Converting PPM to magnitudes - Kepler Data

Affiliation
American Association of Variable Star Observers (AAVSO)
Sun, 08/26/2012 - 18:50

I have been doing some research on KOI-13 in preparation. The Extra solar Planet Encyclopaedia. Exoplanets.org data base and several recent papers list a transet depth of 0.008 magnitudes. The Kepler project Planet Candidate Data Explorer lists the transit depth as 4646 parts per million. If I am converting correctly this works out to be about 0.005 magnitudes, which makes me suspect that I am not doing the conversion correctly.

I am using the following conversion delta mag =-2.5*LOG(1-4646/1000000) = 0.00506. Can anyone confirm that this is the correct calculation.

If that is correct then there are very significant differences between reference sites one normally uses for exoplanet information. There are other significant differences between the kepler site data and the other sources concerning KOI-13, relating to things such as error limtis on transit epochs and periods. Does anyone know whether the Kepler Planet Candidate Date Explorer has the best data for Kepler objects? I would assume it does, but I have been surprised many times in the past by seemingly obvious conclusions that turned out to be bad assumptions, particularly concerning the merits of data sources.  It may be possible that this site doesn't contain updates from follow on observations but the smaller error limits on the Kepler site argue for the Kepler site being more current.

I am a visual observer of…

I am a visual observer of variable stars with absolutely no experience of UBV photometry or exoplanet transit observation, and whilst reading Dr. Dennis Conti's AAVSO Notice 755 requesting observations of transits of HD 80606b, noted a given transit depth of 11 ppt, which measurement meant absolutely nothing to me. At first I guessed it might mean 'parts per trillion', but parts of what, I do not understand. Neither do I understand why the transit depth measurement could not have been given in plain magnitudes, or millimagnitudes, so that everybody - especially the citizen scientists the Notice is directed towards - knows what is being talked about. It was whilst attempting to learn what is meant by a transit depth of 11 ppt that I discovered your post with its conversion formula.

The only two other pieces of salient information I have found on transit depth measurements in ppt are from Dr. Conti's A Practical Guide to Exoplanet Observing: 'Because flux and magnitude at these small amounts are approximately the same, relative changes in flux are often used as a substitute for magnitude when measuring and referring to the transit depth. In fact, more recent surveys such as TESS are now expressing transit depths in “ppt” (parts-per-thousand)'; and his TESS Follow-up Observing Program Working Group (TFOP WG) Sub Group 1 (SG1) Observation Guidelines: 'It should be noted that the predicted transit depths in the TTF are in parts-per-thousand (ppt) rather than milli-magnitude (mmag). This is because TESS depth units themselves are in parts-per-million (ppm), so the ppt values can be easily directly compared to the ppm values (namely, by just applying a factor of 1000). Furthermore, because 1 ppt = 1.0863 mmag, ppt and mmag are sometimes used interchangeably for shallow transit depths.'

All that I understand of any of that is that 1 ppt is equal to 1.0863 mmag. So 11 ppt is equal to 0.0119 mag, which is all I wanted to know with respect to Dr. Conti's Notice. I am sceptical that such a transit depth is detectable by a typical amateur observatory with a 150mm aperture telescope with a digital camera as has been asserted, but as I say I have never tried to detect an exoplanet transit before.

Substituting 11 ppt into your equation - which I don't really understand either - then, yields a magnitude of -2.5×log(1-11/1000) = 0.0120 mag, just one tenth of a millimagnitude fainter than calculated with Dr. Conti's simple conversion formula. So I expect your formula is accurate enough, if not more accurate than Dr. Conti's.

Affiliation
American Association of Variable Star Observers (AAVSO)
mmag

for a 150mm refractor and camera, you may well be able to see a 10 mmag or 10ppt dip if the star is bright enough, maybe magnitude 9  or brighter for that scope.

The secret is keeping it on the same couple pixels all night and doing a little sensible averaging.

 

Ray