Star Analyser Diffraction Grating

Spectroscopy of Moon, Capella and Sirius in Lichfield 18-19/2/2019 – Andrew Thornett & Nick Rufo – CCDSPEC slit-based spectroscope and Star Analyser spectroscopy grating

Last night was predicted to be clear all night. The Moon was bright, so it seemed to be a good opportunity to do something other than observing. Nick and I are both interested in spectroscopy so he bought around his Star Analyser on his camera and I took outside my Sky Watcher 120mm Equinox on EQ6 mount with CCDSPEC spectroscope. Of course, it did not turn out to be clear all night but nevertheless Nick and I were able to do some good work & enjoy ourselves……apart from when I dropped his camera lens on the floor – Andy strikes again – ahhh!

The Star Analyser has advantage of being quick to set up whereas the Equinox/EQ6/CCDSPEC was lot of faff to set up – Nick was photographing spectra well before me!

Successful procedure for aligning EQ6 mount/Sky Watcher Equinox 120mm telescope/CCDSPEC & taking spectra:

In fact, last night was a very positive experience for me because I got the procedure of taking spectra with tracking mount working properly for first time – hitherto my spectra have been on undriven Manfrotto mount with Sky Watcher Equinox 80mm.

Process that worked last night was:

  1. Balance EQ6 using heaviest eyepiece I have – 20mm Explore Scientific 100 degree eyepiece in 2″ diagonal – last night I also had finder scope for 120mm on scope and also Ed Mann’s wonderful powered and heated laser finder. I used 2 counterweights on the EQ6. Do NOT pull dew shield out during balance process.
  2. Perform 3 star alignment using the eyepiece. This is NOT easy to do with CCDSPEC eyepiece so use the heavy Explore Scientific 20mm eyepiece for the 3 star alignment. Need finder and laser to help with alignment.
  3. Exchange eyepiece for CCDSPEC. In CCDSPEC use illuminated reticule eyepiece – this has cross hairs that enclose the slit at centre helping to get stars on slit. Focus CCDSPEC using its eyepiece so star or Moon even better focused on slit – this involves racking focuser right out – hence why balance an issue as moment on the balance point of mount is changed – so need to use heavy eyepiece initially. Pulling dew shield out at this point helps with balance of scope as eyepiece exchanged for CCDSPEC.
  4. Slew to object of interest. Laser pointer and finder help if 3 star alignment not perfect.
  5. Use illuminated eyepiece to slew object onto slit as likely to be slightly off in field of view.
  6. Once object hits slit it will spread out from point light source into tiny spectrum.
  7. Take photo of spectrum using QHY6 camera on CCDSPEC and Nebulosity software – choose ASCOM camera in camera choice drop down list and then QHY6 camera in drop down menu that follows in Nebulosity from choosing ASCOM camera.
  8. Analyse spectrum in RSPEC.

Spectrum of the Moon:

The following is a photo of spectrum on Star Analyser showing the Moon to the left and its spectrum to the right taken by Nick with his Canon camera on undriven mount with Star Analyser grating. I love this photo – which can only be done with the Star Analyser – on the CCDSPEC you don’t see the Moon in the same shot!

In the following image, I have graphed the spectrum taken with CCDSPEC of the Moon last night against a reference solar spectrum (CCDSPEC pointed at cloudy sky in day) taken by myself 1/8/2018 (below). The spectrum of the Moon as taken by the QHY6 camera is shown on the left and a graph of this in RSPEC on the right, together with the reference solar spectrum. It shows that the lines on the spectrum from the Moon match those on the spectrum from the Sun – this is because the spectrum from the Moon is in fact the spectrum of reflected sunlight bouncing off the Moon which does little to alter it as it has no significant atmosphere.

Spectrum of Capella:

I was really pleased when I could slew the EQ6 to Capella and within two attempts get spectrum of this star. The laser pointer REALLY helped to compensate for problems in my poor 3-star alignment.

In the screenshot from RSPEC below, Capella’s spectrum is on the left as it comes out of the QHY6 and on the right this spectrum is graphed against the same solar spectrum as above. Some but not all of the lines match, showing that the two stars differ in composition.

Spectrum of Sirius:

Nick took a spectrum of Sirius using his Canon DSLR/Star Analyser/Canon kit lens:

Some lines are visible in centre of graph (dips) – to determine what these are we would need to calibrate the graph. Turned out calibrating the Star Analyser spectra requires a bit more work on the light used – my CCDSPEC slit easily uses just about any light with clear identifiable lines but we need to point or at least narrow light source for the Star Analyser which we did not have available tonight……a job for Nick to make himself one!

Calibrating the spectra:

I have not got around to doing this yet – but this process involves identifying lines with known wavelengths so that the pixel measurements above can be replaced with wavelengths.

To this end, I took a spectrum last night of a 12V Compact Fluorescent bulb using same set-up as above. For some reason the graph is the wrong way around and needs to be inverted left-right but I seem to be having difficulties getting RSPEC to do this on the data set for this spectrum, hence why I have not yet calibrated the above spectra!

I will be able to identify the lines using this graph below: