On the night of Sep 23/24 2018, I once acquired data through bright but clear skies on the likely black-hole system MAXIJ1820+070, (also known as ASASSN-18ey ).
I increased exposure time to 2 minutes, which helped to improve the measurements. A small change to the altitude of the mount reversed the drift, so I guess my adjustment was too large and overshot the proper position.
The main setup was:
Notes from the night:
This optical and X-ray and radio transient is likely a black hole accreting material at a higher-than-usual rate. It has been the subject of many observers over the past few months -- see the trail of telegrams that include
The object is located at
RA = 18:20:21.9 Dec = +07:11:07.3
A chart of the field is shown below. The size of the chart is about 22 by 18 arcminutes.
I've marked the location of several comparison stars, which also appear in light curves below. Stars C, D, and E are mentioned by the Tomoe Gozen team in ATel 11426, but all three are rather red, with (B-V) ranging from 1.14 to 1.37. Star B is one of the bluest nearby bright stars, with (B-V) = 0.52.
star UCAC4 B V ---------------------------------------------------- B 486-079513 12.975 12.454 C 486-079608 13.968 12.830 D 486-079523 14.637 13.272 E 487-077858 14.637 13.272 ----------------------------------------------------
I ran the camera at -20 C. Nothing out of the ordinary.
The sky value shows no clouds at all. Very nice.
Here's a record of the telescope's drift. The drift to the NORTH as the telescope moved west of the meridian is opposite to the drift in recent nights. My small adjustment of the altitude knob to CW (1/4 turn?) evidently overshot the proper position.
The number of objects detected -- I required 80 objects for an image to be included in the ensemble.
I used an aperture with radius 5.0 pixels.
I discarded images which had obvious trailed stellar images, using a threshold of "round > -0.45", (removing 48 of the 93 raw images). I discarded 2 more images as well, due to double exposures.
Using aperture photometry with a radius of 5 pixels (binned 2x2, each pixel is 1.34 arcsec, so a radius of 6.7 arcsec), I measured the instrumental magnitudes of a number of reference stars and the target. Following the procedures outlined by Kent Honeycutt's article on inhomogeneous ensemble photometry, I used all stars available in each image to define a reference frame, and measured each star against this frame.
Sigma-vs-mag plots show that the floor was about 0.005 mag overall, which is good. MAXI stands out as an outlier at differential mag 3.0.
Here are light curves of the variable and the field stars.
I used the UCAC value for the V-band magnitude of star "B" = UCAC4 486-079513 to shift the ensemble magnitudes to the standard V-band scale.
Here's a closeup on the variable. I'll connect the dots to make its behavior a bit easier to see.
You can download my measurements below. A copy of the header of the file is shown to explain the format.
# Measurements of MAXIJ1820+070 made at RIT Obs, UT 2018 Sep 24, # in good conditions, # by Michael Richmond, # using Meade 12-inch LX200 and ATIK 11000. # Exposures 120 seconds long, V filter. # Tabulated times are midexposure (FITS header time - half exposure length) # and accurate only to +/- 1 second (??). # 'mag' is a differential magnitude based on ensemble photometry # using a circular aperture of radius 5 pix = 6.6 arcseconds. # which has been shifted so UCAC4 486-079513 has mag=12.454 # which is its V-band magnitude according to UCAC4. # # UT_day JD HJD mag uncert Sep24.01354 2458385.51354 2458385.51399 14.020 0.017 Sep24.01657 2458385.51657 2458385.51702 14.031 0.019 Sep24.01806 2458385.51806 2458385.51851 13.996 0.019
Last modified 9/24/2018 by MWR.