andrew s

Mirrorless cameras have built in lens correction including vignetting so it may be due to that. Not sure if it's done in all formats or if you can turn it off. Regards Andrew

Prism diagonals can't be CA free. Try without it. Regards Andrew

Yes it's a complex issue and depends on many variables and as with all things a balance is required. Which noise component dominates is an interesting question. Many of the "standard" answers were derived in the early CCD days and don't reflect the reality of modern CMOS detectors with their small pixels. They suffer significant telegraph noise which is not included in the standard noise equation used by, for example, astroimagej. I have never even seen this discussed recently other than on C Buils spectroscopy web site. It is, however mentioned in the pioneering work of Jansick but was a minor issue compared to others at the time. Another area is scintillation noise in short exposures. Regards Andrew

Astroimagej is very good and I use it for exoplanet photometry. Not the simplest to get into but very easy to use once set up. Regards Andrew

This is not always best practice. If you have bright stars they are often defocused to avoid saturation rather than shortening the exposure. In addition for optimally exposed stars fixed pattern noise dominates and can be exacerbated by switching between a few pixels if guiding is not perfect. It can be reduced by having the star cover more pixels to average it out. Sky background noise can be reduced by by using a good sized annulus. Regards Andrew

I should have made myself clearer. The light from the sky is essentially from infinity and follows the design light path through the optics. The light from the light panel is from just above the corrector and is somewhat diffuse this will will illuminate the camera field and dust particles differently to the sky. This results in different vignetting as well as differences in how the dust shades the camera and hence differences in the motes. I am not clear if you see shifts in the position of the motes or just not fully removed. I would still do the first test just to prove the processing worked as expected. Gumption traps abound in my world. Regards Andrew

If this were my problem I would try the following. Take a set of flats with the ota pointing up. Make a master flats. Then take a few flats in each filter and process them as if they were target images images. Are the vignetting and dust motes removed? If no then either the filter wheel is not accurately positioning the filters or the processing is incorrect. If ok take flats with the ota horizontal in at least two ways to maximise and drop or flop. Again process separately as lights if vignetting and motes not removed movement may be an issue. If vignetting and drop removed then my bet would be different light path through ota for sky and panel as I proposed before. I would investigate this next even if it's not a longterm option. Regards Andrew

Why should the focus star shift if you are using a add-on focuser to focus? It is more likely to be play in the focuser or camera side of it than the mirror moving especially as it's locked down. Regards Andrew

Then I don't think you will solve your problem. Why need it be 3 times the size? The angular size of the c11 field is quite small. Try some twilight flats or just experiment. If you won't change anything how do you expect to solve the problem? The high focal ratio of the c11 makes it sensitive to changes in light path. Regards Andrew

Rod , I feel your frustration! For flats to work the flat light path needs to match the light path from the sky as closely as possible. I suspect you are getting additional light from the Flatman panel either from reflections or flooding in the C11. Not sure where you have the panel in relation to the corrector but the further away the better to match the sky. Regards Andrew

Advise him against getting one it's an incurable disease that leads to licking, sniffing and other disgusting habits as @JeremyS will testify to. You/they have been warned. Regards Andrew 🙃

It's important to remember the statistical spread is not due to the "classical" measurement error but due to quantum nature of the microscopic world. Regards Andrew

Not sure what calibration you used but noise in images goes through 3 major stages. 1) read noise, 2) photon shot noise, 3) fixed pattern noise and finally saturation. 1 and 2 have reduced effect with increasing signal but 3 is proportional to the signal and they only way to remove it is with a good flat. Not sure it's relevant. Regards Andrew

In a thread I can't now locate I was discussing with @ollypenrice that spacetime was seen as geometry. I came across this which may be of interest. He uses the term model for what I would call an empirical model as contrasted to a theory based model. He is a mathematical physicist specialising in string theory but I won't hold that against him. Regards Andrew

I had a good paper on this which of course I can't find now. Shift invariance is fundamental to all linear imaging theory. Hence the point sampling of Nyquist. Things like MTF, convolution and optimal sampling don't formally apply with integrating areal detectors. I do have a paper showing this via simulation for slit spectrographs if your interested. I doubt that with pixels getting ever smaller and the over sampling it makes possible that there will be any practical difference. Regards Andrew PS "Theoretical Bases And Measurement Of The MTF Of Integrated Image Sensors" is behind a pay wall. However the abstract makes my point "By analogy with optics, the spatial resolution of image sensors is generally characterized by the Modulation Transfer Function (MTF). This notion assumes the system being a linear filter, which is not the case in integrated image sensors, since they have a discrete photoelement structure. These sensors must in fact be considered as integral samplers. Their response to any irradiance distribution can thus be computed, knowing the pitch of photoelements and using a characteristic function. This function is more or less similar to the MTF. Once exact theoretical foundations have been defined, a computer simulation enables the various MTF measuring methods to be compared this makes it possible to rule out er-rors inherent to experiments. The most accurate and reliable method appears to be the knife edge method, applied with a relative displacement of the sensor and of the image. This avoids the occurence of aliasing phenomenon. Experimentation of this method for measurement of the CCD sensors characteristic function, which we call MTF as agreed, is described. This method also makes it possible to evaluate the transfer inefficiency of shift registers."

Optical theory would say yes you can. Regards Andrew PS On reflection that's not quite true as a CMOS or CCD sensor has finite sized pixels and is thus not shift invariant. That it it depends exactly where the point source (star) fall on the pixel. However, for all practical purposes it does as other methods like MTF are similarly impacted.

I think you should get yourself a planesphere, a beginner's star atlas or the maps from an astronomy magazin or a similar app based map. That way you can identify them yourself and begin to learn your way a round the sky. Regards Andrew

The expansion of the Universe does indeed result in a tension pulling things apart. However, it is orders of magnitude weaker than the electromagnetic force holding atoms and rulers and other objects together. Similarly, gravitational bound systems like the solar system and cluters of galaxies are bound by orders of magnitude more strongly than the expansion induced tension. It's only when object are much much further apart that the tension is greater than the gravitational attraction and they move apart with the expansion. Regards Andrew

I like this from William Blake: "To see a World in a Grain of Sand And a Heaven in a Wild Flower, Hold Infinity in the palm of your hand And Eternity in an hour." Regards Andrew

If the Universe is spatially infinite then it always was infinite and this is the case in the currently accepted concordance LCMD model. If you think of @vlaiv grid then it always extended to infinity. What is changing is the distance between the dots. This is called metrical expansion. Regards Andrew

I have no idea what Newton actually thought but his theory is one in which space and time can be coordinated with a Euclidean geometry. In retrospect I was reflecting the modern view of Newtonian spacetime rather than his. I am not familiar with Westfall but I know his absolute spacetime was challenged at the time from many angles but its success at predicting the motion of the planets rather confounded them. Even Einstein's formulation of Special relativity and General Relativity along with Bohr's quantum mechanics are now seen as outdated. It reminds me that what we think of as fixed changes and morphs with time. The idea of the election of JJ Thompson in 1897 is very different to that of QED. This thread has weak anti-parallels with Plato and his theory of forms. From Wiki "The theory of Forms or theory of Ideas is a philosophical theory attributed to Plato, that the physical world is not as real or true as timeless, absolute, unchangeable ideas." Regards Andrew

Newton viewed space and time as absolutely (the same for all observers) Einstein viewed them as relative (different for observers in motion with respect to each other) however, both are geometry. They are models, useful representations in different circumstances. No fabric to rip or weave just a branch of mathematics. In this sense the physicists spacetime is as real or unreal as the mathematicians pi. Philosophy has dined out for decades on debating what terms like real and nothing mean without any accepted progress. Regards Andrew PS how can nothing be contained by anything? For if it could it would be the something contained by the anything and so not nothing any more.

I already answered that. It doesn't look like anything 😕 Regards Andrew

Lets keep it simple. Following Einstein, time is what a clock measure and keeps everything from happening at once. Space is what a rulers measure and stops everything being in the same place. No clock no time, no ruler no space and no clocks or rulers no space time. Regards Andrew I will refrain from trying to debunk the misunderstanding about light, photons and time etc. Life is too short and I have flies to catch using the Mr Miyagi chop stick method.

They are thought to exist inside a black hole. Depending on the size of the hole you could pass the event horizon without noticing anything untoward. It is belived that black holes can have charge as well as mass and spin. So at least the EM field and the gravitational field exist. Regards Andrew