Using an MGEN v2 Autoguider on a Losmandy mount with 492 Digital Drive System

Why I bought a Losmandy GM8 mount

Skywatcher EQ5 guide star drift
Figure 1: Analysis of guide star positions when using the MGEN autoguider on a Skywatcher EQ5 Pro mount. The green crosses indicate the offsets in arcsec of the guide star in RA, while red points show the offsets in DEC. The latter shows very large drifts caused by mechanical shortcomings of the mount.

For some time already, I was looking for a stable and accurate mount as basis for astrophotography when traveling with big tele lenses (in the range of 400-600mm at f/4). I have previously owned Skywatcher’s EQM-35 Pro and EQ5 Pro mounts, but was never really happy with them. Not only they had large non-sinusoidal periodic errors, but also they were unsuitable for long exposure times, mainly due to low quality declination axis. These mounts often show large jumps in declination that are too large to be handled by autoguiders (see Figure 1). I invested much time to re-grease, adjust backlash, ideally balance the lens/camera, but nothing really helped to resolve the issue. Thus, I finally decided to buy something else that suits my purpose. As Losmandy mounts have a good reputation, and I was lucky to get a used one (GM8) with tripod, stepper motors, the 492 Digital Drive System and polar scope for less than 800 EUR, I decided to give it a try. If I had known about the trouble the 492 control box has with Lacerta’s MGEN autoguider (which is my favorite autoguiding system), I probably had made a different decision. So, here is the story about how I modified a Losmandy 492 control box to make it work with Lacerta’s MGEN v2 autoguider.

Servicing an old Losmandy GM8

The Losmandy GM8 I got was in an overall good condition, with only minor signs of use. However, I have recognized that it was really hard to lock the axes using the clutch knobs. This is a well-known behavior that is also reported on the manufacturer’s website. Hence, the first thing I did was to disassemble, clean and re-grease the mount (see Figures 2-3). One can easily find instructions for the procedure on the internet, so I do not give more details about it here. Once finished, the mount was ready to be loaded with a small telescope.

The Periodic Error and other shortcomings

Using my MGEN v2 autoguider (with firmware 2.61) attached to a f=240mm guide scope a first test of the GM8 under night sky was performed. After polar alignment with the polar scope, the mount was pointed towards Epsilon Hya, a suitable star near the celestial equator for measuring the mount’s periodic error (PE). Without actually autoguiding (performing corrections), the PE was recorded using the “Savepos” option (to be found under “more” on page 5 in MGEN’s autoguiding menu). The measurements are shown in Figure 4. The PE is best seen after smoothing the data and its amplitude is roughly +/-8 arcsec. However, there is another high-frequency signal present with a much larger amplitude of +/-20 arcsec. Given the large amplitude and short period length this seems more problematic for long exposure times. For that reason, I decided to make some improvements: exchanging the worm bearings and sanding the worm bearing blocks. That way, the amplitude was reduced to approximately +/-10 arcsec (see Figure 5). I describe the whole procedure of reducing the PE in another post, since this article is only about how to modify a Losmandy 492 control box to work with MGEN.

MGEN fails in guiding a Losmandy GM8 with 492 Digital Drive System

At this point, it is important to note that Losmandy’s 492 Digital Drive System (DDS) is incapable of processing simultaneously occuring ST-4 signals. The axes can be moved only in one direction at a time. For that reason the MGEN has a setting called “Exclusive AG out” (which is found in the “Misc” menu under “Mode settings”). This switch must be checked in order to separate signals in time for each direction.

Now I was eager doing astrophotography with the mount, but soon I noticed the next drawback: The MGEN’s calibration procedure ended either with an error or very poor “ortho” values. Also, the calibration took unusually long (few minutes). One would expect ortho values between 95-100%, but what I got was far from that (see Figure 6). And when turning on the guiding, the stars would drift far off the limits (see Figure 7). What was going on? I realized that the autoguider is programmatically sending correct ST-4 signals, but the mount was not reacting to it. When using the MGEN as handcontroller (via its “Manual” mode on page 2 of the autoguiding menu), the mount was not moving in any direction (RA+, RA-, DE+, DE-). However, intermittently at least one or two directions were working.

I was first checking connections: the ST-4 cable from MGEN to the HC/CCD socket on the 492 Digital Drive System and the cables from the mount to the motors. But they were totally fine, the problem must be caused by something else.

Analysing the problem between MGEN and Losmandy 492

Some time and many “trial-and-error” attempts later, I could tell that both devices, the MGEN and the 492 control box were properly working on their own: Controlling the mount with the handcontroller (HC) worked just fine, but when connecting the MGEN instead of the HC only some or none of the directions were functioning. On the other side, the MGEN was properly working with other mounts, e.g. with my Skywatcher NEQ6. Additionally, I have tried another MGEN and also another 492 box. All tests led to the same failure. Either none or only some of the directions were working when controlling the GM8 with the MGEN.

Finally, using a y-connector cable plugged into to the HC/CCD socket, I measured the voltages on the four ST-4 pins (RA+, RA-, DE+, DE-). With the MGEN attached and no button pushed, the level was 2.5V on all four pins, which is totally fine for CMOS HIGH level. But, once a button was pushed the voltages were going down to 1.7V on the three unrelated pins and 0.4V on the activated pin. On the other side, with the HC attached and one button pushed, the voltages on the unrelated pins remained constant at 2.5V and the activated pin showed exactly 0.0V.

Looking into the datahseet of the main chip used in Losmandy’s 492 device, I figured that, while 0.4V should still be recognized as LOW, the EPROM needs at least 2.0V for HIGH. Given that with the MGEN attached, only 1.7V are provided on three pins, the logic would have trouble to interpret the input.

So, what causes the voltage break-down when using MGEN instead of the HC? I knew that the MGEN v2 is internally using opto-couplers for DC isolation. Thus, it seemed obvious that the opto-couplers have a relatively high internal resistance, or likewise the pull-up resistors in the 492 box have relatively low values, causing the 2.5V to be split accordingly. An examination of the 492 board revealed an array of resistors labelled “RP1”. I reckoned that these are the pull-up resistors and indeed, their values were very low, with only 470Ω.

Solution: Exchanging pull-up resistors in the Losmandy 492 DDS

Replacing RP1 on the 492 board with a resistor network in the range between 4.7kΩ and 10kΩ, e.g. this one, should thus solve the problem of voltage drop. However, before ordering something, I wanted to make sure that this was indeed the final solution I was looking for. Since I did not have a 8-pin resistors network on hand, I just used “normal” 4.7kΩ resistors and soldered them together (see Figures 8-12). After that, the MGEN v2 was capable of moving the axes of my Losmandy GM8 with 492 control box in all directions. And “ortho” values during calibration are now 99-100%.

Conclusion

Finally, I am very pleased with my GM8. Its tracking curve with approximately +/- 10 arcsec peak-to-peak values can easily be guided with MGEN (see Figure 13). This mount thus provides a great basis for astrophotography.

Additional Reading:

The modifications described here were previously discussed in German on astronomie.de, see links below.
https://forum.astronomie.de/threads/problem-mgen-2-mit-losmandy-492.291130/ and https://forum.astronomie.de/threads/losmandy-gm8-schneckenfehlermessung-mit-mgen-hohe-frequenzen-mit-hoher-amplitude.280712/

Efficient Turbulence Driven Lyα Escape in the Lyman Alpha Reference Sample

I am happy to announce that our paper about the molecular gas and dust in the Lyman Alpha Reference Sample (LARS) is now available through arXiv. Our main finding is that galaxies with high gas fractions and very long gas depletion times (>10Gyr) are at the same time those with the highest Lyman Alpha escape fractions (>20%). After analyzing observations carried out with numerous telescopes (APEX, IRAM 30m, Herschel. IRAS, AKARI, WISE), we conclude that turbulence is the main physical driver behind these high Lyα escape fractions (see Figure below).

Lyman Alpha Escape Fraction vs. Total Gas Depletion Time
Relation between Lyman Alpha Escape Fraction (y-axis) and the total gas (HI+H2) depletion time for LARS galaxies (Puschnig et al. 2020).

Dense Gas Toolbox – This is for all radioastronomers out there!

Dense Gas Toolbox

I am happy to announce that the first online version of the Dense Gas Toolbox (DGT) is now available. DGT allows to derive gas densities and temperatures from observed molecular lines. The toolbox contains a novel set of radiative transfer models which take into account that observed molecular line intensities usually arise from a multi-density gas rather than from a single zone. Hence, the models allow for more realistic interpretations (in terms of gas density and temperature) of observed molecular lines.

The models build up upon RADEX, i.e. they use non-LTE LVG (expanding sphere) escape probabilities and molecular data from the Leiden Atomic and Molecular Database.

The models were calibrated using data of the EMPIRE survey, i.e. the abundances and line optical depths are fixed based on observations of local star-forming disk galaxies. In the current version (v1.2) of DGT the following rotational transitions are implemented in the frequency range between ~88 and ~345GHz:

  • 12CO (up to J=3)
  • 13CO (up to J=3)
  • C18O (up to J=3)
  • C17O (up to J=3)
  • HCN (up to J=3)
  • HNC (up to J=3)
  • HCO+ (up to J=3)

Recognition of the lunar rhythm under light polluted skies

Circa-monthly activity conducted by moonlight is observed in many species on Earth. However, due to the steadily increasing amount of artificial light at night, the periodicity is progressively disrupted. In a recently published paper, we investigate for the first time in a quantitative way the relationship between light pollution and the recognition of the circalunar variation.

Our main result (Figure 9 of the aforementioned paper), is a linear relationship between the mean zenithal night sky brightness (<NSB>) and the amplitude of the circalunar variation (see Figure below).

Relation between light pollution and circaliunar amplitude
Relation between light pollution and circalunar amplitude (Puschnig, Wallner & Posch 2019)

Eine kurze Geschichte des Lichts – von der Feuerstelle zur globalen Lichtverschmutzung

Am 13. November 2019 lädt der Verein Homo heidelbergensis von Mauer e.V. zu einem Vortrag zum Thema Lichtverschmutzung.

Wo: Heid’sches Haus, Bahnhofstraße 4, D-69256 Mauer

Eintritt: frei

Im Vortrag werde ich zunächst auf die Geschichte der künstlichen Beleuchtung eingehen: von prähistorischen Lampen bis hin zu modernster LED Technologie. Als Astrophysiker werde ich dabei auch immer wieder Bezug zur Astronomie nehmen. Der Schwerpunkt des Vortrags liegt aber bei den Auswirkungen der künstlichen Beleuchtung auf Mensch und Umwelt und zum Schluss zeige ich noch einige Beispiele aus der Praxis.

Mercury’s crust revisited

Airy vs. Pratt isostasy

Figure 1: Pratt (left) vs. Airy (right) isostasy. There are two main ideas how mountain masses are supported. In Pratt’s theory (left), the density changes and less dense crustal blocks “float” higher, whereas the more dense blocks form basins. In Airy’s theory (right) the density is constant, but the crustal blocks have different thicknesses. Higher mountains have deeper “roots” into the denser material below. Image credit: Shih-Arng Pan

In today’s volume of the “Earth and Planetary Science Letters”, Michael M. Sori from the “Lunar and Planetary Laboratory” of the University of Arizona (US) writes about how he used data obtained with the MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging) orbiter to re-measure the crust thickness of Mercury. Crust thickness is an important geophysical parameter, which allows to further constrain terrestrial planet formation scenarios. And since Mercury is always good for a surprise, the new calculations show that Mercury’s crust is only 26±11 km thick, i.e. much thinner (and also denser) than previously thought.

First estimates of the Mercury crust thickness were published by Anderson et al. (1996). Their estimates were based on data obtained with the Mariner 10 spacecraft. They concluded that the crust is 100–300 km thick. Almost ten years later, with a wealth of new instruments on-board MESSENGER to create gravity and topography maps, Padovan et al. (2015) concluded that Mercury’s crustal thickness is on average 35±18 km. The authors assumed topography was predominantly compensated by Airy isostasy, where columns contain equal masses. The equal mass approach was now shown to overestimate the thickness of Mercury’s crust, and instead an equal pressure approach (first described by Hemingway and Matsuyama 2017) should be used. In the next paragraphs, further explanations follow, describing the meaning of isostasy and the equal-mass vs. equal-pressure approaches.

Airy vs. Pratt isostasy and the “equal mass” vs. “equal pressure” assumptions

Mercury grain density

Figure 2: Grain density measurements on top of a MESSENGER image of Mercury. Image Credit: Michael M. Sori (2018)

Mercury grain density vs. elevation

Figure 3: The data shows that Mercury is inconsistent with Pratt isostasy (red dashed line), because no correlation between density and elevation is observed. Image Credit: Michael M. Sori (2018).

Isostasy is a fundamental concept in Geology, meaning that lighter crust floats on the denser underlying mantle. It thus explains why mountains and valleys are stable over large timescales. This is called isostatic equilibrium (this equilibrium can be disturbed by erosion or volcanic activity). There are two main ideas how mountain masses are supported (see Figure 1). In Pratt’s theory, the density changes across the surface and less dense crustal blocks “float” higher, whereas the more dense blocks form basins. On the other side, in Airy’s theory the density is constant, but the crustal blocks have different thicknesses. Higher mountains have deeper “roots” into the denser material below. Thus, in case of Pratt isostasy, one would expect a correlation between density and elevation across the surface of a planet, with mountains having lower densities.

In the study, Sori (2018) shows grain density measurements across several regions of Mercury (see Figure 2). Using MESSENGER’s topography maps, the author could then look for a correlation between density and elevation. As shown in Figure 3, such a correlation does not exist. Thus, it can be assumed that Airy isostasy is a better description for the topography of Mercury.

Now we come back to the meaning of “equal mass” and “equal pressure” approach. The latter one was used by the author of the study. This is the crucial difference that finally led to the new lower value for the crustal thickness. First, it is important to know that the gravitational potential is typically not constant across topographic lines (=lines of constant altitude) of a planet. This is due to variations in density. However, lines of constant gravitational potential (equipotential lines) can still be calculated. One such line of constant gravitational potential is the zero-level (on Earth roughly the sea level) and is called geoid. The quantity called geoid-topography ratio (GTR) thus reflects variations in density. And finally, the GTR is used to calculate the thickness of the crust. The main question is how equipotential surfaces are calculated. As shown by Douglas J. Hemingway and Isamu Matsuyama (2017), the spherical geometry of the problem must be taken into account when calculating equipotential surfaces (which will affect the crust thickness calculation). And here is the problem. Previous publications have assumed a constant width of the crustal blocks (in cartesian coordinates). This is what is called the “equal mass” approach, but in fact one would need to take into account the spherical symmetry (polar coordinates) and thus cone-shaped blocks that put different “pressure” on the surface (compare Figure 1 and Figure 5). This is why the newly calculated thickness is roughly 25% lower than previous results. Note that, the same issue will also affect previous calculations of other objects in the solar system. However, since the difference is larger for smaller objects, Mercury is affected most of all planets, since it is the smallest planet in the solar system.

Mercury Crust Thickness

Figure 4: Geoid-topography ratios (GTRs) as a function of crustal thickness (for an Airy isostasy). The “equal mass” (red) and “equal pressure” (blue) approach are compared to each other, showing that equal pressure reduces the derived crust thickness to the published value of 26km. Image credit: Michael M. Sori (2018)

Airy vs Pratt in polar coordinates

Figure 5: Airy vs Pratt in polar coordinates. This is the same as Figure 1, but showing the crustal blocks in polar coordinates. It can be seen that the crustal blocks are not constant in width, but cone-shaped. The bottom of the cone is the area where pressure is put on the underlying surface. An equipotential surface is then found along lines of “equal pressure” rather than “equal mass”. Image credit: Johannes Puschnig

As explained, the equal pressure approach is a better representation of a state of equilibrium. This is also supported by the fact that the new average crustal thickness value of 26±11 km agrees well with other MESSENGER based models and observations, e.g. with Mercury’s crust being of magmatic origin or excavation of mantle material onto the surface, which was proposed by Padovan et al. (2015).

With this publication another issue of Mercury could be resolved, but many things are left unknown and Mercury still keeps scientist busy. The next large step forward is likely to come when BepiColombo finally orbits Mercury in 2025.

Spotting the zodiacal light in spring

The zodiacal light is a nocturnal phenomena that is revealed only to those who dare to escape the city lights. In spring, after sunset and once twilight fades away into a dark and moonless night, a gentle luminous band opens up when looking towards west. Its majestic cone then seems to stand high above the horizon, as if it was trying to guide the observer. In fact, the zodiacal light directs us to the very beginning of the solar system, roughly 4.5 billion years ago, when our Earth and the other planets were formed from and within a circumsolar dust disk. Although the solar wind steadily sweeps away dust, new dust grains are formed through outgassing comets and minor planet collisions. Most of these objects orbit the sun in a relatively well defined and narrow plane, which is called the ecliptic, i.e. the plane of the Earth’s orbit. As a result, the ecliptic is continuously fed with fresh dust and gas, which causes the redirection of sun rays through reflection and scattering, which are then captured as zodiacal light by some enthusiasts on Earth. Although zodiacal light can be seen all year round, spring and autumn are best suited for observations from mid latitudes, because then the path of the sun crosses the horizon at a steep angle, making the twilight zone short.

zodiacal light

Zodiacal light observed from Roque de los Muchachos Observatory, La Palma, Canary islands, Spain in April 2016.

SQM night sky brightness measurements at 26 locations in Eastern Austria

I am glad to announce that our recent light pollution paper entitled Systematic measurements of the night sky brightness at 26 locations in Eastern Austria will be soon published in JQSRT. In the article, we show that a correlation between light pollution and air pollution (particular matter) exists. We examine the circalunar periodicity of the night sky brightness, seasonal variations as well as long-term trends. Novel ways to plot and analyze huge long-term SQM (‘Sky Quality Meter’) datasets, such as histograms, circalunar, annual (‘hourglass’) and cumulative (‘jellyfish’) plots are presented (see example below).

Hourglass plot

Hourglass plots. The x-axis is a time axis, containing the months of one full year. The y-axis is a time axis as well, but covering the hours (and fractions of hours) of the individual nights. A colour scale is used to denote the measured night sky brightness in units of mag arcsec-2 at each time of the night and of the year. The circalunar periodicity or a lack of periodicity can be well recognized in the plots. Also other features emerge, e.g. the natural variation of the night lengths, which creates the ‘hourglass’ shape.

Jellyfish plot

Jellyfish plots. The x-axis is a time axis indicating hours, the y-axis is the night sky brightness in units of mag arcsec-2. These plots show measurements throughout one full year (here: 2016) and the colour indicates the number density of measurements in the (hour, brightness) plane. Here we show urban, light-polluted sites, which are characterized by two clustered regions, that have little to do with the lunar phases, but correspond to clear nights with moderate skyglow on the one hand and overcast nights with strongly enhanced scattering of the city lights.

The Lyman Continuum Escape and ISM properties in Tololo 1247-232 – New Insights from HST and VLA

As of April 19, 2017 our paper entitled “The Lyman Continuum Escape and ISM properties in Tololo 1247-232 – New Insights from HST and VLA” is accepted for publication in Monthly Notices of the Royal Astronmical Society (MNRAS). In the paper, we report on our work based on data from the Hubble Space Telescope (HST) and the Karl G. Jansky Very Large Array (VLA). Using an advanced data reduction procedure for our COS (Cosmic Origins Spectrograph) spectra, we confirm weak LyC flux emerging from the central region of the galaxy, corresponding to an escape fraction of less than two percent, i.e. the lowest escape fraction reported for the galaxy so far. We further study far ultraviolet absorption lines of Si II and Si IV, as well as 21cm hydrogen radiation and bring them into context of physical processes that drive the LyC escape in the galaxy.