Fw: TN89 and hints on optical vignetting and stray light

["Richard Miles" <rmiles@baa.u-net.com>]

Michael,

Just an idea re. the repeat analysis of image data (as per TN89) but using
only dark-subtracted images.

It would be best to be selective and limit the magnitude range of the Tycho
stars used in the analysis since the fainter stars will have too large an
error bar due to photon statistics and the occasional bright star will be
affected by non-linearity / saturation of the CCD.  You don't mention in
TN89 whether you did restrict the mag range - maybe you did.

At present, my money is on stray light reflecting off the walls and other
odd surfaces/edges around as the source of the problem, in which case lining
with flocked paper so as to further reduce internal reflectivity should show
a direct improvement.  We'll see.

Just recapping on the situation we have.  The dew-shield / light-shield
looks to be fine and will not add to any vignetting (if present).  The most
constricting baffle is the one at 250 mm from lens face, diameter 160 mm.
This provides an acceptance angle of +/-6.8 deg.
For stellar photometry, the point source images are created by focusing
light collected over an acceptance angle of +/-2.8 deg (for the diagonal
dimension of the CCD) so there is an argument that the baffles are a little
too wide at present and are letting in more stray light than necessary.
Provided there is no slop greater than 10mm at the end of the dew-shield
then the 160mm baffle could be reduced to 142mm, and the 140mm could be
reduced to 125mm.  However, suggest waiting to see how well the flocked
paper works first before trying this.

Tom also indicates that there may be no light baffles between the lens
assembly and the CCD or filter assembly.  If the inside of the tube and the
dew-shield are indeed shiny black, then these surfaces will reflect stray
light (arising from lots of sources) entering the collecting aperture at
angles much greater than the +/-2.8 deg light contributing to point source
images.  Indeed, light entering at +/-45 deg will even make it's way down
the tube adding to the background sky flat thereby distorting the flatfield
correction.  I would be interested to see the optical design / layout since
I am concerned that the possible absence of light baffles from inside the
tube would be a serious shortcoming vis-a-vis stray light.

Just returning to the flat-fielding problem and TN89, I should reiterate
that using point sources to calibrate response across the CCD field is an
independent way of arriving at a solution, and is one that is virtually
unaffected by stray light.  Flatfielding methodologies of (a) adding sky
flats and (b) lightboxes or screens both potentially suffer from stray light
perturbation, and this is an increasing problem, the wider is the field.
Actually, there is an argument, which I shalln't go into here, that as field
size / CCD size increases, the point-source flatfield calibration technique
must become the most accurate method at some point or other.  Whether this
is the case for any particular optical system is best determined empirically
by direct measurement using all three methodologies.  It's a hard life!

Looking forward to seeing the results of your reanalysis of the 7x7 image
data.

Cheers,
Richard