Comparing spectra when spectroscope pointed at garden and at sky above garden (DIY Spectrometer)

I decided to compare the spectra when I pointed the spectroscope at the grass in the garden as opposed to pointing it at the sky. The sky gives a solar spectrum (cloudy today).

Please note that after posting the following, I later discovered that there were some significant errors in the calibration values obtained here, although this does not affect the main lessons of the post re spectra from pointing at garden vs. sky – see


The following spectra were taken using the optical cable from the DIY Spectrometer connected to its modified Cheshire attachment but WITHOUT either beam splitter or telescope being used.

They both show the solar spectrum but the intensity on the y-axis is nearly 20x as high when pointed at the (cloudy) sky than at the grass – I did not expect this degree of difference as although I say it was pointed at the grass it was only vaguely in that direction and I would have expected large amount of light to have got in any way. The modified Cheshire does only allow reasonably collimated rays of light to get in to the optical fibre which would amplify this effect. The higher intensity solar spectrum when pointed at the sky is also a lot cleaner with less noise.

With optical cable pointed at the grass in the garden:

With optical cable pointed at cloudy sky above garden:

The following spectra were taken using the optical cable from the DIY Spectrometer connected to its modified Cheshire attachment which was then attached to Sky Watcher Equinox Pro 80mm via beam splitter (so this is with telescope).

With the telescope and beam splitter, I needed to use longer exposure times. Note that with cable alone above I only used 50 ms. The exposure time when using the telescope and beam splitter and with the combination pointed at the garden required a lengthy 60,000 ms to obtain the spectrum below. I got away with a lower 500 ms when pointing the telescope at the sky.

Although requiring a longer exposure time with telescope than with optical cable alone, when pointing both at the sky excellent clean solar spectra are obtained.

The telescope pointed at the garden does have the features of the solar spectrum in it but they are largely swamped by noise.

Note that these spectra are all taken during DAYTIME.

With telescope pointed at garden:

With telescope pointed at sky above garden:

Calibrating DIY Spectrometer+beam splitter+Sky Watcher Equinox Pro 80mm combination 26/8/2018 using compact fluorescent light bulb

I calibrated the combination of DIY Spectrometer + beam splitter + Sky Watcher Equinox Pro 80mm OTA using Compact Fluorescent light bulb.

Please note that after posting the following, I later discovered that there were some significant errors in the calibration values obtained here – see


Calculation spreadsheet for higher order polynomials Excel Andrew Thornett 260818@1237 (Excel spreadsheet)

Calibration-spectrum-CLF-DIYbeamEquinox-80-260818@1237.png – note this shows pixels on x-axis:


Calibrated-spectrum-of-CFL-DIYbeamEquinox-Pro-80mm-260818@1237.png – note this shows wavelength on x-axis now:

Some solar scope calculations

I have wondered about the resolution I get with my solar scope, so here are some fag-packet calculations:

Telescope diffraction limit (in radians)= 1.22 X wavelength in cm/scope dia. in cm

Telescope diffraction limit (in arcseconds)= 206265 X 1.22 X wavelength in cm/scope dia. in cm

Ha light  wavelength = 6562 angstroms = 6562 X 10^-8 cm

Lunt LS35 THa is 3.5 cm dia

Diffraction limit = (206265 X 1.22 X 6562 X 10^-8)/3.5

= 4.7 seconds, or about 5

Solar diameter is around 30 arcminutes, or about 1800 arcseconds

In this image the distance between the 2 spots is around 0.135 X the solar diameter, or around 250 arcseconds

The lines or striations around spot 2720 at their thinnest, are around 1/50th of this distance, or around 5 arcseconds.

So. my Lunt LS35THa images could, in fact be diffraction limited!

The GONG cameras have an aperture of 2.8cm, so they are similar.

Conclusion, to get images like this one (


  1. Win lottery
  2. Buy Lunt LS152 THa
  3. Get bigger window-sill

Of course, a Quark fitted to a 80mm scope should be pretty good! (Andy)


Sunspot 2720

Sunspot 2720 has now appeared and seems to be developing quite rapidly.

“Spaceweather” says:

“New sunspot AR2720 is not only large, but also strange. Its magnetic polarity is reversed. The North and South ends of its enormous magnetic field are backwards compared to the norm for sunspots in the current solar cycle, decaying Solar Cycle 24. Could AR2720 be the first big sunspot of the next solar cycle, Solar Cycle 25, popping up now in the middle of solar minimum? Stay tuned for more information about this intriguing possibility after the current geomagnetic storm is over.”!

Spectra of lights in lounge using DIY Spectrometer and Sky Watcher Equinox Pro 80mm and beam splitter 25/8/2018

The following photo shows the DIY Spectrometer (blue box) attached to my Sky Watcher Equinox Pro 80mm telescope via a beam splitter. In the photo, I am recording a spectrum of a blue LED torch:

The book shown in the photo above “Using commercial amateur astronomical spectrographs” by Jeffrey L. Hopkins has an excellent chapter on the DIY Spectrometer which effectively acts as a manual for its use and for the use of the software that comes with it, “Spectrum Studio”.

The following spectra are taken of a compact fluorescent light bulb in the following fitting on the wall of my lounge, at night with other lights turned off and curtains drawn closed (to minimise extraneous light from other sources, although note that there will be some light from LED lights in hall and computer screen itself will also contribute some light):

The following spectrum was taken THROUGH the Sky Watcher Equinox Pro 80mm telescope with beam splitter/DIY Spectrometer combination. The spectrum was taken of the above compact fluorescent light bulb:

The next spectrum was taken immediately afterwards of the same light, but this time the optical fiber was disconnected from the telescope and the end of the optical fiber pointed towards the light:

The spectra are similar and peak frequencies do not change BUT the intensity recorded in a lot greater WITHOUT the telescope.

Possible reasons I can think of include:

  1. Unlike in astronomical observations where the telescope is used to collect star-light from a great distance away, the light is in the same room and is bouncing off walls/floor/ceiling and exposed optical fiber can pick up the reflected light to increase intensity whereas telescope is only collecting light through the lens of the optical tube.
  2. The optical fiber is not properly collimated with the axis of the optical tube assembly reducing light collected.
  3. There is no collimating lens just before the optical fiber to ensure that light entering the optical fiber does so as parallel waves (focused at infinity – when this is not the case efficiency of light throughput to the spectrometer will decrease) whereas the exposed fiber can collect more light that happens to be in correct orientation.

I don’t know which, if any, of the above are correct explanations.


Spectra of coloured LED torches & white LED & Compact Fluorescent Bulb taken using DIY Spectrometer, ATM beam-splitter telescope-optical fiber connector, Sky Watcher Equinox Pro 80mm

I connected the DIY Spectrometer to my Sky Watcher Equinox Pro 80mm telescope for the first time using my ATM beam-splitter telescope-optical fiber connector.


Spectrum of white light LED ceiling light:

Spectrum of white light LED wall light – similar spectrum visible but intensity significantly reduced as fainter light:

Spectrum of Compact Fluorescent ceiling light:

Spectra of variety of coloured LED torches. Each of these torches are white light LED torches. I have used coloured balloons over the end of the torches as filters. Note that the frequency of the peaks on the spectrum moves to higher pixel readings on the x-axis as the colour moves from blue to green to orange:

Blue torch:

Green torch:

Orange torch:

Pink torch – pink and red do not register on the spectra below because their wavelengths are outside the range of the spectrometer:

Red torch – red is outside the range of the spectrometer but an element of colour is being picked up on the spectrum due to the fact that the balloon is not a perfect filter:

Mid-monthly RAG meeting 24/8/2018

Lee and Nick talked about 6 objects to observe in night sky and various telescope finder devices including Lee’s homemade version of a device similar to a Telrad – a beefy version with 50mm binocular lens in it, 1/10 wave mirror and in-built heater! He is suggesting a workshop to make our own versions of the same but improved over the commercial original- sign me up!

After Lee and NIck’s talk, I gave a brief demonstration of the DIY Spectroscope – thankfully it worked producing instant spectra for the lights in the room!


Nick Cox (left) and Lee Bale (right) give their talk at RAG:

Rhys tries out Lee’s homemade version of  Telrad finder:

Spectral response of the DIY Spectrometer & why I can’t detect red

The following is a series of tests performed tonight of the spectral response of the “DIY Spectrometer”.


The “DIY Spectrometer”:

Feed in to the DIY Spectrometer:

No telescope tonight – direct feed into the end of the device below that I made below using a beam splitter and modified Cheshire eyepiece.

Spectral Response:

Spectrum of Compact Fluorescent light bulb through above device attached to DIY Spectrometer (without a telescope) with and without the illuminated eyepiece on the above device switched on (see below).

Spectrum of Compact Fluorescent light bulb WITHOUT telescope and with illuminated eyepiece turned OFF (below):

Spectrum of Compact Fluorescent light bulb WITHOUT telescope and with illuminated eyepiece turned ON (below):

The above show that the fiber optic cable does not pick up the illumination from the red LED on the illuminated eyepiece.

I initially thought that this might be due to mis-alignment between to fiber optic cable and the LED light from the illuminator.

I tried covering the telescope aperture on the device and pointing the guiding aperture on the device at the compact fluorescent bulb and could replicate the spectrum above – hence light through the guiding aperture DOES get to the fiber optic cable.

Why I am not picking up the red LED light from the illuminator on the illuminated eyepiece in the guiding port on my ATM device:

I then worked out the reason for my inability to pick up the red LED.

The following spectrum was seen when I pointed the device at the blue LED on my printer (with all other lights switched off in room):

Notice the peak at about 465 pixels, consistent with detection of the blue LED.

The spectra above have not been calibrated. However, the following spectrum of a Compact Fluorescent Bulb was taken by myself using the same instrument on 9/8/2018 and I calibrated it at the time (see result of calibration below). This shows that the instrument’s response, with its diffraction grating as currently positioned, is 323-612nm.

The relevance of this can be seen below on the table of wavelengths of visible light from Wikipedia (, accessed 23/8/2018):

The colours of the visible light spectrum[5]
Colour Wavelength
Red ~ 700–635 nm ~ 430–480 THz
Orange ~ 635–590 nm ~ 480–510 THz
Yellow ~ 590–560 nm ~ 510–540 THz
Green ~ 560–520 nm ~ 540–580 THz
Cyan ~ 520–490 nm ~ 580–610 THz
Blue ~ 490–450 nm ~ 610–670 THz
Violet or Purple ~ 450–400 nm ~ 670–750 THz

The wavelength of red light is outside the wavelength response of my DIY Spectrometer’s diffraction grating as I currently have it set.

This is a potential advantage is it means the illuminated eyepiece will not interfere with the spectra produced in use. However, I may need to change the setting of the diffraction grating (not an easy task) as main colour in nebulae is red and infra-red. A possible job for the future…..

In addition, being able to pick up the LED in the spectrometer would allow me to align illuminated cross hairs to fiber optic cable by adjusting one or other’s position in my device until maximum intensity in red line achieved – which I can’t do if the spectrometer does not detect red colour,

Spectrum from DIY Spectrometer when pointed at white computer screen:

This shows two peaks (blue and green) – remember red missing as outside the range covered by the spectrometer currently.

Residual spectral response when all light excluded from DIY Spectrometer:

I have not been able to work out what this peak at 460 pixels is – it persists even when light excluded but is swamped when any reasonable source light enters the spectrometer – but does not completely disappear even then.

Any one got any ideas?