andrew s

Takahashi refractor owners tend to drone on so just a few low frequencies from the fft. Regards Andrew

I scanned the paper and I think they used the known lens mtf in the reduction. I would think any measurement we could make will be for the system. Optics plus sensor. Maybe with a theoretical subtraction of the sensor mtf. Regards Andrew

The attached gives the current state of play.Martin-Delgado_2020_Eur._J._Phys._41_063003.pdf Regards Andrew

Not now it is defined to be c = 3.xxxxxxx m/s exactly. The metre and second are now derived values from c. Regards Andrew

The issue is we only measure the two way average of the speed of light. The challenge is to measure it just one way. Within relativity the issue of simultaneity makes this theoretically impossible if you use something equivalent to the Einstein synchronisation protocols. Now c is defined and length and time defined relative to it. Regards Andrew

I suspect that these approaches use some form of numerical fft or its inverse possibly without understanding the limitations of these with the data sets used. Numerical methods are full of traps if you don't have a good understanding of what the constraints and assumtion are. Regards Andrew

The difference in arrival time is easily measured. The issue will be how the "distance" is known without assuming the two way measurement of c. Regards Andrew

Can you explain how? Regards Andrew

It will never work. Regards Andrew

@dweller25 I think you are right, we are into counting angles on the head of a pin. Tonal colour is most likely due to different absorption in the glass and the anti-reflection coatings. Regards Andrew

Not withstanding the salt, remember these are geometric optics traces and exclude diffraction. Not even Takahashi can alter the fact that the size of the Airy disk is a function of wavelength. Regards Andrew

Waiting for warmer weather. I have confirmed I am a fair weather visual observer. It will be moon and planets in the warm weather. Regards Andrew

I see we are getting to the bottom of your handle fetish . Regards Andrew

The plots show the results of ray tracing the optical design. The x axis is distance across the pupil (often the lens) and the y axis a measure of the deviation of whatever aberration is being plotted. If you look just at the spot diagram it is best to compare it to the size of the diffraction limit circle. For the Tak 100 f7.4 it has a diameter of about 9.4 microns if the spot fits in it's diffraction limited. Regards Andrew Looking again at @JeremyS diagram it looks like the x and y are the other way round.

Very nice. I have one too. Regards Andrew

I am not a visual expert but, given good practice, then magnification is probably the dominant effect on visibility. Regards Andrew

Do you mean visually or imaging? In both cases both collect the same amount of photons ( give or take few) so it depends on what else your doing. For dim galaxies field of view is not an issue. @alacant point is specific to the circumstances. Regards Andrew

I have no experience of either system. In fact I had not heard of Roddier analysis before your mention of it, it looks interesting thank you. Regards Andrew

@jetstream this is their NA agent https://www.innovationsforesight.com/product-category/wave-front-analyzer/ Regards Andrew

Have a look here http://www.alcor-system.com/new/SH/Shackscope40x40.html for what it can do. I think it best suited to an optical test lab. You would also need a large high quality flat unless you had a very long line of sight to a pin hole . Failing that you would be at the mercy of the weather and seeing. Regards Andrew

Well I did get a quote of the Skack Harrtmann telescope setup of €4700 to which you have to add import duty and VAT so out of my league. Pity regards Andrew

Flares yes, budgie smugglers no (only ever in French swimming pools). Regards Andrew

That's one HAL of a joke @JeremyS. My top two are: 1) 2001 A Space Odyssey - which I went to see with the then Miss Ely who was wearing a very small pair of hot pants. 2) Star Wars A New Hope - which I went to see with my then wife of two years but I don't recall what she was wearing . Regards Andrew

I was wondering if, in addition to the expert prize, we could have a pole for the SGLer audience winner? No actual prize just kudos and fun for us non entrants. I realise it might be hard to do if lots of entries. Regards Andrew

That's ok, just contribute your eyes for the trials. 😩😭 Regards Andrew