Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.


  • Content Count

  • Joined

  • Last visited

Posts posted by RobB

  1. Currently away from home and haven't applied any flats to the image.  The processed image above was not from the FITS files but from the jpg.   I need to install a FITS application but as I'm connecting my laptop via my mobile I'll wait until late tonight when I get home.   Maybe the lines will go away.   Not too worried myself but it would be good if they could investigate the issue.

    I'm using the course as a v.small taster of doing a longer AstroPhysics course.  i.e. can I find the time to study.  Maybe the PGDip Astronomy from York.

  2. The standard for VLBI is Hydrogen Maser.  GPS is then used to timestamp the data streams.   Rubidium can be used below 1GHz as the errors due to ionosphere path delays are greater than any timing errors due to the Rubidium, and can also be used as a backup.  Not sure about a GPS disciplined Rubidium oscillator.

    Hydrogen Maser drift: < 10 to the power -15 Hz per day

    Rubidium drift: 10 to the power -13 Hz per day  [how can you superscript here?]

    And if your beam happens to encompass a pulsar you can use that...it will be a lot more accurate!  Unfortunately our dishes won't be big enough :(

    p.s.  Trying not to sound like a know-it-all....I've just read this in section 9.5.3 of "Interferometry and Synthesis in Radio Astronomy:)

  3. It's a bit long but have a look at this YouTube video - GPS Disciplined Crystal Oscillator.  The way GPS works is based on very accurate clocks so you can 'measure' the distance from yourself to a number of satellites. Trigonometry does the rest.  To do this the GPS satellites transmit a very accurate timebase/clock.  As a side effect the receiver (the one with you) can provide a very accurate 1 second 'tick', the rubidium or crystal (or even cesium) oscillator fills in between each tick.  So the oscillator gets a very small correction every second removing thermal drift and aging errors.

  4. Coto - I think the H142-One receiver has a GPS device to stop any frequency drift from the internal local oscillator.   i.e. when you looking at 1420MHz it really is 1420MHz your looking at and not drifted to 1420.10MHz once it's warmed up.  The clock defines the sampling rate of the SDR.  The 'disciplined' part of 'gps disciplined' removes long term drift effects of temperature etc on the local oscillator, while the local oscillator handles the short term stability.  And the discipline part needs to done in a way that doesn't cause phase noise (i.e. correcting frequency in sharp jumps).

    For radio interferometry it will do the same job, i.e. the frequency drift is reduced to almost zero.  However, as you guessed, it does another role and therefore needs to be a lot more accurate and reproducible.  Very accurate timestamp so when we recombine the signals at the computer the signal coming from the same direction we're interested in are constructively summed (need to delay one data stream in software to match the other due to path difference) whereas signals from other directions are not.  I'm still trying to work out the accuracy required (% of wavelength time.. for 21cm this is 0.7ns) to give a small enough phase error for an acceptable result.  Also, is the timestamp from GPS accurate enough (the 1pps output, is it at the same time at both sites)?

    Also need to accurately plot where the two aerials are in relation to each other to a fraction of a wavelength.  Not so easy for us amateurs if not within a few hundred meters.  Military GPS receiver or averaging civilian gps position over an extended time?  Anybody a surveyor here who can tell me it's easy?

    GPS is of course derived from atomic clocks (in each satellite and at the ground base station).  

  5. I had a quick chat with David Lonard and he has used a GPS disciplined clock for 50MHz but didn't go very far, although did prove it can be done for an amateur.  At 1420MHz the issues will be magnified.  He's now concentrating on locally separated receivers (i.e. RF comes back to one point with one central clock).

    I'm not convinced the pure Intensity Interferometry (no clock synchronisation) is the way to go (ready and willing to be corrected on this), so solving the clock on a budget is the gateway to VLBI for amateurs.  Interestingly he did point me towards XTRX Octopack which includes a GPS and can timestamp the data stream.  GPS needs to be married to another technology as I assume the phase noise and jitter will be too much (p.s. all new to me).  Rubidium maybe (I have 3, all of which need a bit of tlc).


  6. That might be my confusion.   Been investigating a fair few different VLBI solutions over the last few days. 

    The solution that David Lonard was looking at included a step whereby the feeds were synchronised (don't have the full details yet) in his python FX Correlator code prior to running FFTs in the GPU.  This was with a system where both RF ends of the baseline were coming into a single dual channel LimeSDR.

    He also started looking at GPS Disciplined Clocks.  See VLBI Oscillators.  My assumption is that, even with a perfectly synchronised timebase, a separate Local Oscillator for down converting would introduce an unknown phase shift at the remote site unless the LO is phase aligned with the synchronised timebase.  I'm getting a little (huge understatement!) out of my comfort zone here but that's what learning is all about :)

  7. Another item to note.  No synchronisation of Local Oscillators required as long as you are direct sampling. I.e no LO required.  So 10GHz might be difficult between sites if required.  

    Time stamping of the IQ feeds needs thinking about. I say IQ because you want to time stamp before a non-real-time operating system gets involved.

    I have both the books I listed above.   NOT light reading.  Lol.

    David Wilmer’s YouTube videos are particularly good.  One thing I do think is critical is the calibration of the gain of the system which needs to be carried periodically during a scan to remove temperature issues.  Also different size dishes (different beam width) would need adjusting for.

  8. Been wanting to expand my mind into Radio Astronomy and Astro Physics for a while.  Attended a IET lecture recently that nudged me to do something about it.   So what is required to build a radio telescope that can do a bit more than tell you that we have a sun, moon, Jupiter and a tantalising glimpse of possible spiral arms?  i.e. How to get higher resolution on an amateur budget (£2k-£3k).

    Have started looking into what is required for Radio Interferometry.  Scouring the internet has turned up quite a lot of info. However, some of the more interesting amateur works seem to have gone dormant.  Started this thread to try and pull together the information sources, if only for my own benefit, but preferably others as well.

    Seems to be two types of interferometry open to amateurs:

    1. Locked Local Oscillators (LO) and matching phasing harness for the antennas
      • requires antennas to be in close proximity
    2. Intensity Interferometry - No LO issues, useful over longer baselines.  Fair amount of Maths.
      • may require a fair amount of data traffic and processing power.
      • could be expanded to other amateurs (see project ALLBIN)

    My preference would be to go down the "Intensity Interferometry" route and build on the work by others where I can dig it up.

    Internet Sources (to be added to):

    European Radio Astronomy Club -

    • Basics of Interferometry  - no LO synchronization necessary (slide 23). 
    • Based around a project called ALLBIN.
    • Link to membership or message board no longer valid.  All gone very quiet.

    David Lonard

    David Wilner - Harvard-Smithsonian Center for Astrophysics - Very good YouTube videos

    Edinburgh University

    University of Amsterdam


    • Interferometry and Synthesis in Radio Astronomy - Thompson - Third Edition
    • Tools of Radio Astronomy - T.L Wilson - Sixth Edition


    • Like 1

  9. Just found this thread having been reading up on 1420MHz Hydrogen line imaging (I'm a radio amateur and professional software developer).   Great amount of info here...many thanks to everyone :)

    I have noticed that some people are adding Satellite boosters after the pre-amp?  Normally the pre-amp should provide enough gain to overcome any feeder losses down to the receiver and also alleviate receiver sensitivity issues, so I'm not sure why they are required?   Also the boosters seem to be 75ohm which means a mismatch and consequent signal loss unless there is some matching being implemented? I'm sure there is a good reason just can't understand it at the moment.   A lot of good work going into this though.

    I'm looking into doing the same but extending to 2 dishes with interferometry, although finding a location for them (I'm in South Devon) may be problematic (back garden, I've been strongly informed, isn't appropriate.....).

  10. Specify a briefer exposure time than the shooting interval". Which means for every 6 minute exposure I have to wait 6mins+ to take the next one.
    I also hit this until I realised that the 'shooting interval' is time is the period between each shutter 'opening'. Therefore it must be longer than the exposure otherwise you'll be opening the shutter again before its closed!

  11. Ahhh is that what they were :) Actually no I did't see them. sorry Merlin. didn't notice them at all if they were there they weren't very prominent.
    They were there...I think on the lower floor demonstrating the spectrum of a light bulb. Grabbed some leaflets as they were busy. They also had an SBIG guy on the stand as well.
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.