[ATM] Corrector/reducer for a fast Newtonian

[fhh@npgcable.com]

David,

As someone whose day job is building CCD cameras for
astrometry, I want to comment on one of the assumptions
in your project, namely the CCDs in the cameras you
mention:

> I know of one design problem that I don't know how to solve: wide
> field imaging performance. A coma corrector is necessary of course,
> but the pixel scale (with 9 micron pixels) is 0.6"/pix with the extra
> x1.15 factor from a Paracorr. That suggests 2x2 binning, which would
> render the field of view awfully small with an ST-8XME (never mind an
> ST-7XME!). There isn't room to add a focal reducer between a Paracorr
> and the focal plane, and I'm not sure how well a focal reducer would
> work with an f/4.2 incoming beam anyway.

I suggest that the choice of CCD cameras, containing the
KAF-0402ME (ST-7XME) or the KAF-1603ME (ST-8XME), should
be looked at in the context of your wanting to do
astrometry.  Both of these CCDs contain "E"-series CCDs
with microlens arrays in front (hence the designation "ME").

A Kodak "E"-series CCD has an image area in which each pixel
is divided into two regions of markedly different quantum
efficiencies (QE).  One part of the pixel is covered with
polysilicon, and has a similar QE response versus wavelength
to the "old" Kodak CCDs predating the "E" and "ME" types.
The other part of each pixel is covered with indium-tin oxide
(ITO), which is much more transparent to shorter-wavelength
(violet, blue, green) photons.  In a poorly sampled optical
system, say one in which your typical seeing disk diameter
(FWHM = full-width half-maximum) covers less than two CCD
pixels, this variation in intra-pixel QE will potentially
offset the perceived astrometric position in the vertical
axis of the CCD.  I do not personally know of any serious
astrometric endeavor that uses Kodak "E"-series CCDs, unless
well sampled spatially.

The Kodak "ME"-series CCDs takes the "E"-series CCD and places
on it the microlenslet array, in an attempt to boost the QE
by focusing most of the incoming photons onto the portions
of the pixels covered by the ITO.  I do not know of any
professional literature indicating that any serious astrometric
endeavor has been attempted with any CCD utilizing a microlenslet
array.

When you calculate an image scale of 0.6"/pxl, that would be
fine sampling for the CCDs in the ST-8XME if your seeing does
NOT go better than 1.2" FWHM, the 1.2" being 2 pixels wide.

I have proposed at work that we purchase Kodak "E" and "ME"
CCDs to explicitly test for astrometric and photometric
accuracy, mostly with the idea of informing the serious
amateur community regarding what performance they can expect
from such CCDs.  No go so far.  We are about to host Arne
Henden of AAVSO as a guest observer, and he intends to try
some commercial CCD cameras on the scope, but with a 7 meter
focal length he will not be testing a poorly-sampled optical
system.  Weather permitting, Arne will have taken his data
within the next week, and I hope he will report back to the
community soon.

Rather than assume an SBIG camera and then making the optics
fit, how about an alternative CCD camera that better suits
your goals with your existing primary and Paracorr?  Perhaps
a camera containing an E2V CCD47-10 thinned back-illuminated
CCD with 13-micron pixels giving 1.03"/pixel, with a field
that is 17.5 arcmin on a side?  A pricier chip, but one that
is guaranteed to return good astrometry (and photometry).
If a custom coma corrector / focal reducer is going to cost
bucks, maybe spend that money instead on the detector?

Respectfully yours,

Fred Harris
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