444/544 Advanced Lab Manual Astronomy

Transcription

1 444/544 Advanced Lab Manual Astronomy INTRODUCTION The purpose of this lab is to familiarize the student with contemporary astronomical methods. Astronomers use CCD detectors to measure light variations in stars over time - a method called photometry. Currently, CCD detectors can accurately measure differences in magnitude on the order of under ideal conditions. However, under the worst conditions our light polluted skies limit us from about ±0.02 magnitudes down to ±0.005 under the best conditions. LAB #1 Data Collection 1. After the instructor has set up the telescope for observing, take a seat at the telescope control desk and select the window labeled DFM Telescope Control System 2. Select Telescope/Movement then the Slew Position tab and enter the coordinates for the star you have been assigned in the Set Slew Position boxes. Make sure the Slew Epoch is set to Click Apply then Start Slew and close the box when you are finished. 4. Select the window labeled Maxim DL. 1

2 5. Navigate to File/Settings and select the FITS Header tab. 6. Highlight Observer and enter your last names (i.e. Smith/Jones) in the value box. Then click OK 7. Find the Maxim CCD control window and select the Expose tab. 8. Set Type to Light then select the V filter and enter a value of 30 seconds. 9. Click Expose 10. Once the image has downloaded, verify the stars in the field are correct by using the finder chart provided for your target star. Hint: It s easier to try and identify geometric patterns with the image and the finder chart side-by-side. 11. If the field needs to be moved to get all the stars you need in the frame, (i.e. program star and comparison stars) then use the hand paddle on the desk. Looking at the image; North is up and East is left so to move the stars in the field to the Right you will need to move the telescope east. Small movements with the telescope can be made by pressing the corresponding directional button for about 2 seconds and releasing. Larger movements can be made by pressing the directional button and the Set button simultaneously. 12. After moving the telescope, you will need to repeat steps 10 through 12 until the required stars are properly framed in the image. Note: If the directional buttons are depressed too long, the telescope might move too far putting the program stars out of the frame. If this happens, return to step #1 and re-slew the scope. 13. Once the program stars are properly framed, you will need to determine your exposure settings. 2

3 14. Under the Expose tab confirm Type is set to Light, enter 60 seconds, and filter is set to V 15. Click Expose 16. After the image is downloaded, select the Information window and ensure the Mode is set to Aperture 17. Set the aperture radius to completely encircle your brightest program star in the image by right clicking then select Set Aperture Radius 18. Move the aperture to each of the stars you plan on using (i.e. variable and all comparison stars) 19. While holding the aperture over each star look at the information window. At the top left next to Maximum make sure the number is less than for each star you plan to measure. Note: This is the full well capacity of the chip therefore the image is saturated and cannot be used for photometry so reduce the exposure time until it is less. 20. After adjusting the exposure times try to keep the Max-Pixel values under 65535, write down the exposure for later. It is also important to note here that the only stars you should be concerned with are the variable and the comparisons. It is not important if other stars in the field are saturated if you do not intend on measuring them. 21. Switch to the B filter and repeat steps 19 through Once the exposure times have been determined, select the Sequence tab and enter the name of the variable in the Autosave Filename box. (example: V1815_Cas) 23. Select the Options button and Setup Sequence then Enable the check boxes on two rows, setting Type to Light, selecting the B 3

4 and V filters and entering their respective exposure times. Ensure Binning is set at 2 and enter 200 for Repeat then click OK 24. Click the Options button again and select Set Destination Path and navigate to Drive:F/ 25. Create a new folder under F:/ and name it for the current date using the following convention: YYMMDD-(D+1) Example: For the date 11/06/07 type Create two subfolders; one named Calibration and the other named for the variable star (i.e. V1815 Cas). 27. Highlight the folder named for your variable and click OK 28. Under the Sequence tab Click Start then sit back and collect your data. Important: Monitor the images periodically making sure the program stars are properly framed and none of them are saturated. If they are, stop the sequence and re-adjust the exposure times and/or re-position the telescope then re-start the sequence. LAB #2 Data Reduction CCD images have natural imperfections that need to be taken into account. At the end of an observation run calibration frames are taken and applied to the images of the variable so accurate measurements can then be made. CCD s have three major layers that need to be removed from the raw image, biases, darks and flats. Biases are from an offset voltage that is applied to the chip for charging the well, usually around 5 volts, but it adds a value to the count that needs to be subtracted. Bias frames are taken by exposing the chip 4

5 for 0 seconds. Darks are from dark current or thermal noise, it is from electrons that are floating around on the chip at random adding value to the count as well. Dark current can come from many sources but it mainly comes from the hot electronics inside the camera, either way, it needs to be subtracted from the image. Darks are taken by exposing the chip with the shutter closed for a period of time equal to or greater than the longest exposure of the evening. Flats are images of how the light falls on the optical system. These are taken by pointing the telescope at a uniformly illuminated and featureless surface and exposing the chip to between half and two-thirds the full well capacity (in our case between 30,000 and 40,000). Flats are an image of all the defects in the optical system, they include dust particles (or donuts) and dead pixels but unlike biases and darks they are divided from the image instead of subtracted. Luckily, the software we are using will do the image math for us but here is how it works: Flats and darks need to be calibrated as well. Darks contain bias information that needs to be subtracted and flats have bias and dark current information so before the calibrations can be applied to the image they must be calibrated separately. 1. Your data will have been transferred to one of four other computers in the Telescope Control Room (Mercury, Saturn, Jupiter or Uranus). Your instructor will assign you to a computer based on what star you were observing. 2. Find the folder on the desktop named Advanced Lab and locate the data folder(s) containing your observations. 3. The instructor has provided the files named bias.fits, dark.fits and flat_b/v.fits for you in your folder named calibration. 5

6 4. Open Maxim DL and select Process/ Set Calibration. After the dialogue box for calibration appears ensure the Dark Frame Scaling is set to Auto Scale and check the Apply to Flats box. 5. Under Bias click Select Files and navigate to the folder containing your calibrations. Select all 10 of the Bias.fits files then click OK 6. Under Dark Frames select all 10 of the Dark.fits files then click OK. 7. Repeat these steps by selecting all the flat frames (all 10) from the calibration folder then click OK Important: You must use the flats from the same filter as the image files you are calibrating. Flat_B.fits should be used for Image_B.fits. 8. Click OK to close the calibration dialog box. 9. Select File/Open and Navigate to the folder containing your variable star images then open all the files from the same filter you will be calibrating. 10. Once all the images have opened, select Process/Calibrate All 11. After all the images have been calibrated select File/Save All (If the computer prompts you to stretch the images before they are saved, uncheck the box and proceed WITHOUT stretching. Ideally, this will not be the case and all files will save automatically without any additional efforts). 12. Repeat these steps until all of your data has been calibrated. LAB #3 Data Analysis 6

7 1. With all images from one filter opened (either B or V) select Analyze/Photometry 2. Under Mouse Click Tag As: select New Reference Star make sure all the check boxes below are checked (Act on all images, Use star matching and Snap to centroid) then select a star in the image that will be used for reference. 3. Referring to your finder chart, found at the bottom of the Advanced Lab web page, enter the appropriate value for your reference star in the box marked Ref Mag 4. From the Mouse Click Tag As: box select New Object then apply the aperture to the variable star. (this should show up as Obj. 1) 5. Select Mouse Click Tag again then New Check Star Select another suitable star in the field for use as a Check Star (this will show up as Chk1. The check star is used to give reference to the behavior of the atmosphere for the nights observing or in other words, how turbulent the atmosphere is. 6. Click View Plot you should see a graph of your variable. Important: sometimes images will have such a low signal to noise ratio that the software cannot verify the stars in the field from one image to the next. In this case a dialogue box will appear during star selection stating that Not all images were acted on if this happens, those images will need to be Excluded from the plot. Highlight the bad image in the Image List and select Exclude 7. With your instructor s approval, print a copy for the observatory records. 8. Select Save Data and make sure the file will be saved to the Advanced Lab folder. 7

8 9. In the Filename box enter the date of the observations, name of the variable including filter and add the extension.csv (for some reason it won t be saved as.csv unless it is forced) Example: _V1815-B.csv 10. Then click Save and repeat these steps for all your images. 11. Open Microsoft Excel then click File/Open and navigate to the folder where your csv s are stored. 12. Open one of your csv files. You should see multiple columns containing all the reference stars you selected along with the variable star. The far left column contains a large number labeled T(JD) called the Julian Date Note: Julian Date is the number of days since January 0, 4713 BC. This is commonly used in astronomy for calculating time between dates. It s much easier than using the Gregorian calendar convention. 13. At the top of the document, highlight the T(JD) column and right click the lettered box and select Format Cells select Number and set the number of Decimal Places to 5. ( days = 51.8 seconds) 14. The column titled Ref1 may be deleted at this time. 15. Highlight the Obj1 and Chk1 columns and set the displayed precision to 3 decimal places. Hint: you can hold down the CTRL key and select multiple columns to save time. 16. Move the Obj1 data to column B and the Chk1 column to D leaving column C open. 17. Create a new column heading named Error under column label C 18. The position of the columns from left to right should be: T(JD), Obj1, Error, and Chk1. 8

9 19. Go to the bottom of the Chk1 column and find the average for all the data and write it down. Do not leave the equation in place. 20. In the first box of the error column type (=) to create an equation and enter: =ABS(D2-{Avg of Chk})/2. Then click the green check to accept the equation. 21. Re-select the box with the new equation and use Ctrl+C to copy it. Highlight the rest of the cells beneath for all your data and use Ctrl+V to paste. 22. Highlight the Obj1, Error and the Chk1 columns and set the displayed precision to 3 decimal places. Hint: you can hold down the CTRL key and select multiple columns to save time. 23. Your spreadsheet should look like Figure 1. Note the format for the equation (Red Arrow) Figure 1 9

10 24. Save this file as a spreadsheet for future use or reference. Click File/Save As then xls and OK Note: Make sure to save it in your own data folder. 25. Click File/Save As and select file type as Text (MS-DOS) keeping the same file name (i.e V1815_V.txt) and click save. Remember to save it to YOUR data folder. Note: Microsoft Excel will ask if you want to keep this format. Click Yes 26. You will need to repeat steps 12 through 25 for EACH separate night and EACH separate filter you have. 27. Close Microsoft Excel. 28. Double click to open the program icon on the desktop named PERANSO 29. Click File/New 30. In the upper left hand corner of the ObsWin#1 box click the + icon. 31. Select Import Data and navigate to your data folder containing the *.txt files and open the Star_V.txt file to work with. You should see four columns containing data named: Time, Mag, and MagError and Ignore (Chk1). 32. Repeat step #27 for each V magnitude data set you have. Note: You may use the radio buttons at the top to toggle between each observation set. 33. Click the ObsSet Properties button at the top of the ObsWin#1 window. 34. Click Star Coordinates and enter the variable star name then select Get Coordinates or enter the coordinates manually if the program doesn t recognize the star. Click Apply 35. Select File/Save and save your data to your folder. 10

11 36. Look at each light curve and compare it to the sample curves from the advanced lab web page. Identify the type of variable and make a note of it for future reference. 37. If you have determined the variable is a pulsating star then you will need to measure its Maximum Extremum or time of maximum. If your curve is indicative of an eclipsing binary then you should find the Minimum Extremum (time of minimum) for the primary eclipse. 38. Select the Zoom on First ObsSet radio button. 39. In the ObsWin select Set/Unset Left Margin Cursor button and click on the curve where it intersects a horizontal line on the graph about middle of the curve. 40. Repeat step 35 for the right margin cursor then select the next button Find Extremum (Kwee-van Woerden) and select maximum or minimum depending on the variable type. 41. Write down the time of minimum/maximum along with the error for your lab report. 42. Repeat step 34 through 36 for each ObsSet you have by selecting the Zoom on Next ObsSet button and noting the times of minimum or maximum along with the errors. 43. In the next exercise you will determine the period. Period Determination for Variables 1. Select Period Analysis/ Lafler-Kinman 11

12 2. Enter 0.1 for a Start value, 1 for an End value, set Resolution to Once the calculations have finished, click the Set/Unset Frequency Cursor button. 4. Note the P= value and click the Refine Period Button (the P button) and enter a value for Start of 0.1 less than the previously calculated period and a value of 0.1 more than P for the End. 5. Set the resolution to 1000 then click OK 6. After the calculation has finished, click the Set/Unset Frequency Cursor button again and note the calculated period. 7. Click the Refine Period button once again and enter values for Start and End that are 0.01 in difference and set the resolution to After the calculation has finished, click the Set/Unset Frequency Cursor button again. This time the value of the period should not have changed very much (not more than ). If the value has not stabilized, continue with period refinement until it does by increasing the resolution value. Note: Increasing the resolution to more than 3000 will take longer to calculate. 9. Once the calculation has stopped click the Set/Unset Frequency Cursor once again then click PhaseWin at Frequency Cursor Value button. A phase window will appear showing the data. If you wish you may click the Double Phase View button for a clearer view. 10. Click the Info button at the top of the Phase Window and note the period, period error, and epoch for your report. 11. Finally, click File/Save and save the Peranso file to your directory using the name of the variable. 12

13 NSVS Data Analysis 1. Return to the Advanced Lab web site and select the star you have been assigned from the list at the bottom of the page. 2. Click on the link labeled NSVS Data. This will bring up a text file. 3. Highlight the data and use Ctrl+C to copy. 4. Open up Notepad from the start menu then use Ctrl+V to paste the text. 5. Save the file in the Advanced Lab folder using the star name and NSVS. Example: V1815_Cas-NSVS.txt 6. Using PERANSO, select File/New and click the (+) sign in the upper left hand corner to add the data. 7. Click the Modify Format radio button and uncheck the box marked Skip first then OK 8. Select Import Data and navigate to the Advanced Lab folder where the NSVS data was stored and select Open 9. Click OK (Chances are you will see a sparse distribution of data points) 10. Select Period Analysis / Lafler-Kinman and enter a start and end range that brackets your previous period estimate using a resolution of After the calculation has finished, click the Set/Unset Frequency Cursor button then the Phase Win at Frequency Cursor Value button. 12. Continue to refine the period until you are satisfied the error is as small as it will get. Write it down on your report sheet. 13