Alpha Crux - four stars system

[jsmoraes (Jorge)]

Acrux is a four stars system located 321 light-years from the Earth. Only two components are visually distinguishable, Alpha A and Alpha B, separated by 4 arcseconds, with surface temperatures of about 28,000 and 26,000 K respectively. Their luminosities are 25,000 and 16,000 times that of the Sun and have minimum separation of 430 astronomical units, the period is at least 1,500 years.

Alpha A is itself a spectroscopic binary star, with its components orbiting in only 76 days at a separation of about 1 AU

The third star has movement that seems to be gavitationally bounded. Its luminosity is low for its class and suggest that it is far away and it may be only apparent bounding, although. It is separated by 90 arcseconds from A B stars.

[Robert9 (Robert)]

Lovely and sharp Jorge with good clear separation.
Robert

[SadisticSarz (Sara Freeman)]

Hi Jorge

I am enjoying your images. do have a question. how are you getting your measurements? is it a program your using that can measure distance?

[JimsShed (Jim)]

Yes I enjoy these images too. This image alone teaches me that beautiful images can just be stars.
Also, what are you using to make the star spikes?

Jim

[glend (Glen)]

I was just separating those tonight, thanks for the images. My MN190 can separate A and B easily with a good clear gap but A and A1 elude me so far but I think I can see an apparent bulge so will keep trying.

[jsmoraes (Jorge)]

Thank you very much for comments. I am glad to know that all you are liking these not VIP and fashion images.

From Sara: how are you getting your measurements ?

I do this with Photoshop. You can measure how many pixels you have between two known distant stars. For example Alpha star A and Alpha star B have 4 arcseconds of separation. Therefore 4 arcseconds / 13.37 pixels = 0,323 arcsecond per pixel.
The photo with the measurements above was resized with 200 %. Therefore, with the native resolution of Canon T3 you have 0,323 x 2 = 0,646 arcseconds per pixel.

Many others measurements that I did show similar value: 0,65 arcseconds per pixel for Canon T3 with my set of acessories: Coma corrector GSO + filter MoonSkyglow.

This value is different if you use only the telescope and Canon T3. Coma corrector and filters may add some barlow effect.

You can use the site http://www.12dstring.me.uk/fov.htm to find FOV and resolution of your optic system.

Or...

You can use the formula:

arcseg/pix = (size of pixel in microns / Focal Length of telescope) * 206.3 * binning factor

notes:

1) DSLRs haven't binning ressource, therefore its value is 1

2) it is very difficult to have the exact Focal Length if you use accessories with barlow effect. The Focal Length may change with distance of barlow element and sensor of camera.

3) Actual Focal Length = Nominal Focal Length * barlow factor


For planetary you can use the formula:

arcsecg/pix = size of object (in arcseg) / size of image (in pixels)

note: some cartes, like Carte du Ciel, WinJupos, Stellarium , says the size of planet or objects for the day and hour you are seeing them.

From Glen: My MN190 can separate A and B easily with a good clear gap but A and A1 elude me so far but I think I can see an apparent bulge so will keep trying

Be carefull. The system has 4 stars. Some texts use different notations to identify them. I used A, A1, B and C.

A and A1 are impossible for visual separation. Only with spectographic method. They have around 1 UA (distance of Sun and Earth) of separation.

Your telescope has a theoric resolution of 0,74 arcsec. This value doesn't consider distortions of refraction and Airy disk. Despite it you have a telescope that can separate those stars.
Many people think that C is the star B. Attention to don't be in mistake with this.

formula:
resolution in arcsec = 139.7 / aperture of telescope in mm

From Jim: what are you using to make the star spikes?

The spikes are natural from my newtonian telescope. I am embarrassed, because some exacerbation of spikes on very bright stars is due to big DAMAGE in the lens of my Coma corrector. Those lens have more than some scratch ... they have Canyons !

This image alone teaches me that beautiful images can just be stars.

I love doubles. I will do many photos of them.

From Robert: with good clear separation.

We must test many expositons and ISO to get the better sharpness and clear image.

[glend (Glen)]

Thanks Jorge, I'll have a look at them with my 16" dob (0.34 arc sec resolution). To determine which separation I was seeing with the MN190.

Regarding your resolution arc/sec calculation, my scope is advertised as having 0.29 arc sec resolution but your formula says 0.34, so something is wrong somewhere.

[Atmos (Colin)]

Originally Posted by glend

Thanks Jorge, I'll have a look at them with my 16" dob (0.34 arc sec resolution). To determine which separation I was seeing with the MN190.

Regarding your resolution arc/sec calculation, my scope is advertised as having 0.29 arc sec resolution but your formula says 0.34, so something is wrong somewhere.

That all comes down to what wavelength you're using within the calculation. One is the Dawes Limit with is calculated by 4.56/16 (your telescope being 16") which comes to 0.285 resolution. Doing calculations for red, green, blue and visual average will all give different resolution numbers. Blue light having a shorter wavelength allows for a VERY slightly higher resolution than red.

In truth though, this is the maximum resolution that the telescope can achieve, the atmosphere will NEVER allow you to reach this point. Under the best skies on the planet you'll at best get about 0.5 resolution so in the end, anything larger than an 11" telescope becomes redundant for resolution.

[glend (Glen)]

Thanks Colin that clears it up for me, and I had seen mention of the wavelngth being part of the calculation and we know the manufacturers will use the best one as their resolution figure. Agree entirely with 'Seeing' being the real limitation, which begs the question about how Jorge resolved all four visually.