andrew s

Yes. It's a bit like trying to design a perpetual motion machine. Cannot be done. Of course the impossibility is now build into the SI system of units. Regards Andrew

All I can say is the consensus is it can't be done. Without a detailed operational analysis of how exactly it would be done it's impossible to say. For example, the slots are moving in a rotating frame of reference so how does that impact the angle? How would you check it? It's beyond my abilities to compute the effect. Have a look a the section on "experiments in which light follows a unidirectional path" here . Regards Andrew

There was a experiment to measure the speed of light based on a toothed wheel where the light passed through a tooth and was reflected back and was either blocked or passed as the wheel was spun at varying rates. In your scheme there are three key measurements. The separation of the two wheels, the rate of rotation and the angle between the two slots. The trick is to say how you would make these measurements without assuming something like the Einstein synchronisation convention. To be honest it not obvious to me where the problem is. However, I would guess at the angular separation of the slots. Regards Andrew

An interesting topic. As a sometime control engineer may I add some comments. Firstly, feed forward control uses a model of the system to calculate corrections. This requires an excellent system (not just the mount) without hysteresis. It can have "issues" as long as they are smooth and repeatable. Very few if any systems don't have some misalignment and or shift due to the changing position with respect to gravity. They need good measurements of all the relative quantities to keep the model updated. Due to varying refraction, optic axis shift etc. both Dec an RA corrections will be needed. They can't correct for unmodeled factors like seeing Feedback systems rely on measuring the control variable (star position) and correcting it based on the error. They can be very good but are limited both by the measurement, inertia of the rig and the speed of the loop. The highest quality pro systems use both feed forward model based system and adaptive optics where they measure the wavefront errors and correct them in a fast closed loop. Quality costs and there is no one perfect solution. Regards Andrew

I have had had a 694 and now use a 294mm on my ODK16. I do scientific imaging so always use flats and darks with both. The only issue with the 294 is you should not use scaled darks. On gotchas two come to mind. 1) The old ASI drivers had a memory leak which caused them to hang if you took a lot of images. The good news is it was fixed in the January release of new drivers. 2) the 294mm is prone to the formation of ice crystals on the sensor when cooled to below zero. The fix is to cool it very slowly. I run at -10 and use a script that cools it over 30mins. One final point small pixel cameras suffer from enhanced random telegraph noise. The 294mm is no exception. If you run it at full resolution then a median filter will fix it without reducing the resolution of your images. Regards Andrew

Have you lost your head? 😊 Regards Andrew

It won't but it will shift the images on the camera. This will impact the reported tracking. Regards Andrew

Not so. They account for the average refraction they can't account for the image shift due to random local "seeing cells". Your correct hysteresis can't be modeled out. Regards Andrew

I think it depends on the seeing. No model can compensate for that. Regards Andrew

It would be correct to say "light per unit time per pixel" ignoring transmission loss through the corrector. Regards Andrew

I have my scope at PixelSkies and would highly recommend them. Happy to answer any specific questions you might have. Regards Andrew

It includes the size of the field as well if you want to get technical. Roughly it is the totally amount of light a telescope can capture. For the same aperture the scope with the larger field will have the higher etendue. Regards Andrew

It even has a fancy French name @ollypenrice "etendue" 😊 Regards Andrew

Ok my personal two would be my homemade Newts the first with a 8" Hinds A mirror from Fullerscopes followed by a 300mm Zerodur mirror from OO and a my worst, horror of horrors, a Tak Sky 90. My final and current favourite is obviously the one I have now a 400mm ODK at PixelSkies. As they say if your not with the scope you love love the scope your with. Regards Andrew

I know I have had my 3 but it's an impossible task. I forgot about all the scopes that opened up new wavelength, in space in the IR, Xray and gamma ray, microwave and radio waves on earth. Not to mention gravitational wave. As we have probed each new area new objects and insights have revealed themselves. Regards Andrew

The Hooker 100 inch must be on the list. Not sure it has a name but the telescope at the Mullard Radio Astronomy Observatory just outside Cambridge with which Jocelyn Bell discovered pulsars. Finally the 48" schmidt telescope that did ground breaking surveys which other scopes exploited. Regards Andrew

That's all there is then. Sorry if I misled you. Regards Andrew

Try FW Udate tab https://astronomy-imaging-camera.com/software-drivers Regards Andrew

Very happy to have a discussion on astronomy and cosmology but your position seems to be that you don't belive any of our existing "knowledge and theories " and we know very little about the subject. This makes a discussion one about personal beliefs rather than astronomy the core of this site. I think that was what @ollypenrice was getting at. We need specifics to have a debate about. For example, one can discuss "tired light" v LCDM as we can look at the studies done on it. Current evidence from supernova supports LCDM rather than tired light. " Two new papers provide the best direct evidence yet. The first, slated to appear in Astrophysical Journal, measures the brightening and dimming of a certain type of supernova. Thanks to Einstein's theory of relativity, if distant supernovae are speeding away from us, they will appear to flare and fade at a more leisurely pace than close-by ones. A team of scientists led by Gerson Goldhaber of the Lawrence Berkeley National Laboratory in Berkeley, California, has shown that this is, indeed, the case with 42 recently analyzed supernovae." From here https://www.science.org/content/article/tired-light-hypothesis-gets-re-tired Regards Andrew

Science is never about 100% certainty. I am personally very happy to accept we have very good current theories. I also look forward to better ones. They get harder to find as they need to encompass an ever growing set of data. I hope JWST opens up new insights as most new instruments have in the past. Regards Andrew

We know in just the same way we know two lumps of stuff in our hands are the same. We make measurements and compare the results. In the case of stars it's their spectra. Regards Andrew

Given it was once said we would never know what stars are made of I think we know an amazing amount. I guess it's a matter of perspective. Regards Andrew

They don't. They make assumptions for example the "cosmological principle" and build theoretical models based on them. These are shaped by observation but put to the test but seeing how well there predictions match experiments. This is difficult for astronomy as we can't directly experiment but it can still be done. As an example, it is often said that space becomes discrete at the Planck scale but time of flight observations of light of different wavelengths from very distant astronomical sources does not support it. Similarly tired light is not supported by observation. On the cosmological red shift there are strong observations that support it. Tests of general and special relativity, the Layman forest the CMB and more. Alternative, theories do get published, say to avoid dark matter, but none as yet as been accepted as they don't explain other observational facts. In the end a scientific theory stands or falls on how it fits the data and its predictive power. Regards Andrew

A minor correction @ollypenrice we can see objects receeding faster than the speed of light. If I recall correctly the limit is currently about 3c. It's complex issue which is difficult to put in a post. The best I can offer is this paper https://arxiv.org/abs/astro-ph/0310808 The diagrams and maths are a challenge but the text is approachable if you accept the mathematics is correct - which it is. As per another threads real understanding takes hard work and definitely requires the wearing of underpants. Regards Andrew

By the way I think the age is right but the distances are wrong. From NASA: https://imagine.gsfc.nasa.gov/educators/programs/cosmictimes/educators/guide/age_size.html Age: 13.7 Billion Years Size: 94 Billion Light Years The most distant objects in the Universe are 47 billion light years away, making the size of the observable Universe 94 billion light years across. How can the observable universe be larger than the time it takes light to travel over the age of the Universe? This is because the universe has been expanding during this time. This causes very distant objects to be further away from us than their light travel time. For additional information, see Ned Wright's Cosmology FAQ. http://www.astro.ucla.edu/~wright/cosmology_faq.html#ct2 Regards Andrew