Periodic Error Correction
Periodic Error Correction
1. Turn off PEC in The Sky->Telescope->Options->More Settings by unchecking the "Apply PEC correction" box.
Figure 1: PEC window in The Sky. In More Settings window accessible from the Telescope->Options menu.
2. Open PEMPro. In the Setup tab (one of the lower tabs), connect to the scope and the camera. Switch to the Acquire Data tab, and set the exposure such that the bright star you are pointed at is bright but not saturated. Press Start to begin acquisition. Once the image is exposed, check Maxim DL to make sure it found the star, and not just a hot pixel. Acquire data for a large number of periods, until you have a good looking periodic curve.
3. When you have enough periods, click Stop. Save the data, and you will be prompted to create a PE curve, which you should do (if for some reason you do not, at a later point you can load the data into the Analyze tab and click Create PEC Curve there.) Do not worry about any syncing messages. A separate Create PE Curve window will open, and a curve will be fitted automatically. Check to see if any frequencies need to be added; you can reference the Frequency Spectrum from the upper left pull down menu. The RMS error in the upper right may be of interest. This window, along with the analyze data tab in the main window displaying the raw data collected (with the correction turned off) on 06/01/2010 are shown in figure 2. Save the curve by clicking the 'Create Paramount PEC curve' button (Note: throughout the procedure, save and open only text files). It will ask you if you want to open the curve data in a text file, which is unnecessary at this point. NOTE: if a this or any point it asks you to invert the curve, be careful. We determined that we do in fact need to invert the curve, but this is confusing (it is hard to tell exactly what each of the graphs represents: error, correction, telescope offset, star offset, etc.) and it is important to keep track of any inversions. You do not have to invert when prompted, since we can do it later in the Refine window.
Figure 2: Raw data (with correction turned off) from 06-01-2010 in the analyze tab of the main menu, and the PEC curve creation window fitting a curve to the data.
4. Close the Create PE curve window. In the main PEMpro window, switch to the Refine tab and and load the curve data (not the original data) you just saved by clicking the 'Curve 1' button. Assuming the curve was not earlier inverted, click the Invert button next to where you loaded Curve 1 (trial and error taught us that we need to invert the curve). We determined (06/02/2010) that when we save a file here, and copy the data into the actual Periodic Error Correction window in The Sky (which does the actual correcting and actively follows the phase of the worm gear) there is a phase offset of 600 units (~66 seconds). On top of that, the zero phase point of the worm gear, as read from the The Sky window, corresponds to about the 15s phase point in PEMpro, requiring and additional 139 unit offset (the math: the worm gear cycle is divided into 1250 units, and has a period of 134.63 seconds. This corresponds to 9.3 units/second). The original offset was found by putting a spike in the The Sky correction curve, recording this spike in PEMpro, and then shifting the recorded curve fit and pasting it back into the sky until the spikes matched up. The 15s phase offset we noticed while acquiring data, by seeing that the The Sky phase mark line passed its zero point when in the PEMpro window the Current Worm Phase was at 15s (you can verify this). So the total offset we decided on is 740 units. So: using the Shift Curve section at the bottom of the window, shift the curve (make sure the pull down is on the correct curve) to an offset of +740 units (to the right. The maximum shift for each step is 50 units, so this will take some clicks. the recorder of the shift may wrap around to negative values, so you may have to keep track of how far you shifted to the right yourself.) The curve fit from our 06/01/2010 data is shown in the Refine tab in Figure 3, along with its shifted and inverted version. Save the curve from the Save section , again selecting the right curve from the pull down. In the save window, select 'Paramount PEC files (*.txt)' from the 'Save as type' pull down.
Figure 2: Curve fit from 06/01/2010 in the refine window. Curve 1 is the original curve, Curve 2 is inverted and shifted +740 units (Note that the offset says -510; this is the wrap around).
5. Open Notepad, and open the file you just saved. copy the entire thing, and switch back to the The Sky PEC window (Figure 1). Here, click the past button. Your curve should appear in the table window. Click upload to save this curve. To apply teh correction, disconnect PEMpro and Maxim DL from the camera, and back in the The Sky window switch to the "MKS 4000 options" tab, and in the reboot section click Normal. Click OK. The telescope should slew to home; you can send it yourself to make sure. Reopen the Telescope->options->more settings window, and go the PEC tab; click the Apply PEC correction box. The Preiodic error should now be correct! You can check this be Acquiring data in PEMpro, which should look much more flat, or by guiding and looking for periodic shifting.
Status(06/03/2010): With correction applied, PEMpro measures a much reduced but still extant periodic error, of about an arc second, more extreme in one half of the period than the other. Guiding in X shows a similar, still present periodic behavior, but it is less extreme and has many fewer jumps.