Exploring Double Stars of Varying Difficulty During Summer Twilight.

The Twilit skies of central Scotland in June, looking north. Image captured at 27 minutes past local midnight on June 11 2019.

June 19 2019

June is a month that usually brings mixed blessings here in Scotland. On the one hand, I have more leisure time but it also accompanies nights that never quite get dark. Indeed, any where north of the midlands, true darkness never returns until the first week in August. ’tis the season of twilight.

And while nighttime temperatures are very mild, they are often accompanied by legion midge flies which can be a source of great annoyance, especially on still, humid nights.

To add insult to injury, these last several weeks have not been good for observing,  with endless low-pressure weather systems which have brought thick rain clouds to our shores. I lay at least part of the blame at the feet of that fiery furnace at the centre of the solar system. Our star is bereft of spots. Indeed, I have not recorded a single sunspot with my 20 x 60 binocular since the afternoon of May 17 last!

But despite these drawbacks, I have savoured the odd clear night, like the one I encountered on the evening of June 9/10 last, where I carried out a number of observations of double and multiple stars with my tried and trusted 130mm f/5 Newtonian reflector, with which I have enjoyed great success, owing to its excellent optics and superior light gathering power to any other grab ‘n’ go telescope I have had the pleasure of using in past years.

Plotina: my wonderful grab ‘n’ go companion under the stars.

A nefarious forum individual, hell-bent on de-railing my findings with this particular telescope(remember Mr. Bad Smell?) has made the claim that such a telescope cannot act as a true grab ‘n’ go instrument since it takes a bit of time to acclimate to ambient temperatures. But it doesn’t take more than a moment’s reflection(excuse the pun) to counter that claim. If there are large temperature differentials between inside and outside, the solution is to begin observing at lower powers and gradually increase the power as the telescope nears full equilibration.


This has been my custom any time some acclimation is required, especially if I’m in a hurry, such as on cold, winter nights. That said, during the mild nights of summer, little or no acclimation is needed; certainly no more than 15 or 20 minutes for even the most demanding of targets.

Here I wish to show readers how you can go from low-power, low-resolution targets to higher-power, higher-resolution targets simply by choosing the order with which those systems are examined!

Polaris A & B: My first target is easy and beautiful; 2nd magnitude Pole Star, the closest Cepheid varibale star to our solar system, which has a delightful companion easily picked up in twilight with the 130mm f/5. With a power of 118x, Polaris B is seen as an 8th magnitude spark wide away from its primary.

Albireo: This lovely colour-contrast double is easily tracked down in twilight high in the eastern sky around midnight in the beak of the Celestial Swan. Any small telescope presents this stellar duo very well, but I’m very grateful for the increased light gathering power of this reflector which renders their colours especially vividly; a soft marmalade orange primary and a royal blue secondary. 118x frames the system very well, darkening the sky sufficiently enough to make the observation worthwhile.

O^1 Cygni: This comely system is a famous binocular double in Cygnus, but its majesty is greatly increased with the power of a telescope. Like a more widely spaced version of Albireo,  the 130mm Newtonian at 118x frames a stellar trio, consisting of an orange, magnitde + 3.8 primary and turquoise secondary a full magnitide fainter. In addition, the telescope easily picks up a closer 7th magnitude blue companion close by the ochre primary.

Cor Caroli: Yet another easy target for small backyard telescopes, Cor Caroli, the brightest luminary of Canes Venatici, is a stunning sight in the 5.1-inch Newtonian at 118x. Presenting with similar hues (white) to my eye, the brighter component shines at magnitude + 2.9, whereas the secondary shines more feebly at +5.9 wide away. Both stars lie about 115 light years from the solar system.

Epsilon 1&2 Lyrae: Moving up to a more challenging system, we take a visit to Epsilon 1 & 2 Lyrae, easily found near the bright blue-white luminary, Vega, in Lyra. Charging the telescope with a power of 270x, the two stars seen in my 6 x 30 finder ‘scope are transformed into a quartet of suns, giving rise to its more famous name, the Double Double. The generous light gathering power of the 130mm Newtonian allows me to easily make out subtle colour differences between these stars, with each pair presenting as nearly at right angles to each other. While some ar pure white, another is creamy and yet another is lemon tinted.

Delta Cygni: One of my favourite summer doubles, the 130mm f/5 makes light work of this system at 270x. The intensely blue-white primary( magnitude +2.9) presents with a  close in companion of greatly reduced glory( +6.3). It is a fine sight whenever the sky is tranquil. Both stars orbit their barycentre every 900 years.

Pi Aquilae: More challenging still is this very faint pair of white stars (both magnitide 6+), best visited later in the vigil when they rise a little higher in the eastern sky during June evenings. Separated by 1.5″ I find they are best seen by charging the telescope with very high powers. On this occasion I got particualrly good results at 405x (using a 4.8mm T1 Nagler and 3x Meade Barlow). Fainter, near-equal pairs are definitely more challenging than their brighter counterparts, especially during strong summer twilight.

25 Canum Venaticorum: This challenging system was kindly brought to my attention by Welsh amateur astronomer, Rob Nurse, a few weeks back. The primary shines at magnitude +4.98, while the secondary is considerably fainter, at magnitude + 6.95, but their separation is only 1.7″. That sounded like quite a challenge, at least on paper, but once I learned how to track it down in the western sky around local midnight, I was delighted to see that the 130mm f/5 Newtonian handled this system quite well at powers of 354x and 405x. Testing my visual skills, I noted the orientation of the secondary relative to its primary, which was almost exactly due east. Then, when I checked with the position angle data Nurse provided me, I was deligted to see that the secondary was indeed located almost due east(094 degrees) of the primary.

I made a quick pencil sketch(shown below) of all these systems as seen in the little Newtonian telescope. Vigil ended at 00:15UT

Simple pencil sketeches of the systems examined on the night of June 9/10 2019 with a 130mm f/5 Newtonian reflector. All systems were examined on an undriven Vixen Porta II mount, equipped with slow motion controls on both axes.

Well, I hope that this short blog will encourage you to go outside and observe the twilit heavens. With a liitle resourcefulness, you can always find something interesting and beautiful to observe.

Clear skies and thanks for reading.


Neil English has penned several hundred published articles on observational astronomy and telescope testing over the last 25 years. He is also the author of seven books on telescopes, astronomical history and space science.




De Fideli.

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