LowSpec amateur spectroscope

Check spectra obtained with LowSpec here

Check other entries related to spectroscopy here

I have been looking for different aspects of astronomy since the time I started to consider astronomy as a hobby. Spectroscopy was also one of this aspect, but for a long time only in theory. Amateur spectroscopes were present in the market as commercial devices (quite expensive) or ATM constructions (not an easy task). However in the 3D printing era it was only a question of time to apply this technology to amateur spectroscopy. I even started to model my own spectroscope project, but then I found out Paul Gerlach project called LowSpec at thingiverse platform. It is amateur spectroscope based on not expensive optic parts.

Main features of LowSpec device:

• 125mm FL collimator lens and 100mm FL camera lens
• 25x25mm reflective grating (range from 100 till 1800 l/mm). I have chosen 600 l/mm blazed reflective grating from Edmund Optics
• OVIO reflective slit wheel
• guiding available

Current project is labeled as version 3.0 with a replaceable grating holder and improved slit holder design. But my construction is version 2.0. I printed all spectroscope components with Creality Ender 3 printer using Impact PLA filament. Grating holder was adapted to fit 9mm thick grating from EO (originally was designed to 6mm thick grating). I also replaced threaded camera and telescope printed rings to custom CNC rings, because I was not sure if printed plastic elements could hold that stress.

Collimation

Spectroscope assembly is quite straightforward, all elements fit perfectly. Then I spent some time on collimation. This process has few parts.

Guider collimation. It requires adjusting of the guiding mirror. First thing to do is to put guiding camera into the spectroscope, adjust focus, so the slit disc is visible, and then adjust guiding mirror, so the slit is in the centre of camera field of view.

Collimator lens adjustment. It needs to be placed at the exact FL distance from the slit, so the light that falls on the grating is a collimated beam. To achieve this I used DSLR camera with the lens focused manually to infinity. I removed the camera lens from LowSpec, backlighted the slit with an LED lamp, and placed the DSLR camera at the position of the main camera. Then using live preview and 10x magnification I focused slit image.

Mirror adjustment. I used laser collimator that I put into the telescope T2 threaded inlet. I verified if laser beam points into the slit on the slit disc, and then if it also points to the guiding camera. Then I removed the slit disc holder and adjusted the mirror that way, the beam falls into the collimating lens center and then the grating center.

Grating holder adjustment. Next step is to adjust grating holder position, so the laser beam points into the centre of camera lens, and then centre of the chip.

First tests

After these steps, I performed the first tests using workshop fluorescent lamp. I put colour IMX244 camera as a main camera and I illuminated slit using fluorescent lamp. I needed to adjust micrometer to place visible part of the spectrum into the small camera field of view. 600 l/mm grating gives quite high dispersion, so only about 80 nanometers of spectrum is visible in IMX224 sensor FOV. Then I focused the image and was extremely happy to image my first spectra 🙂 Then I moved micrometer to another position of wavelengths range and after five attempts I composed a mosaic of full spectrum of fluorescent lamp. It turned out to be mercury based lamp. Due to field curvature and achromatic lenses the focus plane is changing over the wavelength, so the focus needs to be adjusted after micrometer change.

Next day I pointed spectroscope input into the sky and I recorded spectrum of the Sun. It was really spectacular view, and many of lines could be identified and measured.

To process Sun spectrum images I used VSpec software. First plot below is a red part of Sun spectrum. Blue line is raw plot, green line is plot with continuum removed. Orange lines indicates Ca I wavelengths. The deepest absorption line is hydrogen alpha.

Next plot presents Sun spectrum area with well known magnesium triplet line. Triplet is resolved, so the spectroscope resolution is quite decent as for that grating size.

Spectroscope resolution

While waiting for last spectroscope element (focusing bolt) I tested another thing – the Relco SC480 starter spectrum. This specific model is well suited for spectroscopy amateurs, because it contains many emission lines over the whole visible part of the spectrum. Other starters contains fewer lines, or some visible regions miss any line at all, so calibration in this area is impossible. I put SC480 starter bulb in the front of spectroscope slit and recorded spectrum.

Emission lines cover whole visible part 400-700nm, and most of them comes from neon and argon gases, so they are well documented. And are suitable for calibration.

Another thing that I measured during SC480 tests was spectroscope resolution. Using SimSpec excel sheet I calculated LowSpec spectroscope resolution with 600 l/mm grating. Calculations gave resolution R=800 for blue spectrum part and R=1100 for red part. Measurements of emission lines FWHM in BASS software gave me better resolution results. For blue part resolution was in the range 1100-1300 and for the red part it was 1400-1800. I am very happy with this results, especially for the first spectroscope. Such resolution already allows to split many lines, but also to measure line position with accuracy about 0.1nm (1 angstrom). To process Relco starter spectrum I used BASS project software. It is very well suited for amateur use, and contains many useful and easy to perform options and functions.

Now it is the time to mount spectroscope to the telescope and make some real work 🙂

Clear skies!