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Minimum aperture for Betelgeuse

astrophotography
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#1 edhuff

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Posted 25 February 2020 - 03:22 PM

Hi folks,

              I'm wondering what the minimum aperture would be required for resolving the color of Betelgeuse

without the chromatic aberration I experience with my 60mm refractor? You can see by the attached .jpg

that a hint of color is visible but the distortion is more than visible! Thanks! 

Attached Thumbnails

  • Betelgeuse.jpg


#2 photoracer18

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Posted 25 February 2020 - 03:59 PM

You do realize that the star is basically a point source to all but the largest professional scopes and they have to use special methods to resolve the disc. Better optics, good skies and optics that have reached thermal equilibrium can eliminate all or most of the distortion (atmosphere causes most of them). On that basis an 80mm Fluorite doublet or ED triplet might be enough. Or maybe even a good long focus achromat that has reached ambient temperature. Your image looks like astigmatism which is one type of thermal distortion or bad optics. You don't want to image unless the scope has been out for an hour or so and the images are not dancing around.


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#3 TOMDEY

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Posted 25 February 2020 - 04:00 PM

Seems like it would be a function of telescope type, rather than raw aperture. Like an entirely apo all-reflective telescope. That is, if you are only interested that one star, and not e.g. the entire constellation... then you would need no significant field and not even resolution (given that any star is an unresolved point source to amateurs). Just put your camera on a Newtonian telescope and collect an exposure series to get the color of the star. Also worth trying intentional defocus, so that the (doughnut) of light will integrate the color over hundreds of pixels, unsaturated. Then you could analyze the color quite accurately, e.g. x,y,z chromaticity or even a bunch of bandpass filters for extended color NIR, R, G, B, NUV. The epitome of spectral analysis would, of course, be --- a spectroscope, with the star on the slit!  And that should sure be interesting... to look for changes in the spectrum as it wanes and waxes. Obviously, the professionals are doing that sorta stuff right now, and we amateurs who are into spectroscopy could also enjoy the ride. Thankfully, Betelgeuse is bright enough to do stellar spectroscopy with a modest amateur telescope!

 

I think that answers what you are getting at?      Tom

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  • 41 betelgeuse-emission-spectrum-dr-juerg-alean.jpg

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#4 sg6

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Posted 25 February 2020 - 04:24 PM

In all (?) amateur scopes it will remain a point. They have imaged Betelguese but that was a somwhat bigger professional observatory scope.

 

The CA is caused by the scope and a 60mm sounds like an achro and if a fastish one that is the cause of the CA.

 

So the cure for CA would be an apo. An "slowish" ED may adaquate, just depends on the level.

Oddly if good you might get diffraction rings around Betelguese, usually very low intensity and lost in the star image, which will have a size and no longer a point. Just the way optics work. All scopes add aberrations.

 

A newtonian should not suffer CA, but you get a less sharp image and a few diffraction spikes. Most mirrors are not really high precision, which is why a good hand figured mirror like Zambuto cost 4x to 5x the cost of a GSO.

 

Reads that you need a better scope.


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#5 edhuff

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Posted 25 February 2020 - 06:33 PM

Seems like it would be a function of telescope type, rather than raw aperture. Like an entirely apo all-reflective telescope. That is, if you are only interested that one star, and not e.g. the entire constellation... then you would need no significant field and not even resolution (given that any star is an unresolved point source to amateurs). Just put your camera on a Newtonian telescope and collect an exposure series to get the color of the star. Also worth trying intentional defocus, so that the (doughnut) of light will integrate the color over hundreds of pixels, unsaturated. Then you could analyze the color quite accurately, e.g. x,y,z chromaticity or even a bunch of bandpass filters for extended color NIR, R, G, B, NUV. The epitome of spectral analysis would, of course, be --- a spectroscope, with the star on the slit!  And that should sure be interesting... to look for changes in the spectrum as it wanes and waxes. Obviously, the professionals are doing that sorta stuff right now, and we amateurs who are into spectroscopy could also enjoy the ride. Thankfully, Betelgeuse is bright enough to do stellar spectroscopy with a modest amateur telescope!

 

I think that answers what you are getting at?      Tom

 

 

Thank you Tom. A lot to digest! I also have homework to do!



#6 BenKolt

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Posted 25 February 2020 - 06:55 PM

Imaging of star "surfaces" or "atmospheres" is often done through interferometry, where two or more telescopes are employed to resolve objects at far greater detail than would be possible with a single telescope.  If you are interested, take a look at the Wikipedia page on the technology.  The baseline distance between the scopes forms essentially a large aperture and the various scopes are imaging the incoming wavefronts with slightly different phases.  These phase differences are then used to form fringes from which an image can be constructed.  There's a lot more to it than that, but I hope this explanation hits the basics.

 

Some day (maybe soon?) we will be able to image Betelgeuse with our single amateur scopes after it has gone nova and expanded enough to be resolved like we do now for the Crab Nebula.

 

Best Regards,

Ben


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#7 smccully

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Posted 26 February 2020 - 08:19 PM

Imaging of star "surfaces" or "atmospheres" is often done through interferometry, where two or more telescopes are employed to resolve objects at far greater detail than would be possible with a single telescope.  If you are interested, take a look at the Wikipedia page on the technology.  The baseline distance between the scopes forms essentially a large aperture and the various scopes are imaging the incoming wavefronts with slightly different phases.  These phase differences are then used to form fringes from which an image can be constructed.  There's a lot more to it than that, but I hope this explanation hits the basics.

 

Some day (maybe soon?) we will be able to image Betelgeuse with our single amateur scopes after it has gone nova and expanded enough to be resolved like we do now for the Crab Nebula.

 

Best Regards,

Ben

Seems very unlikely at this point,

http://www.astronome...org/?read=13518




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