splitting double stars

[Chippy3476 (Danial)]

Hi all,
I am just after any advice on splitting double stars with my 8" dob, what magnification would be needed and so on. Some sites say very high magnification and others say no more than a 100 magnification? any help would be appreciated.
Thanks
Dan

[barx1963 (Malcolm)]

Depends a lot on the stars and the atmosphere. Factors include the width of the distance between the stars (which can vary over time), the brightness, the difference in brightness between the 2 (eg although Sirius A and B are quite well separated, the glare of A make B very hard to pick up). Th stillness of the atmosphere and elevation also make a huge difference.

Malcolm

[GeoffW1 (Geoff)]

Hi,

I'd put a little effort into collimating your dob. I like star tests as the final step after lasers, cat's-eyes etc. Here's a good read

http://legault.perso.sfr.fr/collim.html

However there are many strong opinions on the best collimation methodology.

Then, a decent EP makes a lot of difference, even in an ordinary scope. I recall once with a SW102 refractor, looking at Albireo, trying first a Baader Hyperion (not a rubbish EP at all), then a new Teleview Ethos. The difference was so great I was gobsmacked.

Now, you don't have to rush off and spend $800 on one EP, there is good glass for less, especially here in IIS classifieds. Maybe a good second-hand EP or 2 there?

Magnification, well, as with so much other astro, less is more. I think that going for x400 or whatever is more trouble than it is worth. Image quality deteriorates, it jumps around in the FOV, and the object becomes more difficult to get centered. It is a matter of preference I believe.

The most influential thing I think is the one least in your control - the seeing. If you get a steady crystal clear night you may be able to split Sirius!

Cheers

[Renato1 (Renato)]

There are thousands of double stars in the sky. When I was using Sky Atlas 2000 with my C8, I'd just aim somewhere in the sky and look at all the double stars plotted there. One in two would show up without difficulty at 100X. Generally speaking though, given atmospheric conditions, I found it easier to split the plotted 5th and 6th magnitude stars. Blobbiness and the bigger star diffraction rings in a reflector relative to a refractor, made some of the brighter ones harder to split some nights. Also, you telescope (assuming it has been collimated) will usually show better images after it has been outside for a few hours.

The biggest disappointment for me was having read books describing colours of the various double star components. Yes I could see orange and red ones, but darned if I could see all those other colours. Turns out a lot of those colours seem to have come from the ingredients that went into making lenses for refractors in the late 19th and early 20th century. Plus imagination on the part of people doing the subsequent observing who'd read the previous descriptions.
Regards,
Renato

[Robh (Rob)]

As a general rule of thumb (depending on seeing and transparency), if the stars are 10" or wider then low magnification will suffice (x60). Between 4" and 10", use medium magnification (x120), less than 4" higher magnification (x180 up). However, there are provisos. If the magnitude of the B component is a lot less than the A component e.g. Sirius, mag -1.4 and 8.3, 10.3" separation, you will need higher magnification due to the glare off the primary. In smaller apertures, a dimmer secondary star may not be achievable as the star is too weak to be picked up by the scope in the glare off the primary.

It is rare that a close double (<1.5") can be easily split (or split at all) simply because the seeing conditions are not good enough. Bumping up magnification will just make the view worse. In fact, it is rare that magnification over x250 will give anything more than a jittery blob.

It is a misconception that a refractor is better at splitting doubles than a reflector. Only if they are the same aperture size. Most amateurs own a reflector much larger than their refractor because they are cheaper per inch of aperture. The ability to split close doubles (<1.5") not only depends on seeing but also on aperture size. Smaller apertures produce a larger airy disk and diffraction rings for stars so it is harder to get a gap between the stars. Conversely, a larger aperture will produce smaller "points" of light for stars as the airy disk is smaller. Here the gap between the stars is wider.

As far as colours of stars, I would agree with Renato. I myself, have never seen a green or purple star as sometimes mentioned in references. Whether it was due to the optics and coatings of the day or a case of individual colour perception is debatable. Sometimes two people can give a different colour for the same star in the same EP and telescope. A bright star of one colour can make a secondary appear different to what it actually is e.g. a bright orange primary can make a close dimmer white secondary appear dull orange. In any case, modern references often quote older sightings and perpetuate the colour errors.

I find the colours of stars run quite close to the spectral type. B0 to A9 tending to be bluish to white. F0 to G9 going from a light yellow to a deeper yellow. K0 to M5 from yellow-orange to a deeper orange. I have only really seen red in the Carbon Stars.

Regards, Rob

[The Mekon (John Briggs)]

It is a misconception that a refractor is better at splitting doubles than a reflector. Only if they are the same aperture size. Most amateurs own a reflector much larger than their refractor because they are cheaper per inch of aperture. The ability to split close doubles (<1.5") not only depends on seeing but also on aperture size. Smaller apertures produce a larger airy disk and diffraction rings for stars so it is harder to get a gap between the stars. Conversely, a larger aperture will produce smaller "points" of light for stars as the airy disk is smaller. Here the gap between the stars is wider.

It is a misconception that it is a misconception that a refractor is better at splitting doubles.

They are!!!

After using a Astro-Physics 130EDT for 22 years mostly on doubles, and now using an 18" SDM with a fine mirror, I find that the refractor was way better.
With the 130 I could usually tell at a magnification of 115x whether a star was double or not. Increasing the magnification gave better view and would confirm the split. The 18" can do all that the refractor can BUT requires far more cool down time and much more sensitive to poor seeing conditions. I find I'm using a 7" off axis mask to get that clean look the refractor used to give.

[Steffen]

Originally Posted by Robh

It is rare that a close double (<1.5") can be easily split (or split at all) simply because the seeing conditions are not good enough. Bumping up magnification will just make the view worse. In fact, it is rare that magnification over x250 will give anything more than a jittery blob.

This may be the case with large aperture telescopes, but in smaller ones (like the 6-inch I'm using) 1 arcsec doubles like Xi Sco are perfectly and frequently doable, even from my backyard. The thing with seeing is that is changes quickly and all the time. Even with mediocre (on average) seeing you could still get to see a clear split for, say, 1 out of 10 seconds. You just have to be patient and observe the double for a long period of time. I've sometimes stared at a double for over ten minutes before a moment of stillness allowed me to see an elusive fainter companion.

As for the required magnifications, I know that for targets like Ascella (0.5 arcsec) for example I need well above 300x to detect elongation, looking best at 600x in my 6-inch.

Extreme magnifications require high quality optics, spot-on collimation and a solid tracking mount. I would probably find it way too hard to use 300x or above with a hand-nudged Dob.

Cheers
Steffen.

[Steffen]

Originally Posted by Robh

I myself, have never seen a green or purple star as sometimes mentioned in references.

I think simultaneous colour and contrast effects play a role there. For example the spectrally blue Antares B has always looked green to me, next to big, bright, orange Antares A.

Cheers
Steffen.

[doug mc]

Just like the previous poster said with his 18inch dob, you can make an off axis mask to place on or in the front of your scope. You should get close to 80mm with your reflector. Star diffraction paterns will be easier to see and there is a lot of doubles to be seen with that aperture.

[Renato1 (Renato)]

Originally Posted by The Mekon

It is a misconception that it is a misconception that a refractor is better at splitting doubles.

They are!!!

After using a Astro-Physics 130EDT for 22 years mostly on doubles, and now using an 18" SDM with a fine mirror, I find that the refractor was way better.
With the 130 I could usually tell at a magnification of 115x whether a star was double or not. Increasing the magnification gave better view and would confirm the split. The 18" can do all that the refractor can BUT requires far more cool down time and much more sensitive to poor seeing conditions. I find I'm using a 7" off axis mask to get that clean look the refractor used to give.

But if you are trying to split some 15th magnitude double star, your 18" will be vastly superior.

Like you, I made an aperture mask for my 14.5" dob, but with 4",5" and 6" holes - as I cut them out, I stuck velcro on them so as to use them as covers for the holes not in use. I don't use it often, but it did come in useful a couple of times with miserable seeing.
Cheers,
Renato

[Robh (Rob)]

Originally Posted by The Mekon

It is a misconception that it is a misconception that a refractor is better at splitting doubles.

They are!!!

After using a Astro-Physics 130EDT for 22 years mostly on doubles, and now using an 18" SDM with a fine mirror, I find that the refractor was way better.
With the 130 I could usually tell at a magnification of 115x whether a star was double or not. Increasing the magnification gave better view and would confirm the split. The 18" can do all that the refractor can BUT requires far more cool down time and much more sensitive to poor seeing conditions. I find I'm using a 7" off axis mask to get that clean look the refractor used to give.

You can't compare a refractor under good seeing to a larger reflector under poor seeing. Or complain about cool-down time. The reason your refractor might seem to give a sharper image is its shorter focal length compared to the 18" SDM. Probably about half. In which case your EPs give a much lower magnification, which does not magnify poor seeing conditions as much. You probably need to bring down the magnification in the 18" if the seeing is that bad.

It is a fact that larger apertures have better resolving power. The Dawes limit for a 5" scope is about 0.9" but for an 18" scope it is 0.25". There is no way you can split stars in the 5" scope to the level of the 18". I will give that diffraction spikes can be sometimes be a problem in detecting a secondary star e.g. Sirius B. But this is only an occasional nuisance.

Regards, Rob Horvat

[Robh (Rob)]

Originally Posted by Steffen

This may be the case with large aperture telescopes, but in smaller ones (like the 6-inch I'm using) 1 arcsec doubles like Xi Sco are perfectly and frequently doable, even from my backyard. The thing with seeing is that is changes quickly and all the time. Even with mediocre (on average) seeing you could still get to see a clear split for, say, 1 out of 10 seconds. You just have to be patient and observe the double for a long period of time. I've sometimes stared at a double for over ten minutes before a moment of stillness allowed me to see an elusive fainter companion.

As for the required magnifications, I know that for targets like Ascella (0.5 arcsec) for example I need well above 300x to detect elongation, looking best at 600x in my 6-inch.

Extreme magnifications require high quality optics, spot-on collimation and a solid tracking mount. I would probably find it way too hard to use 300x or above with a hand-nudged Dob.

Cheers
Steffen.

This is not my experience. You must have really good skies in your area! I regularly check beta Muscae as a gauge of seeing. The two stars are separated by about 0.9". Only on rare occasions do I see a clean split. More magnification simply makes the pair look like a bigger blob.

Regards, Rob

[Steffen]

Originally Posted by Robh

This is not my experience. You must have really good skies in your area! I regularly check beta Muscae as a gauge of seeing. The two stars are separated by about 0.9". Only on rare occasions do I see a clean split. More magnification simply makes the pair look like a bigger blob.

The sky at my place is typically light-polluted and has so-so transparency, but the seeing can get good.

Thanks for the Beta Mus tip, I've added it to my observing list. I've got a big tree blocking my southern view (which managed to survive Saturdays freak storm) but Beta Mus should peak out during certain times. Based on results with similar doubles I should be able to split it (or at least see two touching spurious discs with figure-8 shaped first diffraction rings), but it will likely require well over 300x.

I think the T-Rex may be too big for high-mag work, being affected by seeing too much

Cheers
Steffen.

[The Mekon (John Briggs)]

Originally Posted by Robh

This is not my experience. You must have really good skies in your area! I regularly check beta Muscae as a gauge of seeing. The two stars are separated by about 0.9". Only on rare occasions do I see a clean split. More magnification simply makes the pair look like a bigger blob.

Regards, Rob

Thank you Rob,

You have reminded me that I could regularly split beta Musca with my Astro-Physics 130EDT always before midnight and the scope outside for less than 1/2 hour. The 18" has yet to split this double as I don't spend enough time outside for it to cool/settle down enough!
Plus with either scope I rarely use more than 230x. Most commonly with the AP it was 7mm nagler yielding 147x Now I use either a 9, 13, or 17.3 in the 18" yielding 228x, 158x or 118x.
I sold the AP in September. The 18" is great for faint diffuse objects, but cannot match the AP on even doubles as yet.