andrew s

Two example would be: Galileo's dynamics subsumed as a low speed case of Special relativity. Newton's gravity being a limiting case of General Relativity. There was also the ideas Plank introduced to avoid the problem of black body radiation curve but I can't recall the name of the original theory That's strange or odd s the life blood of science @JamesF. Regards Andrew

While it does well with galaxy rotation (that's what it way aimed at) it does less well with clusters of galaxies and as the Wikipedia article notes it still needs dark matter to help it out in such cases. Regards Andrew

I use an optec Pyxis LE rotator its 2" nose piece forces me to put it directly into the focuser which is ideal fir my setup. I would always go for having it before any reducer, filters etc so they move as a set reducing any shift between them. I have not noticed any tilt issues but my application is not sensitive to tilt. Regards Andrew

Not sure if any one is interested but I have had a look at the literature on GR v alternative theories of gravity this is from a 2012 review: "From a theoretical perspective, model building is an important part of understanding and explaining existing data, as well as making predictions for the future. Tight constraints already available on solar system and astrophysical scales, however, mean that model builders are presented with a choice: They can either study minimal deviations away from General Relativity, or must otherwise look for mechanisms that hide modifications to gravity on the scales probed by experiment. The former of these has value for understanding the special nature of General Relativity, and the consequences of moving away from it, while the latter provides an exciting opportunity to try and solve some of the cosmological puzzles that have arisen with the discovery of dark matter and energy. Modified gravity necessarily involves additional fields, extra dimensions, or broken symmetries, since we know that GR is the unique diffeomorphism invariant theory of a single rank-2 tensor that can be constructed from the metric variation of an action in four dimensions. An important consideration is then whether or not these deviations manifest themselves at the level of the background cosmology, or merely at the level of perturbations. Of course, if we wish to account for dark energy, or solve the cosmological constant problem using modified gravity, these deviations must be manifest in the solutions of the Friedmann equations. We must also require, however, that they do not spoil the successful predictions of the standard cosmology, such as the abundance of light elements, the peak positions of the CMB acoustic spectrum, or the predictions for baryon acoustic oscillations. This requires the background FLRW cosmology of the modified theories to closely mimic the standard evolution of ΛCDM from nucleosynthesis through to matter domination." I have underlined two key section of the discussion. Removing Dark energy and matter come at a cost. There is no free lunch to be had. The extremely long and technical review is available here Modified gravity and cosmology I admit I just scan read it and don't understand it all. The most recent review I found is 2014. Regards Andrew

Some will be scattered, some will be absorbed and re-emitted. In both cases momentum will be transferred. Regards Andrew

While it looks like polar poor alignment you won't get good tracking close to the horizon due to refraction which modifies the apparant sidereal rate. Regards Andrew

That's a fine image and I love the colour. You can feel the nebula evaporating from the emission from what I assume are embedded stars. Looks like there is a bubble centre stage in your crop. Regards Andrew

Here the synopsis of a as not yet peer reviewed paper on Modified Newtonian Dynamics. Posted on ArXiv.org where per published papers are collected. The global stability of M33 in MOND Indranil Banik (Bonn), Ingo Thies (Bonn), Graeme Candlish (Valparaiso), Benoit Famaey (Strasbourg), Rodrigo Ibata (Strasbourg), Pavel Kroupa (Bonn, Prague) The dynamical stability of disk galaxies is sensitive to whether their anomalous rotation curves are caused by dark matter halos or Milgromian Dynamics (MOND). We investigate this by setting up a MOND model of M33. We first simulate it in isolation for 6 Gyr, starting from an initial good match to the rotation curve (RC). Too large a bar and bulge form when the gas is too hot, but this is avoided by reducing the gas temperature. A strong bar still forms in 1 Gyr, but rapidly weakens and becomes consistent with the observed weak bar. Previous work showed this to be challenging in Newtonian models with a live dark matter halo, which developed strong bars. The bar pattern speed implies a realistic corotation radius of 3 kpc. However, the RC still rises too steeply, and the central line of sight velocity dispersion (LOSVD) is too high. We then add a constant external acceleration field of 8.4×10−12 m/s2 at 30∘ to the disk as a first order estimate for the gravity exerted by M31. This suppresses buildup of material at the centre, causing the RC to rise more slowly and reducing the central LOSVD. Overall, this simulation bears good resemblance to several global properties of M33, and highlights the importance of including even a weak external field on the stability and evolution of disk galaxies. Further simulations with a time-varying external field, modeling the full orbit of M33, will be needed to confirm its resemblance to observations.%We finally discuss a novel observational test using the warp induced by the external field in nearly edge-on galaxies. It shows science at work. MOND models do well at galaxy rotation curves. Regards Andrew

It might be worth considering that proposing new "stuff" is quite common. Sometimes it works out and the theory holds up. Examples would be neutrinos, anti matter, Neptune, and Higgs boson . Sometimes it doesn't work out. Examples being the ether, celestial spheres and Vulcan. Then new theories are needed. This is just how science works. Regards Andrew

Putting aside dark matter for a moment GR has been tested in a wide range of circumstances and passed. I don't know of any test where it has failed. If it is not the final answer ( which is quite possible even likely) then any new theory has to make the same predictions as GR. There have been and continue to be new theories of gravity put forward but they have not yet made an real impact. So the best theory we have requires dark matter to explain a range of observations. As far as I know there is no competing theory that can pass the tests GR has passed (where dark matter is not involved e.g frame dragging and gravity waves ) and explains the "dark matter" effects but without the dark matter. If anyone knows of one then please give me a reference as I would live to follow it up. Regards Andrew

There is a thread here on this Regards Andrew

@JeremyS this is it for now Regards Andrew

It made me think of ley lines through McDonald restaurants. Regards Andrew

@JeremyS here is a surface plot. No one view does it justice so it a fumble into animation next! Regards Andrew

Fine image @Rodd, I am always in two minds as to if the H alpha overpowers the rest of the image or not. Do they out shine the blue knots of massive stars. Clearly personal preference. Regards Andrew

There is a constant flow of achademic papers on modified gravity. So far they tend to explain one observation e.g. galaxy rotation curves, but not others. GR has been tested over a very wide spectrum from frame dragging in Earths orbit to neutron star and black hole merges. It is this foundation that make the prediction of dark matter plausible Just as Netwonian mechanics allowed the prediction of Neptune this can be a good strategy but it can also be wrong as with Vulcan. As more observations and tests are made we may develop a better theory of gravity or refine out understanding of dark matter. Regards Andrew Ps an example from to days new papers https://arxiv.org/abs/1610.07805

Optically I don't think it will make any difference. Mechanically it might be better to avoid the focuser where the tube is already under some stress. Regards Andrew

@blinky I don't know the answers to your question but the information here https://britastro.org/node/16553 may help you decide. The BAA recommends astroimagej for analysing exoplanet transits. Regards Andrew

Yes I am looking at surface plots now but matplotlib is a challenging beast a d I have to fumble my way forward. Regards Andrew

Here is the result of the filtering. Still working on how best to display the data. On this 3D plot you can see the H alpha decaying slowly back after the flare. Regards Andrew

I agree on testing but it is worth a read just for the myths it busts on resolution, central obstructions etc. etc. Yes in the end its what you see or photograph but that does not mean understanding of the various effects is wasted in my opinion. Regards Andrew

@rickwayne is correct I gave the wrong advice. Regards Andrew

Looks like amp glow from the camera. Applying flats frames should be able to remove it. Regards Andrew

Normally, turbulence makes them distort, shimmer, boil. Not quite sure what you mean by facets but stable linear features are more likely due to something protruding into the light path. Regards Andrew

Can you post some images of the key parts so we can help you. Regards Andrew