Time domain astronomy and fastest eclipsing binary ZTF J1539+5027 (+20 mag, 6.91 minutes): How to measure its minimum brightness?
Unless I've done my maths wrong, the period of total eclipse is about 18 seconds.
The CHIMERA camera at Mt Palomar, the instrument which followed up the discovery of this system, can take exposures at up to 8 full 1k$\times$1k frames/second and considerably higher if windowed on an object. There is no need for photon-counting equipment for such a slowly varying source. Indeed, the CHIMERA observations used 3 second exposures.
The problem in time resolution I suspect is just how faint the object is. So moving to photomultiplier technology, which would mean a hit in terms of efficiency, is not going to help here. The hot primary has $g = 20.38 \pm 0.05$. Given the the ratios of radii and temperatures in the discovery paper - $R_1/R_2 = 0.5$ and $T_1/T_2 &gt; 4.9$ - then just a simple bolometric scaling suggests the flux from the secondary is $&gt;144$ times smaller (or 5.4 mag).
It is quite challenging to accumulate much in the way of signal-to-noise on a magnitude $\sim 26$ object in $\sim 10$ seconds, even on the biggest telescopes on Earth. Probably the best bet would be to use a standard CCD camera on say the Gemini-N telescope (or Subaru) and just take a ten second exposure exactly at the predicted time of the minimum and then exposures either side (to confirm you were in the right place!). This can be repeated every 6.91 minutes for a whole night if necessary.
It doesn't look hopeful though. I used the Gemini-N integration time calculator with a $g=26$ source of spectral-type $\sim$A0V (ok for a white dwarf) observed in the darkest skies. It would take $215\times 10$s exposures to get a signal-to-noise ratio of 5, and the the noise is dominated by sky background at that magnitude. So basically unfeasible.
Determining the effective temperature of the unheated side of the secondary would be interesting because it might give you a constraints on its intrinsic luminosity, its cooling age, and hence on the evolutionary pathway that led to the less massive, but larger secondary helium white dwarf forming before the hotter primary C/O white dwarf.