Splitting Close Doubles: Are YOU up to the Challenge?

[Rodstar (Rod)]

What's the closest double star you can split in your scope?

I am interested in your experiences, and any tips you may have.

I have been getting into doubles in the last 6+ months, especially at times of the month when Lunar drowns out the deeper objects. Over time I have slowly been whitling down the separation I can comfortably split, as seeing conditions allow.

I have decided that close doubles where both stars are around the mag 8-9 mark are the easiest to split in my scope. If the secondary is more than about 2 mag fainter than the primary, I find the closer doubles very difficult. Separation often needs to be at least 2.5 arc" in those circumstances.

I made a hexagonal aperture mask a few months ago to help (enables close secondaries hiding in the glare of the primary to be identified between the diffraction spikes thus created), but my experience so far is that this method is only helpful with getting around very bright primaries.

A critical aspect to double splitting is making sure your scope has properly cooled. It is quite incredible how even 3 or 4 arc" splits can be difficult if the scope is not at ambient temperature. I have my scope out for 2 or 3 hours before observing now to make the most of my 10 inches of aperture.

Last night I spent some time looking at doubles in Triangulum Australe. The seeing was superb, so I decided to push the limits with my scope, and cranked up the magnification to achieve clean splits of the following:

1. HJ 4809 (SAO 253344), sep 1.2", constituents 6.5 & 8.5 mag, at 495x clean split.

2. I 333 (SAO 253588), sep 1.2", constituents 8.0 & 8.5 mag, clean split at 195x

This separation of 1.2 arc" is the closest I have achieved so far. I am bucking to try to break the 1 arc" mark. Last night I tried I 332 (SAO 253115), separation 1.1 arc", constituents 6.5 & 9 mag, but I could only achieve elongation, not a clean split.

What's the closest pair you have split? Do you have any tips for squeezing the most out of your scope?

[ving (David)]

on ya rod!

gee, my closest double... i might have to get back to you on that. I have done some pretty close ones in orion (closer than 1.5"). I'll have to check my PDA when i get home

I have used a trianglular mask to get some hiding secondaries. different shape masks work well

[jjjnettie (Jeanette)]

I'd like to try splitting Antares. Any tips will be most welcome.
I thought that I'd done it once with the wobbletronic, but it was probably just chromatic aberation.

[ving (David)]

an aperture mask will sometimes help by cutting the glare down. this together with a haxagonal mask over that or whatever other shape works for you

[mickoking]

Originally Posted by jjjnettie

I'd like to try splitting Antares. Any tips will be most welcome.
I thought that I'd done it once with the wobbletronic, but it was probably just chromatic aberation.

I split Antares last year with my 120mm refractor and last week with my old 250mm Dob. Antares companion is not particually close it is just the large difference in brightness between primary and secondry. Steady skies help with Antares

[ausastronomer (John Bambury)]

Gamma Circini is my best split.

Separation is about .85" and the components are mag 5.1 and 5.5. It takes exceptional seeing and a good scope of at least 8" aperture to split this pair.

I have tried this split on countless times and achieved it only twice. Albeit I have only tried it in scopes up to 10" aperture. On both occasions the split was achieved in my 10"/F5 dob with a 6mm UO HD ortho in my Orion 2X Shorty Plus barlow under superb seeing conditions.

Rod, Do you recall the night at Kulnurra when 5 of us stayed after the rain and everyone else departed? That night when the weather cleared the seeing was superb and we had wonderful views of Jupiter and Saturn in my scope at 450X. Those are the type of conditions you need to split Gamma Circini, sub arc second seeing with a well collimated well cooled scope, which doesn't happen all that often. I have no doubt had we tried to split Gamma Circini that night in my scope, we would have split it.

CS-John B

[Rodstar (Rod)]

Originally Posted by ausastronomer

Rod, Do you recall the night at Kulnurra when 5 of us stayed after the rain and everyone else departed? That night when the weather cleared the seeing was superb and we had wonderful views of Jupiter and Saturn in my scope at 450X. Those are the type of conditions you need to split Gamma Circini, sub arc second seeing with a well collimated well cooled scope, which doesn't happen all that often. I have no doubt had we tried to split Gamma Circini that night in my scope, we would have split it.

CS-John B

John, I do indeed remember that night!....let's make a mental note to try to achieve that split next time we have good seeing (and when Circinus is suitably positioned).

[syzygy]

Speaking of Gamma Circini, there's another bright testing pair not that far away. Gamma Lupi is a brilliant bluish white pair mags 3.5 and 3.6 but just about 0.7" apart. Glare would be the major problem with this one.

From my notes made in June 1992: "Barely elongated by 170x, needed 300x and good seeing conditions to show the Airy disks almost in contact. A good test object." That was with a C8.

[Rodstar (Rod)]

At the risk of departing from the original topic, does anyone have a strong view as to whether a UO orthoscopic EP or a Televue Plossl EP would do a better job at high power to split close doubles? In the hope of securing the greatest throughput of light and higher contrast, I am thinking the ortho may be slightly better. Am I right? Obviously FOV is low on the list of priorities for double star observing, which might otherwise have been a reason to favour a TV Plossl.....

[ausastronomer (John Bambury)]

Rod,

Optically there is little between these two eyepieces. They are very close in sharpness, contrast and light throughput.

For a given focal length, the UO HD ortho will have slightly longer eye-relief making it easier and more comfortable to use. In addition the ortho has cool neutral colour reproduction, as opposed to the TV plossl which is quite warm and adds a "coffee" tint to the target, which some people (not me) prefer. At the shorter focal length ranges there is more availability of focal lengths with the UO HD orthos. 5,6,7,9,12 whereas the TV plossls are only available in 8mm and 11mm. I own a full set of UO HD orthos if you would like to try them before buying.

In short, they are both outstanding eyepieces and you would be very happy with either.

CS-John B