[X] My Assistant

[Juramike]

Here is an animated GIF of the May 5th 2009 star occultation sequence from the CL1 VIO image set:


(click to animate)

For this stack, the limb of Titan was used as the reference for alignment.
The contrast has also been balanced between the images of the sequence.

[Jason W Barnes]

For this stack, the limb of Titan was used as the reference for alignment.
The contrast has also been balanced between the images of the sequence.

Sweet. You should probably be asking me this rather than the other way 'round, but what's that star in occultation?

- Jason

[volcanopele]

That's Alpha Eridani, a 0.45 mag blue-giant star in the constellation Eridanus.

Attached thumbnail(s)

 

[Juramike]

Layer stack aligned:



(Not sure of the reason for the slight curve in the track, it might be due to the limb size changing due to changing distance)

[Juramike]

Control points added to contrast balance the images (within 1/255 units):


Two of the images were selected and averaged, and the region from around the star grabbed from the image without a star to back a background control. This will make the Background Model for the CL1/VIO images.


Subtraction of the Background model from the balanced image gives only the starlight coming through the haze layers.

[Juramike]

For a spatial reference point, I'll use the outer edge of the detached haze layer. This feature seems to show up in both the CL1 VIO and the CB2 CL2 filtered images.

So the mapping of the haze layers will be downward from the outer edge of the detached haze layer.
(Anyone got a reference for the altitude of the outer edge of the haze layer from the surface?)

Using the recent Planetary Photojournal images (and reported pixel scales), here are two crops of the haze structure from PIA11485 and PIA11468.

Lining them both up and assuming similar structures, the measured pixel values translated to distance seems to match up. The detached haze layer is about 60 km thick, and the distance from the outer edge of the detached haze layer to the outer edge of the inner thicker haze layer (but still blue scattering) is about 150 km.



Lacking an absolute scale, I'll assume the haze structure is the same in the May 5th images and that the distance from the outer edge of the detached haze layer to the inner thicker haze layer is still about 150 km.

[remcook]

The detached haze layer is at 520 km
link (hope it works for you)

[remcook]

The curvature of the path might be due to refraction due to the atmosphere, which is affected by density (lower altitudes refract more)...
picture: http://gwest.gats-inc.com/sofie/refraction...occultation.jpg

[Juramike]

Been doin' some readin (Thanks for the pointer, remcook!):

If I read the graphs and different plots correctly (see references below), the thickest part of the detached haze layer is located at 490 km - 570 km. With the thickest part centered at about 520 km.


References:
Lavvas et al. Icarus 201 (2009) 626-633. "The detached haze layer in Titan's mesosphere". doi: 10.1016/j.icarus.2009.01.004.
(Specifically Fig. 1 is a summary of the studies below)

Liang et al. The Astrophysical Journal 661 (2007) L199-L202. "Photolytically generated aerosols in the mesosphere and thermosphere of Titan."
(Fig 2 is a summary of UVIS theta-Scorpii occultation data)

Porco et al. Nature 434 (2005) 159-168. "Imaging of Cassini from the Cassini spacecraft." doi: 10.1038/nature03436.
(Fig 11 gives plot of I/F with altitude)

Fulchignoni et al. Nature 438 (2005) 785-791. "In situ measurements of the physical characteristics of Titan's environment." doi: 10.1038/nature04314.
(Fig. 2 is the HASI instrument plot from Huygens descent probe, giving the density measurement. Although not imaging, it provides density [=temperature] evidence for the existence of haze layers)

[Juramike]

Corrected graphic with values assuming an inner edge of the detached haze layer at 490 km:

[Jason W Barnes]

Corrected graphic with values assuming an inner edge of the detached haze layer at 490 km:

This is pretty cool -- more than I've seen yet from ISS on the occultation stuff, anyway. VIMS is in spectrum mode for these, and so once it shifts more than 1 pixel we lose the star.

- Jason

[Juramike]

Here is the sequence of leveled and aligned (shifted) images using the CL1 VIO filter.

Attached thumbnail(s)

 

[Juramike]

For each CL1-VIO subtracted image, I used the Photoshop integrated density measurement of alpha Eridani's image.
The integrated density was normalized to the integrated density in the initial image in the sequence.

Here is the plot (aligned with an image of Titan's haze)



-Mike

[ugordan]

Mike, if you're doing what I think you are, you might be over-analyzing the raw data. These raws have individual contrast stretch applied to them which means relative star brightness from frame to frame cannot be compared because the star is point-like and could be clipped to max brightness (so you lose integrated brightness accuracy as it's saturated) and the camera 8-bit encoding is very probably nonlinear LUT. That, coupled with histogram stretch would mess with your frame subtraction.

[Juramike]

I was worried about that:

I set 4 control points in dark middle-gray portions of the image for one of the images. (Off the top of my head, for the CB2-CL1 set the pixel values were: 25, 33, 61 and 207). These were in the haze layer and on the "twilight" portion of Titan's disk.
After alignment, each image was then adjusted so that the values of the control points were within 1 pixel value.
(It turns out the raw uncorrected values were all very close to begin with (within about 5 values of each other)

If the images were clipped during creation of the raw images, I think they may have been clipped consistently the same way.

You are totally right, there is no way the data I'm playing with could be used to derive extinction coefficients. But if the data was processed (clipped) consistently it should be possible to provide a relative detector response. (Qualitative, not quantitative)