II. Data Acquisition

Before any photometry can be done on the star image, you must first take suitable images. The following describes a procedure using Meade AutoStar software and a DSI Pro monochrome camera to acquire photometric images.

Open the Meade AutoStar Suite software. The first window shown is the Planetarium program. From the Image pull down menu select DSI Imaging.

Figure 1
Meade AutoStar Suite

The AutoStar Envisage window will then open. If there are problems and the camera image doesn't show up, check the USB connection. You should use a powered USB 2.0 interface, even though this will work marginally with USB 1.0. If problems persist, try another cable.

Before going further, you may wish to set the settings for the data acquisition program (Envisage) for your setup. Select the Settings pull-down menu.

Figure 2
Selecting Settings

For the Image Directory and Dark Frames Directory, use the default or create new ones. To avoid confusion, it is suggested to use the default directories. The Temperature boxes are merely a Fahrenheit to Celsius conversion and do not do anything else. Click OK when done.

Figure 3
Settings Window

Open the observatory, set up the telescope and tracking, turn on the camera and start cool down/stabilization. Allow 15 minutes or more.

Taking Dark Frames

The following should be done each evening prior to starting the imaging. Let the equipment stabilize and adjust to the ambient temperature for at least 30 minutes before doing this.

1. From the Image Process menu select Take Darks.

Figure 4
Selecting Take Darks

Note: The default display in the blue area changes for the one displaying the Min Quality %, Evaluation Count and Combine check box to First Exp, Last Exp, Avg Exp and Del Existing Darks check box. Take Darks gets put in the Object Name field automatically.

2. Set the range of exposure times. The shortest is 1.0 seconds. Set the First Exp (in this case 1.0), set the Last Exp (in this case 30.0), This will take dark frames for exposure times 1.0 through 30 seconds. Use the default value of 5 for Avg Exp and select the Del Existing Darks check box. An estimated time for taking the darks will be shown (for this case the estimated time for taking the dark frames is 8 minutes and 32 seconds.

Note: There will be one master dark frame for each exposure 1.0 second or greater (for the times used). You cannot take dark frames for exposure times less than 1.0 second and indeed they normally are not needed for such short exposures. Filters do not enter in for dark frames. The filter selected does not matter. There may be some light leak through the filter wheel so best do this when it is dark.

3. Click the Start button. A Take Darks window will popup telling you to cover the telescope objective.

Figure 5
Take Darks Window

4. Cover the front of the telescope and click the OK button.

5. The computer will take the dark frames automatically . When completed, a Dark Frames Complete window will popup reminding you to uncover the telescope.

Figure 6
Dark Frames Complete Window

6. Uncover telescope. You now have a set of master dark frames for the desired exposure times. These are stored in a folder called Darks located in the Meade Images folder (unless you designated a different folder).

Taking Stellar Images

1. Find stars to be imaged. Position the B filter in the optical path. If using a telescope with a mirror lock and electric focuser, set the electric focus so the range in about midway between extremes. Do a coarse focus with the manual focuser. Turn CCW past the focus and then CW back to the focus. Lock the mirror. Focus with electric focuser for sharpest images. Check the focus in the other filters. I have found that the I filter seems the most critical. If that is focused well the other filters are usually acceptable.

2. Determine exposure time that allows a reasonable maximum counts for the stars of interest, but less than 65,535 counts. This includes both the comparison star and program stars and in each filter. Do not worry if some of the other stars are saturated. As long as they are not of interest and do not overlap those that are of interest, it will not matter. Do not pay any attention to the Histogram or other items in the Stats area. Remember, different filters can have different exposure times.

Figure 7
AutoStar Envisage screen prior to starting the imaging

3. Set the exposure time.

Note: Live exposure times can be set shorter than 1.0 seconds and Long Exp can be 1.0, 1.4, 2,0, 2,8, 4.0, 5.7, 8.1, 11.3, 15, .... seconds.

Figure 8
Exposure Setting

Note: You can use the Live exposure time for short exposures ( up to 15 seconds). For times greater than 1 second I suggest using Long exp. Leave the Live button clicked for now.

4. Check dark field subtraction box (Dark Sub).

Note: Leave Gain and Offset at their default values Gain= 100, Offset= 50).

5. In the Image Process box select Deep Sky.

Figure 9
Deep Sky Image Process

Note: CCD photometry will create an enormous amount of data quickly. Each image will be over 1.2 MB. It is very important to develop a systematic approach to handling the data and naming files. The following is what is used at HPO and is just a suggestion. You can develop your own technique as long as it works for you.

6. In the Object Name field , change the name from Deep Sky to the name of the image file for the exposure, e.g., "Star Name"- B- exposure time-1-, e.g., TORI-B40-1- .

Note: For a 4.0 second exposure and for the times with a decimal, you cannot use a period in the name so just specify the time without the period, 28 is 2.8, 40 is 4.0, 57 is 5.7.

Figure 10
Naming Image Files

Note: The software will automatically add a sequential number to the end of the file name (e.g., TORI-B40-1-1, TORI-B40-1-2, TORI-B40-1-3, the next set would be TORI-B40-2-1, ...).

7. Click the Save Proc... button. The Save Process window will be shown. Make sure for File Type that Fits is selected. For Save Options make sure Normal Operation is selected. Leave the Single Shot and Web Mode buttons unchecked. Click OK.

Figure 11
Save Process Window

8. Unless the sky is exceptionally steady, leave the image quality (Min Quality %) at 30 and evaluation frames (Evaluation Count) at 5. These default values seem to work well. Make sure images are saved as FITS files and Normal Operation is selected (Save Procedure button). Check the Combine box.

Figure 12
Quality and Evaluation Count

9. Now we set up a reference star that will be used to align the images in the stack of images that are combined. While viewing the real time image (with the DSI1 folder tab selected and Live box checked) draw a small box around an isolated star to be used as a reference guide star . If an Alt/Az mount is used, a second box can be drawn around another star to derotate the field when stacking (combining) the images.

Figure 13
DSI1 Folder Tab

Figure 14
Tracking Box

Note: Again, do not worry about anything in the Stats area at this time. That is mostly useful for astro imaging.

Figure 15
Stats Area

10. If a long exposure (more than 1 second) is used, make sure the Long exp check box is selected. Set the Long exp time. Unselect the Live box. If you do not uncheck the Live button, that will be the exposure time regardless of what the Long exp is set at. Make sure the Dark Sub box is still checked. For the DSI Pro, the Mono check box cannot be changed.

Figure 16
Long Exp and Live Selection

11. Select Start.

Figure 17
Start

12. After at least 10 images have been combined, select Stop. If longer exposure times are used, a lesser number can be stacked.

Note: Only images meeting the Min Quality % will be included in the stack. If seeing is bad, it may take several minutes to get 10 good images even with exposures of just a few seconds.

13. Repeat step 11 and 12 two more times to produce a set of three images that will have their data averaged.

14. Select the V filter and repeat steps 1 -13. Name the file "Star Name"-V"Time"-1-."

15. Select the R filter and repeat steps 1 - 13. Name the file "Star Name"-R"Time"-1-."

16. Select the I filter and repeat steps 1 - 13. Name the file "Star Name"-I"Time"-1-."

Note: You should now have three combined images for each filter (total 12 images). When each filter's image data are averaged one data point for each filter will be produced .

17. Repeat steps 1 - 16 for each additional set of data points.

Note: Unless changed the image files will be stored in a folder called Meade Images. Find it and make a shortcut to it and put the shortcut folder on the desktop for easy use.

Flat Frames

Flat frames provide a calibration for the pixels of the CCD chip. Flat field images are taken just like regular star images except the telescope front end is evenly illuminated with a white light.

There are several ways to do this,. A simple light box seems to work best.

One set of flat fields (stacked/combined) must be obtained through each filter. Take as many images per set as practical (10 to 100 is ideal). The brightness of the illumination and/or exposure times may need to be adjusted to get good flats. Adjust the exposure times and/or brightness for at least 10,000 counts on the Histogram but less than 65,000 counts.

Note: It is very important that the optical train is not changed between imaging and taking the flat fields. This means no moving the camera relative to the telescope and only minor focusing. This is why it is usually best to take flats at the end of a session. If the optical path is not identical for the flat images and star images, the flat fields will be of no value as they will calibrate the wrong pixels.

Figure 18
Sample B Filter Flat Field

Take a flat field image for each filter. Take at least 10 (the more the better) and combine them for each filter. Name them FB, FV, FR and FI.

Created 1 February 2007

Modified 14 February 2007

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