Re: The Crab Nebula in 1844

Jonathan Silverlight <jsilverlight@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
wrote in news:3bL4RBnkZorDFwJF@xxxxxxxxxxxxxxxxxxxxxxx:

> John Schutkeker <jschutkeker@xxxxxxxxxxxxxxxxxxxx> writes
>>Jonathan Silverlight wrote
>>>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/51115
> 9/ 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.
>According to Wikipedia, speculim metal was used from

The link you gave describes the following disadvantage to speculum metal

"seem to suffer a 'spectral selectivity' in their reflectivity. Many
green spots on Mars were seen by telescopes using speculum mirrors."

The following link says that the first mirror (Newton used a prism) in a
telescope was used by James Gregory, in 1663. It suggests that he used
speculum metal.

http://www.bpccs.com/lcas/Articles/telehist.htm

Wikipedia says that speculum metal tarnishes rapidly, a process I would
not expect to occur uniformly, to the great detriment of an Rosse's
drawing.

And this link says that it is notoriously brittle

http://www.classicscience.com/speculum/speculum_metal1.html

I think 1844 technology would make it very difficult to polish the
material to a sufficiently flat surface, with finish that wouldn't
damage your observation.

It seems that Rosse had the bad fortune to be using a 180 year old
technology that was of a very low quality, and just on the verge of
replacement. Considering all the problems listed above, I wouldn't
expect it to make sense to try to reproduce the conditions that led to
his drawing.
.

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