Record of flatfield exposures with HDI

Michael Richmond
Dec 11, 2015
Oct 16, 2017


Flatfield lamp and exposure guide

After HDI was returned to KPNO in late 2014, measurements indicate that the response to light may be non-linear above roughly 30,000 counts per pixel. See HDI Tech Note 8 for the details.

We recommend flatfield exposures with mean values less than 30,000 counts.

On Jan 21, 2014, RIT students Sravani Vaddi and Kayla Emerson experimented with the flatfield lamps to figure out the appropriate lamp settings and exposure times to yield a decent signal in each of the common filters. Their results are shown below; their goal was to find combinations which would yield around 25,000 counts in an image.

Jan 2014

Filter Lamp setting Exptime (sec) Mean counts
U High 100% 13 24,000
B Low 100% 25 25,000
V Low 100% 12 25,000
R Low 100% 8 26,000
I Low 100% 8 27,000
 
Hα (6580) High 50% 11 24,000
Hα+40 (6620) High 50% 11 25,000
Hα+80 (6660) High 50% 11 25,000
Hα+120 (6700) High 50% 11 27,000
Hα+160 (6700) High 50% 11 26,000

Con Deliyannis took dome flatfield exposures during Feb, 2015. His measurements are as follows; I suggest that you use them to compute an appropriate exposure time which yields somewhat lower count levels. I've subtracted a constant 3000 ADU from the values he reported to account for the bias, so please treat these as rough estimates.

A very rough summary of these results is that the U-band exposures appear to require MORE time than the Jan, 2014, images, while the BVRI exposures appear to require slightly LESS time. One might interpret them to mean that the high-intensity lamp is a bit less powerful than it was in 2014, but the low-intensity lamp is slighly more powerful.

Feb 2015

Filter Lamp setting Exptime (sec) Mean counts
U High 100% 40 41,000
B Low 100% 30 39,000
V Low 100% 15 42,000
R Low 100% 9 41,000
I Low 100% 9 41,000

Con measured the following times for flatfields on Dec 8, 2015.

Dec 2015

Filter Lamp setting Exptime (sec) Mean counts
U High 100% 18 27,000
B Low 100% 18 26,000
V Low 100% 9 27,000
R Low 100% 5 26,000
I Low 100% 5 25,000

Kaitlin Schmidt and Ekta Shah of RIT acquired dome flatfield exposures during Jan, 2016. Be careful to verify that the telescope is aligned properly with the dome-flat spot! The measurements are as follows.

Jan 2016

Filter Lamp setting Exptime (sec) Mean counts
U High 100% 20 23,000
B Low 100% 28 27,000
V Low 100% 12 25,000
R Low 100% 8 27,000
I Low 100% 6 23,000
6580 High 50% 16 26,000
6740 High 50% 14 26,000
Gunn Z Low 100% 15 25,000

Thanks to Douglas and Follette for the Gunn Z value, added 6/20/2016

In Oct, 2017, Allyn Smith checked the flatfield lamps. His measurements are shown below, but his executive summary was the low intensity lights do require more time now than previously, but the high intensity lights seem to be functioning at about the same level.

Oct 2017

Filter Lamp setting Exptime (sec) Mean counts
Harris V Low 80% 12 13,000
Harris R Low 80% 10 17,000
Harris I Low 80% 10 20,000
Wash M Low 80% 16 16,000
Wash C High 75% 9 18,000
Wash 51 = DDO 51 High 75% 5 20,000
 
Hα (6580) High 50% 20 25,000
OIII High 90% 12 24,000
 
SDSS u High 90% 75 21,000
SDSS g Low 80% 17 15,000
SDSS r Low 80% 10 15,000
SDSS i Low 80% 10 15,000
SDSS z Low 80% 16 14,000