Is splitting the double-double a good test?

[rikyuu]

hi,

I managed an hour out last night, despite the thin cloud covering, to further test and set up the C8 SCT I acquired recently.

I decided to look around the Vega and Lyra region and turned my attention to the double-double, the closer pairs of which, I have not split before.

Using a Bahtinov mask, I managed to nail the focus easily, but collimation was still slightly out, noticeable at higher magnifications. I tried one of the doubles with a 6mm eyepiece, (333x), but it wasn't a clear split. Checking the collimation; inside and outside of focus, the central obstruction appeared central, but when moving close to in-focus, one of the edges was definately brighter. So a quick fiddle with 'Bob's knobs' soon rectified this. I refocused with the mask. What a difference this small adjustment made. There they were, one of the doubles clearly split into 2 well defined stars.

I decided to up the game with a 2xbarlow. This brought the magnification to 666x. Above the scopes 400x theoretical limit.

A little fine adjustment on the featherlite focuser and there they were. Again well defiined, with a clear split between them (I can only describe the separation distance in the eypiece as appearing to be about 5mm). 2 or 3 airy discs were apparent around each one. Even at this magnification, there was only slight image shift when moving the focuser one way then the other. But it did affect collimation (at this high magnification), so the same rotation direction had to be used as was done during collimation.

Now back to my original question, is this a reasonable test of a

telescope's performance?

Thanks

[brianb]

Hmmmm .... I can remember splitting both components at x112 with a poor quality 60mm refractor! Not that there would be a "5mm" space between the components, they were just about touching.

Anyhow from the sound of it you've got the collimation nailed.

[rikyuu]

sounds like it might be more of a test of collimation then.

[Mark at Beaufort]

Over the years I have found splitting the double double a very good test. It is also interesting that whenever you read a review on a telescope the author tends to mention the double double when comparing the quality of different scopes which speaks for itself.

Reading your notes it sounds that you have a good SCT and the collimation is spot on.

Mark

[acey]

Splitting the double-double is a good basic test of telescope performance, but a fine test only for scopes of small aperture. In still air, a 60mm will indeed split them, and any 8-inch should have no trouble at all.

The separation of the pairs is 2.6" and 2.3". There are various ways of defining resolution, but an 8-inch scope can theoretically reach to around 0.5" resolution. There are few nights when the air is still enough to allow details that small to be seen - testing the resolution of an 8-inch is also largely about testing the limits of the atmosphere.

If you want to really push things then you need to try a pair with a separation of less than 1". Sorry, I can't suggest any currently visible, but you could try hunting here:

Eagle Creek Observatory -- Double Stars

Alternatively you could try the Galilean moons of Jupiter, they have an angular diameter of around 1", so resolving their discs is a good test for an 8-inch (I've done it with mine), certainly better than the double-double though still not "the ultimate".

[mdstuart]

I agree with all the above. My 10 inch dob is however better for fuzzies than doubles but I have no trouble with the double double. It is a good test as you should see 4 lovely star dots with black sky between the doubles..

I can't split below about 1.5 secs of arc. There is a good tight double in draco near the head of the dragon that is about 2 secs which I find a good test.

Mark

[Mark at Beaufort]

I checked and I find that Rho Capricornus has a separation of 1.3" arc seconds with the 2 stars being mag 5 and mag 6.9. My double star manual indicates that a 200mm scope should split down to 1.1" arcseconds with a 2 mag difference. With Capricornus being well spaced it might be worth pushing your scope to the limit and giving it a try.

mark

[arad85]

My double star manual ...

What book/web page is that then?

[Mark at Beaufort]

Double Stars for small telescopes by Sissy Haas. Covers 2100 stellar gems and produced by Sky and Telescope. The book gives a general introduction on observing doubles plus info on showcase doubles. The catalogue goes by constellation with general comments by Sissy Haas. Plus what Thomas Webb, Smythe etc saw. Good info and not too expensive - I obtained it from Amazon.

Mark

[Mark at Beaufort]

This is the link to Amazon

Double Stars for Small Telescopes: More Than 2,100 Stellar Gems for Backyard Observers Stargazing: Amazon.co.uk: Sissy Haas: Books

Mark

[arad85]

Mark - brilliant - thanks...

[rikyuu]

Excellent, thanks for the suggestions, I'll give them a go.

[jcougar1]

I have always heard that if you could split the double double it was going to be a good night observing. I was just wondering if you gave the telescope time to adjust to the outside temperature? That would always play havoc for your seeing capabilities. Correct me if I am wrong, but that 8" Catadioptric probably take a hour or so just to become equilibrated.

[albedo0.39]

I'll be honest and say that even in my 16" Lightbridge, I have not always found it easy to split this star, but with a 9mm EP then the magnification hasn't been much either (203x). I've since learned how to nail collimation to a level that shows airy discs so I'm much more confident.

Rumour has it that large apertures in a dob aren't always the best to see this split but the Dawes limit suggests it should be easy peasy.

I've not had perfect seeing up to now, but the very last time I tried, I could easily split the double double - even at a miserly 200x. I'd be interested to hear how well that 60mm scope splits it at 200x ! What are the typical magnifications people use ?

[Izar]

The view with my 60mm scope was superb at 200x the other night.

I have never had any trouble splitting the Double-Double with my 60mm scope.

With my 4 inch scope I have split at around 60x before, but I prefer the view at around 150x.

[Merlin66]

If you're into double stars you MUST get a copy of Paul Couteau's "Observing Visual Double Stars", MIT press, 1982

This is the Holy Grail of books on double stars.

Paul says " a 56mm refractor will show the double double"

[albedo0.39]

So why so much trouble on a 400mm then ?

[Doc]

I have split this in my 16" but it's not easy.

My TAL 100 a 4" refractor splits doubles loads better then my dob.

Dob's IMO are not made for spitting doubles.

[Izar]

So why so much trouble on a 400mm then ?

If collimation is good, then sounds like your large mirror is being troubled by the atmospheric conditions. Have you tried

using an Off-Axis aperture mask to see if that gives an improved image?

Just been observing this star again tonight with the 60mm refractor.

At 80x, all 4 stars were seen, but not cleanly split.

A clean split was obtained at 96x. High powers views of 160x, 200x and 320x were used to good effect.