I’m new here, but I’m not a newbie to astro. I’m a chemist with some experience in spectroscopy and have access to two spectrophotometers. Therefore, some time ago I started to verify the parameters of astronomical filters. During almost two years of measurements, I collected a lot of spectra. Moreover, I have found manufacturers’ declarations often differ enormously from reality. In this thread I will present you the results of my measurements from time to time.
SPEC1 - Shimadzu UV-2101PC (the better one)
SPEC2 - Hitachi U-2900
1. Warming up of the spectrophotometer, especially of both lamps - 15 minutes.
2. Automatic calibration - baseline measurement over the entire spectrum measurement range.
3. Measurement parameters:
- temperature 20 ⁰C
- SPEC1, standard measurements: 300-850 nm, resolution 0.2 nm, slit 0.2 nm
- SPEC1, high resolution measurements: resolution 0.05 nm, slit 0.05 nm
- SPEC2, standard measurements: 200-1100 nm, resolution 1 nm, slit 1.5 nm
- SPEC2, high resolution measurement: …………………………….
4. Four-fold measurement of the empty spectrum (T = 100%) and two-fold measurement of the spectrum for the masked detector (T = 0%). These spectra are used to calibrate the filter transmittance measurements more precisely.
5. Control measurement of the spectra of my two filters: Optolong SII 6.5nm and Optolong L-eX. These measurements are used to check the correctness of the wavelength scale. If the difference in the position of the transmission bands is greater than 0.1 nm compared to the measurements taken as the reference*, then all measurements from a given session are appropriately rescaled.
Maximum wavelength error for standard measurements using SPEC1: 0.1 nm.
The above measurement calibration procedure allows me to obtain repeatable measurement results for a given filter and a reliably compare the spectra obtained during different measurement sessions.
* Reference measurements: the spectra of the selected two filters were measured on two different spectrophotometers with the maximum possible resolution, minimum slit width and minimum scanning speed. Both devices were controlled by separate computers with different control software. The obtained results were fully consistent, i.e. the position of the transmission bands was the same with accuracy of 0.05 nm. This procedure was done twice. In this situation, I assumed that these measurements were correct and the parameters of the selected filters determined in this way would constitute a reference for all subsequent measurements.
6. Angular measurements and filter efficiency simulations
In order to simulate the efficiency of the filter depending on the lens speed and the size of the central obstruction of the telescope, it is necessary to determine the effective refractive index. For this purpose, I measure the spectra in high resolution (0.05 nm) for the filter tilted by the given angle: 8, 10 13 and 15.2 degrees and again in the same order to minimize the measurement error. I count the refractive index to two decimal places with an estimated error of +/- 0.02.
The relationship between the shift of the transmission bandwidth and the refractive index of the filter is given by the formula:
(I can't paste the graphic, so I attach as a separate file)
In the efficiency simulation model I developed, I divide the entire surface of the lens into many concentric circles and I count the shift separately for each of them. The widths of these circles are calculated automatically so that each gives an shift greater than the previous one by a value equal to the spectral resolution of the measured spectrum (0.2 nm). Thanks to this approach, it is very easy for me to count the resultant, shifted spectrum in Excel, because I have no problem with the changing (shrinking) wavelength scale. The steps present in the simulations are an artifact resulting from the spectral resolution of the spectra (0.2 nm). Higher resolution spectra and calculations would result in a smoother curve, but its averaged course would not change significantly. You can also smooth the resulting curve, but any differences in the results are insignificant.