UT Jun 05, 2021: Looking for ZTF21abdwdwo

Michael Richmond
Jun 05, 2021

On the night of Jun 04/05, 2021, under good conditions, I acquired images of the field of the transient source ZTF21abdwdwo, as part of a "homework assignment" for the Kilonova Catcher collaboration.

My images didn't show anything at the location of the transient, which was no surprise (it was possibly 18th or 19th mag at its peak). However, due to an accident at the end of the run, the filterwheel's USB port has been broken.

ZTF21abdwdwo

The target is one of the (many) candidates discovered by the Zwicky Transient Facility. Information on the object can be found at

• the Fink Science Portal

These observations involved:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• ATIK 11000 CCD camera
• clear and R filter, 30-second exposures
• focus in clear at 0.884 yielding FWHM 2.2 pixel
• images binned 2x2
• both guided and un-guided images

Notes from the night:

• no Moon at all
• sky clear, but it seemed bright; maybe pollen in the air
• temperature 75 F / 23 C at start (9:34 PM), dropped to 69 F / 21 C at end (12:41: AM)
• had to change the focus considerably, using the silver knob on the telescope.
• initial use of the guider, with 7-second exposure times, yielded poor results -- quite a bit of East-West motion. I turned it off for most of the unfiltered sequence. When I started the R-band images, I tried decreasing the aggressiveness in X from 0.5 to 0.2 -- and that worked pretty well. Keep an eye on performance and this lower level of aggressiveness in the future.
• see note on the accident below.

The accident came as the result of a sequence of events.

1. while trying to find the field, I accidentally entered the wrong HD number of a star on the handpaddle. This caused the telescope to slew far to the north, so that the camera pushed up against the base of the fork. Uh-oh.
2. I think that as the telescope motors tried to continue turning, but the scope couldn't move, the controller lost its sense of where the telescope was pointing.
3. I slewed the telescope south away from this position, then re-synced on a bright star, and continued with the observations.
4. at end of night, when I issued the "Park Telescope" command, the telescope ran away to North and East. I carelessly didn't notice until it was too late, and the cables had wrapped very tightly.
5. I turned power off. Then, I turned power back on, went through the usual three questions, and slewed manually to un-wind cables and return to normal position.
6. I then re-sync'd the scope to a bright star, told it to Park, and it did so normally.

But the damage was done: when the cables were wrapped tightly, the USB connector was torn from the filterwheel; what was worse, the USB port on the filterwheel was damaged, so that it can't be used any more. Argh.

I've written to ATIK, explaining the situation. I hope that it may be possible to repair the item; if not, a new wheel will cost around $600.

As for images of ZTF 21abdwdwo, a brief summary is:

• median combination of 50 x 30-second clear images shows the galaxy just East of the target clearly. No sign of the candidate. Stars of mag R = 18 can be seen faintly.
• median combination of 140 x 30-second R-band images does show the galaxy due East of target, but with lower signal than in the clear image. No sign of candidate. Limiting mag is probably around R = 16.

The poor limiting magnitude is due in part to the bright sky -- due to pollen??? -- but also due in part to an awful PSF. The FWHM is around 4.0 - 4.5 pixels = 5.0 - 5.6 arcseconds. Ugh. In the future, it might help to discard images with bad PSFs before combining via median.

Last modified 5/31/2021 by MWR.