We   would like to investigate what the possibilities are for broadening the scope of WET targets to include other stars with potential for asteroseismology. Amongst these are the rapidly oscillating Ap (roAp) stars. Because these stars are much brighter than white dwarfs, there are some technical challenges which must be addressed before committing to observing a roAp star. So the following procedure describes a test to assess the stability of your photometer and the sky above your observatory to see whether roAp stars can be observed with your setup.

Why should there be doubts? The answer is that in observations of white dwarfs, a multitude of small instabilities may be hidden beneath the photon noise from these faint stars. Rapidly oscillating Ap stars are much brighter so photon noise is almost negligible.

Also, they are *much* lower amplitude (1 per cent peak-to-peak is a large amplitude star), so instrumental and atmospheric stability are of utmost importance.

Please find below links to 4 stars. Each link contains co-ordinates and a finding chart:

Please observe one of these targets. A B filter is preferred, but if you don't have one, a V filter will be ok. Smaller telescopes (1-m) with a B filter should shoot for one of the first 2 stars, larger telescopes (2-m) for one of the last 2 stars. One of each pair is accessible in the evening, the other in the morning.

Try and choose a star that gives you about a factor 2.5 smaller than the count rate limit of your tube (e.g. if your tube is limited to a million counts per sec, try and choose a target that will give you as near as you can get to 400 000 counts per sec). Obviously, where this is not possible, choose a fainter or brighter target. Try not to go below 100 000 counts per sec if you can avoid it. If you do, photon and sky noise will be detectable and the test will not be as sensitive. It is, in fact, best if you can stay above 250 000 counts per sec.

(If you don't know the count rate limit of your tube, be sure to find it out before you try this as you could damage your tube. A million counts/sec is a typical limit but some red-sensitive GaAs tubes have lower limits, so be careful).

Observe only if you are sure it is photometric, and guide as carefully as you can. You need to get millimag accuracy data, from one point to the next.

Get one hour of data with 10-sec integrations. Use a 30 arcsec aperture, not less. Use a larger aperture if you consider it necessary. Sky background is not a problem here so your normal instincts to choose the smallest aperture consistent with losing no light do not apply. The detectable skirt on the point spread function of stars this bright extends *much* further than for white dwarfs, and you need to avoid "clipping off" even 0.0001 of the light. Therefore using a larger aperture than you are accustomed to lets in more sky, but that is less important than making sure you lose no detectable starlight.

Send the data you get to HQ as usual. Please send information about the setup *along* with the data file so this information and the data file are always together. The setup information needed includes: (1) the telescope; (2) the kind of photometer; (3) the filter used; (4) the photomultiplier tube you used; (5) the target star; (6) the observer.

Best of luck...

Darragh O'Donoghue and Don Kurtz

PS. We would also like you to measure the dead time of your photomultiplier tubes because with stars as bright as this, dead time count losses are significant. Remarkably, variations of dead time losses as the star's brightness changes between maximum and minimum are detectable. Thus, an accurately known dead time is important. Here are two procedures to determine deadtime:

With a constant light source
Using twilight sky