[vsnet-chat 2498] Re: ST6- IR Blocking filter

["Paul Warhurst" <p.warhurst@auckland.ac.nz>]

Hi Stan et al.,
                        to comment on a couple of messages at once:

Rafael Costero wrote:

>Dear Stan and Arne:

>Regarding the halo Stan gets on I images: couldn't it be just fringing?
>We do have that problem with a back-illuminated 2K Thomson CCD when using
>narrower band-pass filters at 700 nm and longer wavelengths. Maybe the halo
>is produced by the ST6 itself and some near-infrared lines: There is a Na I
>doublet, VERY strong, in high- and low-pressure Sodium lamps at 818 nm (see
>Fig. 3 in Osterbrock et al., 1976; PASP, 88, 349). If there are such lamps
in
>Awanui and Kaitaia, that could by the cause of the halo.

    Fringing at longer wavelengths is common in back illuminated devices,
but I haven't heard of it occurring in any thick frontside chips like the
ST6's TC241 chip.

Stan Walker wrote:

>Paul,

>I don't have a flat field around that time. But if this were a flatfielding
>problem it would show up in the BVR filters as well.

If there were some flatfielding problem with the I filter it would not show
up in the other filters, this is why each filter needs it's own flat. Not
only do the dustspots obviously differ from filter to filter, but the global
sensitivity of the chip varies considerably with wavelength

>I think that you need dark skies to notice the effect - it's only about 50
>counts over 30 seconds - and whilst it may be present in unfiltered
>photometry the I signal is then only about 10% of the total.

If the count rate is low then the brightness of the sky may well be an
important factor, but rather than a bright sky being a problem for
determining the cause it could help if the problem is sky-related. The
signal is quite high if it is 10% of your background. A longer exposure
would be instructive, trailed star images is not a problem as it is the
background which is under study.
    Is the problem repeatable? The file header says the image is from March,
do recent images show the same effect?

    Happy New Year everybody, especially to the VS-Net team, for all their
excellent work, long may it continue.

    Paul Warhurst

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