SVT Local News Report: Light Pollution in Stockholm

The Swedish public broadcaster SVT recently made a short report about light pollution in Stockholm, for which I was interviewed (German and Swedish).

Swedish readers might find this story about light pollution in Stockholm interesting. Jonatan Loxdal, reporter from the Swedish news website “kit.se”, interviewed me this week about our recent results based on our nightsky brightness measurements.

Nikkor Lens Comparison for Astrophotography AF 80-200 f2.8 ED vs. AF-S VR 70-200 f2.8 ED

I bought two used Nikon lenses, both very similar in their specifications, which is not surprising as the Nikkor AF 80-200 f2.8 ED is a precursor of the Nikkor AF-S VR 70-200 f2.8 ED (see Ken Rockwell’s history page). With the highest bid, I got the 80-200mm for 280 EUR + 20 EUR shipping from the big bay and for the 70-200mm a private seller in Sweden claimed 8200 SEK, equivalent to 870 EUR. Then I asked myself if it is really worth spending 570 EUR more on the new lens, especially when doing astrophotography, where vibration reduction (VR) and fast autofocus is not needed.

To answer this question, a simple startest was performed. Both lenses were mounted to a Nikon DF (FX format chip: 36mm x 24mm). The camera was then fixed on a tripod without any further star tracking. The scenery was a strongly light-polluted sky. At ISO 1600 an exposure time of 4 seconds was chosen and the camera was pointed towards NE. The startest was performed using an aperture of 2.8 and 4.0. The resulting images are shown further below for reference. The cutouts (center, top right, top left) are 100%, when the images are viewed in their original size.

Conclusion

This Nikkor AF 80-200mm f/2.8 ED I got from Ebay is bad for astrophotography, no it’s terrible! The star images at focal lengths of more than 80mm are frustrating, producing very strong coma effects more or less all over the image and even stopping down to f/4 does not make a big difference. Interestingly at 80mm the quality is OK and the coma disappears in most parts of the image. I am really surprised by this result, because the lens operates really good under daylight conditions when sufficient light is available. This is proven by the testshot below, which is taken at 200mm f/8 and a shutter speed of 1/800 at ISO 800.
On the other side, the Nikkor AF-S 70-200mm f/2.8 VR is superb. It shows only little coma over all focal lengths, with slightly better results when stopped down to f/4.0. Note that the reason for elongated stars is due to the rotation of the Earth during the 4 second exposure and not necessarily coma.

Although really bad for astrophotography, the Nikkor AF 80-200mm f/2.8 ED is really good under daylight conditions; here a shot at 200mm f/8 with a shutter speed of 1/800s and ISO 800. The image shown is a cutout without further processing of the full frame image.

Although really bad for astrophotography, the Nikkor AF 80-200mm f/2.8 ED is really good under daylight conditions; here a shot at 200mm f/8 with a shutter speed of 1/800s and ISO 800. The image shown is a cutout without further processing of the full frame image.


Startest at f/2.8

Startest of Nikkor AF 80-200mm f2.8 ED

Testing Nikkor AF 80-200mm f2.8 ED at 80mm f2.8

Startest of  Nikkor AF-S VR 70-200mm f2.8 ED

Testing Nikkor AF-S VR 70-200mm f2.8 ED at 70mm f2.8


Startest of Nikkor AF 80-200mm f2.8 ED

Testing Nikkor AF 80-200mm f2.8 ED at 135mm f2.8

Startest of  Nikkor AF-S VR 70-200mm f2.8 ED

Testing Nikkor AF-S VR 70-200mm f2.8 ED at 135mm f2.8


Startest of Nikkor AF 80-200mm f2.8 ED

Testing Nikkor AF 80-200mm f2.8 ED at 200mm f2.8

Startest of  Nikkor AF-S VR 70-200mm f2.8 ED

Testing Nikkor AF-S VR 70-200mm f2.8 ED at 200mm f2.8


Startest at f/4.0

Startest of Nikkor AF 80-200mm f2.8 ED

Testing Nikkor AF 80-200mm f2.8 ED at 80mm f4.0

Startest of  Nikkor AF-S VR 70-200mm f2.8 ED

Testing Nikkor AF-S VR 70-200mm f2.8 ED at 70mm f4.0


Startest of Nikkor AF 80-200mm f2.8 ED

Testing Nikkor AF 80-200mm f2.8 ED at 135mm f4.0

Startest of  Nikkor AF-S VR 70-200mm f2.8 ED

Testing Nikkor AF-S VR 70-200mm f2.8 ED at 135mm f4.0


Startest of Nikkor AF 80-200mm f2.8 ED

Testing Nikkor AF 80-200mm f2.8 ED at 200mm f4.0

Startest of  Nikkor AF-S VR 70-200mm f2.8 ED

Testing Nikkor AF-S VR 70-200mm f2.8 ED at 200mm f4.0


Observing Comet C/2013 US10 (Catalina)

Comet C/2013 US10 (Catalina) was first discovered by the Catalina Sky Survey on October 31, 2013. It originates from the Oort cloud, a vast spherical reservoir of comets far beyond Neptun. By chance, gravitational perturbations can push Oort objects into the inner solar system, where they are eventually discovered. In some cases, comets get bright enough to be observed by naked eye or with small amateur telescopes or binoculars. The latter one is true for Comet C/2013 US10 (Catalina).

On Jan. 17, 2016 the comet passed its closest point to Earth at a distance of 110 million km. Using my 10-inch Newtonian telescope, I have imaged the comet that day from a suburban location. The result shown below is a stack of 17 frames of 120 sec. exposure time each. The inverted version on the bottom clearly shows the two tails of the comet.

Comet C/2013 US10 (Catalina)

Comet C/2013 US 10 (Catalina) imaged in L with a GSO 254mm f/5 Newtonian telescope and an ATIK 383L+ mono CCD camera

Comet C/2013 US10 (Catalina)

Comet C/2013 US 10 (Catalina) imaged in L with a GSO 254mm f/5 Newtonian telescope and an ATIK 383L+ mono CCD camera

 

The artificial Skyglow above Stockholm and Why we care about it

On October 24th, 2015 I was invited to give a talk at the Swedish astronomy day (“Astronomdagarna“) in Uppsala. You can find the video stream of my presentation below. I talk about the impact of light pollution on wildlife, ecosystem, human health and well-being and further show how light pollution can be minimized. I present zenithal night sky brightness measurements performed in Stockholm using a Sky Quality Meter and discuss a first analysis of the data, including influence of different environmental conditions, the circalunar rhythm and how auroral activity can alter the nightsky brightness level. Finally, the results are compared to similar measurements performed in Berlin and Vienna.

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Stephen Hawking about “Quantum Black Holes”

Stephen Hawking is currently attending a historic conference in Stockholm – organized by the Nordic Institute for Theoretical Physics (Nordita) and Laura Mersini-Houghton, a professor at the University of North Carolina at Chapel Hill. As an astrophysicist I was especially grateful having been able to hear one of the most brilliant scientists of our time speaking about “Quantum Black Holes” – hypothetical tiny black holes, for which quantum mechanical effects play an important role.

Stephen Hawking giving a lecture in Stockholm

Stephen Hawking in Stockholm, August 2015 talking about “Quantum Black Holes”

 

Aurora Borealis – observed from Vallentuna

It is an amazing spectacle when a solar storm of charged particles hits the Earth’s magnetosphere. The particles following the magnetic field finally collide with particles of the Earth’s atmosphere – mostly oxygen and nitrogen. These collisions lead to either an ionization or excitation of atoms/molecules at an altitude of around 100 km. Subsequently, the recombination is responsible for the emission of a photon. The typically green colour arises from oxygen. On March 17, 2015 a strong solar storm hit the Earth and even at the relatively low latitude of Stockholm one could follow this energetic event.

Aurora Borealis observed from Vallentuna (Stockholm, Sweden)

Aurora Borealis observed from Vallentuna (Stockholm, Sweden)

Aurora Borealis observed from Vallentuna (Stockholm, Sweden)

Aurora Borealis observed from Vallentuna (Stockholm, Sweden)

 

Measuring Linearity, Readout Noise and Gain of a ATIK383L+ CCD camera

Today I did some tests with my 2-year old ATIK383L+ CCD camera. I am using the camera regularly and you can find some results at my astrophotography page. The camera uses the well-known KAF-8300 17.6mmx13.52m CCD chip with a resolution of 3362×2504 pixels and a pixel size of 5.40µmx5.40µm. The manufacturer promotes it as a camera with very low read-noise of 7electrons, great linearity and with ideal Gaussian shaped bias frames.

So let’s see if that is true for my model.

Linearity

This is the easiest of the performed tests. Thereby the camera was mounted on the focuser of my telescope and the aperture was illuminated with my Flatfield panel. Then several images were taken with increasing exposure times as seen in the table below. Subsequently, the so gained flatfields were corrected for bias and darkcurrent contributions. The resulting mean counts in each individual image were tabulated and plotted against the exposure times showing the linearity of the CCD.

Exp
Time [s]
ADU Poisson
Error [%]
Fit 1
Error [%]
Fit2
Error [%]
Fit3
Error [%]
Fit4
Error [%]
1 650 3,92 135,0    
2 1222 2,86 61,7    
3 1598 2,50 51,8    
4 2108 2,18 36,4    
5 2611 1,96 27,4    
6 3106 1,79 21,5    
8 4111 1,56 13,7 15,8  
10 5098 1,40 9,3 11,3  
14 7073 1,19 4,2 6,1  
18 9045 1,05 1,4 3,2 7,3
22 11009 0,95 0,4 1,4 4,6
28 13911 0,85 1,8 0,0 2,3 4,1
36 17771 0,75 2,9 1,2 0,5 1,8
44 21635 0,68 3,6 1,9 0,7 0,2
52 25353 0,63 3,5 1,8 1,0 0,2
60 29083 0,59 3,5 1,9 1,3 0,6
70 33737 0,54 3,5 1,8 1,5 1,0
80 38357 0,51 3,4 1,7 1,5 1,1
90 42887 0,48 3,1 1,5 1,4 1,1
95 45068 0,47 2,8 1,1 1,1 0,9
100 47214 0,46 2,5 0,8 0,8 0,6
105 49310 0,45 2,1 0,4 0,4 0,2
110 51431 0,44 1,7 0,0 0,2 0,0
115 53484 0,43 1,3 0,4 0,2 0,4
120 55549 0,42 0,9 0,8 0,6 0,7
125 57491 0,42 0,4 1,4 1,1 1,2
130 59456 0,41 0,1 1,9 1,6
150 61486 0,40 11,4      
<error> [%]   14,70 2,68 1,56 0,95
ATIK383L+ linearity test

ATIK383L+ linearity measurement. Linear regression was performed iteratively narrowing down the datarange. Best photometric performance was found in the range above 15000 ADU and below 55000 ADU.

A Linear regression was performed iteratively narrowing down the count range (ADU) in order to find typical errors introduced due to non-linearity of the CCD. A fit to all data points (red) gives a poor result, as expected, with a typical error of roughly 15 percent (see last row in table above). When limiting the range to values above 1000 ADU and below 60000 ADU a correlation coefficient R of 0.9996 (orange data; note that the value in the plot is R2) is found, which is still worse than the number given by the manufacturer (0.9998). Thus, I definitely cannot recommend doing photometry within the full range of 1000 to 64000 ADU as suggested on the ATIK website. Nevertheless, the linearity seems to be good within a range of 10000 to 60000 ADU (yellow). In that range typical errors are below 2 percent only. The best performance is found between 15000 ADU and 55000 ADU, which is the range I would consider as the photometric one.

Thus, highest photometric precision is possible for countrates above 15000 ADU and below 55000 ADU.

Gain Measurement

There are several methods available to measure the gain. I have chosen the method described by Michael Richmond. So the following steps were performed:
1) a pair of L-flats was taken with varying exposure times (2,4,8,16,32,64 sec)
2) a set of 3 dark frames with the same exposure time was taken
3) individual darks were combined using the average value (masterdarks)
4) masterdarks were subtracted from the appropriate flats
5) sum of each pair of flats was calculated and devided by 2: sum*.fits
6) difference of each pair of flats was calculated: diff*.fits
7) gain is the inverse of the slope of the plot: mean vs. variance, where the variance is: RMS2/2.

IMAGE Mean   IMAGE RMS Variance
sum2.fits 1524 diff2.fits 89.0 3957.8
sum4.fits 2747 diff4.fits 114.1 6509.4
sum8.fits 5516 diff8.fits 159.5 12720.1
sum16.fits 10975 diff16.fits 218.5 23871.1
sum32.fits 21841 diff32.fits 306.0 46818.0
sum64.fits 43342 diff64.fits 426.4 90908.5
ATIK383L+ gain measurement

ATIK383L+ gain measurement. A linear regression (green curve) was conducted on the measurements of the mean response in ADU vs. the variance. The gain is then the inverse of the slope.

The gain is then the inverse of the slope when doing a linear regression of the mean vs. variance plot.
A gain of 0.48 e/ADU was found for the ATIK383L+.

Readout-Noise

In order to measure the readout-noise of the CCD camera, bias frames taken throughout several hours of observations were used. The readnoise was measured following the procedure described here:
1) 9 bias frames were taken and combined using the median (masterbias)
2) the masterbias was subtracted from each individual bias: Rdnoise*.fits
3) readnoise is the standard deviation in these images (see table)
4) finally the masterbias was analysed using the histogram and a Fast Fourier Transform (FFT)

IMAGE NPIX MEAN STDDEV MIN MAX
Rdnoise1.fits 8529394 0,6629 19,56 -124,5 1885
Rdnoise2.fits 8529394 3,382 19,84 -123,5 1046
Rdnoise3.fits 8529394 -2,212 19,49 -262 644
Rdnoise4.fits 8529394 -1,562 19,57 -129 5063
RdnoiseB1.fits 8529394 -0,4971 20,17 -118 9360
RdnoiseB2.fits 8529394 -0,7299 19,89 -122 518
RdnoiseB3.fits 8529394 0,8872 19,95 -163 426
RdnoiseB4.fits 8529394 -0,467 19,89 -114 523
RdnoiseB5.fits 8529394 1,106 19,95 -116 709

I noticed that the bias-level can vary slightly (order of 20 ADU) during several hours of observations. Therefore, it is the best to keep track on the bias level and take some bias once an hour. For this test, I had to create two individual masterbias frames in order to extract the correct readout-noise. The result is not affected by that.

The average value of the readout-noise for my ATIK383L+ is: 19.8 ADU. Using the gain calculated before, a readnoise of 10 electrons was measured.

Thus, the level of the readout-noise is indeed very low with only 10 e. However, it is not as low as promoted by the manufacturer.

In a next step, the quality and homogeneity of the masterbias was examined using the histogram and a FFT (see below).

Histogram of the masterbias

Histogram of the masterbias showing a Gaussian-like and thus random noise dominated distribution

FFT of masterbias

Masterbias image (left) and FFT of the masterbias (right). A column of an increased bias level can be easily identified in the image. Additionally, the FFT indicates the existence of a non-random large-scale noise pattern.

An examination of the historgram reveals that the bias is of course not an ideal Gaussian as promoted by the manufacturer. However, it is Gaussian-like and thus noise dominated. Furthermore, a column of increased bias level can be identified in the image and after performing a Fourier Transform, it can be seen that some non-random pattern exists. This is indicated by the “cross” in the middle of the FFT image. However, the relative value of the pattern is very low.

I would conclude that the camera’s bias indeed is noise-dominated and very smooth, certainly allowing for faint details to be detected.

Finally the performed measurements show that the ATIK383L+ is a CCD camera that can be used for scientific photometric applications when limiting the linearity range as suggested. Due to its smooth bias it is furthermore an excellent tool for high quality astrophotography.

Comet C/2014 Q2 (Lovejoy)

Comet C/2014 Q2 (Lovejoy) is bright (around 5mag) enough to be easily seen with binoculars or small telescopes in constellation Eridanus. On really good sites one should be able to spot it even with naked eye. However, currently the observing conditions are hard, because of today’s full moon. Additionally in the northern hemisphere the comet’s elevation is very low. I have spotted it two days ago from Vallentuna (near Stockholm, Sweden) when it was only 18 degrees above the horizon at maximum. I have taken some pictures with my CCD camera and was really impressed how fast the comet moves on the sky (see video).

The really good news is that the observing conditions are going to be better during the next days and weeks. C/2014 Q2 (Lovejoy) will climb further up, reaching constellation Taurus around 9th of January, just two days after it reaches the closest position to Earth at a distance of around 70 million kilometers.

Ringsystem um Asteroid “Chariklo” im äußeren Sonnensystem entdeckt

Die Europäische Südsternwarte (ESO) berichtet in Ihrer heutigen Pressemitteilung von der überraschenden und erstmaligen Entdeckung eines Ringsystems um einen Asteroiden. Der Planetoid Chariklo wurde von mehreren Observatorien aus – darunter auch La Silla in Chile – beobachtet. Grund der Beobachtung war eine bevorstehende Sternbedeckung durch den Asteroiden. Solche Ereignisse werden genutzt, um die Größe von Kleinplaneten abzuleiten. Dabei wurde überraschenderweise kurze Zeit vor sowie kurze Zeit nach der eigentlichen Bedeckung, ein Helligkeitsabfall des Sterns registriert. Daraus kann zweifelsfrei geschlossen werden, dass sich um Chariklo ein Ringsystem aus Staub und Eis gebildet haben muss ähnlich wie man es von Saturn, Uranus oder Neptun kennt. Letztere wurden ebenfalls durch Sternbedeckungen entdeckt.

Die Scheibe aus Eis und Staub könnte dabei das Resultat aus einem Impaktereignis sein. Weiters ist nun anzunehmen, dass Chariklo noch kleinere Begleiter hat, welche das Ringsystem aufrechterhalten.

Ring um Planetoid entdeckt

Image Credit: ESO