Aligning a satellite dish for astronomy. - Page 1
Forum section: electronics / rf-microwave

author: medical-nerd
author_rank: Regular Contributor
timestamp: October 29, 2017, 08:46:46 am
content: HiyaI have an approx 2m solid aluminium dish with its polar mount and a substantial ground mount - all weighing over 100kg. I have looked at endless sites and youtube videos relating to setting a dish up for satellite reception. I have also looked at sites describing modifying a polar mount for eme. I have no access/experience to the equipment needed to change the mount hardware since it is galvanised steel a few mm thick and would require cutting/welding - all my tools are for hobby electronic work.What is the best way to align this dish for drift scanning for radio astronomy? I presume that the main axis would have to be pointing due south (with correction for magnetic north). It is in an area where it would have to be pointing almost straight up because of surrounding buildings and trees but have a +/- 45 degree lateral movement and still see the sky. What do I need to set the elevation/declination to in order to have a drift scan? I want it to track the sky during the year and still have the ability to maximise area of observations within the small area of adjustment I have. I have become very confused by the information seen so far. I don't even know if the main axis is correct anymore for drift scans.The dish is in the NE UK.Any advice?Cheers

author: Rerouter
author_rank: Super Contributor
timestamp: October 29, 2017, 09:14:29 am
content: I'm not an astronomer, just googled it,It appears for radio drift scanning you want to be aligned to lines of meridian, tilting up or down half a beam width each spin of the planet. meaning you should only need a single axis of tilt to accomplish it. over the course of a year you would sawtooth your lateral motion, building up a drift model.

author: medical-nerd
author_rank: Regular Contributor
timestamp: October 29, 2017, 09:33:19 am
content: HiyaYes - lines of meridian means that the main axis would be north-south, so I can rotate around that axis +/- 45degrees. The elevation angle would have to be fixed at nearly 90degrees because of the site of the telescope but what do I do with the declination - also I would like to use radio-eyes software and would need to know where the dish is pointing. This is also where I get confused since I seem to have a mental block in how to convert the position of the dish to celestial co-ordinates. AZ-EL software is available but I can't find any that would use values from a TVRO polar mount.Cheers

author: Rerouter
author_rank: Super Contributor
timestamp: October 29, 2017, 10:11:18 am
content: Again, not an astronomer,from what i can see, there are 2 types of telescope mount, Altazimuth, and Equatorial, as your talking about elevation, i presume your is equatorial,http://3.bp.blogspot.com/-Hg2Zsk7lTrw/VXS0H-wMK4I/AAAAAAAACwA/QF-yTvYi8Cs/s1600/Mounts.gifThe goal of an equatorial mount is by having the rotational node of the telescope mount aligned with the rotation of the planet, they can easily be cancelled out, so to follow a star you just change your right asencion as the planet turns.However you in this case would not be changing your right ascension to follow the stars, but simply changing your declination day to day to sweep a new line of the sky.

author: Howardlong
author_rank: Super Contributor
timestamp: October 29, 2017, 11:07:44 am
content: I’m not an asronomer either, but I do have plenty of experience in setting up and operating satellite antennas for non-geostationary satellites up to K band. Almost all of these have been using Az-El rotator mounts, but there are also polar mounts which can work in certain orbits and situations, including I would have thought for star tracking. Polar mounts for a 2m dish typically use screw actuators. A decade or two ago these dishes and actuators were two a penny on the second hand market as C band TVRO was disbanded, I don’t know what the market is like nowadays.Whet frequency band are you interested in? Accurately tracking at higher GHz is actually quite difficult. If it’s hydrogen line, (ISTR ~1400MHz) then it’s not so hard, beamwidths are reasonably large, about 5 degrees for a 2m dish.

author: medical-nerd
author_rank: Regular Contributor
timestamp: October 29, 2017, 11:22:25 am
content: Hiya@RerouterIn that case my polar/equatorial mount axis would be E-W, rotating about that axis to change the declination during drift scans in the N-S axis. With the existing declination of the mount set to zero and the elevation at 90degrees which is opposite of what the mount is designed for. Which is why I am confused. I want to maximise the area of sky in my limited area that I can drift scan. I apologise if this leads to confusion, the questioner is confused.@HowardlongThe frequencies of interest are 1420MHz and 1680Mhz. The dish has a single rotational axis using a linear actuator with the elevation and declination fixed by bolts once adjusted (just as a US TVRO). The solid dish and mount were bought from a decommisioned setup in a university used as a fixed satellite base-station - the mount alone would cost me $600 in the UK last time I looked.Cheers

author: Rerouter
author_rank: Super Contributor
timestamp: October 29, 2017, 11:46:33 am
content: Sorry, but i thought the idea with drift scanning was for the earth to do the rotation, and your telescope just records along that elevation, with the mount aligned to normalise the arc as much as possible.To me its a flipped application of solar tracking math, the sun traverses arcs in the sky from horizon to horizon, which reflects the tilt of the planet. and your location on the tilted sphere, by just rotating on the RA each day, and fine tweaking the declination each night, you can keep a panel perfectly pointed at the sun,In a drift application i would imagine the RA would be fixed, and you would just change the declination as the planet rotated around the sun, if you wanted to instead actively scan the sky, then i could see you scanning declinations while maintaining the same RA each night to get the widest scan pattern possible, however i am not sure how long you need to point at a spot on the sky to get a good reading,

author: CD4007UB
author_rank: Contributor
timestamp: October 29, 2017, 12:53:22 pm
content: Quote from: medical-nerd on October 29, 2017, 08:46:46 amWe have a small radiotelescope (for student use) on the roof of one of our buildings - it's the SRT that was originally developed by MIT's Haystack Laboratory and is used by many universities to detect the 1420MHz signal from galactic hydrogen. You can find the details athttps://www.haystack.mit.edu/edu/undergrad/srt/index.html(under old SRT) along with some useful notes and software (Java based).The SRT has an altazimuth mount and can be used in drift mode or for tracking sources, using motors that drive the 2 axes. A polar mount, as described athttp://www.satsig.net/polmount.htm,is designed for quickly moving between geostationary satellites on the ecliptic. That may not be useful if you are restricted to high elevations. Can you set the main axis to be at or near vertical or horzontal? You would then have one motorised axis of an altazimuth and could (presumably) adjust the other axis manually. At 1420MHz, the beam width is ~7 degrees, so the alignment at that frequency is not super-critical. At ~10GHz (Ku-band satellite-TV frequency) the beam would be much narrower.The sun is a good test source for a drift scan. With a very restricted view, you may have to wait until the UK summer for it to be high enough in the sky. (You can, however, observe the sun using a standard 80cm satellite-TV dish with a Ku-band LNB and a satellite finder - seehttps://arxiv.org/abs/1601.02982.)

author: Kire Pûdsje
author_rank: Regular Contributor
timestamp: October 29, 2017, 03:42:48 pm
content: In the L-band in the past I used the geostationary Afristar satellite for calibration at 1478 MHz, Orbital position 21 deg east.

author: CD4007UB
author_rank: Contributor
timestamp: October 31, 2017, 12:27:16 am
content: Thanks for the tip on the 1478MHz satellite. Unfortunately, I think that's outside the tuning range of the digital receiver on our SRT (and the pre-amp also has a 1420MHz narrow-band filter to reduce interference).