Castor A and B

I have a 80mm refractor and I split Castor A and B clean at 100 and 120X.
It is said to be 3.3 " separation.
Is this a good split? It looks to me that I would be hard pressed to do much
better than 3". Does that sound like
good resolution?

MS

Mike S wrote:
I have a 80mm refractor and I split Castor A and B clean at 100 and 120X.
It is said to be 3.3 " separation.
Is this a good split? It looks to me that I would be hard pressed to do much
better than 3". Does that sound like
good resolution?

Under excellent seeing, an 80 mm long-focus refractor should be able
to do somewhat better than that. If your telescope is f/10 or so, you
should be able to split tighter than that. Try, for example, epsilon
Lyrae, the well-known Double Double.

If you have an 80 mm f/5 (aka the short tube 80), the chromatic
aberration might present a problem in trying to split tight doubles.
I'm not sure. I have a Tele Vue Ranger, a 70 mm f/6.8. The tightest
split I've ever done through that one is to "bread loaf" zeta Bootis,
which I think is in the 0.75-arcsecond range. But it has less color
than an 80 mm f/5, and "bread loafing" is not splitting cleanly.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Mike S wrote:
I have a 80mm refractor and I split Castor A and B clean at 100 and 120X.
It is said to be 3.3 " separation.
Is this a good split? It looks to me that I would be hard pressed to do much
better than 3". Does that sound like
good resolution?

Under excellent seeing, an 80 mm long-focus refractor should be able
to do somewhat better than that. If your telescope is f/10 or so, you
should be able to split tighter than that. Try, for example, epsilon
Lyrae, the well-known Double Double.

If you have an 80 mm f/5 (aka the short tube 80), the chromatic
aberration might present a problem in trying to split tight doubles.
I'm not sure. I have a Tele Vue Ranger, a 70 mm f/6.8. The tightest
split I've ever done through that one is to "bread loaf" zeta Bootis,
which I think is in the 0.75-arcsecond range. But it has less color
than an 80 mm f/5, and "bread loafing" is not splitting cleanly.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

On Thu, 12 Feb 2004 05:47:20 +0000 (UTC), (Brian Tung)
wrote:



If you have an 80 mm f/5 (aka the short tube 80), the chromatic
aberration might present a problem in trying to split tight doubles.
I'm not sure. I have a Tele Vue Ranger, a 70 mm f/6.8. The tightest
split I've ever done through that one is to "bread loaf" zeta Bootis,
which I think is in the 0.75-arcsecond range. But it has less color
than an 80 mm f/5, and "bread loafing" is not splitting cleanly.

Chromatic aberration makes a bigger difference in the detection of
relatively close doubles of strongly unequal brightness. In such
cases, a green of minus violet filter should make all the difference.

DC

On Thu, 12 Feb 2004 05:47:20 +0000 (UTC), (Brian Tung)
wrote:



If you have an 80 mm f/5 (aka the short tube 80), the chromatic
aberration might present a problem in trying to split tight doubles.
I'm not sure. I have a Tele Vue Ranger, a 70 mm f/6.8. The tightest
split I've ever done through that one is to "bread loaf" zeta Bootis,
which I think is in the 0.75-arcsecond range. But it has less color
than an 80 mm f/5, and "bread loafing" is not splitting cleanly.

Chromatic aberration makes a bigger difference in the detection of
relatively close doubles of strongly unequal brightness. In such
cases, a green of minus violet filter should make all the difference.

DC

Under excellent seeing, an 80 mm long-focus refractor should be able
to do somewhat better than that. If your telescope is f/10 or so, you
should be able to split tighter than that. Try, for example, epsilon
Lyrae, the well-known Double Double.

If you have an 80 mm f/5 (aka the short tube 80), the chromatic
aberration might present a problem in trying to split tight doubles.
I'm not sure. I have a Tele Vue Ranger, a 70 mm f/6.8. The tightest
split I've ever done through that one is to "bread loaf" zeta Bootis,
which I think is in the 0.75-arcsecond range. But it has less color
than an 80 mm f/5, and "bread loafing" is not splitting cleanly.


What is the best non bread-loafing you have done? I have f10 and the
seeing was
average at best under maybe a mag 4 sky. If 3" is the best clean split I
can get, what
does that mean about my optics?

Under excellent seeing, an 80 mm long-focus refractor should be able
to do somewhat better than that. If your telescope is f/10 or so, you
should be able to split tighter than that. Try, for example, epsilon
Lyrae, the well-known Double Double.

If you have an 80 mm f/5 (aka the short tube 80), the chromatic
aberration might present a problem in trying to split tight doubles.
I'm not sure. I have a Tele Vue Ranger, a 70 mm f/6.8. The tightest
split I've ever done through that one is to "bread loaf" zeta Bootis,
which I think is in the 0.75-arcsecond range. But it has less color
than an 80 mm f/5, and "bread loafing" is not splitting cleanly.


What is the best non bread-loafing you have done? I have f10 and the
seeing was
average at best under maybe a mag 4 sky. If 3" is the best clean split I
can get, what
does that mean about my optics?

Mike S wrote:
What is the best non bread-loafing you have done? I have f10 and the
seeing was average at best under maybe a mag 4 sky. If 3" is the best
clean split I can get, what does that mean about my optics?

If the seeing was really only average, it doesn't say much at all.
You'll need to try repeatedly to get a good fix on how good your
optics are. There really is no substitute for patience.

I don't recall off the top of my head the best I've been able to
resolve through the Ranger. But it's definitely done the Double
Double, which have separations of 2.3 and 2.6 arcseconds.

The Airy disc of a 70 mm refractor is about 1.75 arcseconds; that's
probably around as tight as one could expect to separate without
noticeable overlap. That of an 80 mm refractor is about 1.6
arcseconds.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Mike S wrote:
What is the best non bread-loafing you have done? I have f10 and the
seeing was average at best under maybe a mag 4 sky. If 3" is the best
clean split I can get, what does that mean about my optics?

If the seeing was really only average, it doesn't say much at all.
You'll need to try repeatedly to get a good fix on how good your
optics are. There really is no substitute for patience.

I don't recall off the top of my head the best I've been able to
resolve through the Ranger. But it's definitely done the Double
Double, which have separations of 2.3 and 2.6 arcseconds.

The Airy disc of a 70 mm refractor is about 1.75 arcseconds; that's
probably around as tight as one could expect to separate without
noticeable overlap. That of an 80 mm refractor is about 1.6
arcseconds.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

What is the best non bread-loafing you have done? I have f10 and the
seeing was
average at best under maybe a mag 4 sky. If 3" is the best clean split I
can get, what
does that mean about my optics?

I am not going to venture an opinion about your optics....

There are many things to consider besides the optics, your experience, seeing
and location all make a big difference. The eyepiece makes a difference as
well.

Another important factor can be the stability of the mount and drives. A
shaky mount can make it difficult to focus and center the target and the mount
can vibrate from the drives.

I have a Pronto and a Celestron , 80mm F5. I live in San Diego and seeing is
pretty decent here except when the Santa Ana winds kick up like last night.

Both those scopes will split Castor nicely and do the double-double as well.
The other morning I spotted Vega so I thought I would give the double-double a
try, I was able to split it cleanly at 80X with the ST-80.

I think focal length is not a big factor when splitting double stars with small
scopes, surely F10 is better than F5, but at 100X, the exit pupil is small
enough that false color is not an issue except when splitting something like
Rigel where the primary is magnitude 0.30 and the secondary 10.30 even though
the separation is 9.2 arc-seconds.

Reading your post, it does seem that you are experienced, what doubles have you
tried and been unable to split? Have you split them before with other scopes?

Jon