Tomoe Note 007: Full reduction of 20160411 = April 11, 2016

Michael Richmond
Apr 28, 2017

Executive Summary

The dataset for 20160411

The dataset "20160411" is based on images acquired during UT 2016 Apr 11. The images cover the span of time

    2457490.00034  ≤   JD  ≤ 2457490.27145

which cover about 6.6 hours.

The raw Tomoe "composite" files have names that look like:

    chip 0:   TMPM0108830.fits      -     TMPM0110080.fits
    chip 1:   TMPM0108831.fits      -     TMPM0110081.fits
           ...

    chip 7:   TMPM0108837.fits      -     TMPM0110087.fits

I'll refer to each of these "composite" files as a "chunk". For example, chunk 108840 is the second set of 360 FITS images collected by chip 0. A reference like "chunk 10884x" will refer to the data collected by all 8 chips during the period of time covered by chunk 108840.

There are 126 chunks for each chip, (11008x - 10883x), and so 126*8 = 1008 chunks in total for the night. The total size of the raw Tomoe composite files for this dataset is 1.7 GB.

"Weather" plots for the 8 chips

After running the pipeline to reduce the data, clean the images, find and measure stars, calibrate them astrometrically and photometrically, I used a script to look at properties of the data over the course of the night. You can read more about the "weather" in another note.

Below are links to the graphs produced for each of the 8 chips.

• chip 0
• chip 1
• chip 2
• chip 3
• chip 4
• chip 5
• chip 6
• chip 7

Below is an animated GIF showing all 8 graphs in succession; it loops over them repeatedly.

What do we learn from the "weather" graphs?

The primary -- and good! -- conclusion is that the properties of images in all 8 chips vary together: when one reports better seeing, they all report better seeing, for example. That confirms that the hardware is working properly.

Let's go through the panels of these graphs, one by one, and comment on features of interest.

• number of images: a full "chunk" should consist of 360 images. There are some cases in which fewer than 360 images were produced by the camera (or perhaps fewer than 360 were saved to the composite FITS file).

  There are two times when just a single chunk has zero images; those may mark times when the telescope was moved, or the camera was briefly turned off, or ... ? Note that these cases appear in all 8 chips at the time times.

• zp mag: the "zero-point" shown here is related to the difference between the instrumental magnitudes of stars in each image, and the calibrated magnitudes of those stars:

  
  
                 zp  = (instrumental_mag) - (calibrated_mag)
  
       

  Large positive values indicate extinction due to the atmosphere or clouds.

  One can see that all the chips follow the same pattern: the zp decreases in the middle of the night, then increases near the end. No surprise.

  However, note that the "central" chips 1,2,3,4,5,6 all have slightly lower values than the "outside" chips 0,7. This suggests that there is a small amount of vignetting in the optics; the difference is about 0.2 to 0.3 magnitudes.

• sky value: this is simply the typical number of counts per pixel in the photometric annulus around stars. It is largest near the ends of the run, when the sky is bright. Makes sense.

  Note again evidence of vignetting in the "outside" chips 0,7: their sky values are somewhat smaller than those in the other chips.

• FWHM: the typical Full-Width at Half-Maximum for stars in each image, converted from pixels to arcseconds. We see similar patterns in all chips: the seeing is best in the middle of the night.
• number of stars: this is the total number of detections of stars in all the images for a given chip, during each chunk. In other words, if one bright star appears in all 360 images of that chunk, it will contribute 360 units toward this total.

  The number increases toward the middle of the night, reaches a peak, then decreases as the sky grows brighter and the seeing grows worse. Makes sense.

  Note that the peak of 80,000 detections per chunk corresponds roughly to (80,000 / 360) = 222 stars in each image. The area of each image is

  
  
                  2008 x 1140 pixels  =  1701 x 966  arcsec
  
                                      =  1.64 x 10^6  sq. arcsec
  
                                      =  456          sq. arcmin
   
                                      =    0.127      sq. degree
      

  The typical distance between stars is about 86 arcsec, which means that the fields are sparse, not crowded. Aperture photometry should be fine for most stars.

Other information from the night

I made a histogram showing the distribution of magnitudes for all detections during the night. This is not a count of unique stars: if a star appeared in 360 images, it will contribute "360 units" to the graph below. But the distribution of magnitudes is clear.

An independent method of determining the limiting magnitude for one chunk of data (in the transient-finding routine ) typically estimates a limiting magnitude of 14.5 to 16.0. That agrees pretty well with the data shown here.