andrew s

A truly fascinating discussion. Not that I have fully understood it but I at least have a feel for the issues. Funny how blue is "cooler" and red, I assume, "hotter" as it's the exact reverse with stellar temperature! Blue stars are hot and red cool. Regards Andrew

Whatever software you use you need to be in the right region of ADU values. CCD and CMOS have 4 main regions. With increasing signal the they are, read noise limited, shot noise limited , PRNU limited and non-linear (saturated). For flats you want to be in the PRNU region where the error is proportional to the signal level. So you want as high an adu you can get while the response is still linear I.e. no saturation. Regards Andrew

I think it will depend on the detailed circumstances. You get blue reflection nebulae with back scatter. I don't know what the primary source of IFN is. True colour photometry might help decide what it is. Back or forward scatter, or absorption of transmitted light. If the surrounding light does not pass through it then it should have no effect. It would require something to scatter additional light into our line of sight to change it. Regards Andrew.

Interstellar reddening is very real and well known. Can someone send @vlaiv a set of data to calibrate in the way he proposes then we will know. Certainly, the IFN will redden any light passing through it. Regards Andrew

No. I just multiplied the 128 by the ratio of the absorption. I did say it was simple. Regards Andrew PS I just use a mono camera to take spectra no fancy colour processing.

I tried a very simple experiment. I created a grey rectangle in powerpoint. I then reduces the green and blue by the factors calculated from the atmospheric absorption from the 45 deg plot I posted. Here is the result. While I should have integrated over the filters response, I just used the centre wavelengths. However, to first order it's obvious the atmosphere will make grey look brown if you don't compensate for it one way or another. Regards Andrew

If finding was the fun what would you expect to replace that now with goto? If goto can give you that then that's your route if not then dob dob dob on. Regards Andrew

Linear units. I was just about to add that! The atmosphere cut off very steeply below 3900 A Regards Andrew PS Theory here http://www.astrosurf.com/buil/extinction/calcul.htm implenented in his ISIS software.

Here is a plot of atmospheric absorption from 4000 to 7000 A linear scale 0 to 1 at 45 deg and 90 deg from the horizon. It uses a typical AOD (aerosol optical depth of 0.13 for France.) Regards Andrew

You can and and spectroscopists and photometrists do correct for atmospheric absorption. It is strongly dependant on zenith angle. Good enough models are available which could be used at a subs level to correct a stack taken over an extended period. However, is it not normally taken into account when colour balancing the stars in an image? I don't know. Regards Andrew

Could not resist this Regards Andrew

They are "uniform" and stable during a nights observations do only simple air mass corrections need be made. Regards Andrew

Yes even us amateur spectroscpists aim to get as close as possible to the arrival rate of photon per resolution unit above the atmosphere. This involves careful calibration of the instruments and images using well known methods and more often than not working differentially using standard or secondary spectroscopic standard stars. Similarly with photometry it is done mainly differentially as few of us have photometric skies. I am currently trying to improve the way this is done for slitless spectra which have particular issues with flat fielding as the response is wavelength dependent. Regards Andrew

I feel @vlaiv proposal is as authentic as it can get but the artist still chooses the framing and exposure etc. However, I like @saac I see astro images as acts of creation albeit from "nature" . Regards Andrew

Have you seen some of the entries in the FLO IKO images processing competition ? I know it has been narrow band so far but my eyeballs are still recovering. 🤩🤩 On a more serious note, as a non imager, it's about the craft of rendering what the processor (artist) wants to draw from the data and its relationship to the intended impact on the viewer. Regards Andrew

Glad it's fixed. Regards Andrew

It can be whatever you want it to be. The more important something is to you the harder it is to pin down and the more passionate the disputes. What's a Farm? Even the US government could not pin that one down. Does an allotment count? A small holding? You get the idea. It's a hobby enjoy. Regards Andrew

Ok Jim, it was the 4 green dots that confused me. The tangential velocity is as you describe for points on a rotating solid body. For an identical inclined free orbit about a spherical planet it will have its axis of rotation perpendicular to the plane of its orbit and have the same angular velocity about that axis. The maximal launch boost at the equator is due to the rocket being on the earth. Hope I have understood now. Regards Andrew

Hi @saac struggling to understand what your saying . If the blue orbit is just a tilted white EQ orbit how can it intersect at the green points? Can you add an arrow to show what you mean by the tangential velocity? Is it the tangent to the orbits or some other projection. Regards Andrew

Thanks, I do also worry about the rolling friction v stability any thoughts on this? Teflon pads? Regards Andrew

If Saturn was spherical then all orbits about its centre would be equivalent. However it has an editorial bulge which tends to pull particles into its plane. A combination of inelastic collisions and the conservation of angular momentum does the rest in a hand waving kind of way. Regards Andrew

You could try this Saturns rings. It's complicated! The link is to a pdf that downloads. Section 2.2.2 is the relevant one. Regards Andrew

Hi @Gina if the cameras can freely rotate in the two plates what stops them slipping back and forth? Maybe it's just not shown. Regards Andrew

Looks like diffraction . What kit do you have? Anything crossing or intruding into the optical path? Regards Andrew

Velocity is relative but acceleration is not. The earth is not an inertial frame hence accelerating. You can choose coordinates in which the earth is not spinning but the equations of motion then require pseudo forces e.g the coriolis force. Regards Andrew