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Maxim DL photometry

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Maxim DL photometry

I am trying to figure out a couple of things in Maxim DL Pro 6 and the online manual does not have this information.  How do I determine the apparent and absolute magnitude of our target and check stars?  I used the photometry tool and saw that it had "magnitude" listed as a measured parameter, however it is unclear whether this is apparent, instrumental, or standard magnitude?

Also I realize that there is a parameter called "transformation" that is needed in order to determine the absolute magnitude, but how exactly do I find the variables necessary in order to calculate this and how is this term calculated?  Lastly what is the difference between standard and absolute magnitude, it would appear as though they are identical and just have different names.

Any help is greatly appreciated!

tcalderw's picture

I think you have some terms mixed-up.  Absolute magnitude is a value that can only be established with knowledge of a star's distance.  All the magnitudes AAVSO observers report are apparent magnitudes.  A standard magnitude comes from transforming an instrumental magnitude (peculiar to your telescope/camera) to a standardized system, like Johnson or Sloan.




TRE's picture
Hi Melanie

Hi Melanie

Dig into Tom's notes here. Take the CHOICE courses that you see near the top of the home page. Take the CCD or DSLR course, the VPHOT course, and a TG/TA course if they ever offer it. 

Start with just apparent (differential) magnitude with your MaxIM software. Differential is just a measure of apparent magnitude that is made by comparing a comparison star to your variable star. The comparison stars are found on the Variable Star Plotter (VSP).  It's in the Observing menue on the AAVSO home page.  Use the photometry plot to find the names of comparison stars that you may need to input to photometry software.

Figure out how to make MaxIm work. Submit it with the AAVSO WEBOBS. Use the AAVSO Light curve Generator (LCG) to compare your observations with others for a popular star like SS Cyg or TT Ari. You may gain some confidence that you are doing photometry just like the rest of us.

That is the route I took, and it took me a few years. It takes some perserverance to get all the tools and software lined up and working. Keep asking questions. 


hjw's picture
Maxim DL

Hi Melanie,

I know what your going through. I'm for the most part self taught in astronomy since I built my first scope at the age of 12. Ditto on what Ray said. Take some of those CHOICE courses, CCD, Exoplanet,VPHOT, and TG/TA. They each have something to offer to broaden your understanding of Photometry. You will also meet alot of people in the same boat as you and get to ask some specific questions as you go. These folks are a wealth of information! When you,ve done that I can help you through some of Maxim DL if you'd like as I use it quite abit as well as VPHOT. There are also some You tube video's on using MaximDL you can watch also. What equipment are you currently using?

John HJW

HQA's picture
MaximDL photometry

Hi Melanie,

Congratulations on wanting to use MaximDL to do photometry.  Lots of the AAVSO users have MaximDL, and it does a fine job of measuring the brightness of stars in a CCD image.  However, like many complex programs with different target audiences, it can sometimes be confusing to use.

If you just point-and-click using the Information window, it uses a predefined measurement circle, counts up all of the pixel values inside of that circle, and reports the total counts and a "magnitude", which is just -2.5 log (counts).  This is an instrumental magnitude.

MaximDL has the option of generating an AAVSO Extended Format Report.  For submission to the AAVSO, you need to convert the instrumental magnitude into a useful value, as a raw instrumental value from you would be different from the raw instrumental value from someone else with a different telescope.  To do this, you need to compare your target magnitude against a "comparison star" in the same field of view.  The comparison star is one that has been chosen (usually by the AAVSO) to be constant and that has a known "standard" magnitude.  The formula for this is

Target_magnitude = (Target_instrumental - Comparison_instrumental) + Comparison_magnitude

Where "Comparison_magnitude" is the value found on an AAVSO variable star chart (see "Variable Star Plotter" on the main pulldown menus).  A magnitude calculated in this fashion is called a "standardized magnitude".

The concept of transformation is a step farther down the path, where you realize that your filter is not quite the same as those used to create the standard Johnson/Cousins system, and so you transform your results onto the standard system using coefficients determined by imaging a standard cluster.  I wouldn't worry about this right now.  And, as reported earlier, absolute magnitude is a special term reserved by the astronomical community and doesn't come into play here.  Don't you love terminology?

The best starting point is the AAVSO CCD Photometry Manual:

which I recommend highly.  It might be a good read before even looking at the MaximDL photometry chapter, as it gives background on what you are trying to do and why the AAVSO requires submissions in the format that it does.

In general, there are no bad or stupid questions.  Feel free to ask whatever you need to make things clear.  Good luck - it is a lot of fun to watch your selected variable star change brightness!


HBB's picture
MaximDL Photometry

Welcome to photometry Melanie!

Everyone has given you some great advice.  I strongly suggest the CHOICE courses, but since they are only offered a few times a year I would suggest signing up for an AAVSO mentor.  The mentoring program is a great way to get started with immediate help right at your fingertips. You can read about it here: or send an email to


GFB's picture
MaxIm Photometry

Hello Melanie,

I found MaxIm's photometry tool to be limited, tho the photometry it produces is good.  The steps I would suggest are:

Find and print the VSP chart for you target field.  It will identify the approved comp stars for you and the photometry table that is attached(if you choose to print it) will give you the standard magnitudes for those comp stars in the various band passes.


Open your image(should be a calibrated image), locate on your image the comps stars on printed chart.  Click on some of the comp stars one at a time and enter the standard magnitude in the dialog box for you filter band, such as the V magnitude.  The dialog will ask you to identify what type of object it is comp or target.


Doing this tells MaxIm what the standard mags are, you can then run the photometry to calculate the target's mag.  As Arne says, it will also produce the AAVSO report.


Unfortunately, when I last used MaxIm for photometry, none of the work is savable; unless others here have found a way.  So you have to reenter it all again if you do the target a second time.  There are other packages out there that have more complete approaches for our work.


The comments above about CHOICE courses is also a great idea.  The steps above might get you started and add to your understanding taking the course.



Thank you so much to everyone

Thank you so much to everyone for your advice and help!

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