to go flare somewhere else.  THEN — I thought I saw a speck of light.  And I do mean just a speck — the kind your mind could invent.  It was there, it was gone, it was there, it was gone again, etc., etc.  I kept working very carefully and gingerly with the fine focus knob and finally I got it to the point where I could see that terribly small speck about 75% of the time.

Now I really had to look hard to see it.  And I do mean hard.  My eyes were screaming for a break.  They’re my friends in this endeavor, so I agreed and looked off into the distance at the clouds and stars for about thirty seconds, then slipped up on the eyepiece again, and IT was still there — but just very barely.  IT was so small and IT was almost touching the bright orange rim of the primary, and IT was just so darned faint!

Hmmm,  “need to confirm, need to confirm,” — I kept hearing that voice in the back of my head — “better be sure, don’t be stupid, be sure.”  Considering that the seeing was noticeably less stable in the 12mm compared to the 15mm, I didn’t want to jump too far forward, so I swapped a 10mm Tak LE (100x) into the focuser, and leaned forward to look again.  IT was still there, still very briefly, still very small, but actually not quite as easy to see as in the 12mm Brandon.  All that move had accomplished was to create more doubt.

The Powerful Plössl at rest.

So, in cases like this, I revert to my secret weapon, the old Celestron 7.5mm Powerful Plössl that has bailed me out many a murky night.  I always handle this small piece of metal and glass with reverence, and this time was no different.  First, out with the 10mm Tak, then face the southwest in the direction of Gemini, raise the Powerful Plössl to the sky, take a deep breath — and take a long sip of hot tea.  And then place the little Plössl in the diagonal with real reverence.

And that’s what I did.  And this is what I saw.  An orange glow hopping around all over the place.  Back and forth, sideways and edgeways, up and down and all around.  BUT — every now and then it would hold still — for something like a tenth of a micro-second.   But my eyes are used to that.  When it would stop — or maybe, to be more accurate, when I could catch it in between hops — that dinky little speck of dim light was there.  And clearly very close to and yet very separate from the primary.  A bit more than hair split, but only by a hair — at about 257 degrees, give or take a couple tenths of an arc minute.  😉

But that kind of bouncing is bad for balance — not to mention the eyes —  so I went back to the Brandon.  And I could still see that dim dot of light.  And IT was hardly hopping  at all at 83x.  And my eyes appreciated that.  And I just looked for another fifteen minutes or so.  And finally the clouds took IT away.  And I sat there for a few more minutes with that image of IT still in my mind, grinning from my southeast ear to my northwest ear.

Now there were two things that really made this effort a success — besides the seeing, which was at least decent, but not spectacular — I’ll rate it a three, as in Roman numeral number  III.  The first thing was the ten to one reduction knob — AKA the fine focus knob — on the focuser (which, by the way, was a Baader Steel Track model that I added to the scope).  I could not have done this without it.  And second was the small group of faint stars on the opposite side of the primary from the secondary — which would be the east side of it, at a PA of about ninety degrees.  I found that if I very slowly turned the fine focus knob until as many of those faint stars as possible popped into view, I had the focus I needed to catch that very evasive secondary.  When I turned the focus knob just enough to lose a few of those stars, I would lose the secondary also.  I’ve used this technique before, but this time it was critical for seeing the secondary.  There was virtually no tolerance here for focus error — it was either 100% perfect, or it was 100% hopeless.  Nothing at all in between.   Take away number one or number two, and for me Propus would still be an alleged double.

Some folks — in fact, most I would guess — would attack this one with brute force, meaning with as much magnification as possible.  But neither Greg nor I see that kind of seeing except for maybe one or two nights out of the year.  So that pretty much limits us to separations of about two arc seconds on doubles of unequal magnitudes, such as this one.  You’re rarely going to read that I split anything at an atmospheric shattering 300x — it’s just not likely to happen here.   What that means is that if I’m to have any success at all, I need to banish brute force and go for finesse.  And in this case,  between the Brandon — and the focuser — and the aid of those faint stars east of the primary — I think I finessed all the photons possible out of this one.  A few less photons, a bit more haze, and I would have met abject failure again.

So, now I’ve got a new friend in the 12mm Brandon.  And I’ve pried a piece off of Propus.  And survived the sucker-hole hops to share the adventure.

Clear skies, Star Splitters!  😎

Update 1/31/2016:  In case you’ve wondered what you’re missing, here’s an enticing image of Propus taken by Mark McPhee (west is at the left, north is at the top):

Propus, Mark McMphee


8 Responses

  1. Fantastic account detailing so well the trials, tribulations, and triumphs of intrepid double star observers. All hail fine focus!

  2. Reading this piece on Propus (which I dont think I have read
    before) reminded me of how I have seen its companion, its there
    its not there, its there, that is until last Friday when the seeing was
    fantastic. I had made my way up to Propus from Orion and was just
    about to call it a night when I deceided to have a quick look . And boy
    was I glad that I did, at 100x there was something there and when I
    moved up to 275x the companion stood out, not just stuck on the side
    of Propus but well seperated and visible 100% of the time. To me
    it seemed to be a greyish blue colour . I watched it for maybe 15 minutes then the clouds rolled in, as it was about midnight it was
    time to hit the sack and dream of a small greyish blue dot next to
    a bright orange star.


    • Hi Pat and Mark,

      Pat, your experience with Propus is very similar to mine. Haven’t had a chance to look at it this season, but the few times I’ve been back to it since I wrote the above article, the seeing was miserable, so no luck. Should have a chance sometime in the next few months to get another shot at it, hopefully with the six inch refractor.


    • Pat:
      Lately I have endeavored to image Propus with my 15 inch reflector–what a task that is turning out to be! Anyway, the bit of the companion that is not hidden by the primary in these crude pictures has an unmistakably blue tint to it.
      I think I see some blue for the secondary in the image in the eyepiece with my 15 inch reflector at 395x, but really cant say what the color is when using the 8 inch reflector at 340x–I’m just happy I finally got a good split of it with the 8 inch!
      I really didnt know much about Propus until I was confounded by its difficulty a few days back. This lead me to John’s fine writeup, and well, now I’m obsessed with damn thing…

  3. Hi Mark, mostly when I was looking at Propus I could not make out
    any color other than a greyish dot just visible some of the time, it
    took the fantastic seeing we had last friday to show any real color.
    I have found Propus very difficult to get a good split in the 6 inch
    reflector. John has a way with words, he can put into words what
    we are thinking and seeing but cant get down on paper, there is not
    one of his posts which I did not enjoy reading. You can get obsessed
    easy enough with double stars so just enjoy the ride.


    • I think someone asked me if I could resolve Propus with a 100mm scope, so being foolish and naive, I gave it a try. Under good seeing, it should certainly be possible. The sane thing to do in regard to Propus is attack it with a minimum of six inches of aperture.

      But then again, sanity doesn’t play much of a role in this obsession.


      P.S. — I updated the position angle and separation to match the latest numbers in the WDS.

    • Hi Pat and John,

      I just posted a few words and pictures of Propus over at the Yahoo Double Star Imaging forum in case you are interested. The secondary is indeed bluish. What a fine little gem this tantalizing binary has turned out to be.

      • Great image, Mark. I’ve added it at the end of the post above so those who aren’t members of the Double Star Imaging forum can see it.


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