The preparation for this event

For the calculation of signal and contrast ratio in video records all relevant curves in the useful spectrum have to be known. The diagram below shows such curves.

Detailed notes about the curves

-  (1) Albedo of Jupiter full disk with a maximum reflectance of 52.56% at 680nm and only 4.11% at 895nm.
-  (2) Sensitivity of WAT-902H2 Ultimate CCD (CCIR ICX429ALL or EIA ICX428ALL) with a QEmax of 70% at 600nm.
-  (3) Methane band filter Edmund Optics 880/50nm with a maximum of 55% at 895nm.
-  (4) Spectral density of a star with spectral class A7 and 8000K relative to the maximum value of 1.
-  (5) Blue filter from RGB filter set Astronomik Profi typ 2c with a maximum of 95% in the range of 420nm - 440nm.
-  (6) Sensitivity of WAT-120N CCD (CCIR ICX419ALL or EIA ICX418ALL) with a QEmax of 54% at 500nm.
-  (7) Methane band filter Custom Scientific 890/18nm with a maximum of 97% at 885nm.
-  (8) Spectral density of a star with spectral class A7 and 8000K but now calculated for a +6.0mag star normalized to
       Jupiter Albedo with a total brightness of -2.8mag in the visual wavelength range of 400-700nm.
-  (9) Transmission of Earth atmosphere relative to the maximum value of 1.
- (10) Methane band filter Dr. Anders laboratories 891/17nm with a maximum of 87% at 890nm.
- (11) Spectral densitiy of a star with spectral class G2 and 5777K (like our sun) relative to the maximum value of 1.
- (12) Brightness of Jupiter disk lit by a star with spectral class G2 and 5777K (like our sun).
- (13) Baader U filter with a maximum of 76% at 360nm.

The non integrating video camera WAT-902H2 Ultimate has a double sensitivity in the 890nm methane band in comparison to the integrating WAT-120N. But this can be compensated mulitple times with the integration feature of the WAT-120N. So if needed time resolution allows the final advantage will be at the WAT-120N.

Calculations using the values from the diagrams
To get a rough view about what combination of cameras and filters could bring good results some calculations are needed. I did a simple superimposing of all curves involved in every combination. And a final integration delivered the estimated electrical signal amplitude coming out from the camera. In this way five different combinations were calculated and the results are collected in the table below.

As we can see, without any filtering the signal from Jupiter is much stronger than the signal from the star Cap. 45. So the star would be outshined by Jupiter during disappearance and reappearance. To avoid this we have to change the signal ratio. A very good result would be a signal (contrast) ratio in the range of 1. This means nearly the same brightness of Jupiter and the star on the record and no danger of outshining.

In principle there are two ways to change the contrast ratio. The use of a blue filter to support mainly the signal from the bluish A7 star CAp. 45 or the use of a filter that can reduce the light coming from Jupiters surface. The blue filter can be an alternative if no methane band filter is available and/or if the used telescope/camera system is not very sensitive.

Fortunately there is a wavelength range around 890 Nanometers called the methane band where Jupiters surface has a minimum Albedo of only 4 percent. If we mainly would use only the light around this wavelength range we would get a much weaker signal from Jupiters surface. So called methane band filters with different FWHM values and curves from narrow to wide are available for this. But beside the FWHM also the maximum transmission values of the methane band filters are different.

Using methan band filters we get a better contrast ratio but in generally a much weaker signal from the star too. And this can become a limiting factor depending on the used camera and telescope.

Table with calculation results

This table can give a rough overview about the signal levels with different combinations of cameras and filters. It should be noted that both cameras are calculated as working in non integrating mode, this means normal video timing. So the basically lower sensitivity of the WAT-120N is clear visible here. But the use of this camera in integration mode and with a narrow methan band filter will give a good signal/noise ratio and a good contrast.

But we also have to take in account that the CCD pixel size, the focal length of the used optical system and the seeing can affect this table results very much. Especially bad seeing conditions can bring a system to the limits. This I could see also during my test records that will follow below in the next part of this page.

20. Juli 2009

July 15, 2009

July 06, 2009

Test images by courtesy of Bernd Gährken, recorded in July, 2009 with the 80 cm telescope at the observatory in Munich.

© Bernd Gährken

I want to thank Bernd Gährken for providing his test images here on this page.