Omega Centauri

[avandonk]

Here 20MB

http://d1355990.i49.quadrahosting.com.au/2014_06/OCent_L.jpg


Both 47Tuc and O Cent were test images to evaluate a new method for producing accurate flats.


Bert

[Astroman]

wow loads of stars there... The little Jpeg has loads of compression artifacts, the larger image is well worth it. Excellent image Bert. How do you combat the light pollution? How bad is it from your observatory?

[trent_julie]

Wow, Thats nice. Is there a hint of nebulosity there? What are the details of these "test exposures" ?

Trent

[strongmanmike]

Awwww com'on Bert get that beauty under a perfect dark sky pleeeease :prey: just imagine.....

Mike

[cosmophoton]

Amazing deep exposure. I liked the colors too, we frequently see other images full with bluish small stars in the core, which is not right to me. Great shot!

[Paul K]

Awesome shot that ! :)

[KenGee]

They are called blue struggles.

http://en.wikipedia.org/wiki/Blue_straggler

[SimmoW]

Wow, dense image Bert. Exposure details please?

[Eden]

A great shot -- nice to see some galaxies in the background there. What equipment/exposure times on that one?

[cosmophoton]

Hi Kenny,

I'm not referring to these. Blue stragglers constitute only a minority of the stars there, and they aren't enough to make blue as the predominant color.

http://www.fromquarkstoquasars.com/astronomy-picture-of-the-day-110513-the-omega-centauri-globular-cluster/

[avandonk]

Here is an animated gif of the same region from my image and an image taken with an AP175 @ F8.3. This is at the full resolution of the Astro Physics refractor image.

40MB

http://d1355990.i49.quadrahosting.com.au/2014_06/OC_L.gif

Once the animated gif has downloaded into your browser window, just left click to see at full resolution.


The AP175 image details here.

http://astrophoton.com/NGC5139-2.htm

This image by Christoph Kaltseis et al was an APOD.


His exposures were only two minutes mine were six minutes.


Bert

[cosmophoton]

Interesting comparison, Bert!
Luiz

[avandonk]

Yes, here are some hard numbers.

RH200: FL 600mm F3 with PL16803 9 micron pixels

AP175: FL 1400mm F8 with PL29050 5.5 micron pixels


Both with Astrodon filters. Both images from southern hemisphere on top quality mounts. His sky would have been far darker.

The RH200 is about six times faster than the AP175 at F8.3. The equivalent sub exposure on the RH200 would be 20 sec for the 2 minute exposures of the AP175.

Bert

[Eden]

How many 6 minute subs did you take on this image, Bert?

[avandonk]

12x6 min for each of RGB. 36 in total.

Bert

[avandonk]

Here is the drizzled version. 40MB!

http://d1355990.i49.quadrahosting.com.au/2014_06/OC_DRZ_L.jpg


The Galaxies are far clearer?


Bert

[SimmoW]

wow, I looked at your large images for the first time, amazing, so many galaxies. Either one looks great, though I think the second one just feels better?? One of my faves from you

[gregbradley]

A 200mm aperture scope is not 6X faster than a 175mm scope. Its about aperture not F ratio in that regard. See the Fratio myth paper by CCDstack author Moore. F ratio is more about FOV, a wider FOV allows more flux in from a wider part of the sky.

Greg.

[avandonk]

Yes Greg I am well aware of all the myths that astrophotographers are devotees of.

My estimate was based on sound Physics. I do have a degree in Physics.

If you consider a single pixel detecting the flux from a bit of sky.

This pixel is accumulating signal as well as noise.

For a shorter focal length optic at some given aperture this pixel will collect more photons in a given time from the same evenly emitting extended object than a longer focal length optic at the exact same aperture. Both will collect the same amount of noise for the same exposure length.

Assuming perfect diffraction limited optics the longer focal length optic has a larger Airy disc. Let us just neglect this obvious problem for now.

The shorter focal length optic has higher signal to noise for any exposure length. That is why very faint stuff is recorded above the noise.


The focal ratio myth is a combination of scaling down optics AND pixels.

Reciprocity failure with film also helped this 'intuitive' myth.

Most of real science is counter intuitive. It takes a depth and breadth of knowledge to navigate it.

Bert

[gregbradley]

Thanks Bert. Well described.

Greg.

[avandonk]

Yes Greg. The focal ratio myth is really a myth!

It is all down to lack of understanding or carefully defining what is really being discussed.


If I was to prove it totally, the Airy Disc is immaterial for an infinite evenly emitting object imaged by an optic of a given aperture.

The real world is not like that.

In the real world all point sources look like Bessel Functions in our best optics with a circular entrance aperture.

The Airy Disc is merely the first maximum peak of the Bessel Function.

For an extended object of even emission the resultant image is just all the Bessel Functions of each point overlapping to produce a what looks like an even field of illumination.

Bert

[Eden]

Nice work, Bert. The cluster actually looks like a nice spheroid instead of just a flat disk for a change.

What caused the rainbow arc at the bottom of the image?

[avandonk]

Any bright star outside the recorded field of view can cause internal reflections that are very nicely recorded by this very fast system. It is just photons sneaking past the baffling of the folded optics. If you eliminate this problem by increasing the optic's baffling you end up with a far slower system.

I do have an extended 'dew' shield which limits the amount of sky seen by the optics to just the bit that is being imaged.

I am in the process of designing a far better carbon fibre tube extension with multiple internal baffles and a perhaps a variable entrance aperture.

Bert