First doubles.

[4lefts]

Spent a little bit of time looking at some doubles tonight, for the first time since I got my scope. Just, Mizar/Alcor, which were split into three nice sharp white stars, and Algieba (gamma leo), which was just split into two very orange stars at x120. It made me realise how good doubles are when you have some serious light pollution - I took my dob the back of our new flat to show my mum and dad Saturn, and wow, the street lights out there are unbelievable. It really is like day time, or like being under flood lights. I won't be doing much observing out there. Though I will be spending some time on double/multiple stars.

Though I do now have a question. I have the Cambridge Double Star Atlas, and there is a big list of "showpiece" doubles in there, which lists there separations. How can I calculate the limit of what doubles I can split with my scope at a given magnification?

Thanks.

Stephen.

[lunator]

Hi Stephen

Don't worry about the level of magnification needed.

The seeing which is the steadiness of the air is usually the limiting factor.

What size is your scope as this will also put a bit of a limitation.

as a (very) rough guide most showpiece doubles should be split below x200 mag.

Cheers

Ian

[4lefts]

Thanks. I'll see how I get on. x120 is currently my max mag, but that's because I only have the 10mm plossl that came with my scope. To be rectified soon.

[Moonshane]

I think you may also want to think about Dawes Limit Dawes' limit - Wikipedia, the free encyclopedia

Effectively this dictates that the maximum resolving power of your scope is equal to e.g. 4.56 divided by the aperture in inches.

e.g. my 12" dob can in theory resolve detail to 0.38 arcseconds. of course seeing, optical train quality and the subject come into play. e.g. an extremely bright primary and a very dim secondary (e.g. Sirius) often cannot be resolved/split in the UK no matter what you do.

a more detailed view (I don't really follow this as my brain aches) is Angular resolution - Wikipedia, the free encyclopedia

for now, I would suspect that most of the ones with separation of perhaps 2 arc seconds (2.0") or more. e.g. the two elements of Epsilon Lyrae (the double double) can be tricky and have seperation of about 2.6"and Epsilon Bootis (Izar) has a separation of about 2.8" and then see how you go.

I love double stars and they are unaffected by transparency (unless you cannot see them of course!). a really great way to spend an evening is to work through the doubles in one constellation. see The Constellations for lots of info on some great double stars.

I'd appreciate a review of the Cambridge book you have as I was thinking about getting this sometime.

cheers

Shane

[4lefts]

Thanks for the info Shane - lots of useful stuff.

I really like the Cambridge book, but I'm a total beginner. There's an introduction section and a discussion of equipment etc, and section of 133 showpiece doubles, arranged by constellation with RA, Dec, Mag, Sep and notes on colour, and then there's the maps themselves, which are really good. It's called a double star atlas, and there are lots of those marked, but so are the Messiers, and the a lot of NGC objects etc, so it's a useful all round map. I love mine. And its spiral bound.

[ronin]

What exactly is the scope you have?

The diameter and focal length are needed to determine the maximum resolution of the thing and this determines the doubles that you can split.

[4lefts]

Sorry, should have said before. I have the Ford Focus of scopes - skywatcher 8" f/6 dob.

[Moonshane]

You will be able to split hundreds of doubles with a well collimated, cooled 8" f6.

[lunator]

I have an 8" f6 newt and you can split thousands of doubles.

The limiting factor will always be the seeing.

Cheers

Ian

[chiltonstar]

I bought the Cambridge atlas a few weeks ago, and I would agree - it's extremely good. The general sky maps are also excellent because of the scale, the quality of the paper (thick and tear resistant) and as stated, the spiral bind means you can open it up and lay it flat on a table. It's a good general star atlas, not just for doubles.

Re numbers of doubles, even in nights of relatively poor seeing, I can usually better 2 arc secs with either my 4" refractor or 127mm Mak (and my eyes are no longer young!) so I am sure that with your scope, most of the doubles in the atlas should be achievable. On a good night, both my scopes reach the Dawes limt for doubles that are fairly similar in brightness.

Chris