Hi Wolfgang-
To be clear, I didn't say that a meridian flip wouldn't alter the data
being collected, I said that it shouldn't interfere with the ability to
do exoplanet photometry. I still think that is basically true. The
majority of examples you give are all systematic errors that can be
corrected for (a few, of course, are simply the product of bad planning,
such as moving in and out of moonlight, or dome slot vignetting). There
could be some specific "bad luck" situations, such as an occultation
occurring very close to the flip, or a change in sky conditions
coincident with the flip, but these will be relatively rare.
Chris
*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
----- Original Message -----
From: "Wolfgang Renz" <w_renz@onlinehome.de>
To: "Chris Peterson" <cpeterson@earthlink.net>
Cc: <SBIG@yahoogroups.com>
Sent: Tuesday, February 26, 2008 11:12 AM
Subject: Re: [SBIG] Re: Photometry software
Hi Chris
> I can't think of any reason a meridian flip should interfere
> with exoplanet photometry ...
Really not ?
The meridian flip might not be the cause of the issues, but
it will make any issue that is present visible that might not
interfere without a meridian flip.
Its the experience of many observers, that they partially
get a jump in their mag values when they make a meri-
dian flip with their GEMs due to unresolved calibration
issues. See e.g.:
http://www.aavso.org/tmp/NGC6811.xls
These might influence the photometry by up to 0.2 mag !
A few even have "discovered" "new variables" due to this
issues when using moving averages.
Especially if the variable is relativly far way from the comps,
it will pronounce every calibration issues even for differential
photometry. See e.g.:
Differential photometry using distant comp
http://www.lolife.com/astronomy/bad.png
Differential photometry on the same frames using close
comps:
http://www.lolife.com/astronomy/good.png
The first discontinuity in the "bad" one is due to the meridian
flip. During the meridian flip the scope went from being
bathed in moonlight to being completely shielded from the
moonlight. The second discontinuity is due to some dome
obstruction that was resolved by moveing the dome.
What about having to rotate the camera by 180° after the
meridian flip to be able to place all stars to 100 % (inclu-
ding the guide star) on the very same pixel again (which
is practically impossible with most amateur equipment) ?
What about scattered light issues and background gradi-
ents to e.g. moon, dusk/dawn light that might differ bet-
ween before and after the meridian flip due to e.g. differ-
ent shading of the scope itself (even if its well baffeled!)
or a dome?
These might not spoil the photometry on uncalibrated
light frames directly (e.g. by using concentric sky annuli),
but they for sure can spoil the flat frames and therefore
the photometry on the calibrated light frames too.
If you want to read more about this, take a look into the
'[Aavso-photometry] moon problems' thread in October
2006 at:
<http://www.aavso.org/publications/email/archive_restore/aavsophot.html>
and into '[Aavso-photometry] effect of collimation on
photometry' thread in March 2007 (unfortunatly this was
not restored yet but I could forwardb the posts)
<http://www.aavso.org/pipermail/aavso-photometry/>
What about that the slightest bit of flexture in the setup that
will double the differences when not rotating the camera ?
If one doesn't want or cann't buy or use a derotator, one
often ends up making pre and post meridian flip flats.
Just because it should not occure in a perfect world doesn't
mean that it won't occure in reallity.
Exoplanet photometry is more tempting than other "usual"
photometry and much more than pretty picture imaging in
regards of image calibration. Here one has to fight down
every possible source of errors to as much below 1 % as
possible to be able to detect a typical exoplanet transit
depth of 0.5 to 3 % at all. Even pros have difficulties with
getting they photometric results to better than 1 % and
they usually can just reach this with some additional efford
in baffeling/flocking/blackening the the inside of their sco-
pes, adding light shield /dew caps and improving their flat-
fielding severely. Still then they might not reach their goals
withoutn keeping all stars of interest on the very same well-
behaving pixel of the CCD.
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