Forum der Vereinigung der Sternfreunde

Hallo,
ich möchte Euch auf folgende "kleine Bastelei" aufmerksam machen:
https://arxiv.org/pdf/1611.01596.pdf
Viele Grüße
Christian

Danke für diesen Hinweis, Christian!
Ich habe mir das Paper mal angesehen, es geht hier um ein Breitband-Polarimeter und nicht um ein Spektralpolarimeter, wie ich zunächst erwartet hatte. Das macht aber auch Sinn weil selbst niedrigauflösende Spektralpolarimetrie mit kleinen Teleskopen hoffnungslos ist. Man darf nicht vergessen, dass massereiche Sterne meist Polarisationsniveaus von deutlich unter 1% haben. Das heißt, ein Stern leuchtet in polarisiertem Licht rund 5-7 Magnituden schwächer als normal. Das zeigt sich auch an Abb. 6, in der der Fehler exponentiell mit der Magnitude ansteigt. Selbst breitbandig ist selbst bei polarisierten Standardsternen (also schon sehr viel Polarisation) bei etwa 12 Magnitude Schluss. Deswegen tauchen in Tabelle 4 eben auch nur sehr helle Sterne auf. Wenn man mehr als ein paar Dutzend Sterne untersuchen möchte sind Polarisation und insbesondere Spekralpolarimetrie nur etwas für große Teleskope. Anders ist das z.B. bei Kometen mit bis zu 30% intrinsischer Polarisation. Ich hatte selbst schon überlegt, für unser 80cm-Teleskop ein Spektralpolarimter zu entwerfen. Das habe ich wegen des Aufwands, der Kosten und der limitierten Ziele verworfen.
Gruß, Thomas

It is difficult but still possible to make some interesting spectrapolarimetry measurements with small telescopes and spectrographs though. Christian Buil has been running some experiments

http://www.astrosurf.com/buil/polar2/tuto.htm
http://www.astrosurf.com/buil/polar3/index.html

It is not new science yet of course, but it is still good fun and educating for amateurs interested in exploring the limits of their equipment.

Cheers
Robin

That's right, Robin, it might be a lot of fun!
Because I am pragmatically oriented I need to take as many parameters for this fun into consideration as possible.

• 1. Magnetic fields: Buil made his measurements at a star with 10.000 Gauss. This is a lot and far away from regular values. I once tried to measure the magnetic field of Ori C at 1.6m Mont Megantic Observatory and failed at a threshold of 400 Gauss. Only years later and with an improved equipment a 200 Gauss field could be measured.

• 2. Brightnesses: Buil detected a magnetic field of a 4 mag star via Zeeman splitting. This requires moderate resolving power quite difficult for fainter stars with smaller B fields.

• 3. Prices: I can not find the price of the ferroelectric Liquid Crystal (FLC) modulator used by Bailey et al. But already their rotator costs around 700 Euro. A superachromat retarder from Thorlabs (perhaps used by Buil) is about 2100 Euro. Taking into account all other necessary devices (neglecting a spectrograph), the price for a polarimetric unit is not low.

I just want to point this out to put all these measurements into the correct amateur perspective. Cheers, Thomas

Hello Robin, hello Thomas,
I cannot find the price of the ferroelectric Liquid Crystal (FLC) modulator used by Bailey et al too. But the other components are listed below:
PRM1Z8 - Ø1" Motorized Precision Rotation Stage (Imperial) 785,70 €
GT5-A - Glan-Taylor Polarizer, 5 mm Clear Aperture, Coating: 350* - 700 nm 465,89 €
2 Photosensor module H10720-210 2 x 830.00 $ US

I found the price for the last item in ebay.
I don't know whether the PRM1Z8 is really necessary. Maybe the rotation can be done manually.
For the FLC see
http://www.jeos.org/index.php/jeos_rp/a ... /08019/286
Cheers,
Christian

Hi Christian, when performing (spectro)polarimetric measurements you basically calculate the intensity relations between the ordinary beam (which follows the diffraction law) and the extraordinary beam (which doesn’t) behind a beamsplitter (e.g., a Wollaston or a Glan-Taylor prism) behind the retarder (who shifts the two beams against each other by a certain amount). To do so, you need to use different positions of the retarder (a FLC or wave plates). That means, for one Stokes parameter you need to do more than one measurement (six for all four). The retarder position needs to be accurately repeatable. Hence, the polarimetric measurements are very time consuming and rotating manually makes it even worse and not accurate enough. That is the reason why a PRM is used. You can find the mathematical basics at http://www.stsci.de/pdf/review_spektralpolarimetrie.pdf (I guess I already announced it) at page 36.

In any case, a polarimetry unit is expensive. Cheers, Thomas