Spectre for Imaging Use

High-resolution spectra can be recorded with an astronomical CCD or CMOS camera and the Spectre! As I develop my process I will update this site with more information. The initial tests are positive.

Spectre in standalone mode with ZWO ASI174MM Mini

Testing with ZWO ASI174MM Mini monochrome camera began on July 1st 2020. This sensor package was chosen for the following reasons:

  1. Size, weight, cost, power: The ASI174MM is essentially a planetary/guiding camera and is smaller than most eyepieces that cost $499. (Literally, the size of a 30mm Plossl at a fraction of the weight.) It is powered by USB, and that is the only required cable for this application.
  2. Monochrome is essential for recording complete spectra without camera filter artifacts. Color cameras have color filters with compromised performance in the crossover regions (the colors in between blue/green and green/red.) Use of color cell phone cameras at the eyepiece shows a 3-color spectrum with what appear to be broad absorption bands in between. See image below.
  3. Sensor size: The Spectre produces a large spectrum across the visible band, measuring roughly 12mm across from 400nm to 725nm. The ASI174MM has a sensor that is 11.3mm wide, so it can capture everything I can see at the eyepiece (more, really, as I estimate that <430nm and >650nm are impossible for me to see in low light.) Larger sensors are available at a much higher cost of course, so it will be thrilling to see what amateur astronomers armed with these capture in the near-infrared and near-ultraviolet!
Solar spectrum taken with phone. Note the broad dark band at the green/yellow border. This is an artifact of the camera filters!

My first images using the ASI174MM were of a compact fluorescent lamp (CFL) back-illuminating a slit, spaced about 10 feet from the Spectre. In standalone mode the Barlow lens is removed, converting the Spectre to an integrated telescope-spectroscope instrument. It can resolve detailed spectra of point-like sources at a distance.

Since three of the wavelengths from a mercury-based CFL are known the spectrum can be easily calibrated. These sharp emission lines are 546nm (green), 436nm (blue) and 405nm (violet). I used ImageJ to create a plot of pixel position vs. brightness, and then measured the pixel positions of the emission lines to determine the scale factor in nanometers per pixel.

My result was 0.160nm/pix, which is very close to the design value of 0.161 nm/pix! I do expect there to be some variation from unit-to-unit, so if you want to do anything really quantitative and absolute, please do your own calibration.

Calibration image of CFL source

The next step of course is to image the spectra of stars with my 12.5″ scope. The weather cleared on July 2nd, and I was able to get a few good ones right off the bat!

Arcturus (violet on left, red on right for all images
Vega (poorly focused?) With an eyepiece instead of a camera all I can easily see are the 434nm and 486nm lines. 486 is left of center.

These images are rough: I haven’t used dark, flat, or bias frames to calibrate them. They are each stacks of 100 images stacked in DeepSkyStacker and given a little histogram adjustment. The potential for far better imaging (and SCIENCE) is there! I’m a novice, don’t expect much more from me.

I would like to apply false-color to the images to give a more visual sense of the eyepiece view. It’s stunning! Photos do not do justice to the views through the Spectre!

Wavelength calibration was performed using Vega as the reference. Vega produces hydrogen lines at 397nm, 410nm, 434nm, 486nm, and 656nm. (I am surprised by the near UV performance!) ImageJ was used for extracting the data using Plot Profile. It’s now worth noting that the addition of the Barlow lens changed the calibration substantially, I’m getting a value of 0.108nm/pixel. And this might change from setup to setup, if the camera isn’t inserted to the same focus position each time. For best results, I’ll have to lock down the camera in the Spectre.

I removed the cylinder lens to see if concentrating the spectrum into a line would reduce exposures and yield good results. I found it hard to focus on the absorption line features. Only the broadest, darkest ones could be seen in the camera’s live view. Nevertheless, I obtained a good spectrum of Rasalgethi at a fraction of the exposure time, and some features are overexposed.

Image of Rasalgethi’s spectrum with cylinder lens removed
Spectral plot of Rasalgethi, uncalibrated. All of the features in the previous spectral plot are present, although the horizontal scale is shifted to the near IR a bit.

While I was at it, I put the cylinder back in and retook Arcturus just to see if I could get a prettier picture. I think the results are improved!

Arcturus, July 4th 2020: Improvement on my first try
Plot of above image of Arcturus.

I’ve gone back to Vega many times, noting that the camera sees far more spectrum than the eye. So I decided to take two spectra shifted apart to cover the full sensor range:

Vega in two exposures, from UV to near IR.

Almost all the lines are from hydrogen. One can almost see to the Balmer series limit, which is about 365nm. Hydrogen alpha, 656nm, is the 3rd line in the upper image. The fainter lines further to the right are not hydrogen, so what are they? Are they part of Earth’s atmosphere?