Refraction

[Porthandbuoy]

Further to Neil's astro thread.

Using a bubble sextant through a window will introduce errors due to refraction, okay, but I can't get my head around using an artificial horizon. Would that cancel or compound the errors?

I've resorted to drawing little sketches, but still can't figure it out.

Just to make it more interesting, consider whether or not the artificial horizon is inside or outside of the window.

Answers on a postcard please.

 

[30boat]

Take a reading to something fixed with the sextant behind the window and another outside and then compare.That'll give you the refraction value of the glass.

 

[Porthandbuoy]

Take a reading to something fixed with the sextant behind the window and another outside and then compare.That'll give you the refraction value of the glass.

An empirical solution I'd love to try, but I'm weeks away from going home and trying it out. Just thought someone with a few more functional neurons than me could figure it out.

 

[boguing]

The 'ray' bends once as it enters the glass, and then bends 'tother way when it comes out. So self correcting.

 

[peterb]

The 'ray' bends once as it enters the glass, and then bends 'tother way when it comes out. So self correcting.

Only if the glass is perfectly flat and even. Most cheap window glass introduces noticeable distortion.

 

[johnalison]

An astronomical object will appear slightly below its correct position (ignoring atmostpheric refraction) because light rays from the object outside the window will be bent upwards of the straight path when entering the glass, and bent the other way on leaving. The difference between the paths will be pretty small and depend on the refractive index of the glass and its thickness. Close to the vertex the displacement will be roughly the thickness of the glass. The horizon can probably be taken as producing no refractive error, presuming the windows are vertical. The displacement will be proportional to the glass thickness x sin altitude of the object.

 

[snowleopard]

An astronomical object will appear slightly below its correct position (ignoring atmostpheric refraction) because light rays from the object outside the window will be bent upwards of the straight path when entering the glass, and bent the other way on leaving. The difference between the paths will be pretty small and depend on the refractive index of the glass and its thickness. Close to the vertex the displacement will be roughly the thickness of the glass. The horizon can probably be taken as producing no refractive error, presuming the windows are vertical. The displacement will be proportional to the glass thickness x sin altitude of the object.

Now we open a can of worms. With a 'real' sight, the path of light from the sun is refracted by the atmosphere but the light from the horizon 3 or 4 miles away travels pretty well straight. When using the artificial horizon, bot light paths are from the sun and subject to atmospheric refraction. So how does one allow for that when making the corrections?

p.s. the effect of refraction can be pretty significant. When the centre of the sun is on the horizon it actually appears a whole diameter higher in the sky.

 

[srm]

Now we open a can of worms. With a 'real' sight, the path of light from the sun is refracted by the atmosphere but the light from the horizon 3 or 4 miles away travels pretty well straight. When using the artificial horizon, bot light paths are from the sun and subject to atmospheric refraction. So how does one allow for that when making the corrections?

p.s. the effect of refraction can be pretty significant. When the centre of the sun is on the horizon it actually appears a whole diameter higher in the sky.

Don't think there are any worms here. I would just use the refraction tables in Burtons or Norries. Artificial horizon is replacing earths horizon (as you say pretty well straight). Standard atmospheric refraction varies with altitude, maximum at horizon and zero at zenith. If you want to get pedantic I seem to remember refraction table corrections for temperature and barometric pressure.

 

[peterb]

Don't think there are any worms here. I would just use the refraction tables in Burtons or Norries. Artificial horizon is replacing earths horizon (as you say pretty well straight). Standard atmospheric refraction varies with altitude, maximum at horizon and zero at zenith. If you want to get pedantic I seem to remember refraction table corrections for temperature and barometric pressure.

But the dip is zero. You are measuring against a true horizontal, not a horizon below your eye level.

 

[srm]

But the dip is zero. You are measuring against a true horizontal, not a horizon below your eye level.

Indeed it is, but dip is a separate correction based on height of eye in a sight reduction calculation. That is why I referred to refraction tables and not total correction tables.

However, the standard nautical refraction tables are intended for use at (more or less) sea level. I suspect that there may be a difference if you use an artificial horizon at significantly greater altitudes. Is Bilbow Baggins around as he will be able to help on this?

 

[peterb]

Agreed. My 'but' was to warn against using corrections for dip, whether separately or combined with other corrections.

 

[Lady Campanula]

Indeed it is, but dip is a separate correction based on height of eye in a sight reduction calculation. That is why I referred to refraction tables and not total correction tables.

However, the standard nautical refraction tables are intended for use at (more or less) sea level. I suspect that there may be a difference if you use an artificial horizon at significantly greater altitudes. Is Bilbow Baggins around as he will be able to help on this?

I forwarded the query to our old friend, who is recovering his strength after a long winter sea passage to warmer climes 'Into The West'.

His reply follows....

Although not fitted as standard equipment on an Elfin vessel, one of the RAF's old Bubble Sextants was found in a dark corner of the ship's navigatorium, with some other bits 'n pieces....

If memory serves, the tiny glass plate on a bubble sextant through which one sighted the Object was optically flat. There was also a small card tabulating the refractive correction for that specific glass plate/sextant against Sextant Altitude ( degrees ), called 'Bubble Sextant Error' which was included in the sight reduction calculation. On Mesozoic and pre-Cambrian aircraft, where the sextant hung from an 'ook and peered through a perspex dome, the dome itself was calibrated for refraction - or 'ballpark' values could be taken from a table printed on the inside cover of the Air Almanac.

This differs from the other necessary correction for 'Atmospheric Refraction', which varies as Height Above Sea Level ( height/altitude above sea level ) and Sextant Altitude ( degrees ).

The above-mentioned table for Atmospheric Refraction has 'entering arguments' up to 55,000 feet, and also Temperature. Student navs would - on occasion - use the sextant on an identifiable marine lighthouse or headland, for Distance Off - a 'Depression Shot', so-called in consequence of the usually-disappointing results. Academic arguments would later ensue over what 'corrections' ( or 'fiddle fctors' ) would then be justifiable. On some aircraft, the optical bombsight was considerably more accurate than the depressed sextant...... Geeky, or what?

There was also a table for 'Corrections To Be Applied To Marine Sextant Altitudes', for 'Dip of the Horizon', which accommodated heights/altitudes up to 2655 feet. It is recalled that one student - only one - queried why the table was limited to that lowly altitude, and the dismissive reply from an Aero-Nav Instructor from the high-flying Victors was that the only RAF aircraft known to have used marine sextants were Sunderland Flying Boats ( better than a Rocna! ) and a few traditional lapstrake wooden Shackletons - none of which could climb higher than the said 2655', a Coastal Command record for generations....

As for using 'an artificial horizon at greater altitudes', I met no-one quite mad enough to try to use a bowl of engine oil or elemental mercury on the cabin floor of an aircraft in flight. Some silly blooger would have stepped in it......

Some aircraft, such as Vulcans, had two periscopic sextant mountings. On rare occasions, these were used simultaneously for an Instantaneous Sun/Moon Fix or a Simultaneous Two-Star Sandwich Fix - but that was just showing off.

Where the sextant type included a 'pendulous reference' in replacement for a 'bubble chamber', then certainly some additional corrections were required. As these were AFAIK fitted to much-faster jet aircraft ( Vulcans, Dominies, Victors, Nimrods, Canberras ), then 'Acceleration Errors' ( lateral and longitudinal ) became significant, so procedures and corrections were initiated to minimise the effects.

All this became more and more complex and esoteric. Eventually, some bright blooger said 'Sawd this for a game of soldiers! This is too much like work! I'm gonna go off and invent GPS. Once that's working, I'll get one of the ground-techies to couple it to the autopilot, and then the precious aircrew can spend ALL their waking hours playing with their Wii's."

The sun is high, so I think I'll put my aged and yellowing copy of The Air Almanac back on its shelf now, and get back to my pipe and book of Old Elvish pomes. These days, the stars are for twinkling.

Best wishes to all my good forum friends

Old Bilbo