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Does the methods of taking dark frames or bias frames or even flats also apply to Radio Astronomy ? Like in optical astronomy, where we are hampered by electronic noise of the sensor chip and also stray light within the optical train, in Radio Astronomy we got similar problems. The ambient noise (temperature, yes Radio Astronomers talk about temperature "black body radiation") and also the noise figure of all active elements ( e.g. LNA, filter) including the local processor needs to be removed as much as possible, otherwise the very faint radio signals we want to receive are not detectable. I experimented with various methods of noise elimination, known for a long time : a) switching between two antenna feeds, one in the direction of the target, the other on the back of the mirror b) two complete antenna systems in opposite of each other c) switching the gain (amplification factor) of the receiver d) use two different frequencies, the second frequency some MHz lower to higher. Thanks to Frequency regulations for Radio Astronomy there should be no emitter within as certain limit this approach in fact includes all noise cancelling methods at once (dark,bias,flats). all version subtract the two signals, like using dark, bias or flat frames in the optical telescope image processing. I experimented with a) c) and d), but I got the best results using d) The switching time between the alternate signals is in the range of seconds, so switching in a),b) could be done by using a (UHF) relay. c) and d) is done by the software which controls the SDR dongle. Both signals are added up (integration) and subtracted from each other after a desired time. Based on the earth's rotation and the Antenna opening angle, a few minutes of integration is feasible. I n my case the Antenna is fixed at AZ and EL during a measurement : drift-scan methode. I choose 5 minutes. If the Antenna is steerable and tracking, integration time can be as long as you need to detect the desired signal ( pulsars etc.) My question is : Does someone have other experiences or may be suggestions how to minimize noise temperature of a Radio Telescope ? b.t.w. I am self taught in this field, all I know is coming from searching the internet for publications / examples and making my own experiments. cheers Wanni |
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Hi Wanni, sorry I have no answer for your question, but many thanks for sharing your experience. That is very interesting and educational. Cheers Rüdiger |
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Have you tried Chop and Nod? Chopping and Nodding for Mid-Infrared Astronomy Kevin Volk December 2007. CS David |
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David, thank you for this question! To be frank, no, I didn't know what CHOPPING and NODDING means. Thanks google, I found this paper : https://www.sofia.usra.edu/sites/default/files/709_Chop-Nod_Tutorial.pdf From what I understand, in principle, I am doing similar things using the method d) : CHOPPING: Background removal (wobbling a t high 1- 20 Hz) The Sofia infrared telescope wobbles the secondary mirror between target and nearby patch, under the assumption that the nearby sky background is similar, but without the actual target in it. NODDING : removing telescope thermal pattern Moving the whole infrared telescope every 10 to 20 sec. my "chopping" : ( switching time = 20 ms ) In the Radio wave domain, I just "wobble" to a nearby frequency every 20 ms and integrate/subtract each signal (FFT of 2048 channels /2 MHz bandwidth) separately over 30 times. my "nodding" : not exactly the same , but after 500 times of the wobbling period , I subtract the averaged accumulated signals (20 ms * 30 * 500 = 5 min.) and the result is forwarded to the waterfall image. I also generate a power amplitude , which is the total sum of all 2048 FFT channel every 300 sec. ( red vertical line in my images) But I definitely need to investigate the CHOPPING and NODDING further. CS Wanni |
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David, now I found the article you referred to : https://www.gemini.edu/sciops/instruments/mir/chopnod.pdf CS Wanni |
