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How to determine the normalization value in IRIS software?

Helix9
Affiliation
American Association of Variable Star Observers (AAVSO)
Sun, 06/20/2021 - 16:28

Hi! 

I'm following a beginner tutorial for IRIS from the AAVSO site (https://www.aavso.org/sites/default/files//IRIS_Beginner_1.pdf). I've come to the part of making the master flat field. One of the input fields is "Normalization value". Since there's no deeper explanation about it in the beginner's guide, I went to the official Iris website where it says that normalization value is "the median value of all green pixels is of 20000 ADU in this example - a typical good choice". But it doesn't say is it the median value of the flat field's green pixels or of the star field ones. Also, it doesn't say how could you determine or calculate the normalization value.

Does anyone have any advice regarding this problem?

Any help is welcome! :-)

 

Thanks in advance!

arx
Affiliation
Variable Stars South (VSS)
Sun, 06/27/2021 - 10:19
Normalization value for master flat in IRIS

Dino, I have found an AAVSO document on my computer titled "AAVSO DSLR photometry. Data analysis beginner - IRIS". The document states that it was submitted by bkloppenborg on 10 February 2010.

The section relevant to your question states, for the creation of a master flat:

... set the normalization value to 1000.  After you create the image you should check that the maximum output value of the STAT command (i.e. type STAT into the command window).  This value should be less than 32767.  If it is not, modify the normalization value and repeat until the maximum output value is under 32767.  Before saving the flat, type the command "grey_flat" in the command window to normalize the CFA-flat and then save the flat field to a file called "master-flat". (End of Quote)

I experimented with this with some DSLR flats I'd previously created, and found that a high normalization value (say 20,000) yielded a maximum output value after the STAT command of 32767. A much lower normalization value (say 1,000) yielded a maximum output after STAT of well below 32767.

I don't use IRIS for photometry, but it is curious to me that the minimum, mean and maximum values in the master flat produced as described above can be counts of hundreds or thousands. A flat produced in AIP4Win for photometry of DSLR images will have a minimum between zero and 1.0, a mean of about 1.0, and a maximum between 1.0 and 2.0.

Hope this helps.

Roy