Starting Analysis

Before you start imaging and analyzing your own data, we suggest you use one of our tutorials and sample data sets to learn the steps for obtaining a raw instrument magnitude. We have created two sets of data that include a minimal number of images for this purpose. The Beginner data file was taken under ideal observing conditions and may be reduced by any Final Reduction Method. The Intermediate level data was taken at higher air mass and requires the Intermediate Level Final Reduction method to get good results. Both data sets may use either the AIP4WIN, IRIS, or MaximDL tutorial listed below to extract instrument magnitudes.

If this is your first time going through these tutorials, we suggest using the Beginner Data File. Once you become comfortable with this one, you might give the Intermediate Level Data file a try. Either way, please download a copy of the sample data and extract it (to a convenient directory) before you proceed through the tutorials. Also have the standard stars page open in the background so you can refer to it when needed.

Final Reduction Level	Description	Size (MB)
Beginner	12 RAW Cannon (CR2) files taken as 5-second exposures at F/4.0, ISO 800 on 26 Jan 2010.	74.8
Intermediate	Nine RAW Cannon (CR2) files taken as 5-second exposures at F/4.0 ISO 1600. These data were taken at air masses of ~2 and require the intermediate-level final reduction spreadsheet.	63.1

The tutorials below contain step-by-step procedures for processing digital images to yield instrument magnitudes, which are then used in the next step, Final Analysis. All of the tutorials assume you know how to load your camera’s RAW format images into your computer.

Reduction Software

AIP4WIN

IRIS

MaximDL

After you have finished the tutorial in one of the reduction packages proceed to the next step, Finish your Analysis . Here you will create calibrated visual or V-magnitudes for the variable star(s) in your image.

AIP4WIN: Beginner

This tutorial provides a step-by-step procedure for processing digital images obtained in “Raw” format to yield insturmental magnitudes. The instrumental magnitudes will be used in the “Finishing Analysis” portion of our tutorial. You will need AIP4Win 2.4.0. If you don’t have this version, a free update is available from Willmann-Bell at: http://www.willbell.com/aip4win/AIP.htm. The tutorial assumes that, in addition to a series of sequential star field images, you also have dark frames, flat frames and flat-dark frames. If your camera has automatic noise reduction, you can use this feature when taking your star and flat images and eliminate the need for darks and flat-darks. Before you begin the tutorial the images from your camera (or tutorial data files) should be copied to a directory on your computer. This will simplify and speed up processing.

Using AIP4Win, we will:

1. Convert images in “Raw” format to green-channel grayscale “FITS” images.

2. Setup Calibration

3. Calibrate and stack the images

4. Obtain “instrumental magnitudes” for the variable and comparison stars.

After completing these steps, the instrumental magnitudes are entered into a spreadsheet to yield calibrated magnitudes suitable for submission to AAVSO.

This tutorial was written by Tom Pearson with editing changes by Paul Lloyd, GMara and bkloppenborg.

AIP4Win Processing

Step 1. Convert images in “Raw” format to green-channel grayscale “FITS” images.

1. After opening AIP4Win, go to the menu across the top of the screen and click “Preferences/ DSLR Conversion Settings”.

2. A dialog box will open. Click the button: “BILIN” for the De-Bayerization Algorithm. Click the button: “DeBayer, Convert Color to Grayscale.” Set the red and blue parameters to zero and the green parameter to 1.0. Click on “Save” and “Done” at the bottom of the window.

Now that AIP4WIN is setup to open the green pixel data in grayscale, we need to open and save all images as FITS files.

3. Click “File/ Open Image…” at the top of the screen. A dialog box will open. At the bottom of the box, choose “Digital Camera Files” as the file type. At the top of the box find the folder containing your star images. From the list of images select your first star image to open.

4. Back at the top of the screen, click on “File/Save as FITS…”. Save the image as a “*.fts” file, e.g., “gray3024.fts”. When asked, save the image with the 32-bit floating point option (the default).

5. Close the image window.

6. Repeat this process for each of your star images, darks, flats and flat-darks.

Note: I have tried stacking the images first and then doing a single conversion but this method does not work for some reason. So, I go through the drudgery of opening each of my 40 images (10-star, 10-dark, 10-flat, 10-flat-dark) and converting each to gray scale individually.

Note: If you are taking images using your camera’s automatic noise reduction feature, you only need to use flat frames processed in this step. There is a glitch in the program, however, that requires you to process at least one dark frame. Here's the work around:

1. In the "Setup Calibration" window, load and process one dark frame

2. Uncheck the box at the bottom that says "Subtract Dark Frame".

Step 2. Setup Calibration.

We will use the "Standard Calibration" method in AIP4WIN.

1. At the top of the screen click on “Calibrate/Setup”. A dialog box will open.

2. At the top of the box, select the “Standard” calibration protocol.

3. On the dark tab, click on, “Select dark frame(s)”. A dialog box will open to select your dark frames. The button will turn green when your frames are selected.

4. Click on “Process dark frame(s)”. The button will turn green when processing is complete.

5. Click on the “Flat” tab and select your flat frames as above.

6. Check the box “Subtract flat-darks”

7. Select your flat-darks and click on “Process flat frame(s)”.

8. When processing is complete, all buttons should be green. The “Subtract Dark Frames” and “Apply Flat Field Correction” boxes should be checked. Also, look at the buttons across the top of the screen. The “Calibrate Image” button (red circle around letters CAL) should be illuminated.

9. Close the dialog box.

1. In the "Setup Calibration" window, load and process one dark frame

2. Uncheck the box at the bottom that says "Subtract Dark Frame".

Step 3. Calibrate and stack the images.

1. At the top of the screen click on "Multi-Image/Auto-Process/Deep-Sky". The AutoProcess Multiple Images dialog box will open.

2. At the top of the box, click on “Select Files”.

3. Open the FITS image files to be stacked.

Note: For the images to appear in numerical order, you must select the last image in your sequence first, then hold down the shift key and highlight the first image. Otherwise, the last image in the sequence will be at the top of the “Select Master Frame” list in #5 below. We want the list to be in numerical order to avoid tracking errors while stacking the images.

4. On the “Pre-Process” tab, check the “Calibrate images” box.

5. On the “Alignment” tab, click on the “Select Master Frame” window and choose the image at the top of the list.

6. When the image opens, check the “Two Star” alignment box.

7. Increase the track radius to 10 (or higher if tracking problems are observed in your stacked image).

8. On the “Image Display Control” dialog box zoom out to about 33%.

9. On the “master” image, click on the first alignment star. Then click on the “Star1” button on the “Alignment Tab”. A yellow number 1 should appear next to your star. Repeat the steps for Star 2. For best results the two stars should be on opposite sides of the image.

10. Back on the “AutoProcess Multiple Image” box, click “OK” to start image calibration and stacking.

11. If no tracking problems are observed (i.e., all stars are points and not streaks), save the stacked image by clicking “File/ Save as FITS…”. Otherwise go back to #7 and increase the radius until a satisfactory stacked image results.

12. Close the “AutoProcess Multiple Image” dialog box.

Note: If you have been working with the three images from the sample data file, your stacked image should look something like the example here. Don’t be alarmed by the bright area in the lower right. There was a full moon near Auriga when the images were taken.

Step 4. Obtain “instrumental magnitudes” for the variable and comparison stars.

1. Locate your variable star and comparison stars (three or more) on the stacked image.

Note: I use the free star atlas Cartes du Ciel to help find the stars. This program allows you to create a red frame overlay the size of your stacked image. You can also rotate both the frame and the star chart to match the orientation of your image. Clicking on any star in the atlas produces a box containing the star name and particulars. Other planetarium programs could do much the same, but this is a readily available free one.

2. At the top of the screen select “Measure/Single Image Photometry Tool (SIPT)”. This will open the Single Image Photometry dialog box.

3. At the bottom of the Image Display Control dialog box, zoom in on your variable star.

4. Back on the Single Image Photometry dialog box, click the “Settings” tab and adjust the “radii” so that the aperture (inner circle) is sized approximately as indicated on the image above (See the AIP4WIN Handbook section 10.1.2 for an explanation).

5. Adjust the annulus (outer circles) as appropriate for the aperture. One thing to avoid is getting other stars inside either the aperture or annulus. To show any change you make to the setting(s), click on the variable star – this deselects it – then click on it again; the changes you have made will now be displayed.

Note: Steps 1 through 5 are performed only once for a particular imaging series. These settings can remain unchanged as long as the same stars are being imaged using the same exposure settings, i.e., time, F#, ISO and lens focal length. For a different star field or different camera settings, choose the brightest star (largest diameter) in the set of variable and comparison stars to set your aperture and annulus size.

6. Leave the remaining items on the “settings tab” at their default values for now. Later, you can adjust the “zero point” to make your instrument magnitudes approximate the calibrated magnitudes.

7. Click the “save” button on the “Settings” tab.

8. Now, click on the “Results” tab.

9. On the “Image Display Control” dialog box, zoom back out to about 30%.

10. Shifting to your star image, click on your variable. Record the Raw Instrumental Magnitude that appears at the bottom of the “Result” tab on the “Single Image Photometry” dialog box. Also, check the value of “PV max.” This number should remain well below the saturation level for your camera (4096 on my Canon 20D).

11. Repeat step 10 for each of your comparison stars.

This last figure shows the stacked image with stars used in “Finishing Analysis” labeled.

They are: V-eps Aur, C1- lam Aur, C2 – rho Aur, C3 – mu Aur, C4 – ome Aur, C5 – sig Aur, C6 – 58 Per.

Note: You can also generate a report containing the instrumental magnitudes by using the following steps:

a. On the Single Image Photometry Tool, click “Get Magnitude” at the bottom of the box.

b, Open the AIP DataLog from the menu at the bottom of the screen. Save the data log. The file contains imaging data that includes star labels (V, C1, C2, etc.) and Instrumental magnitudes that will be needed in finishing analysis.

You're done!

Now that you have obtained insturmental magnitudes, you are ready to compute calibrated magnitudes. The DSLR Documentation and Reduction team has created spreadsheets to help you finish your analysis.

IRIS-Beginner

This tutorial provides a step-by-step procedure for processing digital images obtained in raw format to yield insturmental magnitudes which are used in the next step in our tutorial, processing the data using Excel. This process assumes that, in addition to a series of sequential star field images, dark frames, flat frames and flat-dark frames have been taken. This tutorial also assumes that you have already copied the images from your camera to some directory on your computer (this greatly speeds up processing).

NOTE: This is not the "live" page on the Citizen Sky website for the IRIS Analysis Tutorial. Brian must update the main site, please send him an email whenever you make significant revisions to this document. Please regard the below content as a "rough draft" and feel free to edit it to make it current.

IRIS is a powerful image processing utility with a command-line editor and a graphical interface. We attempt to highlight both methods below.

In this tutorial you will

Initialize IRIS
Load images
Perform Bias, Dark and Flat subtraction
Align images
Select the green channel
Perform Aperture photometry.

After these steps, the instrumental magnitudes are entered into an spreadsheet which will yield calibrated magnitudes which can then submitted to AAVSO

This tutorial was written by GMara, bikeman, and Roger Pieri with editorial changes by bkloppenborg and screenshots by bikeman.

Note: We recommend using the latest version of IRIS (5.58 as of the time this document was written) because it implements several new features that make it easier to work with raw images.

Note: When loading a series of images, IRIS will pause for some time to read, transform, calculate and save the images. Only the final result of these calculations will be displayed.

Note: For you Linux users out there, This tutorial was tested under IRIS 5.58 on a Ubuntu 8.10 machine running WINE. The only thing that does not work is the drag-and-drop of images from Windows Explorer. ~~You'll need to write a script to import your files.~~ You have to use the convertraw command in order to load a series of raw images.

Step 1 - Initialization:

Open File>Settings and choose your working path (where you have COPIES of your images), the script path (if you want to use some scripts..! ~~needed mostly for linux users, but that will be written later~~), and the file type (PIC for DSLR RAW). Note, IRIS will convert raw images to CFA images.

Open Camera Settings (the button with the camera picture) and leave the default values except: binning 1x1, model (select appropriate) and raw interpolation method linear. If your version of IRIS has it, make sure the White balance button is NOT selected.

Step 2 - Loading:

Click Digital photo > Decode RAW files. This will open up a window in which files should be dragged-and-dropped from Windows Explorer (the file browser). Drag and drop your raw DSLR images (e.g. for Canon: *.CR2 files) here. (If you are running IRIS on Linux, open up the command line input dialog, the button to the left of the camera button, and type "convertraw input_sequenceoutput_sequence number", without the quotation marks, where input_seqeuence are the images from your camera and output_sequence are the images that IRIS created in CFA format from above.)

This dialogue lets you specify a sequence name. After dropping in your images, press the "-->CFA" button to decode the images.
Repeat this process for star field images (e.g. use the name "img"), dark frames (suggested name "dark"), flat field images (suggested name "flat") and bias frames (optional).
Now you should have a series of images, darks, flats and optionally bias frames in the working directory you configured in step 1a.

Step 3 - Preprocessing > Bias, Dark, Flat:
Unlike CCD cameras, DSLR cameras dont provide the user with the ability to control temperature and apply a master dark while images are being taken. Because of this one should take dark shots during your imaging session which will include the bias pattern in your camera. Although the DSLR dark frames will account for the bias, IRIS requires separate bias frames. Because of this, we can either create a formal bias frame or a fake ("dummy") bias frame, both work equally well, but one of the two must be completed.

3.1.1 Creating a Formal Bias
To generate a Flat master frame go to Digital Photo Tab and select "Make an Offset..." (Bias frame). Type the appropriate sequence name (the one used in step 2, e.g. 'bias') and the number of bias images. Then select the "command line" button (the one on the left from "camera" button) and type:

> Save master-bias (or any name you would like) or use the menu "File/Save..."

3.1.2 Generate a "dummy" Bias
Some DSLR/DSC cameras use a "system offset." This offset is built into the camera's analog to digital convert (ADC). This offset will take the black level (ideally 0) and encode it as a higher value (normally 128 for 12-bit cameras, 256 or 1024 for 14-bit cameras). the result is that generated values can be less than the offset zero position or, in other words, negative brightnesses are permitted! IRIS doesn't handle this parameter, even if the camera types are identified, therefore we need to determine it for generating the artificial bias.

The easiest way to do this is to load one of the dark frames and look at its black level. To measure the black level use the command prompt window and type:

> stat

The output window will show an average value of the whole image. Use this value as the black zero-point reference. If you would rather not use the command prompt, you can select a blackened portion of the image using the mouse. To do this left click on the area, draw a box, right click and select "statistics." IRIS will show the average value in the boxed area. Use this value as the black zero-point.

Now we create the artifical bias frame. Use the command prompt and type

  > fill value

Where "value" is the black zero-point from above. Now, save the master bias by typing:

> save master-bias

You are done.

3.2 Master Dark:
Now we need to create a master dark. Type in the generic name, the offset image (the master bias frame you just created), and the number of images in the dark sequence. Select either the mean or median method (they work equally well), and press OK. Then type the command:

> save master-dark

Or choose File/Save from the file dialog.

3.3 Master Flat:
The flat process is mandatory for eliminating the vignetting of the lens. Vignetting as high as 30~40% are common and could induce large differential magnitude errors depending location of the reference star and the target star.

Repeat the same process for darks except before saving type in the command:

> GREY_FLAT

in the command prompt to normalize the red and blue CFA-flat. Then save the file as "master-flat."

3.4 Improving dark frame subtraction:
This function automatically detects hot pixels above a threshold level. It might take some experimentation to determine the threshold value you need, but a good starting value is between 100 and 200 for 12-bit cameras and 500 for 14-bit cameras.

Open up the master dark frame (created in step 3.2) and type the following in the command prompt:

> find_hot cosme number

where "number" is the threshold value you have slected and "cosme" is the name of the file where the results are recorded (you could chose any you like). Check the output box (this should open automatically). If there is a number of hot pixels not detected, change the threshold accordingly.

3.5 Finish Preprocessing
Go to Digital Photo>Preprocessing. Input the sequence name of you data images (img) along with the offset map name (master-bias), dark map name (master-dark), flat-field map name (master-flat), and the cosmetic file name (cosme). Give an output sequence name (e.g. 'img-cal') and the nunber of files. DO NOT tick dark optimize (as it will take longer to finish).

Step 4 - Alignment of Images:

Go to tab Digital photo>sequence CFA conversion and select the seq name that you gave at step 3e ('img-cal'). Give an output name ('img-cal-conv') and number of your star images and select Color output files type .

Go to Processing tab>Stellar registration and put the sequence name of 4a ('img-cal-conv'), give output sequence name (e.g. 'img-reg') and number. Choose Global matching and Quadratic transformation

Stacking of images: Processing tab>Add a sequence and give sequence name of 4b (img-reg) along with the number of images and method arithmetic

Save the image!

Step 5 - Selecting green channel:

The recent version of IRIS (i.e. 5.57 or newer) include a quick and easy way to pull out the RGB channels:

From the Digital Photo menu, choose RGB Separation. Then just type in the names of the color channel files (i.e. final-r, final-g, final-b) and you're done. Click OK and you're done separating the channels!

Step 6 - Photometry:

IRIS is a little funny when it opens images. It places the (0,0) pixel in the top left corner of the screen instead of the bottom right. So open up the image from 5d and look around for a while. In our sample data, Capella (alf Aur) and eps Aur should be easy to spot. You can increase the visibility of the stars by moving the upper slider in the Threshold Window to the left until stars appear with enough contrast. This is just changing the display settings, not the image itself.

Go to Analysis>Aperture photometry. With our sample data, you can probably use the default values for aperture, but for your own data you may need to change these sizes so you may wish to review how to size an aperture . If you place the circles to any object you will take an output at the output box with values for Intensity and Magnitude (along with other parameters). The important one is the Magnitude (or Intensity which are equivalent). But in order to get an estimation we have to configure the Magnitude constant.

Now, locate all of the comparison stars you wish to use (see the table of comparison stars). Then open the Analysis>Aperture photometry tab (or select Analysis>Magnitude Constant if you have already activated the photometry tool - you should see the circles at the mouse point and a tick -activated- left to Aperture photometry option at the Analysis tab). Now, check your comparison star (i.e. eta Aur) and record the output in the spreadsheet or on paper to three decimal places. Repeat for as many check stars you want to use. Finally you select epsilon Aurigae to take an instrumental value of its magnitude.

Repeat the photometry steps for each image you have taken recording the instrumental magnitudes as you go.

Note:
As long as you are using the same lens and DSLR, you don't have to re-create a master-flat and master-bias for every observation session! You can save them for re-use and then skip the steps 3a and 3c.

You're done!

Now that you have obtained insturmental magnitudes, you need to compute calibrated magnitudes. The DSLR Documentation and Reduction team has setup an Excel Spreadsheet to help you finish your analysis.

MaximDL Beginner

After these steps, the instrumental magnitudes are entered into an spreadsheet which will yield calibrated magnitudes which can then submitted to AAVSO.

This tutorial was entirely written by Nick Long, an undergraduate at the University of Denver, whose contribution we greatly appreicate.

Performing Photometry with MaximDL

Step 1
Upload all DSLR raw or Jpeg files into a directory on your computer. Put your flat, dark, and bias frames into this directory as well. This can be done by using your camera’s software or by setting the camera as a USB drive. Copy the files over to the desired directory. It is recommended to name that folder with the date that the photos were taken.

Step 2
Before we begin further processing you must subtract dark, flat and bias frames from each individual image. Flat and bias frames may not be necessary but it is recommended to take dark frames. The amount of dark frames recommended is the square root of all the images to take.

Step3
To dark subtract all dark frames from each image first open calibration wizard under the process menu. This is the easiest method for dark subtraction. A calibration window will open that will ask you a series of question about the parameters of the dark subtraction. These include: If your camera regulates temperature, how the program will group images, and where your dark frames are located. Once you click finish you will see all images in that folder grouped. Remove any frame that is not a dark frame. Then click add group. Notice that under type you see whether it a dark, flat or bias frame. Once all dark frame have been added click ok.

Step4
Open every image that you wish for a dark subtraction to be performed on. Then go to process and press calibrate all. This automatically performs the subtraction and every image open. Your images should appear cleaner as a result of the subtraction.

Steps 5-10 are optional depending on whether you choose to stack images or not
Step 5
After Maxim DL is successfully installed, open the program on your computer.
If numerous images were taken, it can often be easier to average stack subsets of those images to ease the amount of work done later in a program such as Microsoft Excel. For example, if 50 images were taken, you could stack 10 frames into 5 sets producing 5 final frames.
On the top of the screen, go to process and then select Stack from the drop down menu.

Step 6
From the newly opened stack window, the Select tab should already be open. Click Add Files. Select the first subset of files that you want to stack by highlighting them. Next click on the first frame of the subset, hold ctrl + shift, and click the last frame of the subset. Next, click open to add the files to the stack process.

Step 7
Click the Align tab and under Mode, make sure Auto - star matching is selected. Click compute and wait several seconds for the software to align the selected images.

Step 8
Open the Combine tab, select Average under Combine Method, and select 16-bit Int under FITS Format. To stack the images, click Go. The time required to complete the stack will take several seconds. Once the stack is complete, the stacked image will appear in the background titled group1. Close the stack window and save this stacked image into the same directory as the original frames. Naming the file (date group ##-##), where ##-## is the range of camera image numbers selected in the stack.

Step 9
Repeat steps 2-5 for the remaining subsets of images until all desired images are stacked.

Step 10
Now each separate image or final stacked image is ready to be photometrically analyzed. Click File, select open in the drop down menu, and open up the directory in which the images are stored. Open every image that you wish to analyze. To do this, first select the first image then hold ctrl+shift and click on the last image highlighting every image. Click open to bring up each file.

Step 11
Click Analyze from the top menu and select Photometry from the drop down menu. This function allows the selection of any image opened in Maxim DL and the extraction of instrumental magnitudes. Select the first image that you would like to analyze.

Step 12
If you hover the cursor over an image, you will see three green rings surrounding your cursor. Before extracting data, you must adjust these rings to their maximum potential. Right click and a menu will appear which contains adjustment capabilities for each ring. Aperture refers to the inner circle that surrounds the star. Set the Aperture Radius so that you will be completely enclosing the full diameter of the stars that you will be analyzing. Gap width is the distance between the inner and outer ring. The annulus (outer ring) thickness should be set so that there are no stars on the ring. The annulus examines the star background and subtracts it from the star to achieve accurate photometry. Set it small enough so that no star appears in the rating, but large enough to give the background subtraction maximum information. Sometimes small stars can appear right next to your selected stars do your best to eliminate them from the parameters of your aperture and annulus. A small star may not make much of an impact on accuracy.

Step 13
From the information window instrumental magnitudes (magnitudes without consideration of an offset) can be extracted from the Magnitude value. Hover the cursor over your selected stars and center the star in the inner circle. Record this value from each star and then select your next image and record the instrumental Magnitude for the same stars.