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/

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.

Nikon D90 astromod VS. Nikon DF unmodified

It is fact that Nikon’s DF is among the most sensitive camera’s available on the market today. Its FX format CMOS chip offers 16.2 million pixels. The corresponding pixel size of 7.3μm is thus large compared to most other state-of-the-art cameras (with typical sizes of less than 5μm). As a result the Nikon DF has much better low-light, high-ISO performance.

However, as all unmodified cameras also DF’s CMOS detector is covered by an infrared (IR) blocking filter. This is unsatisfactory for astrophotography, in particular when imaging nearby star-forming regions. The reason is that young, massive stars emit hard UV radiation that leads to the ionization of the surrounding hydrogen. Subsequent recombination of free electrons with ions then produce strong emission lines such as the Hα line at approx. 656nm (in the red part of the spectrum). This wavelength unfortunately is already blocked by the IR filter found in almost all digital single-lens reflex (DSLR) cameras.

For that reason, some companies such as DSLR Astro Tec in Germany recently have specialized on modifying DSLRs. Different modifications exist, the one for astrophotography is basically a replacement of the IR-blocking filter with a clear-glass filter. This modification drastically increases the sensitivity of the camera at the wavelength of the Hα emission line. This modification comes at the cost of the camera’s white-balance, which then needs to be set manually. However, for astrophotography this doesn’t play a role anyway.

Since I own an unmodified Nikon DF and a modified Nikon D90, I was wondering how these two cameras would compare to each other when imaging star forming regions such as e.g. M8, the Lagoon nebula. In order to perform the test, I have used my Nikkor AF-S VR 200-400mm 1:4 lens, operated at 400mm f/4 and took images of the nebula using both cameras. In both setups the exposure time was set to 30 seconds at ISO 800. The result is shown below. Both images were taken in raw format and only brightness and contrast were adjusted in the same way. The result makes clear that an astro-modified D90 clearly outperforms even Nikon’s low-light market leader, the Nikon DF.

Nikon D90 astromod vs Nikon DF unmodified

Observing Comet C/2013 US10 (Catalina)

Comet C/2013 US10 (Catalina) was first discovered by the Catalina Sky Survey on October 31, 2013. It originates from the Oort cloud, a vast spherical reservoir of comets far beyond Neptun. By chance, gravitational perturbations can push Oort objects into the inner solar system, where they are eventually discovered. In some cases, comets get bright enough to be observed by naked eye or with small amateur telescopes or binoculars. The latter one is true for Comet C/2013 US10 (Catalina).

On Jan. 17, 2016 the comet passed its closest point to Earth at a distance of 110 million km. Using my 10-inch Newtonian telescope, I have imaged the comet that day from a suburban location. The result shown below is a stack of 17 frames of 120 sec. exposure time each. The inverted version on the bottom clearly shows the two tails of the comet.

Comet C/2013 US10 (Catalina)

Comet C/2013 US 10 (Catalina) imaged in L with a GSO 254mm f/5 Newtonian telescope and an ATIK 383L+ mono CCD camera

Comet C/2013 US10 (Catalina)

Comet C/2013 US 10 (Catalina) imaged in L with a GSO 254mm f/5 Newtonian telescope and an ATIK 383L+ mono CCD camera

 

Aurora Borealis – observed from Vallentuna

It is an amazing spectacle when a solar storm of charged particles hits the Earth’s magnetosphere. The particles following the magnetic field finally collide with particles of the Earth’s atmosphere – mostly oxygen and nitrogen. These collisions lead to either an ionization or excitation of atoms/molecules at an altitude of around 100 km. Subsequently, the recombination is responsible for the emission of a photon. The typically green colour arises from oxygen. On March 17, 2015 a strong solar storm hit the Earth and even at the relatively low latitude of Stockholm one could follow this energetic event.

Aurora Borealis observed from Vallentuna (Stockholm, Sweden)

Aurora Borealis observed from Vallentuna (Stockholm, Sweden)

Aurora Borealis observed from Vallentuna (Stockholm, Sweden)

Aurora Borealis observed from Vallentuna (Stockholm, Sweden)

 

Comet C/2014 Q2 (Lovejoy)

Comet C/2014 Q2 (Lovejoy) is bright (around 5mag) enough to be easily seen with binoculars or small telescopes in constellation Eridanus. On really good sites one should be able to spot it even with naked eye. However, currently the observing conditions are hard, because of today’s full moon. Additionally in the northern hemisphere the comet’s elevation is very low. I have spotted it two days ago from Vallentuna (near Stockholm, Sweden) when it was only 18 degrees above the horizon at maximum. I have taken some pictures with my CCD camera and was really impressed how fast the comet moves on the sky (see video).

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The really good news is that the observing conditions are going to be better during the next days and weeks. C/2014 Q2 (Lovejoy) will climb further up, reaching constellation Taurus around 9th of January, just two days after it reaches the closest position to Earth at a distance of around 70 million kilometers.

Milkyway and Venus rise (Video)

The following timelapse movie shows the rising Venus and the Milkyway. The individual photographs from which the animated sequence was created, were taken at the Roque de Los Muchachos, La Palma, Canary Islands during an observing run at the Nordic Optical Telescope (NOT).

  • Roque de Los Muchachos, La Palma, Canary Islands, Spain
  • 3rd of March, 2014
  • Nikon D300 f/3.5 ISO 2500
  • exposure time per photograph: 20s
  • Time lapse interval: 120s
  • duration: approx. 1.5h

 

Planeten mit der Webcam

Jupiter

Jupiter

  • Datum: 21.04.2006
  • Zeit: 00h 00m
  • Ort: Sophienalpe
  • Aufnahmetechnik: Webcam: Philips ToUCam Pro (640×480 Pixel); fokal mit 2xBarlow am VC200L
  • verwendeter Filter: IR Sperrfilter
  • Brennweite: 3600mm
  • Blende: f/18
  • Belichtungszeit: je 1/250
  • verwendeter Film: CCD Chip
  • digitale Bildverarbeitung: Addition von ca. 500 Einzelbildern mit Registax, dann 2fache Vergrösserung

Saturn

Saturn

  • Datum: 20.04.2006
  • Zeit: ca. 23h 45m
  • Ort: Sophienalpe
  • Aufnahmetechnik: Webcam: Philips ToUCam Pro (640×480 Pixel); fokal mit 2xBarlow am VC200L
  • verwendeter Filter: IR Sperrfilter
  • Brennweite: 3600mm
  • Blende: f/18
  • Belichtungszeit: je 1/250
  • verwendeter Film: CCD Chip
  • digitale Bildverarbeitung: Addition von ca. 500 Einzelbildern mit Registax, dann 2fache Vergrösserung