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 Post subject: Re: Astrophotography...
PostPosted: Sun, 11-02-18, 21:00 GMT 
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fenerit wrote:
Nice! Now, let suppose one wish to render the tidal disruption/interacting bridges between galaxies: what should be the procedure? Do one need to parallelize a new class of shader objects shaped like arcs, rings, "C", "S", "J", "integral sign" profiles and then call for them by the single peculiar galaxies (e.a like those in the Arp catalogue)?


Hmm. I suppose you asked how one could algorithmically estimate the main size deformations due to the effects of an increasing gravitational pull between galaxies that are approaching each other? Furthermore, would it be possible to create codewise some smallish set of graphical building blocks, the successive composition of which would describe an approximation to the gravitational deformation process?

First of all, we will have to give up the current static description of galaxies. We need at least the relative velocity vector of the 2 galaxies, telling us about the direction along which the inter-galactic gravitational forces are getting strongest. Then we need to estimate the masses of the interacting galaxies which can be easily done via the galaxies' absolute magnitudes. There are nice papers about this method, which Dawoon and I used also in our gravitational Lensing framework for .Sci.
https://www.google.de/url?sa=t&rct=j&q= ... PWfN8I6KbJ

Most importantly, Dark Matter will play a crucial role and thus needs to be included. In this context, the exciting story of the famous Bullet cluster could be a role model! Here is a brief survey: https://astrobites.org/2016/11/04/the-b ... rk-matter/, as a reminder.

This would be another exciting celestia.Sci - based research project together with some knowledgeable collaborators (?)...

Let me know if I misunderstood your question ;-)

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Fridger

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 Post subject: Re: Astrophotography...
PostPosted: Sun, 11-02-18, 22:58 GMT 
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You didn't misunderstood. I would stay on common physics, though: those are millions and millions of years of interacting evolution that should be insane to use celestia.Sci just to simulate how the galaxies were in the past before or how they will be after the merging either. Common physics mean that in cases like The Antennae, seem to me the tidal tails have the shape of an arc of parabola; let say: the galaxies merges completely where the two parabola's axis meets themselves. The tails are residuals of the increasing force of attraction (free fall) which the farther arms and extended regions of the two galaxies can't following having lost coesion with the gravitational potential of the nuclei. Visually the same, but in reverse way, of a freefall spills that start with the water bound together and then ends in droplets. Of course accounting for all phenomena concerning galaxies is near impossible to render.

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 Post subject: Re: Astrophotography...
PostPosted: Mon, 12-02-18, 7:46 GMT 
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Oh nooo, don't do this! Don't start another galaxy rendering improvement project! ;)
The current rendering is outstanding! Just make a release as it is and keep improvements for the next version.


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 Post subject: Re: Astrophotography...
PostPosted: Mon, 12-02-18, 9:18 GMT 
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schreiberste wrote:
Oh nooo, don't do this! Don't start another galaxy rendering improvement project! ;)
The current rendering is outstanding! Just make a release as it is and keep improvements for the next version.


Steffen,

no new project starting ;-) I still have lots of administrative work to do for a release, like giving proper credits, Copyright issues, switching from SVN to GIT etc. This takes most of my time, not some little science fun for a day or two ...Technically, celestia.Sci is mostly ready for a stable release!

Fridger

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 Post subject: Re: Astrophotography...
PostPosted: Mon, 12-02-18, 9:46 GMT 
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Quote:
You didn't misunderstand.

Fenerit,

The real problem is not to fiddle a particular colliding galaxy pair into a more realistically looking one. Rather, one would require a general algorithm to tell the computer at which point of the approach he should start applying gravitational pull corrections to the rendering. Whatever these corrections are. I am affraid this automatism would not work in a static scenario.

Finally, here is some instructive fun for understanding the striking Antenna galaxy pattern better:

It is scientifically supported that gravitational pulls among spiral galaxies effect typically an elongation of outer arms as well as a weakening of their brightness, since the contained matter (gas, stars) is most weakly bound in the galactic periphery. This can e.g. be also observed in Stephan's Quintet (NGC 7320)

With this fact in mind, I just did two little modifications to the Antenna galaxy rendering: I assumed that the colliding galaxies were originally of Hubble type Sc (rather than Irregulars). Moreover, I assumed NGC 4038 to have a clockwise arm rotation pattern, while NGC 4039 remained at the canonical anti-clockwise pattern. These two changes give this rendering:

Attachment:
Screenshot_20180212_100609.jpg
Screenshot_20180212_100609.jpg [ 33.67 KiB | Viewed 733 times ]

Can you see the two antennae now? All it takes finally is to elongate and weaken the outer arms as expected from gravity arguments...

Fridger

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 Post subject: Re: Astrophotography...
PostPosted: Mon, 12-02-18, 10:30 GMT 
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Wow! :shock: That's is definitive for me. 8)

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 Post subject: Re: Astrophotography...
PostPosted: Thu, 15-02-18, 13:42 GMT 
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Finally better weather again!


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File comment: M45 Pleiades,
90 subs of 30 seconds each

M45-Plejaden_2018-02-13_(90x30s-ISO1600).jpg
M45-Plejaden_2018-02-13_(90x30s-ISO1600).jpg [ 1.16 MiB | Viewed 705 times ]
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 Post subject: Re: Astrophotography...
PostPosted: Thu, 15-02-18, 17:00 GMT 
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That is a VERY nice one of the Pleiades (M 45), Steffen!

Do you know that by giving your seconday mirror supports a particular shape (cardboard!) you may reduce the huge spikes of your 4 vane spider in the stars almost entirely...
The process is called Apodization. See also https://en.wikipedia.org/wiki/Apodization

Fridger

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 Post subject: Re: Astrophotography...
PostPosted: Thu, 15-02-18, 20:23 GMT 
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Cool shot!
t00fri wrote:
That is a VERY nice one of the Pleiades (M 45), Steffen!

Do you know that by giving your seconday mirror supports a particular shape (cardboard!) you may reduce the huge spikes of your 4 vane spider in the stars almost entirely...
The process is called Apodization. See also https://en.wikipedia.org/wiki/Apodization

Fridger


The best way to reduces spikes with newtonian reflectors is to leave with them; they are an aestetic feature and their removing will cause the loss of contrast/sharpness. Moreover they are a reference for stacking further exposures taken after several days of pause. Fictional spikes are introduced via softwares by refractors imagers just for that. Pleiades are not an easy subject to shot with reflectors because they are very bright nonetheless. In the great shot above I can see a tad of miscollimation in Maia/Taygeta/Celaeno/Elettra group (center right) and the bad tracking/polar alignment shown by the orthogonal rainbow pattern upon spikes. Once these two, along with a good camera focus, are optimal, the spikes would be reduced a lot. Another thing to do should be to rotate the tube within the rings to set up the spike axis along the vertical: rotating the camera will not change that.

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 Post subject: Re: Astrophotography...
PostPosted: Thu, 15-02-18, 21:04 GMT 
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Thanks for your point of view, fenerit,

yet these spikes are clearly an artefact and on the professional level they are not too much liked. Here is an example statistics: https://www.google.de/search?q=m45&hl=e ... ys08naGJIM:

Under this URL you can see about 40-50 mostly excellent shots of M 45, but only 2/45 have spikes...

Spikes are clearly a matter of taste. Also things depend much on the kind of application and the size of the instrument. E.g. with a suitable, rotatable apodization mask for my C8 it was relatively easy for me to clearly separate Sirius B (a long time ago)!

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Fridger

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 Post subject: Re: Astrophotography...
PostPosted: Fri, 16-02-18, 8:10 GMT 
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Ah, very good, recommendations for improving my results! I appreciate that!

t00fri wrote:
Do you know that by giving your seconday mirror supports a particular shape (cardboard!) you may reduce the huge spikes of your 4 vane spider in the stars almost entirely...
The process is called Apodization


No, I wasn't aware of that. I'll definitely read up on this.

fenerit wrote:
In the great shot above I can see a tad of miscollimation in Maia/Taygeta/Celaeno/Elettra group (center right)


Interesting, could you point me to something specific to look for? Maybe with a crop of the image?
I tried to collimate the telescope using a laser tool, but only on the first of the two observation nights. Maybe I spoilt the collimation by moving the telescope inside and then setting it up again the next day.

(And yes, it took 2 nights to collect 90 x 30 seconds of light...!)

fenerit wrote:
... and the bad tracking/polar alignment shown by the orthogonal rainbow pattern upon spikes. Once these two, along with a good camera focus, are optimal, the spikes would be reduced a lot.


Focus might be an issue here, as I don't have a focusser with reduction, and the seeing was quite bad. The stars where flickering a lot and I had to go back and forth quite some time to get a somewhat good focus.
Also, the inside of the telescope tube is not as black as one might wish. There are a lot of reflecting things, especially the drawtube that is silvery and very far inside the tube. Maybe I should try to trim it and paint it black.


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 Post subject: Re: Astrophotography...
PostPosted: Fri, 16-02-18, 12:30 GMT 
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I've marked the issue in red. Premise: if those are reflections introduced by the optical chain after the secondary mirror, they would follows the camera's rotation; so this is a point of further investigations, because it is most easy to rotate the camera a bit between sessions than moving the mirrors inside the tube. This is doesn't seem the case because the "V" shaped patterns have been remained in their places and just in that side of the shot. Possibly the issue is before the camera, either both mirrors must be twisted a bit or the camera's nosepiece is reflecting inside the draw tube/coma corrector. Usually reflections, even those made by the adapter's vignetting, have an arc shaped form. Of course if you did find the collimation perfect, as long as the laser itself isn't out of collimation, mirrors can't be the cause.
Attachment:
M45-Plejaden_2018-02-13_(90x30s-ISO1600) copia.jpg
M45-Plejaden_2018-02-13_(90x30s-ISO1600) copia.jpg [ 510.48 KiB | Viewed 678 times ]

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 Post subject: Re: Astrophotography...
PostPosted: Fri, 23-02-18, 9:57 GMT 
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While the moon is quite a disturbance when trying to get images of some deep-sky objects, I made the best of it by just imaging the moon instead ;)


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File comment: Moon, stack of 38 images
Mond-2018-02-22_(38 subs).jpg
Mond-2018-02-22_(38 subs).jpg [ 328.46 KiB | Viewed 571 times ]
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 Post subject: Re: Astrophotography...
PostPosted: Fri, 23-02-18, 20:24 GMT 
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Steffen,

this is a pretty good one! Incidentally, did you ever find some time to examine your mirror via a Ronchi grid or perhaps simpler: "mit der Foucault'schen Messerschneiden-Methode" ? From your shots I'd say the mirror must be very good, indeed!

Over the years, I did quite a few Moon shots with my C8, having an effective focal length of 2.0m. Yet I don't think the result was any better than yours. This is mainly due to the much more delicate requests for grinding and alignment accuracy of such catadioptric folded systems. Also --compared to simple parabolic mirrors--Coma becomes even more serious at somewhat larger angles... Unfortunately, my images are all on paper, since they are from "early, pre-digital times".

Fridger

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 Post subject: Re: Astrophotography...
PostPosted: Fri, 23-02-18, 21:02 GMT 
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That's an experienced amateur shot. You did catch the "Werner X" feature!
Attachment:
Mond-2018-02-22_(38 subs).jpg
Mond-2018-02-22_(38 subs).jpg [ 226.71 KiB | Viewed 554 times ]

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