[vsnet-chat 4662] Re: eta Car

[Fraser Farrell <fraser@trilobytes.com.au>]

> Ted Gull, one of two team leaders of our group, has been
> performing a simple photometry on each acquisition image
> taken with the STIS. Two measurements are done with an
> aperture centered on the central bright source and with
> a square aperture array 5 x 5 and 96 x 96 pixels (i.e.,
> 0.25 x 0.25 and 4.9 x 4.9 square-arc-seconds).

Bish,

It occurs to me that you could resample your 96x96 array as 4x4 bins. 
This might let you identify light-echo effects by tracking the change in 
each bin's luminosity with time. Or has this already been done?


I'm sure the majority of southern observers, like me, are using binocular 
or finder scope to observe Eta Car these days. This is Standard Procedure 
for 5th magnitude targets. Although it is naked-eye visible, it's a 
really crowded patch of sky and some magnification is helpful.

The combination looks stellar at typical binocular/finder magnifications 
and it's only when you pass ~50x that it begins to look "odd". The real 
Homunculus shape doesn't become evident at less than ~200x; and a clear 
resolution of the brightest inner bits of Homunculus from Eta Car demands 
a steady atmosphere and a magnification of about 250x. Which puts any 
comparison stars far outside the field of view, significantly reducing 
one's visual accuracy.

And having the greatest nebula in the Milky Way to distract you just 
makes it harder :)

I've also watched many attempts by local CCD gurus to image Eta Car and 
the Homunculus; and their #1 problem is pixel bleed from Eta itself. Even 
IR filters don't entirely eliminate the problem. And if the image field 
is widened to encompass some comparison stars, then there's pixel bleed 
from some of them too. Very short exposures help – but then you lose the 
Homunculus.

You're right – this is a difficult target!


cheers,
Fraser Farrell

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