An Investigation into the Operation and Control ... - Explore Scientific

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standard ST4 interface available on most mounts today. The ST4 interface was developed by the Santa Barbara Instruments Group (SBIG) in the 90’s to provide an automation interface similar to the way a hand controller interface is used when manually making guiding corrections. The ST4 control interface is basically a contact closure interface for each direction to drive the mount: North/South, East/West. Additionally, the ST4 uses a pulse-width scheme similar to the way one would manually adjust the position of the telescope. Figure 1: TDM Encoder Installation As one manually pushes the guide buttons to reposition the telescope on the guide star, a variable amount of ‘push’ is made depending on the amount of difference between the position of the telescope and the guide star in the cross-hair. In the same way, the automation using the ST4 interface provides a variable pulse-width which may range from 10mS to 500mS. There is one other variable available to manage the control response and that is the ST4 Guide Rate. The ST4 Guide Rate is set to adjust how fast the telescope mount moves in making the correction. In most modern mounts, the hand controllers provide for setting this value from 0.1x to 1.00x Sidereal Rate. The Sidereal Rate is ≈15 arcsec/second therefore; an ST4 Guide Rate of 0.75x would equal an 11.25 arcsec/second correction rate. The effective correction to a TE would be a combination of the 2 variables; a rate of 11.25 arcsec/second times a pulse width of 0.05 seconds (50mS) would equal a correction of 0.5625 arcsec. The TDM control algorithm provides correction outputs within the range of the selected TE control range. With a selected control range of ±0.5” arc, the output would be active with a TE > ±0.5” arc, and would be inactive when the TE < ±0.2” arc. The dead-band of ±0.2” is necessary to minimize any over-controlling of the mount using the On-Off control algorithm. There is an additional variable to this correction, and that is the control output update frequency. This frequency is 5Hz in the TDM. The TDM has demonstrated, and is capable of pulse output widths of < 50mS, perhaps approaching 10mS. For any given correction, a combination of pulse-width and ST4 Guide Rate setting results in a response to that correction. The nominal ST4 Guide Rate is recommended to be in the range 0.1x to 0.5x Sidereal, depending on the response of the given mount. 3. Data Acquired The TDM system was ordered by the author at the end of February 2011, and was received in April 2011 at the NEAF 2011 show from Scott Roberts of Explore Scientific, and Tassilo Bohm of Meade. There was a slight delay in the delivery of the EQ6 Pro mount adaptor parts, and they were received in June 2011 from Attilla Mádai. I want to thank all the help that Scott, Tassilo, and Attila provided in obtaining the TDM system. Installation of the system (Figure 1) was completed by the middle of June without complications and per instructions, and first tests were performed at that time. The initial testing showed that the TDM worked as advertised. An initial test was performed without the ST4 input active so as to acquire the data necessary to characterize the mounts PE, or in this case TE. The chart in Figure 3 (at end of document) shows the mounts TE with the ST4 disconnected, and then connected to show the difference in noncorrected versus corrected TE.
Further testing demonstrated that the TDM did in fact control TE to < ±1 arcsec. The measured value for this particular EQ6 Pro mount has been a consistent (95%, 2σ) ±0.7 arcsec. It should be noted that this is equivalent to an RMS TE of 0.12 arcsec as measured by an analysis performed on one of the data files acquired from the TDM. The PE analysis program used in this analysis is called PEC Prep. The website is at: http://eqmod.sourceforge.net/pecprep/index.ht ml. Subsequent to the initial indoor testing, several nights were spent using the TDM at the end of June with great success in imaging various targets. It should be noted that a very accurate polar alignment is necessary in order to image with no guide scope/CCD. This is the ultimate goal at this observatory in order to quickly setup and acquire images of Minor Planets, and maximize the time spent imaging. This goal was obtained on the 4 th night of imaging. During 2010, a total of 18 observations were made of 5 Minor Planets over several months (108 Hecuba, 260 Huberta, 494 Virtus, 713 Luscinia, and 1103 Sequoia). Some of the observations in 2010 were in support of obtaining the observatory code I24. In contrast to this, on 02 July 2011, a total of 16 observations were made of 4 Minor Planets, one of which (2001 LO7) is a NEO (Figure 2) (895 Helio, 2150 Nyctimene, 6613 Williamcarl, and 68348 2001 LO7). This was only possible through the use of the TDM in my estimation. Astrometry and photometry was performed on this data, and observations were reported to the MPC. The Astrometry residuals measured were < 0.2 arcsec, and some were 0.1 arcsec or less. This is a testament to the guiding accuracy and Figure 2: Close-up, Minor Planet (68348) 2001 LO7. 1800 sec unguided exposure: 10 x 180 second stack. ease of use of the TDM/EQ6 Pro mount combination. 4. Further Testing Subsequent to the 4 days of imaging, further testing was performed on the TDM system, and a small average output offset was measured (–0.2 arcsec TE), and was investigated. It is important to understand that this small offset detected in no way effects the ability of the TDM to perform its design function. As previously stated, the TDM provides excellent tracking performance resulting in discarding the need for a guide scope/CCD combination. During the initial discussions with the system developers about this issue, a theory was proposed that perhaps the balance of the load on the mount was responsible for this small output offset. Additional testing showed that this was not the case. Based on further discussions on the control algorithms used in the TDM a working theory was proposed that perhaps the offset was being introduced by the fact that the control system was not forcing the TE back to a nominal zero (0.0 arcsec TE) and this was the source of the offset. Based on data acquired from the original system delivered, an equation was developed which described the behavior of the offset (arc seconds): This equation was based on a least-squares fit of data acquired using different ST4 Guide Rate values from 0.125x to 1.00x. Further discussions led to providing the author with 3 versions of the TDM firmware which introduced an offset (the TDM Offset Value
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