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17th April 2010 - Splitting doubles with a Skymax 127 (and spotting a galaxy too)


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Date: 16/4/2010

Place: NW London

Time: 2130 to 0100

Seeing: average, some periods of calm.

General Visibility: Mag 4 stars visible to naked eye at high altitudes, but limiting magnitude drops towards the horizon, only the brightest stars visible below about 20 degrees due to orange sky glow. No moon. No contrails :D So perhaps clearer at overhead than usual.


Nikon 10x50 bins

Skywatcher Skymax-127 scope (f/11.8)

Various Plossl eyepieces: 9mm (167x), 12mm (125x), 15mm (100x), 20mm (75x), 40mm (37.5x), 2x Barlow

Last time out I spent some time looking at some double stars, I found I could resolve some and not others. This time I wanted to be a little bit more systematic so I made up a list of doubles of varying closeness and set out to see how I got on splitting them.

I started in Cancer. Probably a bad move as I always seem to spend ages looking for anything in this constellation - I don't know what it is but I find it very hard to find my way around.

The one thing I can find is Praesepe (one of my favourite things in the sky, I always take a look), it's very easy to find just now as Mars is just to the NW of it, giving me a good naked eye signpost - the rest of the constellation is invisible when I am :(

While planets were not my focus this evening, I did spend some time looking at Mars. I tried out some filters, yellow perhaps added a little contrast, bringing out a hint of a polar cap, but Mars is shrinking away now, and I didn't really spend enough time studying it to be sure of anything.

The first double I looked at was Gamma Cancri. This system has a separation of 95" and so ought to be easy... I was expecting to see a clear companion at 37.5x, but found I couldn't. I think the reason for this is that the companion is Mag 12.9, possibly beyond the limits of my scope under miasmatic London skies. At 100x the star did appear to resolve into a white primary and very feint red companion. These two were quite far apart, though and I'm not convinced the red star wasn't just something that isn't shown on my atlas (which goes down to Mag 7) - Stellarium shows a Mag 9.8 red star close to Gamma, which is probably what I saw, I am just not sure if it is the companion star! An inauspicious start to what I had hoped would be an evening of scientific enlightenment :) Things didn't get much better looking at Zeta Cancri, the legendary triple star system; it took me much longer to find it than it would had it lain in some reasonable constellation, and I found my self tracking back and forth to Praesepe (my reference point) to start my hunt again, each time the star field looked different I swear! I might have found zeta, I'm not sure. I found a double that I could split at 75x into a white and blue/white pair.

Needing something of a morale boost, I abandoned Cancer and slew the scope over to Regulus in Leo (separation 177.6"). If I couldn't split this, I would almost certainly just gone an had a look at Saturn again :D Fortunately for science, Regulus was no match for my scope and yielded its secrets with barely a struggle. At 37.5x Regulus proved to be a pair of blue/white stars, one considerably brighter than the other.

Onwards, and, literally, upwards ('North' if you must), to Gamma Leonis, aka Algeiba (sep. 4.5"). This double is listed in all the guidebooks and is well established on the tourist trail as one of the most beautiful doubles in the sky. It split for me at 200x, yielding a yellow pair. I thought one a little brighter than the other but not dramatically so. I thought the fainter had a hint of blue, though this was possibly illusory.

From Gamma I hopped NW to Kappa Leonis (sep 2.4"), which I found quite easily. This threatened to split at 200x, but I couldn't get a clean split. Higher magnifications didn't seem to help as the seeing took its toll, what was given in size was taken away in stability. With hindsight I am not fully convinced I was looking at Kappa at all... I think I would have been able to split it cleanly given my experiences splitting nominally closer pairs.

Next, out of Leo and into Coma Berenices, home of my hitherto favourite double, 2 Coma (sep. 3.7"). This is an awfully pretty double, and beats Algeiba hands down for title 'prettiest double in the sky'. Since I had trouble with Kappa Leo I thought I might with this too, since it's a closer pair. In the event though, it was just about split at 100x, and was clearly split at 167x. It resolves into a white/yellow and orange/red pair. Lovely.

24 Coma (sep 20.3") is also a gem, it split at 75x into a blue and yellow pair. Well worth a look if you're nearby, and perhaps want a break from the endless supply of galaxies in the area.

The last few doubles I had listed to look at were all in neighbouring Bootes. Tau Bootis (Muphrid) (sep. 4.8") was my first port of call here. I couldn't split it, primarily I think because the companion at Mag 11.1 is probably just too feint to pick out.

Pi Bootis (sep 5.6") split nicely at 100x into a blue/white and yellow pair. There was also a nearby third star, and wondered if the system is a triple, but I can find nothing in the literature to indicate this is so.

Xi Bootis (sep 6.7") also split at 100x (and would possibly have done so at lower magnification). It is a yellow/white and orange/red pair. The red star is really quite striking, and for me this double ranks alongside 2 and 24 Coma for attractiveness.

My last scheduled target in Bootes was Epsilon Bootis, aka Izar aka Pulcherrima (sep. 2.8"). I had visited this star before and not been able to split it. Someone suggested I should be able to split it, so I had another go. I did manage to split it, though not cleanly, and realising I had the split took some doing. The pair is a dominant white star, with a very close yellow companion. The companion appears as a spot within the diffraction pattern of the primary - in fact it looks a lot like one of the 'floaters' you get in your eyes (which, if I recall are actually diffraction patterns of things floating about on your retina). It wasn't until I had been fiddling about with focus and tracking for a while I realised that the spot always appeared in the same place relative to the primary and so probably wasn't floating in my eye. The split was clearest at 200x, the seeing wasn't good enough for extra magnification, which just yielded a seething mess of twisted Airy discs.

To summarise, here are the splits I achieved, and the magnification used. There is a clear trend - closer doubles need more magnification :D If you are a Skymax 127 user, perhaps these results will help you gauge what you should be able to see.

[B]Star		Sep		Split at[/B]
Regulus 177.6" 37.5x
24 Coma 20.3" 75x
Xi Bootis 6.7" 100x
Pi Bootis 5.6" 100x
Algeiba 4.5" 200x
2 Coma 3.7" 167x
Epsilon Boo 2.8" 200x
Kappa Leo 2.4" 200x ?

It is interesting that Algeiba (and Kappa Leo) took more magnification than similarly separated stars in Bootes. This may be because I did the Leo observations earlier on in the evening when perhaps the seeing was worse or maybe the scope was still cooling.

Beyond the expected conclusion above, I took the following points from my observing session:

1. Make sure you're looking at the right star! A good chart and finder are key!

2. Seeing really matters. When you are looking at a star under very high magnification you will notice the diffraction pattern wobble about a lot. This makes separating close pairs much harder.

3. Moving the scope about a little bit when splitting close pairs helped me and it helped me differentiate between stuff inside my eyes and what the scope was showing.

4. Sometimes you can't split because the companion is just too feint.

5. Not all splits are immediately obvious, some need some good observing time to become apparent.

As a bonus, and given the clear (no contrails!) conditions I had a look for M81 in Ursa Major. It's a Mag 8(ish) Galaxy. I've seen globular clusters before through my scope, including the mag 7.7 M53 but have always failed to find galaxies. Until now that is :) I managed to track down M81 fairly easily, and while I didn't see much detail it was definitely visible. Had it not been getting so late, and me so cold, I would have stayed out and had a better look. I'm pleased I could see it with my scope, and it bodes well for seeing some fainter galaxies when I finally get to a dark sky site :)

[Note: my list of doubles and their separations comes from this site, I do not know how accurate or up to date the data is]

Edited by Jove
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Good report reminds me to plan a few more doubles ... reminds me to plan a bit more [full stop]. :D Also a good indication as to what I can expect with that scope as I'm intending to get one as a more portable one.

I too split Algeiba on the 17th in similar LP conditions (perhaps a bit more steady) to yourself and noted a nice contrast in magnitude between the pair and a slight difference in colour. I could tell it was a double even using moderate powers but best views at 150-200x.

I didn't have any specific plans, unlike on a couple of previous occaisions, for looking a doubles except for that one. LP was odd being quite steady but noticably poorer transparency than normal at lower altitudes. That curtailed me from looking for anything particularly faint, but gave the new TV 32mm Plossl a good run out though.

But as I've found out you can 'discover' doubles anytime as I did with with 2 Puppis last month whilst slewing past to look at M46 and M47. I went back and looked at it (Mag 6.1 and 6.8 separated by 17"). I noted at the time that I felt that there was a greater contrast in magnitude than I was expecting and only noticed later that the secondary was variable and later still an eclipsing binary that I'd caught mid-eclipse.

Also in the area (sort of) you were looking I had previously (oh my back in Feb now) looked at i1 Cnc (Iota) (listed in Turn Left at Orion) a nice colour contrast of brighter yellow Mag 4.2 and fainter blue Mag 6.6 some 31" apart.

The web-site link contains Web based version of the Saguaro Astronomy Club Deep Sky databases Version 7.2. The SAC website has Version 8.1 dated March 2010 at SAC Downloads including Excel and CSV lists of some 2000 doubles that's dated July 2008.


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