Testing a Daystar QPE 0.6A filter with a H spectrum tube.

I LOVE finding out about different ways to appreciate the Sun and light in general. Use this forum to post your info or questions about various outside the mainstream ways to appreciate our life giving star!
Post Reply
User avatar
p_zetner
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1644
Joined: Sun Mar 18, 2012 4:59 pm
Location: Winnipeg, Manitoba, Canada
Has thanked: 166 times
Been thanked: 719 times

Testing a Daystar QPE 0.6A filter with a H spectrum tube.

Post by p_zetner »

Hello Everyone.

To look further into etalon testing methods, I applied the scheme used for the Lunt etalon to my Daystar QPE0.6A filter. The two parameters I was able to measure are the wavelength readout calibration and the fwhm bandpass. The integral blocking filter makes it impossible (?) to measure the free spectral range of this filter since an extensive fringe pattern is not observed.

First, a look at the setup....same as the Lunt configuration.
fig1.jpg
fig1.jpg (112.79 KiB) Viewed 830 times
Interferograms were imaged using different wavelength settings of the Daystar. Here is a compilation of the measured fringe patterns. The wavelength settings are shown (in angstroms) in the figure.
fig2.jpg
fig2.jpg (151.21 KiB) Viewed 830 times
Looking at the series of interferograms, one can guess that the filter is truly on-band in H alpha for a setting somewhere between 6562.7 and 6562.8 angstroms, indicating that the number on the readout and the true centre wavelength (cwl) of the device are very close. To quantify the difference, I carried out the following steps. In figure 3, the intensity profiles along a cut through the 6563.0 A, 6563.2 A, 6563.4 A and 6563.6 A interferograms are shown. These were fitted with Lorentzian profiles used to find the image pixel number of the fringe peaks. The use of Lorentzians is not strictly justified here by theory but served the purpose of locating fringe peaks.
fig3.jpg
fig3.jpg (121.58 KiB) Viewed 830 times
Knowing the locations of the two peaks associated with one particular ring shaped fringe allowed the centre of the circular fringe to be established. Then it became possible to measure fringe locations with respect to the fringe centre (in image pixels) and replot the intensity patterns as a function of squared pixel position with respect to this centre. For such plots, theory shows that that a Lorentzian profile fit is justifiable (to a good approximation) and these are shown in figure 4.
fig4.jpg
fig4.jpg (118.46 KiB) Viewed 830 times
From figure 4, the fringe radii squared can be directly found. In the next step, I plotted fringe radius squared versus wavelength reading. Theory would predict this relation is a straight line and a good straight line fit bears this out. Here is the plot:
fig5.jpg
fig5.jpg (136.67 KiB) Viewed 830 times
The intercept of the straight line with the horizontal axis is a feature of this plot that I was interested in. This gives the readout wavelength for which the fringe radius equals zero, a reasonable indicator of the central fringe. The intercept would give directly the difference between the wavelength readout and the tru cwl of the filter. A value of -0.04 angstroms was obtained which is well within the readout resolution. In other words, I could essentially trust the calibration of the wavelength readout.

Another useful parameter of the straight line fit is the slope of the line. This gives a scaling factor between squared distance (in pixels squared) on the image and the readout wavelength. Such a scaling factor exists because the filter uses thermal expansion to change the size of the etalon gap, D. I'm assuming the readout value is calculated in the filter electronics such that it is proportional to the temperature. In this case, a change in temperature (delta_T) is proportional to the relative change in D (delta_D / D) which theory shows to be proportional to the fringe radius squared. The interesting point here is that, if the fwhm of the Lorentzian fitting peaks in figure 4 are measured in squared radii (image pixel values squared), this can be immediately converted to fwhm in wavelength using the scaling factor from the slope of the line in figure 5.

The results of this exercise gave a mean value (over the 8 peaks in figure 4) of filter fwhm as 0.52 angstroms (std. dev. = 0.023 angstroms). This seems like a reasonable result and, if accurate, is slightly better than the 0.6 angstroms fwhm specified by Daystar. Of course, a direct measurement of the bandpass with a spectrometer seems to be the more obvious method but, although simple in principle, requires that the spectrometer itself be very well characterized, especially in terms of its own spectral resolution which should be much better (narrower) than 0.6 angstroms. I thought the present measurement scheme offers a more convenient and more reliable (?) alternative.

Cheers.
Peter
Last edited by p_zetner on Wed Mar 03, 2021 1:24 pm, edited 1 time in total.


christian viladrich
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 2145
Joined: Sun Jun 14, 2015 4:46 pm
Location: France
Has thanked: 1 time
Been thanked: 2702 times
Contact:

Re: Testing a Daystar QPE 0.6A filter with a H spectrum tube.

Post by christian viladrich »

Excellent Peter !!
I will try this method on my side.


Christian Viladrich
Co-author of "Planetary Astronomy"
http://planetary-astronomy.com/
Editor of "Solar Astronomy"
http://www.astronomiesolaire.com/
User avatar
Bob Yoesle
Almost There...
Almost There...
Posts: 994
Joined: Thu Mar 08, 2012 7:24 pm
Has thanked: 536 times
Been thanked: 806 times

Re: Testing a Daystar QPE 0.6A filter with a H spectrum tube.

Post by Bob Yoesle »

Wonderful results and analysis Peter. :cool:


Diagonally parked in a parallel universe.

Curiosity is the father of knowledge; uncertainty is the mother of wisdom.

Dark-Sky Defenders
Goldendale Observatory
User avatar
marktownley
Librarian
Librarian
Posts: 42120
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Has thanked: 20229 times
Been thanked: 10111 times
Contact:

Re: Testing a Daystar QPE 0.6A filter with a H spectrum tube.

Post by marktownley »

Super analysis Peter!


Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!
LTHB
Im an EXPERT!
Im an EXPERT!
Posts: 433
Joined: Thu Mar 03, 2016 7:31 pm
Been thanked: 581 times

Re: Testing a Daystar QPE 0.6A filter with a H spectrum tube.

Post by LTHB »

Hi Peter,

thanks, this is almost a tutorial! I'll try this method for my Daystar Quantum when I find the time.

Frank


Post Reply