Unlike many more advanced amateur astronomical spectroscopists, I don’t have an observatory, but I do have small kids and a full-time job. Time to set up my spectroscopy rig under good skies does not come so often. So I decided that, for now, any equipment purchased would have to work on a portable rig that I could set up in minutes.
Polarimetry and spectropolarimetry are fascinating subjects in astronomy.
So I bought a QHY550P camera
more information about these types of cameras here:
For about a year I couldn’t get it all to work. I made lots of mistakes and learned a lot (which was the whole idea).
The basic idea of the camera is that rather than a color bayer array RGBG, this camera has a polarization filter array, 0°, 45°, 135°, and 90°.
Astronomical polarimetry is photon starved and poisson statistics are important. If I understand correctly, the fullwell of this camera, at about half the maximum gain, is 2000. So if I stack 100 spectra, pixels that are half full should have, on average, about 10^5 photons which should allow detection of polarization at about 10^-2.5 or +/-0.3%
QHY didn’t provide any way to separate the channels or calculate the Stokes parameters (maybe in the SDK?) The only way I was able to get this to work was the following: debayer, stack and separate the channels with Nebulosity TWICE, once with the 0° channel as the RED channel and AGAIN with the 45° channel as the RED channel. This is a pain, but nothing else worked (Fitswork etc.) I think that interpolating the values for all polarization angles is better than only using the data recorded from each pixel (this gives subtraction artifacts).
Also, as much as possible, I want the light that lands on two adjacent pixels to be different only with respect to polarization and not because of the image (or spectrum). I tried various ways to facilitate this including defocusing the image, dithering a focused image, and allowing the image to drift across the field of view, but I think the best solution was to increase the magnification to somewhat ridiculous levels to spread the light over more pixels.
Here is the Grab and Go portable spectropolarimetry rig:
The mount is an iOptron Sky-Tracker Pro camera mount on a camera tripod with a 3-way geared head.
The telescope is a Borg 55FL (250 mm FL increased to 1250 with a 5X focal extender). I can get nice 3 s acquisitions with this setup (using Sharpcap with darks subtracted on the fly). There is a visual and electronic finder.
The dispersing element is a Star Analyzer 100 set so close to the camera to afford a dispersion of 1.9 nm per pixel!
The spectra for all four polarization channels were debayerd, stacked and extracted with Nebulosity (as above) and processed with RSpec!
The data file was then exported to excel to calculate Q and U (normalized) and the degree of linear polarization (DOLP or just P).
Here are some results for unpolarized stars
Obviously there is some instrumental polarization, but it is more pronounced for the Q channels (0° and 90°) than for the U channels. I think this must have something to do with the light being spread out parallel to the 90° channel and perpendicular to the 0° channel. Does the grating cause polarization? If anyone has any ideas to explain this or help correct for this instrumental polarization, I would appreciate it.
As a first approximation, I just used the polarization from these stars as a measure of instrumental polarization and to correct for this I simply subtracted the values from the other star. The results were as follows:
That is already looking a little better. The paper by Vorobiev suggest that such cameras will have a hard time measuring polarization less than 0.5%. Luckily there are many stars more polarized than this.
I also did the same thing with phi cas, which is a more polarized star (literature value around 0.032 – 0.034). Actually, degree of linear polarization, although widely used, is a biased indicator because it involves squaring values, it cannot be negative. This is why using the normalized Q and U values gives a better indication of the errors.
The result for phi cas was:
I am overestimating the amount of polarization. I think this is to be expected when the sources of error are not well corrected .
I still have many more observations to make and calibrations to consider, but I do think there is some possibility of success for this project. Any comments are welcome. I am especially curious about the effects of the grating on polarization.