andrew s

Nice Robin, it could only be bettered with a colour camera! 😏 Regards Andrew

It a brain mangler thinking on the scale of the Universe. Firstly, we believe the Universe is the same everywhere ( homogeneous and isotropic) on large scales and we have evidence to support this. However, it is true only for observation made at the same time (i.e. age of the Universe by observers co-moving with the expansion of the Universe). Which is a fancy way of saying they would not measure and red or blue shift in the CMB. On earth we do but it is subtracted from the images you normally see. So at the same epoch the expansion will look the same from everywhere and they will measure the same value for the Hubble "constant". Note the Hubble constant is not constant! Secondly, in the past the expansion (as measured by the Hubble constant) was significantly higher than it is now (this has nothing to do with the proposed early cosmic inflation phase). So yes the expansion is slower now than before. Thirdly, the Hubble constant is asymptotically tending to a constant value even with the "acceleration of the universe" . Sound odd but its because its a ratio. Imagine you are driving your car and accelerating you velocity increase but so does the distance traveled. The Hubble is like taking your velocity and dividing by the distance traveled. Fourthly, as you say, the further we look out into the observable universe the further back in time we look. In practice its very hard to account for all this and you need a model of the Universe to calculate distances. This is currently done based on the LCDM concordance model. Regards Andrew

Yes normal astro refractors have a concave focal plane looking from the objective. Regards Andrew

You can get combined rotator/focuses but they are expensive. Regards Andrew

Interesting talk. Well done Jack.

Try checking the file with this Fits checker . I suspect some key information is missing in the header. Regards Andrew

Just hope I am still here to see it reach a conclusion. Regards Andrew

I don't think scientists assume the universality of laws. They may postulate the idea but they then test it by observation. I am not sure where it's up to but an experiment to see if the fine structure constant varies with time is planned / underway. As we get new instruments they do new test e.g. the arrival time of gravitational and electromagnetic waves to test the universality of the speed of "light". Regards Andrew

That's right. When I use The Sky X I give it J2000 it converts it to Jnow and points there. If it plate solves it does it with J2000 catalogues and reports the actual position in J2000 coordinates. Similarly if it needs to offset the interface works in J2000 but the control system converts to Jnow under the bonnet. Regards Andrew

I don't know Carte du Ciel but yes that would be my starting point. Regards Andrew

Thanks @George Jones we desperately need more data to constrain the speculation. Regards Andrew

Fine images and resolve in getting up at 1am I assume. Regards Andrew

@Horwig what the y scale on your plot? Regards Andrew

Plate solving will use a catalogue with J2000 coordinates. Regards Andrew

Well done Dave, as you may know I love diffraction spikes. Keep the images coming. Regards Andrew

That was the best advice I was ever given. Regards Andrew

You have two many telescopes trying one at a time. Regards Andrew

Led lights vary and are broad band. Regards Andrew

Sounds like CDC is using JNOW where the J2000 positions are precessed to now. This is what you have to do for accurate telescope pointing. In, for example, The Sky X it is done automatically. Regards Andrew

@vlaiv Airy first derived an approximate solution (GB Airy, trans. Camb. Phil. Soc., % (1835), 283.) but a modern derivation using Fraunhofer diffraction of classical waves is given in Principle of Optics by Born & Wolf. I can scan the pages if your interested. There is little point in going for a QED solution when the classical field approximations are perfectly adequate. This is the root of the issue. The issue is the Airy patterns are not the same. Increasing the aperture of a perfect circular aperture just scales the Airy disk and MTF. However, adding and obstruction does not do this it is in fact the difference of two "Airy disks" A positive contribution from the" unobstructed" aperture and a negative one from the obstruction. So my question is why does the Fourier transform of an Airy disk have a cut of at all? In Fraunhofer diffraction the PSF is proportional to the square of a Bessel function of the first kind. I see no logical or physical reason for a cut off other than for practical, pragmatic reasons i.'e a convention. I agree you need to do the calculations by that means doing the Fourier transforms of the PSF directly. It seems to me the equivalent methods assume the cutoff for them to be valid. Regards Andrew

@vlaiv I think the issue is the cutoff frequency is defined by convention as 1/(lamda F#) and not on the actual shape of the PSFs in the two case. The comment you object to is not intended as you read it in my view. What he is saying is that the PSF of an unobstructed aperture diameter D is significantly more different to an obstructed aperture D than the obstructed aperture PSF is to the PSF of an unobstructed aperture 10% larger than D with 1/4 wave P-V spherical aberration. Thus the MTF of the obstructed aperture should be closer to that of the aberrated larger aperture than the smaller unaberrated one. I agree with this view. With your logic why should the cut of be the same for the more different PSF of the obstructed and unobstructed apertures with the same diameter? Regards Andrew

Enjoyable thread. I have generally gone to telescope-ootics.net as my main source of information on this area. I was taken by the comment that there is an inconstancy in the analysis which put the cut of frequency in the same place for unobstructed and obstructed apertures. As is pointed out the obstructed central peak is narrower then the unobstructed one and so can't have the same cut-off as the must have different transforms. I did try to find any other references to this but found none. Placing the cut-off in the same place seems to be a convention. The discussion is here Discussion Regards Andrew

I not unreasonable. Regards Andrew

...but it is evident in deep space objects. Field stars can show halos due to CA. Regards Andrew

Mixture of both in fact. Regards Andrew