Re: The Crab Nebula in 1844

In message <Xns973738443C317lkajehoriuasldfjknak@xxxxxxxxxxxxxx>, John Schutkeker <jschutkeker@xxxxxxxxxxxxxxxxxxxx> writes 
Jonathan Silverlight <jsilverlight@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote
in news:tkvOeQj3HmrDFwoO@xxxxxxxxxxxxxxxxxxxxxxx:

In message <492-43ADF415-259@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>, WALTER RAVE
<kn_walterrave@xxxxxxxxx> writes

The drawing may have been so poorly done because he was using a
telescope with a metal mirror.


The problem with that argument is that his other pictures are superb.
Look at the drawing of M51, for instance.
Are there any reports by people using speculum metal mirrors today?


Speculum metal?!  And are you sure he used the same telescope for the crab
as for the other drawings.


All of Rosse's telescopes were made from speculum metal, as were most or all mirrors at the time, because Leon Foucalt didn't start making silver-on-glass mirrors until 1857 (though this link is about metal mirrors and says the process was invented by Liebig <http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/511159/
Main/505146>. One poster was making a speculum metal mirror)

To answer your earlier question, I mentioned red light because the filaments are very bright in hydrogen-alpha. The eye normally isn't very sensitive there - the peak is in green light.
.