Sunday, February 16, 2014

Long-term, High Time-resolution Photometry of Jupiter's Galilean Satellites

Long-term, High Time-resolution Photometry of Jupiter's Galilean Satellites

I will be making photometric measurements of the Galilean satellites at least
over the next several months until about one month (I'm not sure how close to
the sun I can get before light from sunset begins the interfere) before
superior conjunction that takes place on 14 July 2014.  At that point I should
have sufficient data to determine if there's enough "interesting stuff" to
continue this project after superior conjunction.

So this means I have from now (16 Feb 2014) until about 14 June 2014 for my
observations.  This is a maximum of 119 nights.  I will collect data for
at least 2 hours each night.

Assuming a 50% clear night return, and a data collection rate of 522 images per
night (based on 87 images over the course of 20 minutes on 12 Feb 2014), I
should have at that time 31059 data points (this is a rate of 261 measurements
per hour, so if I observe more hours per night, this number will increase) for
at least one of the satellites.

Because of the dynamic nature of the OTA (nightly collimation changes & camera
mount angle), I will have to obtain new flats every single night to ensure
that they properly correspond to the system for that particular evening.  I
will monitor this over the course of several nights to see if there is indeed
a measureable difference.  I suspect that there will be.  Hopefully I can get
this sufficiently routine to not spend more than 30 minutes on this per
observing session.  Hardware improvements listed below will alleviate some of
this tediusness.

The one great advantage I have over "professional" observations is that I have
access to my telescope at any time, and setup time is a matter of minutes.
This means I don't have to write observing proposals for obtaining telescope
time and then hope that the nights I request will be photometric (or at least
clear).  I have no competition for observing time.  I can cover long periods of
time continuously with high temporal resolution -- something that has never
been done (as far as I can tell with an ADS and literature search)
when it comes to observing the Jupiter system.

=========== What am I looking for? ============

1. Photometric light curves

    From what I've read in the literature, I should expect to see fluctuations
    in the light curves.  Simonelli (2002AGUSM.P22A..07S) says "Io exhibits
    probably the most diverse set of different photometric behaviors of any
    object in the Solar System".  That catches my attention.

    These curves should be pseudo-sinusoidal in general, but will probably be
    more complex than this based on phase angle (where it is in relation to
    Jupiter), changes in albedo (surface features), solar angle (as Jupiter
    and it's system orbit the sun), and sudden changes in brightness due to
    unknown events (volcanic eruptions, meteor impacts, solar CME events,
    etc.).  All four moons are tidally locked, which means that they always
    face towards Jupiter (just like our moon does with us).  So observing each
    satellite in its orbit will allow me to measure the entire surface (except
    when transiting or being occulted by Jupiter).

    While it'll be interesting and useful to add to the database of
    observations, what I'm most interested in is

        1. detecting those unknown events
        2. detecting novel patterns

    Once I get into this more, my guess is that the data will help me see other
    things to look at more closely.  This is a very organic project.

2. Distance measurements from Jupiter

    I'm fairly certain that the orbital parameters of these satellites are
    very well known and that I won't find anything new here, but nevertheless
    I will be making those measurements.

3. Mutual Occultations

    These are occultations of one satellite by another satellite.  I'm
    not aware of anything interesting I could get from this information, but
    I will observe them and see if the data tells me to look for anything.

4. Stellar Occultations

    These are occultations of stars by a satellite.  The timings of these
    events will allow for measurements of physical diameters.  Again, it's
    likely that there are things I'm ignorant of and I'll just have to wait
    for the data to suggest what to look for.

5. Jupiter Transit and Occultation timings

    Obtain timing measurments of transits and occultations of Jupiter.
    Once again, I'm not sure if there's anything useful in this data, but I'll
    be able to do it so I might as well measure it.

6. Cosmic Ray detection and correlation

    Most astronomers detect cosmic rays just to eliminate them from the

    images they have.  I think there is something interesting about cosmic
    ray events and the fact that they are so easily detected on CCD cameras
    make them another thing I can take a hard look at instead of just
    throwing away as junk.  So for starters I will count the number of cosmic ray
    events and correlate them to solar activity.  Here once again I'm going to 
    let the data tell me how to proceed by looking for patterns.

============ Things To Do For The Future ============

I am very confident that my current system and observing methods will provide
me with very encouraging results that will justify the following future

1. Get a telescope that tracks

    At the moment, my telescope doesn't track.  This limits the amount of
    data that I can collect.  At the moment, I'm able to get about 8 images
    before the targets move out of field.  I then have to move the telescope
    slightly west in RA which I usually overshoot.  So then I wait several
    images until the targes are back in view.  Using 2x2 binning, the
    duty cycle is six seconds.  This is barely tolerable, especially when it
    comes to occultation timings and spacial resolution (pixel scale)

    This also limits the exposure time to something very short since longer
    the exposure time, the longer the streak caused my earth's rotation.  This
    is not a problem for this project since very short exposures (<0.1s) is
    adequate to obtain seeing-limited data.  However, if I ever want to
    extend this project to fainter targets (Saturn satellites, asteroids,
    variable stars, etc.) I would require a longer exposure time which would
    produce streaks which makes photometry problematic (streaks tend to
    overlap if they're too long).

    The ideal arrangement will be for me to retire my current system that I've
    proudly had in my posession for 30+ years and replace it with a new,
    similarly sized system with all of the modern accurate tracking and
    optical capabilities.

2. Recoat the primary mirror

    Due to non-use and neglect, my 25.4cm primary mirror is in pretty
    desperate need of a recoating.  This diminishes the sensitivity of the
    system, but as long as my calibrations are good, this shouldn't be a
    problem in overal performance.  There would be no need for this if I'm
    able to replace my current system with a new one.

3. Get a new camera

    The two downsides to the camera that I now have (an SBIG ST-7ME)

        1. the array size is small (765x510 Pixels, 9 micron)
        2. the download time is long (6s for 382x255 -- 2x2 binning)

    The array size is actually ok for now since it's a lot easier to keep
    the target in the large FOV.  That limits pixel scale, though.  But
    in truth 9 micron pixels is pretty good so the only way to decrease
    the pixel scale would be to modify the focal length of the system and
    currently that would mean narrowing the FOV which makes it all that
    more difficult to keep the target within the field.  This will change
    once I'm able to track.

    The download time is the major problem with this system and limits the
    time resolution of the data.  Still, a 6-12 second time resolution should
    be adequate for this "first look" project.  A further analysis of the
    data will tell me if shortening the duty cycle would help in any way.
    However, with a system that can download faster (an ST-402ME claims
    800,000 pixels per second using USB -- so that's a 0.48s download time
    at 1x1 binning) I could set the time between exposures which would be
    nice to be able to control rather than having that limitation.

4. UBVRI filters

    The current system has 'Red', 'Green', and 'Blue' filters which are
    pretty wide band and not necessarily useful for scientfic measurements.
    It would be very nice to collect color information which would add
    another dimesion to the photometry.  I would expect the "color" curves
    to be much more complex than single color photometry.

    So I would like to obtain a UBVRI filter set and create an observing
    program that collects data with all of these filters.  Simultaneous
    multispectral photometry would be an ideal situation.

5. Automation

    At the moment, I need to be with the telescope during all observations.
    I can see a time when I could set up an observing "queue" and allow the
    system to run itself throughout the night.  This would increase the
    amount of data throughput by at least a factor of four since most nights
    are at least 8 hours long (excluding time to take calibration data).

6. Networking

    24 hour coverage of the Jovian system is an ideal, but from a single
    location I will average 6 hours a night over the course of a year.
    The ideal sitation would be to have a number of observing locations
    spread geographically over the planet to provide with 24 hour coverage
    with perhaps an hour of overlap for each location.  This means eight
    stations equally spread over the planet.

    This is unlikely to happen, but even finding one or two other locations
    with willing observers would be great.

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