andrew s

The speed of light locally is always measured to be constant i.e. c. It is a velocity. In general over large distances or strong gravity where spacetime curvature becomes significant you cannot do the normal vector addition of velocities as it depends on the path you take. In a locally flat region of spacetime velocity is not a unique quantity as it is always relative to something. I currently have a velocity of zero relative to the sofa I am sitting on but a speed of order 0.8c relative to the muon about to pass through me. The metrical expansion of space does impart a recresional velocity between non gravitationally bound systems (e.g distant galaxies) but all we can observe is the red shift in the light we receive from them. Regards Andrew

Can you 3D print chocolate? Regards Andrew

This agrigation is most likely due to surface electrostatic forces. Google "Particle aggregation in microgravity". Regards Andrew

"There is no dark side. It's all dark" Pink Floyd

I think the issue may well depend on if the guiding optic is smaller or larger than the atmospheric turbulence cells. Below about 300mm Optic the image tends to be displaced by turbulence while above 300mm it causes bluing. I had a different problem when guiding on a spectrogarph hole as the star could spill out one side or another causing a jump in the centroid. I had to use short exposures << 1 s as the star was bright and direct from the main 400mm optic. I countered this by using a very low aggression which essentially integrated out the jumps but countered any long term drift. PE was not and issue for me. Regards Andrew

You must have a humungous printer! 🤔 Regards Andrew

It will be a challenge so you need to optimise everything. As Peter proposed, collimation but also cool-down time and a ADC (atmospheric dispersion corrector) may help. Regards Andrew

Hope it goes well. Any chance we ca get to see your presentation? Regards Andrew

... Coffee that is. I have decided to ship my kit of to a hosting site in Spain so I can do some serious observing before old age prevents me observing any more. I only got 5 reasonable sessions in last year and potentially could get 200 in Spain. I want to do low resolution high cadance (every 20s) spectroscopy of M Dwaf flare stars. But as the flares are nearly random you need a lot of observing time. Maybe also some photometery of exoplanet transits that need long observing runs. To this end I have been reading the kit for full remote operation. First task was to get the PC to auto boot on power-up. Not too difficult once I found the right screen and figured out how to edit the values. Second was to provide power switching over IP. Bought a LIndy IPower switch only to find it did not come with a power lead! Having got a cable via amazon the next challenge was to discover it's IP address. Searched using a IP address software I could not find it in the list. Eventually applying the "If all else fails read the manual" strategy I found that holding down the function key for two beeps got it to read out the number - doh. Inputting the number into my web browser brought up a user log in with name and password - this time I consulted the manual first. (Not completely true I tried to guess a couple of time first, admin 1234 etc.) I have used Radmin for remote access for years and still stayed loyal to it as it works so well. On their site they you can download for free (although you need a Radmin 3 server licence) their VPN software. This installation went easily without having to look at any instructions - phew. So how to test it? It would run over my network but what about over the internet? Hence the trip to Costa Coffee. Coffee bought logged on fired up Radmin VPN selected the telescope computer and... ...it worked. Just need to tidy up and label all the connections and wait for Ian King et. al. to complete their new site in Castilléjar in August. Regards Andrew

I am not sure they would appreciate a radio transmitter in their radio quiet area! Regards Andrew

He had knowingly over spent the budget which was possibly fraudulent. Regards Andrew

@JeremyS is too modest to mention his own excellent record in advanced photometey. Regards Andrew

We have the Russians to thank for saving it and keeping Bernard Lovell in work (and possibly out of jail) with the launch of Sputnik 1. Regards Andrew

I find dividing the amount by 10 or 100 helps! Regards Andrew

@JeremyS I have become convinced you are on a one man mission to keep the astronomy market buoyant 😉 Regards Andrew

@JeremyS I have become convinced you are on a one man mission to keep the astronomy market buoyant 😉 Regards Andrew

Quite right. Another related issue is digital zoom v optical zoom. Discuss. 😉 Regards Andrew

My ex biology teacher still wife assures me most Biology slides would have been wet with water or alcohol ! Regards Andrew

If you biology microscope plates were wet it is likely that it was surface tension holding them together. While good microscope slides are not normally optically polished. Regards Andrew PS Nice video thanks for posting.

I think the main difference is that they are looking at errors in specific measurements e.g line positions, width etc. This I think is more demanding. For example from the paper "the HARPS planet-finder spectrograph (Mayor et al, 2003) uses 3.2 pixels/FWHM". Regards Andrew

Interesting work as ever @vlaiv. In a slightly different context this paper "Detector sampling of optical/IR spectra: how many pixels per FWHM?" found here https://arxiv.org/abs/1707.06455 may be of interest. While its focus (pun intended) is on slit spectrographs planetary features can be pseudo-linear. It quotes sampling in the 3 - 6 pixels per FWHM. Regards Andrew

Well it maybe old hat worm and wheel technology but my Paramount ME II with The Sky X just works. 😊 Regards Andrew

To first order I think it does. The simplest test is to image a star either side of the meridian at the same (low) altitude. You should get the red/blue in the same place wrt the star field. If it is say due to say "wedge" of the optics it should flip 180 with the optics and camera. Regards Andrew

I agree but what I am saying is that if you have the same focal length then with the same Strehl ratio a larger aperture mirror will give a smaller image than a smaller one. Thus you can simply compare mirrors of the same size, but when you compare mirrors of different sizes and focal lengths you need to be aware of difference in Airy disk size. That is all. I could have a 1cm diamter fl 500mm mirror/lens with a Strehl ratio of 99.999 would it be better in resolution that a 100cm diameter fl 500mm mirror/lens with Strehl ratio of 98.9? Not obvious to me one way or the other without doing the calculations. All else being equal the simplest way to increase resolution is to increase the aperture! Regards Andrew

Just be aware that Strehl ratio is dependant on the size of the Airy disk . For a given focal length a small aperture has a larger Airy disk than a larger one so can have a higher Strehl ratio than the larger one even if the linear image size is the same! Regards Andrew