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#1
06-09-2008, 03:14 PM
 Karls48 (Karl) Registered User Join Date: Jan 2006 Location: Sydney Posts: 753
Splitting Alpha Centauri

I have stupid question. How do you imagine close binaries? I can just split Alpha Centauri binary visually with 25mm eyepiece and 120mm refractor but with CCD camera (even it equals about 7mm eyepiece magnification) image comes up as one star. I know it is due to Airy disk size and that the eye got "smaller pixel" size. Anyone knows the formula how to calculate lowest angular resolution for given pixel size of CCD and telescope focal lenght? I have searched Web for split image of Alpha Centauri and I found only one image and it was taken in infrared.
#2
06-09-2008, 04:00 PM
 bmitchell82 (Brendan) Newtonian power! Love it! Join Date: Jul 2008 Location: Mandurah Posts: 2,597
well if your looking for something like that, Do you know the angular resolution formula for telescopes. First find out what the angular resoloution for your telescope is, and im guessing find out what the size of your pixels are.!?

Remember that your telescope should have a greater angular resolution because the lense is way bigger than the human eye.

Angular Resolution R = Lambda/ Distance

Is the formula for this

λ is the wavelength of the observed radiation and D is the diameter of the telescope's objective lense. The resulting R is in radians.

Sources larger than the angular resolution are called extended sources or diffuse sources, and smaller sources are called point sources.

For example, in the case of yellow light with a wavelength of 580 nm, for a resolution of 0.1 arc second, we need D = 1.2 m.

A bit of basic algebraic manipulation of the formula will serve you with the answer you need im guessing.

anybody else ?

Last edited by bmitchell82; 06-09-2008 at 04:01 PM. Reason: spelling :(
#3
06-09-2008, 04:47 PM
 Terry B Country living & viewing Join Date: Mar 2006 Location: Armidale Posts: 2,766
Quote:
 Originally Posted by Karls48 I have stupid question. How do you imagine close binaries? I can just split Alpha Centauri binary visually with 25mm eyepiece and 120mm refractor but with CCD camera (even it equals about 7mm eyepiece magnification) image comes up as one star. I know it is due to Airy disk size and that the eye got "smaller pixel" size. Anyone knows the formula how to calculate lowest angular resolution for given pixel size of CCD and telescope focal lenght? I have searched Web for split image of Alpha Centauri and I found only one image and it was taken in infrared.
The problem with Alpha cent is that it is very bright. It saturates very quickly.
I agree that I get it as 1 star with a 1 sec exposure but can image much closer mag 13 stars and detect the separation.
#4
06-09-2008, 04:55 PM
 Karls48 (Karl) Registered User Join Date: Jan 2006 Location: Sydney Posts: 753
The resolving power of 120mm Skywatcher is 0.97 arcs of second. Resolving power of my CCD and the telescope supposed to be ~3 arc of second. Separation of component A from B of Alpha Centauri is ~14 arc of second. Yet the image of Alpha Centaury comes out as single bloated star. I have tried imagining the Alpha Centaury with 8” Meade LX90 with same results.
#5
06-09-2008, 05:01 PM
 Karls48 (Karl) Registered User Join Date: Jan 2006 Location: Sydney Posts: 753
TerryB, next time when whether clears I will try to take number of very short exposures to see if I can split it .
#6
06-09-2008, 05:56 PM
 Karls48 (Karl) Registered User Join Date: Jan 2006 Location: Sydney Posts: 753
To my way of thinking Alpha Centauri is most important star visible. It is closest 2 stars system to us and both components are very similar to our Sun. We are indeed privileged to see this star as most of Europeans and Americans cannot see it. It is possible to detect a planet orbiting a star even with smallish telescope by recording light curve of that star. It would make great project for IIS members to measure light curve of Alpha Centauri over period of couple years. Anyone with Maxim DL could do it. It would also require some computer-programming wizard to come up with the program that can average observations with different telescopes and CCD cameras. This is main reason why I would like to split Alpha Centaury images. I think that split image photometry would make more sense then bloated image of it. I personally can see this star only for about months a year due to obstructions. So it does not make sense for me to take up this project alone. IIS got now about 4500 members; some of it are very experienced astronomers. If only 40 would take up this challenge – we may get some result. I would be quite happy to follow the lead of someone with better astronomical and organisational skills then mime. Thin about it guys.
#7
07-09-2008, 12:45 AM
 Gama Registered User Join Date: Dec 2005 Location: Melbourne Posts: 1,121
Alpha Centauri is actually a 3 star system.

Try using a decent barlow lens to bring some magnification, or try afocal projection..

Theo.
#8
07-09-2008, 03:38 AM
 kinetic (Steve) ATMer and Saganist Join Date: Jun 2008 Location: Adelaide S.A. Posts: 2,260
Quote:
 Originally Posted by Terry B The problem with Alpha cent is that it is very bright. It saturates very quickly. I agree that I get it as 1 star with a 1 sec exposure but can image much closer mag 13 stars and detect the separation.
Hi guys,

If it is any help, I used Alpha Cent to focus on when chasing the space
station for a hand guided capture attempt.
Without knowing the exposure needed for the ISS and time running out
before the pass was due, I quickly swung over to Alpha.
Very proffessional preparations I know .

But it seemed to get me in the ballpark for the exposure setting.
I was imaging at Newt focus on my 8" F7, with a homemade focal
If I had the modified webcam on LX, Alpha would bloat into an
overexposed blob.
If I switched to Non-Lx I could get two separated stars and focus
easily on them.
For this exposure, the webcam was operating at 1/5sec I believe,
and gain was mid-slider.
RGB sliders were at 'daylight' setting from memory.

On the issue of capturing the wobble I think you would need to
be very high magnification.
I can give you another example which may help.
I decided a few years ago to try and identify Proxima Centauri firstly
using widefield long exposure shots, then I went to 8"F7 shots
once I knew where it was.
I repeatedly took a frame of Proxima over many months to see if
I could make a movie of it's parallax wobble over a two year period.
Consequently, after two years of chasing it, I haven't been able to detect
any significant wobble while imaging at 8" F7.
So either I am getting something completely wrong (likely) or
the angular movement on the sky background is too small to detect
at this resolution.

regards,
Steve B.
#9
07-09-2008, 08:16 PM
 Karls48 (Karl) Registered User Join Date: Jan 2006 Location: Sydney Posts: 753
Hi Steve, thanks for the tip. I will try Web Cam next time. If you want to see parallax of Proxima Centauri over 20 years period have look on this - http://my.hwy.com.au/~sjquirk/images/film/proxima.html
#10
08-09-2008, 11:12 AM
 bojan amateur Join Date: Jul 2006 Location: Mt Waverley, VIC Posts: 6,054
I did Alpha couple of nights ago when playing with MTO-1000A.
The image is from prime focus, canon 400D, ISO100, 1/50 sec exposure.
Atmospheric turbulence is quite visible.. another problem was mirror flip, I should have used mirror lock.
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