UT Aug 01, 2023: Color terms from M11

Michael Richmond
Aug 3, 2023

On the night of Jul 31/Aug 01, 2023, under good conditions, I acquired images of the open cluster Messier 11 in order to determine color terms in our system.

The results were roughly consistent with earlier attempts using Landolt equatorial fields, but contained more stars. I'll adopt these values, for now, but will do more tests with M67 and NGC 7790 over the next few months.

Spoiler: the results. Given notation where capital letters denote catalog values (B, V) and lower-case letters denote instrumental magnitudes (b, v), and the formula

 
     (B - b)  =  delta  +  c*(b-v)

my results on M11 this night, using the 12-inch telescope and ASI CMOS camera, yield the following color terms c.

                                                                Number of 
  filter        equation            color term c  +/-           stars used
---------------------------------------------------------------------------
  B      (B-b) = delta + c*(b-v)           0.218  +/-  0.025       43

  V      (V-v) = delta + c*(v-r)           0.067  +/-  0.027       40

  R      (R-r) = delta + c*(v-r)          -0.362  +/-  0.035       34

  I      (I-i) = delta + c*(r-i)           0.038  +/-  0.031       15
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Color terms using M11

I chose as my target the cluster Messier 11, as the AAVSO's list of standard fields includes a rich set of measurements in BVRI, containing over 400 stars.

The setup was

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• ASI 6200MM CMOS camera running at T = -16 C
• BVRI filters, 30-second exposures
• images binned 4x4
• guiding in V, R, I, but not B (guide star too faint)
• focus positions:
  • V: 0.347 yields FWHM 2.0 at T = 67/19
  • B: 0.366 yields FWHM 3.2 at T = ??/?? (between 19 and 16 C)

Notes from the night:

• sky was clear: only a few clouds near horizon at sunset, none later on
• nearly full Moon was in the trees until the end of my run
• for successful communication between dome computer and 12-inch telescope, use ASCOM.MeadeGeneric.Telescope device and port COM6. The ASCOM Device Hub is a good tool to check the connection.
• guide camera information:
  • best focus was at fine focus = 4.5
  • guider calibration completely without error
  • X-Speed -40, Y-Speed -35, angle = 3 degrees
  • Binning 2 (yielding guide image 480 x 640)
  • Settings: Rotate 90 CCW only
  • don't forget to set the camera's slew speed to 1 before starting to guide!
• I rotated the guide camera in the off-axis housing so that the orientation of cardinal directions lined up with rows and columns. North up, East to right on the MaximDL guide camera display. See photos below. Basically, line up the "R" in "RoHS" with the brass set screw.

  

  

In general, I took sets of 30-second images for each passband (BVRI) in each field: thirty each in BRI, and twenty in V. After cleaning them all in the usual manner, I co-added all the images in each field to yield a single, deep image in B, V, R, I, then measured aperture photometry for that deep image.

The difficult part of the calibration was caused by the very rich nature of the data: there were hundreds of calibration stars, all close together in a single field. Many of the stars were so close together that they contaminated each other's aperture photometry. After lots of work, I found that a good approach was to choose which stars to include in the calibration by visually inspecting each one at high magnification, both image and radial profile. If any neighbor's light was significant at less than 8 pixels (10 arcsec) away from a candidate star, I discarded it.

The final dataset used for calculation of color terms contained 43 stars. The graph below shows the measurements for each field which served as input to the color-term analysis.

Here are the measurements before making any color correction ...

... and here are the results after applying the corrections.

                                                                Number of 
  filter        equation            color term c  +/-           stars used
---------------------------------------------------------------------------
  B      (B-b) = delta + c*(b-v)           0.218  +/-  0.025       43

  V      (V-v) = delta + c*(v-r)           0.067  +/-  0.027       40

  R      (R-r) = delta + c*(v-r)          -0.362  +/-  0.035       34

  I      (I-i) = delta + c*(r-i)           0.038  +/-  0.031       15
---------------------------------------------------------------------------