[Aavso-photometry] In defense of the ST-9 & CCD choice Considerations

[Tim Crawford]

In defense of the ST-9 & CCD choice considerations

The following comment appears to have been a private
correspondence from one observer to another but the
receiving individual reprinted it in an email to our
group and I therefore feel compelled to respond to
what appears to be a “maligning” of a fine photometry
instrument.  I do recognize that it is quite possible
that the original author never intended for his
comments to be publicly circulated, in addition, the
original author, to his credit, did express that his
final thought with “imo.”
 
>The ST-9 despite it's hype is better suited for
astrometry and SN >patrols. The fov, lower noise and
spectral response make the ST8 a >better choice for
photometry imo.

Owning two SBIG products I am some one “tuned” into
the company’s web site and ads as well as a number of
CCD  groups. 

IMO:

1) I have never been exposed to anything that could be
classified as “Hype” for the ST-9, whether from users
or SBIG.  I am curious how the author received this
impression?

2) >is better suited for astrometry and SN patrols
.
That is a strong statement and here again, I would be
curious what foundation exists for such a strong
pronouncement and the “suggestion” that the ST-9 is
not suitable for photometry?

3) It is true, everything else being equal, that the
ST-8 should produce a larger FOV (about 1/3 bigger).
it is also true that the QE of the ST-8 is higher in
the BVR pass bands than the ST-9 (but not the Ic & Iz
pass band).  Both CCD’s have an identical Read Noise
specification, according to SBIG.  Today at SBIG’s web
site the ST-8, with a class 2 chips is selling for
about 60% more than the ST-9 with a class 1 chip,
there is no denying that there are generally
advantages when spending greater sums of money for a
CCD
. But a more expensive CCD does not necessarily
diminish the suitability of a less expensive CCD for
photometry.

4) As to which CCD is a better choice I think one of
the most important considerations for photometry is to
match your local “seeing” such that your scope/CCD
combination will spread seeing out over a range of 2-3
arcsec/pixel.  Fewer than 2 pixels will probably
result in under sampling, which will affect photometry
and should be avoided.  Over 3 pixels will generally
result in over sampling which with differential
photometry is not a big deal, although the more pixels
used the more read noise contributed by them to the
output signal
 there again, under normal circumstances
no big deal.  

Another way of putting  “it” is that 3 arcsec seeing
requires an image scale of 1 - 1.5 arcsec/pixel.

It is a good idea to try and get your local “seeing”
range from others in your area if at all possible then
use one of the online CCD calculators (HQ, has one in
the CCD manual)
 also try goggling CCD calculators. 
The CCD calculators will allow you to shop CCD Specs
to see which unit best suits your local seeing
conditions with your particular scope and f/ratio
setup.

Lets look at some examples:

In my case my local seeing range typically runs from
3.5-5.5 arcsec/pixel.

I have a 12” SCT that I use at f/10.

Below  image scale and FOV data is  from Ron Wodaski’s
CCD Calculator.

If I were to use a ST-7 the image sacle would be .62
arcsec/pixel and the FOV would be 5.3 x 7.9 arcmin
while with the ST-8 the image scale would be the same
.62 arcsec/pixel but the FOV would grow to 10.5 x 15.8
arcmin.  Both the ST-7 and the ST-9 have the same
pixel size, even though the ST-8 has a much larger
chip.

With my local seeing of 3.5-5.5 arcsec/pixel with the
ST-7 or ST-8 then seeing would be spread out over 5.6
arcsec/pixel to 8.9 arcsec/pixels.  This would be way
outside the range of 2-3 arcsec/pixel. (Seeing divided
by image scale of .62 arcsec/pixel).

If I were to use a Focal Reducer and take the system
to f/6.3 then both the ST-7 and the ST-8 would provide
an image scale of .98 arcsec/pixel with seeing then
spread out over 3.6-5.6 arcsec/pixels.  There again,
closer to the desired range of 2-3 arcsec/pixel but
still outside that range, although not a bad
compromise. However, in the case of the ST-8 I would
have to spend 60 percent more money to have this “not
a bad compromise” match.

Ok, lets bin the system at f/10.  This gets me an
image scale of 1.24 arcsec/pixel.  Seeing then would
be spread out over 2.8-4.4 arcsec/pixel.  Much closer
to the desired range and, admittedly, acceptable as
far as the scale goes BUT then it would leave me with
the concern of having to worry about being outside the
linear range provided by being unbinned and related
issues that I do not want to have to deal with when
trying to be accurate with my photometry.

The ST-9 has a pixel size about double that of the
either the ST-7 or the ST-9.

With my 12” SCT at f/10 and using an ST-9 I have an
image scale of 1.37 arcsec/pixel UNBINNED.  Seeing
then would be spread out over 2.6-4.0 arcsec/pixel.
And the FOV would be 11.7 x 11.7 arcmin.

While not ideal, for my local seeing range with my 12”
SCT at f/10, the ST-9 comes closest to matching the
desired range of spreading seeing out over 2-3
arcsec/pixel and not being binned.   

To really match your system to a CCD you need to know
your local seeing value as well as have access to a
“CCD calculator” then you can run all the options to
see what the changes are
 i.e., different size pixels,
different focal ratios, different size apertures,
binning, etc.

‘nough said

Ad Astra

Tim Crawford CTX
Arch Cape Observatory

http://homepage.mac.com/windwalker1/ACObserv.htm