Proposal #723

Calibrating a smart-telescope colour index against a standard system, and measuring how SW Lacertae's colour moves through its eclipse cycle.

I am a high-school student observing with a 30 mm ZWO Seestar S30 Pro from Vancouver, BC. My photometry comes from the Bayer planes of a one-shot-colour CMOS sensor, so my three bands are the camera's own dye curves - broad, overlapping, and on no standard system. I have measured this instrument carefully: my green channel is linear to about 10,000 ADU, my per-night photometric floor is 0.064 mag from non-variable comparison stars in the same frames, and I have just submitted my first TG series on SW Lacertae to the AID under observer code VDBA.

What I cannot do with this equipment is produce a real colour index. A B-V from Bayer dyes is not a B-V, and there is no honest transformation available to me because I have no filters to transform.

I would like BVRI photometry of SW Lacertae so that I can compare a true colour index against my instrumental one on the same star, and so that I can see how the colour changes around the orbit. SW Lacertae is a W UMa contact binary at V 8.5-9.5 with a 0.32-day period, so both eclipses and both maxima fall inside a few hours, and its components differ enough in effective temperature that the colour should vary measurably through the cycle.

HOW OFTEN: one BVRI set - one exposure per filter - roughly every 20 minutes, on any night the star is up, for as many nights as convenient. I do not need continuous coverage of a single night and I am not asking for a whole night on the telescope; scattered sets across several nights sample the 0.32-day period better than one uninterrupted run would, because consecutive nights fall at different orbital phases.

WHICH FILTERS: Johnson B and V are what the project needs. Cousins R and I are welcome if they cost nothing extra, but B and V alone would answer the question.

WHEN TO STOP: once roughly 30 BVRI sets exist spread across the orbital phase, or after three months, whichever comes first.

The star is bright, so short exposures should do. I am happy to take whatever the scheduler finds convenient rather than specifying a telescope.

I will process the images myself in VPhot and submit the resulting photometry to the AID. Any publication using these data will carry the AAVSOnet acknowledgement.