UT Sep 19, 2021: Photometry of V445 Cas

Michael Richmond
Sep 20, 2021

On the night of Sep 18/19, 2021, under good conditions, RIT undergrad Michael Dussault and I acquired images of the eclipsing binary system V445 Cas as part of his capstone project.

We had two problems during the evening. First, moisture started to condense on the dewar window about half-way through the run; I tried operating at a CCD temp of +1 C to avoid the moisture. Second, the camera's connection to the telescope loosened at one point, causing images to be slightly rotated by small random amounts.

Both problems were fixed by the next night, but they may have caused the second half of the data to be a lost cause.

V445 Cas

The main setup was:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• ATIK 11000 CCD camera
• B, V filters, exptime 30 seconds
• images binned 2x2
• no guiding
• focus shows a typical offset: B foc = V foc + 0.060
  • V: 1.281 at T = 67/19
  • B: 1.340 at T = 65/18
• camera ran at T = -20 C at first, but then at T = +1 C to avoid ice crystals

Notes from the night:

• light clouds now and then, no big deal
• full Moon made sky bright, but target to the North
• at 12:50 AM, I noticed the effect of ice crystals appearing in images: a dark circular shadow at center of frame. I tried increasing temperature to +10 C briefly to sublimate the crystals, but they re-appeared with 20 minutes when I cooled to -20 C again.
  So, I set the camera temperature to T = +1 C and ran at that level for the rest of the night. The dark current is much higher, so faint stars disappered. The target is still clearly visible, so photometry from these images _may_ be salvaged.
• But a second issue appeared starting around 2:50 AM, near image 216. One or more cables caught on the mount briefly, leading to trailed images and, apparently, twisting the camera slightly. This loosened the connection to the telescope mount, which
  1. twisted the field by about 2 degrees, and
  2. allowed the camera to rotate by small amounts at random for the rest of the night
  Since my standard reduction pipeline requires all images to have the same orientation, I will have to perform some extra work to save the latter half of the data. I think I can determine the rotation for each image and correct the star positions, then match as usual.
• no skunks :-(

The object is located at

  RA = 00:31:39.65   Dec = +53:13:00.3   (J2000)

A chart of the field is shown below. The size of the chart is about 31 x 26 arcminutes.

I've marked the location of several comparison stars as well. The bright star P is the ninth-magnitude HD 2826.

Label	B	V
A	10.094	9.378
B	10.353	10.202

I'll use star "A" to shift my instrumental magnitudes to the V-band scale.

I took a photo of the finder TV's screen when pointing to this target; this could be a useful reference for the future:

The sky value shows little effect from clouds -- the increased scatter at late time is due to the ice crystals.

The FWHM graph below shows a jump due to crystals, but then settles down again.

Using aperture photometry with a radius of 7 pixels in B and V filters (binned 2x2, each pixel is 1.24 arcsec, so a radius of 8.7 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 0.018 mag after I removed images with large outliers.

The change in zeropoint was driven by airmass, mostly.

Last modified 9/20/2021 by MWR.