[theodog (Jeff)]

Originally Posted by Terry B

Is it easy. NO
I have succeeded with may 5 stars so far.
I tried NGC6752 (mag 5.4 GC)
I could see it easily on the slit with a 5 sec exposure. A 300 sec exposure aimed at the middle of the spectrum didn't reveal anything through my 200mm scope.

Spectroscopy is possibly a little more difficult, but I only think it’s to do with the exposure length.
For bright point objects (stars & asteroids) most of the light (magnitude) will pass through the slit. However for extended objects, only the portion that passes through the slit can be counted. Given that galaxy magnitudes are usually integrated magnitudes –all of the light as if it’s in one point, the magnitude at any pixel would be much lower than a star at the published value for the galaxy. This much lower light level is then spread over the spectrum. Given only a small portion of the light is at each frequency, then the time needed for an integration and signal to noise ratio is much increased. It is not a simple, straight multiplication. The exposure time will increase out of proportion with an increase in magnitude value and size of the extended object.
Does this make sense I know what I’m trying to say, but failing to say it I fear.
Maybe like taking the spectra of a much out-of-focus star through a slit would demonstrate the increase in exposure times needed.