From: Alberto Sadun

To: Dan Bruton

Here are the instructions for reducing already calibrated frames in MIRA. (I assume you already can calibrate, that is darkframe subtract and flatframe your images. If not, let me know and that will entail another discussion.) What we are doing here is aperture photometry. Once you get down the general procedure, it shouldn't take more than a minute or so per image. But, there is a learning curve, so at first it will take longer.

In this letter I'll just go over the basics so that you can get started right away. There are two additional things you may want to look into later, but I might as well mention them now. The first is that in MIRA, besides circular apertures, you may also do elliptical apertures. These take longer, and I do not recommend them for ordinary images. However, if you have large tracking errors and want high accuracy or are doing photometry on a galaxy core or an asteroid track or comet, you may want to use this.

The second thing is that you can make batch files. This is not too well described in the manual, and in fact I am not sure how to do it myself yet. I have assigned a trusted student of mine to look into it over the next couple of weeks. Such batch files would apply to any repetitive set of instructions, such as dark subtraction, flat fielding, and perhaps photometry of the same image. I think this will be very helpful to us later on, after we figure out how to do it, but for now I'll explain manual aperture photometry.

Once you have your image displayed, identify the field and if necessary, perform reorientations to get the image the way you want it. (Rotations and flips can be accomplished by going to "Math" and then to "Geometry" and from there to "Re-orient".) Now that you can identify the field and the comparison stars marked on your finding chart, and are ready to photometer, go to "Measure", and from there go to "Aperture Photometry". You should be getting a number of icons down the left side of the image. Click on "P" to locate the AGN, and P* for each comparison star. The program will display an inner circle which represents the area you want to photometer, and an annulus which represents the area where you want to sample the sky background. The size of the circle and annulus can be adjusted as will be explained later. The red arrow gets you back to the cursor setting. At this point you may want to enlarge one such marked star to see how the circle and annulus are positioned and centered, just for your own edification. As you go through this exercise, the data is automatically recorded in a box that later can be printed out or saved as a data file.

Anyway, for each comparison star (making sure of the order you want to do them as that will be how they are presented in the data file), you will be queried for the magnitude and weight. The magnitude should be taken from the literature I provided you. Make sure you are doing the correct filter. The weight should be the inverse of the square of the error (or standard deviation) as presented in the literature, that is 1/(2. When you are done with the photometry, print out the results in the usual manner.

This gets you all the appropriate fluxes and instrumental magnitudes. To get a zero point, and to get all the true magnitudes, click on the calculator button. This will make use of your inputs for the comparison stars. This brings up a box regarding the zero point, which you click, and then the data file displays new numbers. Print or save this, too. When you print things out, you may need to use landscape mode in printer setup to get in most of the important data. (All the calculations are made in the background, but if you want the theory of the statistical analysis, I can send you a few pages summarizing it.)

I will need the outputs for both the intermediate (that is, relative) calculation, and the final result. The reason is as follows. If we are to agree on the same set of calibration stars, or if one person only is trying to generate a light curve, then the final results are the appropriate ones to use. If, however, we are contributing the data to a database in which different observers use different stars or even different numbers of stars, or use different references for the comparison star values, then it would be better to send the data in terms of instrumental magnitudes only, and let the compiler of all the data handle the differences. We will probably be using both methods depending on the circumstance.

It is very important how you set the aperture sizes, using the icon that looks like a check and labeled "Options". The result is not sensitive to how you set the inner circle. Unless otherwise told (for Seyfert galaxies, one has to agree upon a common diameter, typically 10 arcsec), your circle size should encompass most of the star enclosed. If you want to be specific, you can make a radial profile plot, or click the FWHM button, and use the FWHM as the radius you want to use. If you use too small a circle, you do not get good enough statistics. If you use too large a circle, you get too much background, and degrade your statistics, but as I said, it isn't very sensitive, so you will do pretty well by using the FWHM as the rule of thumb.

You also want to set the annulus radii, and for this you want to be a little more careful. This is set such that you are clearly outside the area of the star (at least 3/2 FWHM radius), and make sure that you don't find that any dim star has slipped into the annulus. (The annuli may overlap, however, which is permissible.) We are sampling the sky value, and even a relatively dim star will artificially inflate this value. A larger annulus is better, but in a crowded field like BL Lac, this may not be possible. I like to keep the inner radius close to the outer one, perhaps separated by 1. The AGN and all comparison stars use the same aperture and annuli, so these precautions apply to all marked objects in the frame.

This should be enough to get you started. There may be some subtleties for particularly difficult objects and images, but what I discussed should cover 95% of the cases. Let me know if you have any further questions.

Take care and good luck,