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First Steps Into Variable Star Photometry


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First Steps Into Variable Star Photometry

 

 

 

 

 

 

 

 

Gary J Hawkins

 

 

 

Version 3

San Diego Astronomy Association

Outreach Special Interest Group

May 5th 2021

 

 

 

 

 

 

 

 

 

 

 

 

Acknowledgments

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The author would like to thank the following people – Dave Decker, Pat Boyce, Kevin Alton, Ken Menzies, and Dennis Conti.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

INTRODUCTION

 

 

 

 

 

 

 

Goals

 

This short white paper aims to demonstrate that it is possible for the average amateur astronomer who is engaging in digital astronomy to step into the world of photometry.  Why might you consider this?  Well, it opens up opportunities for advancing one’s knowledge in the hobby, as well as the potential to participate in valuable citizen science projects, such as the upcoming Exoplanet Watch[1] program. Participating in a course on exoplanet transit measurements hosted by the Boyce-Astro Foundation[2] started my interest in photometric analysis.  I wondered if my modest telescope setup could carry out such measurements. 

 

If you already practice astrophotography or Electronically Assisted Astronomy (EAA), transitioning to photometry is straightforward.  Why these disciplines?  Well, you already have most, if not all, of the required hardware, you are familiar with the use of astronomy software packages, and you have the prerequisites skills to pull the use of this hardware and software together.

What This White Paper Is Not

 

This white paper is not an introduction to photometry theory and the many different aspects of photometric analysis.  There are some excellent books and tutorials that cover these areas that I do not intend to replicate. Instead, it is a primer to help step into the world of photometry, start looking at light curves of variable stars, and see if this branch of the hobby is for you.  Where practical this primer does try to use photometric ‘best practices’ adopted by The American Association of Variable Star Observers (AAVSO).

 

Can you step directly into photometry without having done either astrophotography or EAA?  Of course, but the learning curve is going to be much steeper, and you will likely need a considerable upgrade of your hardware and software.  As such, this white paper is probably not the starting point you require.

Assumptions

 

The reader is assumed familiar with one of the many astro image capture packages such as NINA, Maxim DL, ASI Studio, etc. This paper discusses the use of SharpCap, which is widely used in the EAA community for real-time image capture and processing.

 

You will also need a software tool to undertake photometric processing.  There is a long list of tools that can be used. This text discusses the use of AstroImageJ (AIJ), a free photometry package developed initially for exoplanet transit analysis. The information provided in this paper is based on AIJ version 3.4.0.25.

 

 

 

Astronomy compels the soul to look upwards and leads us from this world to another. Plato

 

 


 

What is Photometry?

Put Simply

 

Photometry is the science of measuring the intensity of light.  In this case, we aim to measure the light coming from a celestial body, and most typically a star. When you capture a digital image of the sky, you capture the intensities of light of the targets within the Field of View (FOV) of your optical train assembly (OTA), otherwise known as the telescope.  These images can be captured with either a dedicated monochrome or color CCD or CMOS camera.  A DSLR or modified-DSLR can also be repurposed for photometric measurement.

 

Astrophotographers and those undertaking EAA likely have the required equipment to start on their photometric journey and are familiar with the software that captures astro images.  Of course, they might not be concerned about such issues as saturation, airmass, or scintillation, but more of that later.

Equipment

 

My equipment for EAA consists of the following – see Figure 1.

 

 

Figure 1

It is a modest setup consisting of a Celestron C8 SCT, Skywatcher EQ6-R Pro German equatorial mount (GEM), ZWO ASI533MC color imaging camera, homemade guide scope, manual 5-position filter wheel, Pegasus Power Box, and a TELRAD. This equipment connects via a USB hub to a Windows-based laptop on which resides the imaging and other software used for EAA.

 

Do you need this specific equipment? No, almost any astrophotography or EAA setup will do.  Is there any specific equipment required for photometry?  Yes, ideally, you should be capable of taking images of one to two minutes in length, so a guide scope is necessary unless your mount tracks exceptionally well. Secondly, a cooled camera allows for better precision when it comes to photometric measurements. Thirdly, if you have a monochrome camera (as you will do if you practice astrophotography) use this rather than a color one. An equatorial mount is preferable to an Alt/Az mount, and an equatorial fork mount is even better than a GEM, as meridian flips are taken out of the imaging equation.

 

But let me stress none of this extra equipment is necessary to start.  Initially, work within the constraints of the equipment you have to hand and determine if photometry is for you before spending money on new, shiny things!

Software

 

For EAA, I use the following suite of software – see Figure 2.

 

Figure 2

At the center of this suite of software is SharpCap, which is a widely used EAA live stacking standard for Windows-based systems. The highly-recommended Pro version includes a Sequencer, plus a whole host of other great features like polar alignment and focusing tools – all for just $15/year.  I usually run the latest Beta version of SharpCap Pro to provide feedback to the developer on features that are under development.

 

SharpCap connects directly to my camera and the mount via the open-source software EQMOD. ASCOM drivers are the glue to bring all the communications between the software packages together.  The free and popular software package, PHD2, is used for guiding.  I use the free Cartes du Ciel sky map package, but there is a wide choice.  Open Broadcast Software (OBS) Studio is a live streaming package used for the online presentation of EAA. It is not needed for photometry, but if you have aspirations about ‘going live’ or recording an evening session, OBS is a great tool to have.  You may not be familiar with ASTAP; it is one of four plate solvers that SharpCap can use for OTA alignment (typically, within one or two arc minutes).  If you do not use a plate solver with your telescope, you overlook a powerful tool that makes target acquisition a breeze!

 

Hardly any changes were required to my setup to start photometry.  On the hardware side, I added a ZWO UV/IR cut filter to the filter wheel.  The IR/UV filter constrains the ZWO ASI533MC spectral response (Figure 3) to 400 – 700 nm. This is required, as I intended to submit DSLR TG, TB, or TR coded results to the AAVSO (more on the meaning of these filter codes later).

 

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Figure 3

It was also necessary to add dew heater bands to the OTA and the guide scope.  While I had some minor dewing issues undertaking EAA, the extended duration of photometry runs meant the potential for ‘dewing’ was higher. Less than a $100 investment resolved the problem since I already had a Pegasus Power Box with dew heater ports mounted on the top rail of my OTA.  If you want an even more cost-effective solution, heater bands are a fun DIY project.

 

On the software side, the additional programs I chose for photometry were AIJ and Dimension4:

 

·        AIJ is for astronomical image analysis and precise photometry. AIJ provides an image display environment and tools for astronomy-specific image calibration, data reduction, time-series differential photometry, light curve detrending and fitting, and light curve plotting. AIJ reads and writes standard Flexible Image Transport System (FITS) files, provides FITS header viewing and editing, and is World Coordinate System (WCS) aware.

·        Dimension 4 is a Windows-based package that uses a low-level internet protocol, called SNTP, to connect with special-purpose Internet Time Servers that have been keeping the web on-time for decades. These Internet Time Servers send the correct time back to Dimension 4, which adjusts your computer’s clock to within a few milliseconds of the actual time.  Photometry requires knowledge of the precise time an image was recorded.

 

You’ll likely find the upgrade path to your equipment just as simple, whatever hardware and software you currently use for astrophotography or EAA. 


Your First Photometry Run

Pick A Star

 

Photometry consists of measuring the apparent brightness (more scientifically known as the flux) of a star.  But how do you know what star to observe?  Well, one great place to start is the Variable Star Index (VSX) hosted by the AAVSO.  You do not have to be a member to access this database or many other resources through the AAVSO website.  If you decide photometry is for you, membership has considerable benefits, including the ability to be assigned an AAVSO mentor.

 

The VSX database contains over two million known variable stars.  As with all the resources available at the AAVSO, there is a detailed manual, but frankly, it’s easier to select ‘Search’ and figure it out.  Initially, look for a target star that offers a reasonable degree of variability in a short period (instant gratification is a good motivator) – thus, eclipsing binaries or rapid pulsators offer ideal candidates.

 

I started with the variable star, V474 CAM. The AAVSO VSX database defines V474 CAM as an EW W Ursae Majoris-type eclipsing variable.  These normally have a period shorter than one day. The depths of the primary and secondary minima are almost equal or differ insignificantly. Light amplitude changes are generally less than 0.8 magnitudes.

 

The period of V474 CAM is 7.88 hours, making it possible to capture a complete light-curve in a single evening of observing.  I picked this star because it tracks in an area of the sky readily observable for long periods from my home during the winter months. On an observing night, I like to check my target star’s position using the Isaac Newton Group of Telescopes, Object Visibility tool, STARALT[3].  Enter your observatory location[4], your time zone, and target ID. The plot below is for V474 CAM imaged on 16th March 2021 – see Figure 4.  As you can see at twilight, the target is already high in the sky, and by 1:30 am local time, it has dropped below 30 degrees altitude (do not image below this).  Thus, on this day the ideal imaging run is limited to about 5 ½ hours.

 

 

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Figure 4

The AAVSO provides finder charts in its ‘Pick A Star’ section.  The finder chart below (Figure 5) was obtained for V474 CAM. The crosshairs in the center of the chart mark the target - the angular size was chosen to match the FOV of my telescope.  The one star marked ‘96’ is an AAVSO recommended comparison (comp) star with magnitude 9.6[5]. If at all possible, use AAVSO comp star recommendations. Requesting the output in the form of a “photometry table” will give you the position of each comp star along with its magnitude and the magnitude error in each bandpass.

Figure 5

The finder chart has two uses.  Firstly, it is reassuring to compare your target image with a known source to make sure you are imaging the correct target.  And secondly, it helps identify other comp stars of similar magnitude (their dots are of comparable size). The online sky map Aladin 10 Lite[6] can then be used to see which of these stars has the closest color match to your target star.  Note your comp stars on the finder chart for future reference.

Image Acquisition

 

Having identified your target and comp stars, you can start imaging. This may start before twilight with the collection of calibration frames: Darks, Flats, and possibly Bias.  You should be familiar with doing this if you practice EAA or astrophotography, but if not, then take a break from reading this paper and do a little research.  There is no great mystery; these are simply image frames taken in a specific way to allow you to compensate for optical errors and noise that will be present in your image or ‘science’ files due to imperfections in your optical chain.  SharpCap will walk you through collecting calibration frames and generating the Master Darks and Flats necessary for photometry.  I have a library of calibration files, so unless I need new ones, my imaging starts after dark.

 

I use two sequences I’ve developed in the SharpCap Pro Sequencer (still in development at the time of writing) to guide me through the image capture process. The first sequence, which is unimaginatively called ‘EAA Startup,’ consists of the following ordered activities:

 

1.      Connect all hardware.

2.     Check counterbalance weights are in place – it is easy with the design of the EQ6-R mount to forget to add the counterbalance weights.

3.     Ensure the computer has connected to WiFi.

4.    Check the computer has the correct time. Dimension4 will ensure accuracies to milliseconds, but you want to make sure you also have the correct time-zone set.

5.     Switch on the mount.

6.    Check the Windows Device Manager, under Ports (Comm & LPT), to determine what communications port the mount (in my case identified as Prolific USB) is connected to.

7.     Ensure the same port is specified in EQMOD Toolbox.

8.    Run Cartes du Ciel (CdC) and connect it to the telescope – this will open the mount control software, EQMOD. Unpark the mount in EQMOD and select sidereal tracking.

9.    Check the CdC Observatory and Lat/Long are correctly specified.

10.                        Check the EQMOD Lat/Long are correctly specified. Force Epoch J2000, as all my other programs, use J2000 referenced coordinates to specify the target RA/Dec.

11.  Run the PHD2 guiding program and connect the guide scope camera. Initiate image looping ready for guiding.

12.  Open the “ZWO ASI533MC” in SharpCap.

 

Your startup sequence may not be identical to mine, but it will likely be similar. 

 

As the Sequencer within SharpCap disables the User Interface (UI), I now have a few manual things to do using the UI.  These are:

 

·        Undertake focusing using the Focusing Assistant in SharpCap [Tools, Focusing assistant]. For photometry, the FWHM routine is more than adequate - you might even have to defocus a little for bright targets.

·        Undertake polar alignment using the SharpCap Polar Alignment routine [Tools, Polar Align].

 

I now start my second sequence, ‘Image VARIABLE Star.’ This sequence takes me through the following ordered steps:

 

1.      Slew the mount to the target star.

2.     Load the camera profile, which sets specific values for SharpCap, including setting the Color Space to RAW 16, setting the save file format to FITS, and selecting 1x1 binning.

3.     Set the camera Gain = 550, exposure =10 sec, and plate solve to place my target in the center of the image.

4.    Set Gain = 200, and exposure = 60 sec, which more closely matches what I can expect to use for the photometry image collection of stars around 11 - 12th magnitude.

5.     Start PHD2 guiding, and ensure the guider is stable.

 

Now, I’m ready to make final adjustments before beginning data collection.  The first thing is to make any movement of the image necessary to ensure the target star and chosen comp stars are centered in the image.  This will ensure they do not drift out of the FOV over the extended imaging run ahead[7].

 

Next, I need to ensure the target star and comp stars are not SATURATED, and I’m operating in the LINEAR region of the ZWO ASI533MC sensor; this is below 50,000 ADUs[8].  Also, since the target star is variable, I must provide some overhead in case the target star flux increases over the measurement run. I, therefore, choose my exposure (typically starting at 60 seconds) and camera gain[9] to ensure SharpCap is not reporting a peak of more than 30,000 for the target star, and more than 45,000 for any comp star. How do I determine this? I take a Snapshot image in SharpCap and open the FITS image in AIJ [File, Open]. Mouse over the brightest of the target or comp stars and check the peak value (reported towards the top right-hand side of the window) is below 30,000 or 45,000, respectively.  If it is not, I reduce the exposure, take another Snapshot, and reexamine the brightness of the ‘overexposed’ star.  I continue until the brightness is at or below the required level.  It may be necessary to change camera gain to achieve this within the exposure range of my Dark library – 32 to 88 seconds.

 

I do not use exposures below 32 seconds to ensure the effects of atmospheric SCINTILLATION are minimized.  If this cannot be achieved at the selected camera gain, I lower the camera gain to the next lowest gain for which I have Dark frames. Defocusing the telescope a little can spread the star’s light function and therefore also be used to reduce its peak value.  Do not defocus to the point where the star becomes a doughnut.  Since, I’m using a Dark library to avoid having to generate Darks on the night, I typically select target stars between 9th to 14th magnitude as these match the range of this library.

 

The precise numbers you use for your system may be different from those described for mine.  The maximum output value and linearity value for your camera can be found in its specifications sheet.  If the specifications do not provide the linearity value, you can either make an assumption (like 80% of the maximum value) or measure it.  A measurement procedure is described in the book ‘The Sky is Your Laboratory’ listed in the Recommended Reading section.

 

With the exposure and gain correctly set, I start collecting science images. If my exposure is shorter than 60 seconds, I set the collection frame rate to one per minute. If using a GEM and a meridian flip is required, store the images after the meridian flip in a separate folder as you will be analyzing this data as if it were a separate run.

 

The images captured by my ZWO ASI533MC are color and have a resolution of 3008 x 3008 pixels at 1x1 binning with an RGGB arrangement. They are stored in a FITS format, and each image is about 17MB in size.  As such, I make sure I have the disk space available before a long imaging run.

 

If I am not going to be with the telescope, I use OBS Studio to stream my laptop desktop to YouTube so I can monitor progress on a tablet or similar device while inside in the warm.  I ensure the desktop has both the SharpCap and the PHD2 guiding display visible.


 

Preprocessing

Visual Inspection

 

You have now collected your raw images that you’re going to use for the photometric analysis.  You can be confident that the target and comp stars are not saturated, but the data may have been corrupted by the passage of high clouds, etc.  Thus, after you move your raw images to a ‘Science’ folder, create two more folders, one called ‘Analysis’ and one called ‘Quarantined.’ Copy all your files in the Science folder to your Analysis folder.  Never touch or process the files in the Science folder.  These are for the time when you accidentally corrupt or delete one or more of the files in the Analysis folder, and you need to start the processing again.

 

I use another home-grown SharpCap sequence call ‘Generate Light Curve’ relating to ordering events in the preprocessing and analysis process - but a list written on paper or in Excel is just as applicable. I strongly suggest using a list of some sort to ensure you do the same photometric analysis steps in the correct order every time.  Modify the list as you optimize your photometric processing.

 

Open AIJ, check for any updates [Help, Update AstroImageJ].  Next, import the image files from the Analysis folder [File, Import, Image Sequence].  You only need to select the first image, others of the same type will be imported, and a dialogue box will confirm the number of imported images. An image window will appear with a slider towards the bottom.  Moving this slider left or right allows visual inspection of all the images in the run.  You will quickly spot any that look suspect.  For example, you might see clouds in the image, or the histogram below the slider may broaden significantly with the presence of unseen high clouds.  If a tree has temporarily obstructed your line of sight, then the stars will fade or disappear.  Note the sequence numbers of the poor images and move them from the Analysis to the Quarantine folder. Close AIJ.

Preprocessing in ASTAP

 

I now use ASTAP to continue preprocessing the data. This package has some excellent batch processing procedures under Tools.  I use ASTAP to undertake two activities:

 

1.      Plate solve the images.  Plate solving will add WCS data to the FITS headers of all the image files, allowing the position of stars and photometry apertures to be identified or specified using RA/Dec coordinates.

2.     Since I’m using a color CMOS camera, I extract one or more of the Bayer matrix pixels to produce Blue, Green, and/or Red raw images. Processed results from these files would be identified as DSLR Blue [TB], DSLR Green [TG], and DSLR Red [TR], respectively, when uploaded in an AAVSO observation report[10]. The resulting image size is 1504 x 1504 pixels.

 

If you are using a monochrome CCD/CMOS camera with a similar sensor size, 2x2 binning might be appropriate to reduce your working files to a manageable size. The processed results would be identified as CCD Clear (unfiltered) reduced to V [CV] or Clear (unfiltered) reduced to R [CR] within the AAVSO reporting upload.


 

Photometric Analysis with AIJ

 

You are now on the home stretch.  Raw images have been collected, bad images quarantined, and preprocessing is complete.  Now, it is time to analyze the data and produce some light curves.

 

Start by cleaning up any unwanted files in the Analysis folder.  Next, import your preprocessed images in AIJ [File, Import, Image Sequence]. The first image appears.  Mouse over the brightest of your target or comp stars in the image window and left-click on the mouse.  Then select [Analyze, Plot Seeing profile …], and a graph will be displayed relating to SOURCE and BACKGROUND aperture settings.  Click [Save Aperture] at the bottom of the screen to select the recommended apertures.  The photometric aperture size to be used for the analysis of this batch of data is now set. Close the first image window.

 

Now, start the CCD Data Processor in AIJ - Figure 6.

 

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Figure 6

Two windows appear - the ‘DP Coordinate Converter’ window and the ‘CCD DP Processor’ window. In the ‘DP Coordinate Converter’ window, enter the target name, and AJI will attempt to get the relevant information from the SIMBAD database. If the target star cannot be found in the database, an error message will be displayed[11].

 

Next, enter the geographic location and elevation of the observatory where the measurements were taken.  With this done, close this window, which will leave just the ‘CCD Data Processor’ window open - Figure 7. 

 

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Figure 7

At the top of this window, select the folder where the aligned images are stored. Then select the first file, modify the file name with a wildcard character to ensure the CCD Data Processor sees all your preprocessed image files.

 

If you did not calibrate your images on-the-fly when capturing your raw images, then specify the location of the Dark, Flat, and Bias[12] frames so that AIJ can calibrate the images now.  Check the ‘Save Calibrated Images’ checkbox.  The default location to save the calibrated images is a subfolder called ‘pipelineout.’ If you calibrated your images on-the-fly when capturing your raw images, leave the ‘Save Calibrated Images’ checkbox unchecked.

 

Leave the remainder of the checkboxes set as shown, and click START. The first image in the sequence will be displayed, along with the ‘Multi-Aperture Measurements’ window.

 

We are now going to define our target and comp stars for the photometric analysis process.  I make it a habit to select five comp stars in addition to my target star simply because I can use the same template when it comes to plotting the light curves.  You should already have a good idea of several comp stars from looking at the AAVSO finder chart and Aladin 10.  The additional ones should be selected based on having a magnitude close to or less than the target star.  Don’t stress about choosing these extra comp stars; a couple of random choices will not harm you.  Ensure the following checkboxes are selected, see Figure 8.

 

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Figure 8

Click ‘Place Aperture’ at the bottom. The first aperture is ALWAYS placed over the target star. Mouse over this star (with the aperture target) and left-click the mouse.  The aperture will locate over the target, and it will be labeled T1.  Next, select your first comp star. Mouse over this star with the aperture target and left-click the mouse. The aperture is located, and the comp star is marked C2.  A ‘Magnitude Entry’ box will appear for C2.  I use the APASS database available through the AAVSO[13] to get the appropriate color magnitude by specifying the comp stars RA/Dec, and searching in a 0.01 degree window.  This usually returns just one or two results and its generally obvious which one is the comp star if there’s multiple returns.  If you are using Green, Blue or Red data, select the Johnson V, Johnson B, or Cousins R magnitude respectively. Since Johnson V data is always available, I typically process the Green channel data.  If you are using a full spectrum image from a monochrome camera, then select the Johnson V magnitude for the star.  Continue identifying the rest of your comp stars, and then save a .PNG file of your target and comp star selections with ‘File, Save image display as PNG…’

 

Next, press ‘RETURN’. All sorts of Windows will appear on your computer screen, and your computer may start beeping as it starts photometric processing.

 

Manipulating the many windows that result from analyzing your data in AIJ can take some practice.  I recommend using AIJ with two monitors, the second working in Extended Mode.  Place the ‘Plot of Measurements’ window, the ‘Multi-plot Y-data’ window, and the ‘Multi-plot Reference Star Settings’ on the Extended Screen as shown in Figure 9.  For variable star analysis, you need nothing to the right of the ‘Bin Size’ on the ‘Multi-plot Y-data’ window, so size this window accordingly.

 

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Figure 9

Keep the ‘Multi-plot Main’ and ‘Measurements’ windows on your primary screen.  

 

You use the windows as described below while viewing the results in the ‘Plot of Measurements’ window:

 

·        The ‘Multi-plot Main’ window is used to select your default x-data (which can be anyone of the date formats, I typically use J.D.-2400000), and define your plot Title, Subtitle, Labels, Legend position, and Scaling.  Make sure ‘Unphased’ is checked in the Phase Folding section.  If you need to redraw the plot, left-click ‘Redraw Plot’ in the bottom right-hand corner.

·        The ‘Multi-plot Y-data’ window is used to select what is shown on the ’Plot of Measurements’ window.  You’ll usually want to plot the normalized relative flux of the target star (rel_flux_T1) and the comp stars’ normalized relative flux (rel_flux_Cn)[14].

·        The ‘Multi-plot Reference Star Settings’ window is used to determine what comp star measurements calibrate the target star measurements.  Comp stars that have saturated or that have measurement values going into the non-linear region of your imaging sensor will display red or orange, respectively[15]. Uncheck any red comp stars and suspect any orange ones. Green comp stars are good.  As your selected comp stars should be of constant magnitude and ideally a similar color to your target star, any flux variation of a comp star can be considered as the effect of a systematic error that will also affect the target star.  Thus, if good quality comp stars are used to calibrate the target star measurements, the error in the calculated target star flux will be minimized.  The good comp stars can be identified in two ways.  Firstly, their normalized relative flux plot will vary within tight bounds and will be well correlated to other good quality comp star flux curves.  Secondly, when individually selected (using the ‘Cycle Individual Stars’ function), the normalized relative flux curve of the target star will remain smooth and of the anticipated shape.  If the selection of an individual comp star results in a ragged or misshaped target flux curve, this comp star is suspect and should be disregarded.  Make a note of what you think is the best comp star, and ultimately select (via their associated checkboxes) what you believe to be the good comp stars to generate the final light curve of the target star and the final measurement data in the ‘Measurements’ window.

 

Save the measurements using the [File, Save As function] in the ‘Measurements’ window.  You will later open this data in an Excel spreadsheet and extract the required information for your AAVSO observation report.  Also, save the ‘Plot of Measurements’ (this will save a as .PNG file)


Submitting Your Data to the AAVSO

 

Having taken all this care and time to collect and analyze the light curve for your target star, it makes sense to make that data available to the rest of the scientific community.  Even though you have not used photometric filters, if you are using a color/monochrome CMOS or DSLR camera, your analysis has been rigorous enough that your data is valuable, and the AAVSO acknowledges this by allowing data submission under the:

 

·        CCD CV, and CR filter categories for a monochrome CCD or CMOS camera.

·        DSLR TG, TB, and TR filter categories for a CMOS color or DSLR camera.

 

All the data you need to submit in the AAVSO observation report is in your notes and the Measurements file you saved at the end of the photometric processing.  The required format of the file submitted to the AAVSO consists of two sections - a header section and a results section. The way I generate the AAVSO data file is with the Excel spreadsheet shown in Figure 10.

 

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Figure 10

In the header, the only update you will require is adding your AAVSO observer code in place of mine.

 

In the data section (line 8 on), you need to do the following:

 

·        Cut and paste the HJD_UTC data from the measurement file opened in Excel to the AAVSO template column B, starting at line 8.

·        Do the same for the:

o   Source_AMag_T1 that is copied to column C.

o   Source_AMag_Err_T1 that is copied to column D.

o   AIRMASS that is copied to column L.

·        Add the target name in column A, cell 8. Copy this down column A to match how far the HJD_UTC information extends down the spreadsheet.

·        Do the same for:

o   The filter ID that is added to column E;

o   ‘NO’ that is added to column F.

o   ‘STD’ that is added to column G.

o   The ID of the best comp star that is added to column H.

o   The magnitude of the best comp star derived from APASS that is added to column I.

o   If available, add the name of the 2nd best comp star in column J.  If you do not have a second-best comp star, put ‘na’ in this column.

o   If available, add the magnitude of this 2nd best comp star you derived from APASS in column K. If you do not have a second-best comp star, put ‘na’ in this column.

o   ‘na’ that is added in column M.

o   The name of the AAVSO finder chart you created in column N.

o   Add any comments you have in column O. Comments could include weather conditions, the presence of clouds, etc.  DO NOT use punctuation in these comments; it may cause the report to be rejected when uploading to the AAVSO.

 

It would be more elegant to do the above with an Excel template and associated macro, but I’ve neither found one or developed one. With a little bit of practice, this manual cut and paste process can be done quickly.

 

Save the complete AAVSO template as a .CSV file.  Then open the .CSV file with Notepad.  In the header section, carefully remove all but one of the commas, so your file looks similar to Figure 11.

 

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Figure 11

Using [File, Save As…], change the name of the file, and save it as a .TXT file.

 

Now open the AAVSO website, log in, and click on the ‘Submit and Access Data’ section. Click on ‘Upload Photometry’ and then select ‘Observation Files’ under ‘Submit’. Choose your saved .TXT file and upload it.  If the data is correctly formatted, the upload will be accepted by the AAVSO, and you have taken your first steps to becoming a CITIZEN SCIENTIST!

 

Hopefully, this is the first of many variable star data uploads you do to the AAVSO.

 

 


 

 

Recommended Reading List

 

1.      AstroImageJ 2.4.1 User Guide plus Getting Started with Differential Photometry.

2.     AAVSO Manual for Visual Observing of Variable Stars.

3.     The AAVSO Guide to CCD Photometry.

4.    The AAVSO DSLR Observing Manual.

5.     A Guide to AstroImageJ Differential Photometry – British Astronomical Society.

6.    The Sky Is Your Laboratory – Advanced Astronomy Projects for Amateurs by Robert K. Buchheim, published by Springer.

 

 

 


 

 

 

 

 

 

 

 

 



[1] https://exoplanets.nasa.gov/exoplanet-watch/about-exoplanet-watch/overview/

[2] http://boyce-astro.org/

[3] http://catserver.ing.iac.es/staralt/index.php.

[4] Note that the observatory Longitude is specified in East coordinates.

[5] The decimal place is dropped to avoid confusion with it being a star.

[6] https://aladin.u-strasbg.fr/

[7] Even with guiding, the image will drift slightly over the course of the night, so any required star close to the edge of the image is at risk of being lost.

[8] ADU – Analog-to-digital unit.

[9] This will correspond to one of the exposure/gains used for the Dark files in my Dark library.  My Dark library has Dark frames exposed at 32, 36, 40, 44, 49, 54, 60, 66, 73, 80, and 88 seconds, with gains of 25, 50, 100, 200, and 400 respectively.  This may seem excessive. but all these Dark frames can be collected in a few hours work, and it allows me to select an exposure/gain for which I already have a Dark frame.

[10] AASVO filter definitions can be found here, https://www.aavso.org/filters. RAW images collected using a monochrome CCD or CMOS camera used in conjunction with a photometric filter, has its spectral characteristics defined by the filter.  These will typically be Johnson and Cousins, or Sloan filters. Such filters are expensive, and if the reader does not have these to hand, it’s recommended they experiment with no filters to determine if this is a path of interest before spending considerable sums of money on dedicated photometric filters.

[11] If an error is displayed, then the target information will have to be manually entered in this window.

[12] Sometimes Flats and Bias are combined, as is the case if collected with SharpCap.

[13] https://www.aavso.org/download-apass-data

[14] Note – if you only have one comp star selected in the Multi-plot Reference Star Settings this comp star does not get plotted. 

[15] These values are set from the ‘CCD Data Processor’ window, Click ‘Set with the circle above,’ and the ‘Aperture Photometry Settings’ window appears. Saturation and Linearity warning limts are defined at the bottom.  Also, input CCD gain, CCD readout noise and CCD dark current per sec at the camera gain you used to take the images.


  • AaronF likes this


4 Comments

This is truly an excellent getting started paper. I can't tell you how much this will help me as I have been wanting to start photometry myself but find myself floundering around a little. A start to finish guide like this is perfect! Thank you so much for all the effort you put into this.

 

cs...John

    • garyhawkins likes this
Photo
garyhawkins
Jun 01 2021 06:34 PM

John,

 

I very much appreciate your feedback.  If I can be of any help as you start photometry, please reach out.  Happy to help.

 

Gary

Nice manual. Well done !

 

I am using aij also. Just one remark : i had some strange behaviour with the "remove stars from background". The flux for some comparison stars jumped between two lines. Results were much better when this tickbox was off.

 

Question: do you extract the green channel after calibration ? The workflow is not exactly clear to me.

Photo
garyhawkins
Jun 03 2021 09:08 AM

Thank you.

 

I've not experimented with the 'remove stars from background' option, but I will now you've brought it to my attention.  

 

I do my Dark and Flat frame calibration on-the-fly in Sharpcap, so the green channel is extracted after calibration.  Even if you do the raw image capture, and then do the Flat/Dark frame calibration in post, you would still do this before channel extraction.  

 

Please let me know if you have further questions on the workflow.

 

CS Gary

 

 

Nice manual. Well done !

 

I am using aij also. Just one remark : i had some strange behaviour with the "remove stars from background". The flux for some comparison stars jumped between two lines. Results were much better when this tickbox was off.

 

Question: do you extract the green channel after calibration ? The workflow is not exactly clear to me.



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