UT Mar 25, 2024: Photometry of asteroid (3) Juno

Michael Richmond
Mar 28, 2024

On the night of Mar 25/26, 2024, under good conditions, RIT students Alex Speyer and Ben Hyman and I acquired images of the asteroid (3) Juno. They are part of the PHYS 373 "Observational Astronomy" class, and their project is to measure the rotation period of the asteroid.

We acquired images in B and V passbands, getting about 80% of a rotation.

Asteroid (3) Juno

This asteroid has been studied for over 200 years. Its rotation period is thought to be about 7.2 hours, but we decided to measure it ourselves.

These observations involved:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• ASI 6200MM CMOS camera
• B-filter 30-sec, V-filter 20-sec images
• images binned 4x4
• no guiding
• focus position on 12-inch:
  • in V: 0.354 yields FWHM 4.9 at T = 50/10
  I moved the focus to 0.374 at 12:15 AM, around image number 181.

Notes from the night:

• sky started clear, but cirrus arrived late in the run
• bright waning gibbous Moon
• camera at T = -20 C
• first three images don't show Juno -- we were in the wrong field

The picture below shows an image of the field of Juno from this evening. The field of view is about 22 arcminutes across.

I've marked the location of several comparison stars.

  star       name                  B          V         
------------------------------------------------------
    A      TYC 839-1199-1         10.99    10.23   
    B      HD 92308               10.36     9.70   
--------------------------------------------------------------------------

 

Here's the sky background over the course of the run. The early clouds disappeared after an hour or two.

The FWHM:

The graph below shows changes in the photometric zeropoint of an ensemble solution of the instrumental magnitudes over the course of the run. Notice the smooth curve for the middle and end of the run, indicating clear skies.