Losmandy HGM TITAN mount tracking speed and PE Measurements

Those of you familiar with this blog will be well aware of the tracking problems I’ve had with tracking speed descreptancies with the Titan mount. I finally decided to quantify accurately the amount of drift, using the free version of Pempro software, so that there can be no question left regarding this issue, as I wanted to send a report to Losmandy. People usual answer to my problem (Titan’s owner who do not use their mount or measure anything mostly) is that surely, I do not know what I’m doing, since Losmandy would never release a buggy mount to the public (yeah right!).  Also, I often read and hear real nonsense on forums regarding  mount’s Periodic Error (PE) and this is a good occasion to quantify it with real and accurate numbers and curves.

1. Background

1.1 Setup

The Observatory was equipped with a C14 EdgeHD on a Losmandy titan Mount in August 2010. The mount is fitted with a Gemini 1 Level 4 controller. The mount is a Titan 1:50 type, fitted with Maxon stepper motors. The observatory is located in the French Alps and its main purpose of the observatory is prime focus CCD astrophotography. The equipment was bought from Optique & Vision, Juan les pins.

[SinglePic not found]

 Figure 1: Setup

In addition to the C14edgeHD, the mount is loaded with an Orion 120/1000 refractor used for guiding in Hyperstar configuration. The counter weights consist of four 20Lbs weights.

1.2    History

From earlier on, despite accurate polar alignment, it was noticed that average auto-guider corrections over a long period of time was not nil, as would be the case, if the auto-guider was only correcting for worm screw Periodic Error. CCD images were recorded at the time, with mount alignment off by a few degrees in azimuth so as to produce a drift on the DEC axis. The images clearly showed that tracking speed was not correct (RA is Y axis, while DEC is X; notice the average drift in RA).

[SinglePic not found]

Figure 2: RA drift, Image Y axis is parallel to RA axis, and X axis parallel to DEC axis

The image above was taken in Sidereal tracking mode at 0° of Declination around the Meridian. At the time, it was thought that the root cause of the drift might be bad Gemini settings, electronics or stemmed from a mechanical problem. After ensuring the settings were correct, the electronics were changed twice and the entire mount once, across a number of months. Finally, a number of precision screws, manufactured by “Optique et Vision” were used. Although Periodic Error, while not formally measured, seemed improved, the tracking speed discreptancy remained. At this point, everything having been tried, no further work was performed in an attempt to solve the problem until summer 2013. The next idea was to see if, using Comet Rate/ User Defined setting, Tracking speed could be improved in order to minimize auto-guider’s corrections during exposures.

 

2  Tests and Measurements

2.1 Method

In order to measure Tracking speed discrepancy accurately, specific software from CCDWare, PEMPro, was used, as it allows accurate measurements using a CCD camera. In this case, an Atik 11000-CM camera, placed at the C14edgeHD prime focus, was used. In addition, ASCOM driver version 6 SP1 and Gemini.net version 1.0.58 were used to control the mount from the PC and perform calibration. Prior to testing, Mount polar alignment was refined and checked with PEMPro. The polar axis is off by 3’ to the West and 3’ to the South as given by PEMPro. These values were confirmed by the Gemini, upon building an accurate model using 15 stars, visible across the entire sky. The tests were made across 4 nights and starting on the 21th of July 2013.

2.2 Initial test on tracking speed error

Initial tests, performed on the 21th of July, only aimed to determine the amount of tracking speed error and see if the error could be cancelled out using User defined tracking mode. PE is not of interest at this point.  PEMPro was calibrated using the Atik 11000-CM sensor, and Sidereal tracking mode was used on the Gemini. The tests were performed pointing the C14 at an area  3° East of Meridian, at 0° Declination.

[SinglePic not found]

Figure 3: Tacking in Sidereal tracking mode

Figure 3 above shows 3 worm cycles (or turns), lasting 15.95 minutes in total (1 turn is 319.1262 seconds). From the beginning of the blue trace to the end of the pink one, there is  about a 20” drift (apparent angular drift of the guide star). The tracking speed is too fast (East is down, West is up on the PEMPro graph). The mount was then put in Comet tracking/User defined tracking mode, as this mode allows for RA Divisor timer adjustment. Figure 4 shows an extract of the Gemini 1 manual, explaining the use of the RA Divisor Parameter.

[SinglePic not found]

Figure 4: Gemini Manual extractdown

In order to slow the tracking (37398 being the RA Divisor value in Sidereal tracking mode) the value of RA divisor was slowly increased. After each increase, a new acquisition was made by PEMPro to evaluate the drift.

[SinglePic not found]

Figure 5: Gemini advanced settings panel

With a RA Divisor value of 37500, it was found that the drift was fairly well corrected (Figure 6).

[SinglePic not found]

Figure 6: Tracking with RA Divisor value of 37500 on the 21th

At the time, it was assumed that 37500 was the correct setting. However, during further testing on the 26th of July the value had to be adjusted to 37485 (Figure 7).

[SinglePic not found]

Figure 7: Tracking with RA Divisor of 37485 on the 26th

Atmospheric turbulence on the 26th was fairly good, allowing accurate acquisition to be made. Testing on the 27th led the RA Divisor to be adjusted to 37475 to cancel drift out. Again, all testing was done using the same area of sky. On the 31st the proper RA divisor value was found to be 37460 (Figure 8). Because previous measurements were all carried out using an SCT telescope (with a clamped primary mirror) which can be subject to flexions, Figure 8 test was done using the 120/1000 refractor mounted parallel to the SCT (refer to Figure1) by means of a a heavy duty H plate.

[SinglePic not found]

Figure 8: Tracking with RA Divisor of 37460 on the 31th, 120/1000 refractor Atik16IC

2.3 Tests on Periodic Error

Tests carried out on the 31st of  July focused primarily on evaluating Periodic Error of the mount. An analysis of curves acquired on the 26th of July (Figure 7) was added at the end as seeing was particularly good that day.

It had been noticed during previous tests that PE did not have the same amplitude when flipping Meridian (hence when the tube is West or East of its peer), while pointing  an area of the sky 3° East of Meridian at 0° declination. It was also noticed that mount balancing had an impact, meaning if telescope tube and counterweights balanced each other perfectly or not. 5 sets of PE analysis were carried out:

  1. PE test1: Tube positioned East, Mount balanced
  2. PE test2: Tube positioned West Mount balanced
  3. PE test3: Tube positioned East, Mount slightly “tube heavy
  4. PE test4: Tube positioned West Mount balanced “tube heavy”
  5. PE test5: Tube positioned East, Mount slightly “tube heavy” previously acquired 26th of July

2.3.1       PE test 1

This test (Figure 9) is performed pointing an area 3° East of Meridian, 0° Declination, Telescope tube East, mount balanced.

[SinglePic not found]

Figure 9: Test 1 Periodic Error

The result of the test is a +5.3/-6.9 arc-second PE. Frequency spectrum of the PE is shown Figure 10.

[SinglePic not found]

Figure 10: Test 1 PE curve Harmonics

2.3.2     PE test 2

This test (Figure 10) is performed pointing an area 3° East of Meridian, 0° Declination, Telescope tube West, mount balanced.

[SinglePic not found]

Figure 11: Test 1 Periodic Error

The result of the test is a +10.1/-10.2 arc-second PE. Frequency spectrum of the PE is shown Figure 12.

[SinglePic not found]

Figure 12: Test 2 PE curve harmonics

2.3.3       PE test 3

This test (Figure 13) is performed pointing an area 3° East of Meridian, 0° Declination, Telescope tube West, mount slightly tube heavy.

[SinglePic not found]

Figure 13: Test 3 Periodic Error

The result of the test is a +10.1/-9.1 arc-second PE. Frequency spectrum of the PE is shown Figure 14.

[SinglePic not found]

Figure 14: Test 3 PE curve harmonics

Please note that stray harmonics amplitude is greatly decreased.

2.3.4       PE test 4

This test (Figure 15) is performed pointing an area 3° East of Meridian, 0° Declination, Telescope tube East, mount slightly tube heavy.

[SinglePic not found]

Figure 15: Test 4 Periodic Error

The result of the test is a +5/-7.2 arc-second PE. Frequency spectrum of the PE is shown Figure 16.

[SinglePic not found]

Figure 16: Test 4 PE curve harmonics

Please note that stray harmonics amplitude has increased.

2.3.5     PE test 5

This test (Figure 17) was performed pointing an area 3° East of Meridian, 0° Declination, Telescope tube East, mount slightly tube heavy.

[SinglePic not found]

Figure 17: Test 5 Periodic Error

The result of the test is a +5/-7.2 arc-second PE. Frequency spectrum of the PE is shown Figure 18.

[SinglePic not found]

Figure 18: Test 5 PE curve harmonics

Please note that stray harmonics amplitude is quite high, even though PE is very good.

2.3.6      Periodic Error Synthesis

Below is a summary of the PE measurements:

PE test

PE (p to p)

PE (RMS)

Tube position

Remarks

1

12.2”

1.208”

East of peer

Mount Balanced

2

20.3”

1.159”

West of peer

Mount Balanced

3

19.1”

1.304”

West of peer

Mount slightly tube heavy

4

12.2”

0.817”

East of peer

Mount slightly tube heavy

5

5.5”

1.237”

East of peer

Mount slightly tube heavy

Looking at the summary table above, it’s obvious that the side of the worm  in contact with the teethed wheel has an impact on PE. That, and also, the amount of pressure on the worm due to Tube/CounterWeight imbalance. In my case I get far better PE measurements when the scope is East of the pear. What’s important, is that, for a given mount, depending on the tube balance and on the position of the tube, PE can vary quite widely. This means, that, whenever performing PE measurement to assess a worm, several measurement should be carried out with, different balance on both sides of pear.

 

3  Conclusion & Analysis

3.1  Tracking speed error

For the author, there was no doubt whatsoever that standard mount tracking speed be it in Sidereal mode or King mode, was not appropriate since a drift occurred. What came as a surprise is that it is impossible to set a correct speed by setting a fixed RA Divisor value, since this value changes from night to night to get proper tracking. Across 4 nights, RA Divisor value ranged from 37460 to 37500… Setup; sky area tested on, was exactly the same every night, the only difference being a slightly different temperature, ranging from 15°C to 20°C. But, looking at the values there doesn’t seem to be a relation between temperatures and RA Divisor value. In any case, it is difficult to explain how the Gemini quartz controlled time base could drift by that amount by a 5°C temperature change.

Also, what seem a bit “strange” is that during testing, while RA Divisor value seemed appropriate for the first couple of worm cycles, the last cycles seemed to show drift appearing again (figure 19).

[SinglePic not found]

Figure 19: Tracking speed drift across time

At first, it was thought that this was caused by the telescope passing Meridian, but, since the mount was “tube heavy” there should not be any change. Also, what is important to consider is that the RA stepper motor is used in closed feedback loop, where an 4096 step encoder ensure that the motor runs at the speed dictated by software. So, unless there is an inaccuracy within the Gemini time base (Gemini controller was changed thrice), which should logically have been spotted by other Gemini Titan 1:50 users, it only leaves one cause for the drift: Atmosphere. Indeed, the observatory is situated on the flank of a large mountain, and it is quite possible those atmospheric temperature gradients are not that of a plain. Since refraction calculations show quite a large impact on object apparent position (minutes of arc) declination wise, it is quite possible that for “non standard” atmospheric temperature gradient to impact apparent sidereal movement. Also, this could explain the difference in RA Divisor values changing in time. Unfortunately, the telescope being at fixed station, it is not possible to move it to a different location and acquire data to ensure this.

But, at the end, the mystery remains… Can minor changes in the atmosphere refractive properties affect visible star movement to a few seconds of arc? I would appear to be so, or, otherwise, Earth rotation on the planet I live is not as regular as my Titan mount would like it! If you have any idea to explain these results, please let me know!

VN:F [1.9.7_1111]
Rating: 9.6/10 (5 votes cast)
VN:F [1.9.7_1111]
Rating: +2 (from 2 votes)
Losmandy HGM TITAN mount tracking speed and PE Measurements, 9.6 out of 10 based on 5 ratings

4 Responses to “Losmandy HGM TITAN mount tracking speed and PE Measurements”

  • Very interesting, I have also noticed variances with tracking on my G11 mount. I notice that the Gemini control panel in the screenshot shows “Gemini Calculates Refraction” is disabled. Did you notice a difference when it was enabled?

    VA:F [1.9.7_1111]
    Rating: 0.0/5 (0 votes cast)
    VA:F [1.9.7_1111]
    Rating: 0 (from 0 votes)
    • admin says:

      High Stephen, and thanks for your comment.
      No, I haven’t tried it! Regarless, I would not expect much difference, as I experimented with King rate (as opposed to comet rate on the screenshot you’re refering to) and it made no difference. I still have no idea what’s causing this discreptancy, as drift amount changed from night to night, apart from a really weird temperature gradient where I live, thanks to the huge mountain to “my back”.

      Rgds

      Serge

      VN:F [1.9.7_1111]
      Rating: 0.0/5 (0 votes cast)
      VN:F [1.9.7_1111]
      Rating: 0 (from 0 votes)
  • Thank you for mentioning that you’d tried King rate. I would expect that King wouldn’t make a difference measured at Zenith since it’s supposed to vary when the telescope is pointing at lower altitudes.

    Might be worth making a matrix of divisors crossed by temperature and barometric pressure. A pattern could be confirmed or ruled out.

    I was directed to your post by Franck at Ovision in response to my questions to him about a gradual drift that I see on my G11.

    VA:F [1.9.7_1111]
    Rating: 0.0/5 (0 votes cast)
    VA:F [1.9.7_1111]
    Rating: 0 (from 0 votes)
    • admin says:

      Stephen,
      thx for the suggestion. I did think of doing something similar, but, because we’ve had so few clear nights I rather take some meaningfull images when I can rather than measure mount inaccuracy! Ideally, I’d need to take the mount to some other mountain free location and check to see whether tracking changes or not. The fact is, “non standard” air refraction path can easily move objects by minutes of arc. As Franck probably told you, We’ve changed everything from electronics to the mount itself, and the problem remains. Send Franck my regards!

      VN:F [1.9.7_1111]
      Rating: 0.0/5 (0 votes cast)
      VN:F [1.9.7_1111]
      Rating: +1 (from 1 vote)

Leave a Reply