Another CMOS: Skylight spectrum with ASI1600MM-Cool

[Thilo Bauer]

Dear all,

Working for a while with an IR modified Canon EOS 60D DSLR for spectroscopy, I'm just switching to CMOS camera ZWO ASI 1600MM-C to extend my work to the fainter objects.

The ZWO camera comes with a CMOS sensor from Panasonic. Light sensitive area of the sensor is relatively large with dimensions of 17mm x 13 mm (WxH) and pixel dimension of 3.8 µm. So it is useful for both, imaging and spectroscopy.

I was curious about the spectral sensitivity of the camera. Unfortunately, the camera manufacturer just presented spectral response for 400-800 nm. I also couldn't find any technical sheets from Panasonic for the monochrome sensor to complete my knowledge about sensitivity in the NIR spectral range. So, I simply took a solar spectrum from the diffuse light with my Alpy through the window of my home. The spectrum is just provided for demonstration of its capabilities in the spectral range. The sensor obviously provides serious capabilities to obtain signal in the visible range, but also in the near UV and IR spectral range. This is good news for those having asked in the ZWO forum about capabilities for the Ca H and K lines and also sensitivity beyond H-alpha.

The image attached was taken with Alpy guiding unit and slit, but no optics mounted to the Alpy spectrograph. The spectrum was simply taken by holding Alpy plus camera at hand towards the diffuse skylight while focussing the device. Therefore, no serious calibration of the spectrum could be applied. The image presented demonstrates full width of the sensor and is intensified by gamma correction of 1.8 to enhance parts of the spectrum with lower intensities. As the sensor has 16 Mio. pixels, the image is scaled down by 4x4 binning to obtain a reliable resolution for presentation in this forum.

Unfortunately, current weather conditions don't allow for nightly astronomical observations since my camera arrived. Weather conditions are rude and change between clear skies and heavy rain currently may occur within 30 minutes. So, other users of the ZWO ASI camera are welcome to add their experience and measures to my post.

Please click on the image to see more features of the full spectrum in the NIR.

Have a nice Sunday!

Thilo Bauer

[Thilo Bauer]

First light at the 20cm Cassegrain

Ignoring the effort to measure boring numbers, it might be interesting to find out, how faint objects may be, if one tries to detect them close to a brighter star. I took my first measure of the Wolf-Rayet star WR 136 at the OHP this year. In the mean while my DSLR used for color photometry is replaced by ASI camera for spectroscopy, equipment optimized and the whole system (telescope, laptop, camera and autoguider) running mobile and powered by a car battery.

With the 20 cm Cassegrain and focal reducer (f=1200 mm) I took single 5 minute exposures of the ring nebula M57, the Dumbell nebula M27 and the bright 7.5 Vmag WR star with slit positioned so that part of the faint outer bubble of the nebula NGC 6888 is placed within the slit. The exercise to have the Dumbell nebula put on the slit was a bit difficult. While the nebula is clearly visible with the eyepiece, it couldn't be detected by the Alpy autoguider with Orion Starshoot guiding camera. The spectrum taken with the ASI camera however was done easy. The camera cooling was set to a temperature of -15°C for M57 and M27 and -20°C for the WR star plus NGC 6888. Gain was set to maximum dynamic range (not unity gain).

Result: The emission lines of faint nebula structures within the field are shown with exposure time of just 5 minutes. For the ring nebula there are first signs of the features of the outer shell found. Faint structures of NGC 6888 are clearly found from the WR star blowing the bubble to the outer shell of the nebula. Few telluric atmospheric lines are also found, as my observation site is relatively dark but not perfect, located close to Bonn and Cologne (Germany) while looking towards south and away from the bright city spots.

Please also note, the images presented here are raw, just scaled to enhance the faint intensities and gamma 1.8 applied to stretch low intensities. No further image calibration was done, i.e. no subtraction of dark current was performed.

From a first experience the camera is especially susceptible for faint object spectroscopy and satisfies my needs.