[Aavso-photometry] CCD Observations of Epsilon Aurigae

[Wolfgang Renz]

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 focis in both cases.
This was probably never used before for photometry, but it might be
worth a try.

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.

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