[Aavso-photometry] CCD Observations of Epsilon Aurigae

Tue Mar 10 00:06:31 EDT 2009

For anyone interested, our Campaign Newsletter #10 can be downloaded 
as a pdf at
http://www.hposoft.com/Campaign09.html

Newsletter #11 is due out the end of March 2009.

The is a rather complete list of UBV data from July 2008 to January 
2009 (my data), plus data from several other observers.

I agree with Wolfgang and suggest you use separate plots for each 
band and expand the magnitude scale to show the variations better.

BTW, zeta Aurigae is in eclipse right now. Very large change in the 
shorter wavelengths and very shallow in the longer. Newsletter #11 
will have more on it.

Jeff

At 20:40 -0700 03/09/2009, Wolfgang Renz \( new \) wrote:
>Hi David
>
>If you would use an a bit wider range, you could more easily verify them:
>http://harmonia.aavso.org/cgi-bin/lcg.pl?name=EPS+AUR&lastdays=&start=2454630&stop=2454930&obscode=TDW&obstotals=on&grid=on&type=png&pointsize=1&width=800&height=600&mag1=&mag2=&mean=&uband=on&bband=on&v=on&r=on&iband=on&unknown=on
>The first three of your obs look a bit off while the last three look OK.
>Do the first three of your obs have something in common ?
>E.g. obs at a higher zenith angle ?
>
>Did you ask Jeff Hopkins too if he has UBV data for that time
>
>Clear skies
>  Wolfgang
>
>--
>Wolfgang Renz, Karlsruhe, Germany
>
>----- Original Message -----
>From: "David Daiku Trowbridge" <David at tinyblue.com>
>To: "'Wolfgang Renz'" <w_renz at onlinehome.de>
>Cc: "'AAVSO-PHOTOMETRY'" <aavso-photometry at mira.aavso.org>; "'Jeff 
>Hopkins'" <phxjeff at hposoft.com>
>Sent: Monday, February 09, 2009 4:22 PM
>Subject: RE: [Aavso-photometry] CCD Observations of Epsilon Aurigae
>
>>  Wolfgang,
>>
>>  Thank you for your insightful comments. I cannot respond intelligently to
>>  them right now; I think they will require a great deal more experimentation!
>>
>>  However, to address your first question: Between Sep 15 and Jan 24, the
>>  AAVSO light curve includes data from five observers. Four of the data points
>>  are mine (TDW), obtained using the methods I have described in the paper. To
>>  see where they fit in, take a look at:
>>
>>  http://www.aavso.org/cgi-bin/lcg.pl?name=EPS+AUR&lastdays=400&start=2454725&
>>  stop=2454855&button_name=Please+Wait...&obscode=TDW&obstotals=on&grid=on&poi
>>  ntsize=1&width=600&height=450&mag1=&mag2=&mean=&bband=on&v=on&r=on&iband=on
>>
>>  David
>>
>>
>>
>>  -----Original Message-----
>>  From: Wolfgang Renz [mailto:w_renz at onlinehome.de]
>>  Sent: Sunday, February 08, 2009 7:04 AM
>>  To: David Daiku Trowbridge
>>  Cc: AAVSO-PHOTOMETRY; Jeff Hopkins
>>  Subject: Re: [Aavso-photometry] CCD Observations of Epsilon Aurigae
>>
>>  Hi David
>>
>>  Nice results.
>>
>>  Did you already compare them with other measurements
>>  during that time ?
>>
>>
>>>  Comments and suggestions are welcome.
>>
>>  I've a few comments on your usage of a Hartmann mask:
>>
>>  It looks like as the holes are placed too far outside !
>>
>>  You said that you use a Takahashi FSQ-106 APO. So the outer
>>  free diameter of the lens should be 106 mm. But the diameter of
>>  the out most parts of the holes are at  2 * (50mm + 17.5 mm  / 2)
>>  = 117.5 mm. So parts of the light through the holes will also be
>>  cut off by the outer edge of the APO lens. This is for sure the
>>  reason why you see triangular stars and diffraction spikes.
>>  But how much additionally light is shadowed depends on the ori-
>>  entation of the stars to the holes in the mask and how far off-axis
>>  the stars are. But this is IMO really not recommendable as this
>>  might not get fully compensated by the flats.
>>
>>  If you would placed the round holes a bit more inside at r = 44
>>  mm minus a small reserve to account for the f#, you should not
>>  see any strong diffraction spikes but could still focus by placing
>>  the three out-of-focus images on top of each other.
>>
>>  If you want to use diffraction spikes for focusing, you could use
>>  equilateral triangels that are placed in a way that their sides are
>>  parallel to each other. This ensures that you don't get a double-
>>  diffraction spike per every side of every triangle when they are
>>  orientated differently (which would make 9), but just 3 3x as bright
>>  ones all together.
>>  But the most effective way to focus is probably the new Bahtinov
>>  Mask (that is in the meanwhile probably the longest thread in the
>>  CloudyNights Forum):
>>  http://www.cloudynights.com/ubbthreads/showthreaded.php?Cat=0&Number=2653381
>>  The classical version of the Bahtinov Mask is now also available
>>  commercially:
>>  http://www.spike-a.com/
>>  http://www.spike-a.com/focusmovielarge.gif
>>  If I would have to choose between the different possible designs,
>>  I would choose the version splitting the aperture of the scope in
>>  two halfs each having parallel lines in the same direction but with
>>  two different frequencies. This gives just two double-spikes, but
>>  as the two halfes have different fequencies, the brightest parts of
>>  the double-spikes are at different distances from the stars. This
>>  way one can focus just by placing two streak pairs (an inner and
>>  an outer pair) to line up which should IMO be the easiest way to
>>  do it.
>>
>>
>>  By using the Hartmann mask you dim the light to (3 * (17.5/2)^2) /
>>  (106/2)^2) ~ 8 %. So you can expose ~ 12.2x longer than without
>>  the Hartmann mask.
>>
>>  You wrote:
>>  "Using three holes rather than one allowed us to maintain resolution
>>  while reducing incident light."
>>
>>  While this might be nearly true for the resolution and for sure for the
>>  reduction of the light, it might not give the same benefit for reducing
>>  the scintillation noise. If you take a look at images of flying shadows
>>  during solar eclipses, you will notice that they usually show a small
>>  scale pseudo-periodic pattern with brighter and darker areas that
>>  are much longer than wide. Their form, orientation and frequency
>>  will depend on the strength and orientation of the winds in the higher
>>  atmosphere. As these brighter and darker areas are neither black
>>  and white nor indefinite small, the scintillation noise probably does
>>  not rise anymore with scopes having a diameter of 5 cm or a bit
>>  less (the common scintilattion noise formulae never considered
>>  such small diameters and converge to indefinit large scintillation
>>  noise for indefinit small diameters)
>>  So an aperture mask with three hole will sample just small parts of
>>  this pattern and should therefore usually not give the same advan-
>>  tage in reducing the scintiallation noise as the full aperture.
>>
>>  But there are three other options to lengthen the exposure time while
>>  maintaining the full opening of the scope. And these shouldn't have
>>  any penalties in reducing the resolution and the scintillation noise
>>  and will probably also give less issues with flats:
>>
>>  A) Defocus the stars
>>  To reach similar long exposure times you would have to spread the
>>  light over a just ~ 3.5x larger FWHM than when in sharp focus. If a
>>  setup is undersampled anyway (and most short FL setups are) and
>>  one doesn't hunt for the faint limit for ppi, one should defocus a bit
>>  for astrometry and photometry anyway to reach at least a FWHM of
>>  ~ 2.5 to 3.0 pixel to avoid issues due to inter-pixel structures and
>>  intra-pixel sensitivity variations.
>>  This works of course just without interferences if the stars of interest
>>  don't have very close companions. Severe defocusing works better
>>  with unobstructed scopes as obstructed scopes show donuts when
>>  they are defocused. With unobstructed scopes it might make also
>>  a slight difference if one defocuses to the inside or the outside of
>>  the sharp focus. In one case one will usually get more PSF-looking
>>  defocused stars, in the other case a brighter outer edge.
>>
>>  C) Use a blurring filter (softeners)
>>  For daylight there exist softener filters that can be used in front of
>>  the camera lenses. They are also available with relative large dia-
>>  meters. But for larger scopes they have probably to be used some-
>>  where in the convergent beam. Using these far away from the focal
>>  might be criticaln as they might not be very plane.
>>  This works of course also just without interferences if the stars of
>>  interest don't have close companions. But it should work equaly
>>  well with obstructed and unobstructed scopes as the blurred stars
>>  are in focus in both cases.
>>  This was probably never used before for photometry, but it might be
>>  worth a try [(e.g. with digicams)].
>>
>>  B) Use a grey filter
>>  The 8% value is in the range of moon filters (e.g. ND 0.9 = 12.5 %
>>  transmission). So you could use one of them in the convergent beam
>>  on the nosepiece or in front of the camera window in the D-block of
>>  an SBIG ST camera. There are also fine polished versions available
>>  (e.g. from Baader), so they should not give issues when used far in
>>  front of the focal point. [This is probably the best way to handle such
>>  bright vars and does also work well if one of the stars of interest has
>>  a close companion.]
>>
>>  Clear skies
>>  Wolfgang
>>
>>  --
>>  Wolfgang Renz, Karlsruhe, Germany
>>  Rz.BAV = WRe.vsnet = RWG.AAVSO
>>
>>
>>
>>  ----- Original Message -----
>>  From: "David Daiku Trowbridge"
>>  To: "AAVSO Photometry" <aavso-photometry at mira.aavso.org>
>>  Sent: Wednesday, February 04, 2009 12:01 AM
>>  Subject: [Aavso-photometry] CCD Observations of Epsilon Aurigae
>>
>>>  I've posted a document describing how I have used a 4" telescope with a
>>>  3-hole mask to do BVRI photometry of Epsilon Aurigae. Also, I've included
>>>  examples of Excel spreadsheets that I've used to determine standard
>>>  magnitudes following the methods of Bruce Gary and Priscilla Benson.
>>>  Standard magnitudes for the comparison star Lambda Aurigae are from Jeff
>>>  Hopkins. Transmission coefficients were measured using M67.
>>>  http://www.tinyblue.com/htm/eps-aur.htm
>>>
>>>  Comments and suggestions are welcome.
>>>
>>>  David Trowbridge
>>>
>>>  Tinyblue Observatory
>>
>>

-- 
Jeff Hopkins
HPO SOFT
Counting Photons
http://www.hposoft.com/Astro/astro.html
Hopkins Phoenix Observatory
7812 West Clayton Drive
Phoenix, Arizona 85033-2439 U.S.A.
(623)849-5889
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