This image shows the spectrum and Lyman-alpha forest of the very distant ultra-luminous quasar SMSSJ2157−3602, recorded with over 18 hours of exposure through a 12.5 inch f/4 Newtonian telescope and a StarAnalyzer diffraction grating.
This extreme object is the most luminous quasar discovered so far, being over 100 trillion times brighter than the Sun. The light from the quasar originates from the accretion disc of a supermassive black hole in the centre of a very distant galaxy 12.23 billion light years away. The black hole's mass has been measured at 34 billion solar masses, which makes it one of the most massive black holes in the known Universe.

The quasar is very faint in the optical range (G magnitude = 18.3) and has a very large redshift of z=4.692. This means that recording its spectrum with amateur equipment is extremely difficult as the peak emission is shifted heavily into the deep red and infrared where my CCD camera is not very sensitive. To make matters worse the diffraction grating also dims incoming light by approximately 6 magnitudes. But by combining 220 exposures of five minutes each through the grating I was able to clearly detect the zero order image of the quasar itself and also a very faint signal of its spectrum.

I wanted to identify the distinctive feature of the Lyman-alpha forest, to verify that I had indeed recorded the faint quasar spectrum. To do this I used RSpec to analyse the stacked image FITS file and create the calibrated spectrum profile. This profile was then exported as a 1D FITS file for further analysis and comparison with a reference spectrum. I downloaded a detailed quasar spectrum taken by the VLT X-Shooter instrument from the ESO Archive Science Portal: http://archive.eso.org/dataset/ADP.2019-08-06T11:10:58.655 and then wrote a python script to plot my observed spectrum against the ESO reference spectrum. While my observation is on the very edge of what can be squeezed from a simple diffraction grating, the result clearly shows a good match with the features in the reference spectrum, particularly the Lyman-alpha forest and the peak near 700nm of the Ly-alpha emission line that has been redshifted from the extreme UV all the way into the deep red.

The Lyman-alpha forest is a series of absorption lines in the spectra of distant galaxies and quasars arising from the Lyman-alpha electron transition of the neutral hydrogen atom. As light from a very distant object travels across billion of light-years to reach us it will pass through many different gas clouds where the Lyman-alpha absorption will occur at different redshifts. This gives rise to a distinctive 'forest' of absorption lines in the spectra of distant quasars.

Image details:
Date: August-November 2022
Exposure: 18 hours 30 mins @ -25C
Telescope: Homebuilt 12.5" f/4 Serrurier Truss Newtonian
Camera: QSI 683wsg with Lodestar guider
Filters: StarAnalyzer 200 grating
Taken from my observatory in Auckland, New Zealand

Many thanks to Dr Christopher Onken, one of the discoverers of the quasar's extreme luminosity, who kindly assisted me with suggestions and finding the detailed reference spectrum to compare my observation against.

Spectrum in ESO Archive Science Portal: http://archive.eso.org/dataset/ADP.2019-08-06T11:10:58.655
ESO blog post: https://www.eso.org/public/blog/behemoth-behind-the-brightness/
https://en.wikipedia.org/wiki/Lyman-alpha_forest