andrew s

It's made of English Cheddar cheese. Regards Andrew

Amazing video https://www.asteroidmission.org/?attachment_id=26458#main Regards Andrew

Oh come on why does such a tnney-weeny telescope need a handle? 👹 Regards Andrew

The way I was taught to think about this was that the earths ocean, being a liquid, want to have an equipotential surface. That is the gravitational force (Newtonian model) is equal everywhere on the surface. If not the net force causes the water to flow towards one. Neglecting the continents etc. this is what you get. Regards Andrew

Re analysis of alma data cast doubt on signal Re analysis Regards Andrew

Have a look at this https://www.google.com/url?sa=t&source=web&rct=j&url=https://phys.org/news/2016-05-earth-survive-sun-red-giant.html&ved=2ahUKEwi1xvLHvcXsAhXIMMAKHWcmBy4QFjAAegQIHRAB&usg=AOvVaw1UUmQyy_6K-mGipibVr4Lq Regards Andrew

The sun will become so hot as it enters helium burning that it will boil off the oceans long before we are engulfed by it. Regards Andrew

While the density and pressure is low in the outer atmosphere it is at about 3000 to 5000k I think we would notice that. Regards Andrew

...and somewhat obtuse. Regards Andrew

I love the centre portion and the fine diffraction spikes, almost ready for an xmas card. Regards Andrew

Yes but the Vandal took the handle so no scope for a joke. Regards Andrew

Just to be clear, I was not questioning the colour balance, rather that it is now being brought to prominence. Regards Andrew

Fine image @ollypenrice. Is it me or has "brown" dust suddenly appeared? I don't recall seeing it until relatively recently. Regards Andrew

Search " observing variable nebula nick hewitt " the pdf may be what you want. Regards Andrew

Works for me. Fine image to my eye. Regards Andrew

I have never tried to use it for this so I can't comment. Sorry I can't help any more other than to say it's well built a d the software sound. Regards Andrew

I have one I use for setting PA but not field derotation. It works well. If you have a specific question I may be able to help. Regards Andrew

@jetstream we came up with Etendue after a similar long discussion on here. I fully support your analysis. It applies equally well to visual and ccd/CMOS imaging. With visual you have to check you eye doesn't reduce the true field with the iris becoming the field stop. Regards Andrew

Aperture determines the number of photons. If the exit pupil from the eyepiece is smaller than your iris then it all gets in. The size of the exit pupil is the aperture of the telescope divided by the magnification. Regards Andrew

Just musing but increased magnification reduces the surface brightness in a linear fashion but the eyes response is logarithmic. So a lower sky background will reduce more than a brighter foreground on the logarithmic scale increasing contrast. Maybe that's why a wider field with more background helps. Regards Andrew This seems wrong on second thought but on third thoughts it is reducing the logarithmic compression at lower light levels compared to higher light levels. Need to think about it more a d do the sums.

@vlaiv I think you mean @AstroTim. Regards Andrew

No you buy them frequently. Regards Andrew

The exit pupil is the image of the objective formed by the eyepiece. Neglecting light loss in the optics all the light from the objective goes through the exit pupil. The bigger the objective the more light through the exit pupil. Regards Andrew

Yes, your correct I was think of the illumination at the exit pupil not the final image Regards Andrew

If you using the same eyepiece the exit pupil will be different. If the exit pupil is the same the magnification must be different. I think, that at the same exit pupil the smaller aperture i.e. with mask will be dimmer. Regards Andrew