photometry. The degree of the problem depends on how the image calibrations
are done and how the photometry is done. Those are software issues.
Systematic errors will occur when the standards and target object(s) are
placed on opposite parts of the CCD. The issue is "Can you get the
systematic offset below the statistical significance needed to detect the
exoplanet transit?" In other words, suppose the photometric needs to be +/-
0.003 magnitude to detect a 1-point dip of 0.01 magnitude. Therefore the
systematic magnitude offset between the East data and West data needs to be
around 0.003 magnitude or less to claim that a difference between the E/W
data sets means there was an eclipse occurring during the time of flip.If
the transit occurs entirely on the East half or entirely on the West half,
then the flip is not important other than to provide a longer baseline for
"outside of eclipse" data against which you must detect the dip during
eclipse. In that case, the more observations you get, the better you can
nullify the offset by aligning the East and West parts of the light curve.
Michael
----- 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.
>
> Clear skies
> Wolfgang
>
> --
> Wolfgang Renz, Karlsruhe, Germany
> Rz.BAV = WRe.vsnet = RWG.AAVSO
>
>
>
> ----- Original Message -----
> From: "Chris Peterson" <cpeterson@earthlink.net>
> To: <SBIG@yahoogroups.com>
> Sent: Monday, February 25, 2008 4:08 PM
> Subject: Re: [SBIG] Re: Photometry software
>
>> I can't think of any reason a meridian flip should interfere with
>> exoplanet photometry, as long as your system is capable of
>> placing the target within a few pixels of its position before the
>> flip. Certainly, a flip should have no effect on your flats. There
>> are types of photometry where I could imagine how a flip might
>> introduce error, and also some astrometry. But not differential
>> photometry.
>>
>> The Paramount is no different from any GEM in how the camera
>> is oriented after a flip. It is capable of a high degree of intrinsic
>> accuracy, but the same pointing accuracy can be achieved from
>> nearly any mount with a bit of help from smart control programs.
>>
>> Chris
>>
>> *****************************************
>> Chris L Peterson
>> Cloudbait Observatory
>> http://www.cloudbait.com
>>
>>
>>
>> ----- Original Message -----
>> From: "Bill Logan" <wb9sat@frontiernet.net>
>> To: <SBIG@yahoogroups.com>
>> Sent: Monday, February 25, 2008 7:14 AM
>> Subject: Re: [SBIG] Re: Photometry software
>>
>>> We are a bit off topic here, but according to the experts such (I
>>> am not one of them), using a GEM for exoplanet photometry is
>>> not desired because of the change of orientation of the star field
>>> when doing a meridian flip.
>>> Well some say that one can simply rotated the camera to accom-
>>> modate the flip, but that would skew the flat field. We have one
>>> member in the exoplanet astronomers group that uses a Para-
>>> mount ME. Apparently it can flip automatically between iterations
>>> while keeping the same orientation, but I don't have any first hand
>>> experience with that mount. I don't have enough SPUs to buy one.
>>> LOL
>>> Sorry about the last question. Sometime the 'puter has a mind of
>>> its own. My question was; if you have a dome, is it possible for you
>>> to image during windy conditions?
>
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
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