CCDSPEC Spectrometer

Spectroscopy from Lichfield with CCDSPEC – Damian and Andy

Damian and I went outside at my house in Lichfield and manually guided the spectroscope to get a bundle of spectrums of stars tonight. Once he had got the technique there was no stopping our boy Damian!

He calibrated the spectroscope using a compact fluorescent bulb – we used linear equation for figures for calibration file tonight.

Comparing images of spectra from brighter stars with online typical spectra – we could immediately find most of main lines generally within 2nm (20A) or real values. Wow!

Sky Watcher Equinox Pro 80mm, Manfrotto alt-az manual mount, hand guided, CCDSPEC, Nebulosity 4 software, QHY6 camera.

Note spectral classes:

Deneb A2 la
Altair A7V
Arcturus K1.1IIIFe
Vega A0V



N.B. With the peak of the Perseids tomorrow night and a reasonably clear night tonight we had expected to see a number of Perseid meteors…….Damian saw one in the nearly two hours we were outside and I missed even that one!


Calibration of the CCDSPEC/QHY6/Sky Watcher Equinox Pro 80mm/Nebulosity setup:

Calculation spreadsheet for higher order polynomials Excel Andrew Thornett 100818

Damian’s calibration below (his measurements circled):

Some spectra were amazing, showing incredible lines:



Spectral type A7V



Some spectra were not so amazing:

How to use the higher order co-efficients with PCSpectra

From: Kenneth Elliott
Sent: 10 August 2018 14:15
Subject: Re: 3rd order polynomials


Hi Andrew

This is how you use the higher order co-efficients with PCSpectra

Original (linear values entered into PCSpectra calibration file):

70.255, 0.6474

How to enter higher order co-efficients, x2, x3 and x4, into PCSpectra calibration file:

70.255, 0.6474, 0.001, 0.000000013, 0.00000000000121

Alternatively, scientific format can be used for co-efficients e.g.

7.0255E1, 6.474E-1, 1E-3, 1.3E-8, 1.21E-12

Pitfalls of using higher order co-efficients in PCSpectra:

There are two pitfalls

1: You need to have lots more lines if you use higher order fits and any fits outside the wavelength range you have got calibration lines may be wildly wrong as the equation has no constraints, so its fine with the Argon lamp.

2: you need lots of precision in the coefficients to get the accuracy, otherwise if it is multiplied by X**4 it could be wildly out.

How to improve accuracy/usefulness of higher order coefficients with PCSpectra:

Three different ways you could do this:

1. Astrosurf French site which has line idents and fitting programs, but I gave up trying to use it as, although the program runs in English, if there is an error then the error messages are in French.

2. Using STARLINK (UK) software – on Linux and needs a fair level of skill to get going.

3. The HST software both on Linux – on Linux and needs a fair level of skill to get going.


Why different image acquisition software programmes result in need to recalibrate CCDSPEC/QHY6

This follows on from Andrew’s post about needing to recalibrate his CCDSPEC/QHY6 when he changed image acquisition software from Nebulosity 4 to EZCAP:

Re-calibrating CCDSPEC/QHY6 combination for photos taken with EZCAP Image Acquisition software rather than Nebulosity software 4/8/2018

Regarding why recalibration is necessary when image acquisition software is changed:

Hi Andrew

It’s quite simple really.

Although the spectrum appears in exactly the same location on the chip, what comes out depends on how he data is clocked out and that depends on the software. Some chips have non sensitive pixels which are clicked out first and disregarded whilst other programs read out the whole chip which may be bigger than the imaging part.

So the pixel number in both X and Y can be different. Hence, it is essential to do the calibration and observations with the same readout program

Hope this helps.


PS I designed and built a CCD camera Electronics and software 30 years ago

Reflection on experience of doing spectroscopy so far

I have now been the proud and excited possessor of a spectroscope for about a month. If you have been following my posts, you will see they clearly demonstrate my intense interest and excitement in this new area (for me at least).

Statting with the fortunatel purchase of a competitively priced and very well designed system thayvallowed me to generste spectrums virtually immediately, it also allowed me to obtain star slwctra quickly and so i did not become put off by a significant learning curve.

Thay learning, i have found, does exist and can be frustrating but rewarding too when i get something to work.

I have discovered:

I can get spectra of daytime objects eaaily and quickly by just pointing the instrument at the ibject in question. This is rally easy to do and will work well for outreach. Unlike other astronomy branches, this one can be done in the day!

I can obtain spectra of bright stars with my 80mm Equinox Pro on alt-az mount also easily and it is not too much if an issue tk hand guide the telelscope to obtain these spectra. This is not mentioned in the manual and very little about it on the internet where guided exposures are the thing but my experience shows it is effective for bright objects which can easily be seen in the 10mm eyepiece in the guiding port.

The eyepiece in the guiding port simply needs to be focused on the slit – do this in the day. Then at night focus the star to a small round dot – as small as possible. When it is over the slit the star divides in two and spectrum evident in the guiding eyepiece. This is only when over the slit. The eyepiece in the guiding port has small field of view. It is really important to ensure the finder is aligned on the scope. I might add illuminated cross hairs to help here. Not on that yet.

The situation becomes more complocayed once i start to calibrate the spectrometer. Calibration helps as it enables accurate determination of the wavelength of peaks on the spectrum amd i can consistently obtain accuracy of calibration, it appears, within 1nm. Compact fluorescent bulbs seen to be good source of light for this exercise. They come with diffuser as part of the design (the glass cover is milky). They are small and portable. I could do with making a portable rig including such a bulb to take into the field as i have found that calonration needs to be done each time I change the telescope or even the image acquisition software! Basically I suspect it is best done every night the spectrometer is used. Comparison spectrum of fluorescent lights are available from Wikipedia together with information on the wavelengths of peaks on the spectrum. Although calibration is tine intensive, it ks rewarding and leads to exciting results when you can identify peaks correspnding to specific molecules or elements.

Although using the spectrometer hand guided with 80mm Equinox Pro was relatively easy, I have found that changing to the 120mm Equinox on EQ6 Pro mount is a lot more complex and time consuming and frustrating although potentially a lot more rewarding. Changing to this set up opens up the possibility of taking spectra of much fainter and difficult to find objects and taking much longer exposures including planetary nebulae and faint stars. I have only used this set up once to date (last night), and this experience demonstrated to me that the driven moint does allow me to take significantly longer exposures without having to keep moving the scope with fine control on handset even when mount is poorly aligned. I found that using the spectrometer eyepiece to align the mount was quite difficult as the eyepiece uses light from the reflection on the highly polished metal around the slit. This is a lot less efficient than a high quality mirror with costing ps and so the image is much fainter than I would normally see in an eyepiece in a diagonal. Two possible solutions to this problem for the future are:

1. Next time I could align the mount without the CCDSPEC in the scope and then insert the CCDSPEC. As the CCDSPEC will need to be calibrated and its weight needs to be taken into consideration when balancing the scope on the mount, in practice this will mean that I need to mount the telescope on the mount together with CCDSPEC in the focuser, calibrate the CCDSPEC/QHY6 whilst attached to the scope, take the CCDSPEC out, then align the mount without rhe CCDSPEC in situ, then reinsert the CCDSPEC and do observations. This leads to quite a lengthy and complex set up process and hence I can see the advantage of having an observatory with fixed and permanently aligned telescope for spectroscopy. In addition, I am worried that the mount will have difficulty being aligned without the weight of the CCDSPEC in the focuser due to balance having been achieved with the CCDSPEC in place.

2. A better suggestion has been made by Damian. He has suggested that rather than taking the CCDSPEC on and off to align the telescope, a preferable option would be to use the excellent new laser pointer made for us by Ed as part of the three star alignment of the mount. Great idea! In addition, changing the eyepiece on both the CCDSPEC and finder scopes to illuminated reticule versions will help.


CCDSPEC spectrum of Vega 4/8/2018

Taken just before midnight 4-5/8/2018, I tried using EZCAP software but then found that Nebulosity 4 gave better images. There was no interlacing effect with images of Vega through the Skywatcher 120mm DS Pro refractor up to 6 seconds in length (I presume because camera is not receiving > maximum tolerated amount of light). Even though the scope (on EQ6 Pro) was not properly aligned, it tracked sufficiently to keep the star in the slit for 6 seconds or longer.

The EZCAP-based calibration file gave incorrect wavelengths on the graph so I resorted to using the Nebulosity-based calibration file from 29/7/2018.

My spectrum clearly shows the Hydrogen Balmer series of lines.


Compare to this typical spectrum of Vega from

Or this one from

Taking a spectrum of Arcturus

A low resolution spectrum of the prototypical cool giant star Arcturus, also known as alpha Bootes (spectral type K1 III). This star has subtle peaks in its spectrum but it is not too far off the expected spectrum seen at links below.


Calibrating CCDSPEC in the field for the first time – using Skywatcher DS Pro 120mm Refractor and EZCAP software/QHY6

My first go at calibrating in the field – using the light from the compact fluorescent bulb in the study – I aimed the telescope at this.

To get sharp peaks, I ended up taking an image of only 67ms (I tried longer lengths initially).

Calibration files download:

Calibration SW120 EZCAP Compact fluor bulb 040818

Photos of spectrum:

Compact Fluorescent Light Bulb spectrum for calibrating spectroscopes with wavelengths of principal lines marked (Wikipedia 4/8/2018) (Spectrometer Calibration, CCDSPEC Calibration)

Compact Fluorescent Light Bulb Calibration Spectrum with wavelengths of principal lines marked – from Wikipedia below, reproduced under Wikipedia commons licence (I have annotated it with the wavelengths also given by Wikipedia although on that website they were in a separate table).


Example of a Compact Fluorescent Light Bulb:

Spectrum of Compact Fluorescent Light Bulb:

Compact fluorescent light bulb calibration spectrum with wavelengths from Wikipedia 040818 (Downloadable PDF File of picture below)