Hi Darron!

At 04:40 AM 6/30/2001 -0500, you wrote:
>1) Focusing my CCD, I usually watch FWHM values in Maxim DL. What values
>should I be able to obtain on good nights? The best I've managed has been
>2.5 pixels.


The actual FWHM values you will obtain are dependent on several factors.
Focal length,
pixel size, and seeing all factor in strongly. Pixel size includes binning
as part and parcel
of it's consideration.

Now - that being said, I will state that your values for FWHM seem about
normal for
an unbinned image for your camera and focal length; for a binned one, you
should be
getting much smaller values.

One way to help yourself out on this is to calculate the number of
arc-seconds per pixel (
or vice-versa) and then see if that matches the seeing pretty closely. To
do this, multiply
206.265 by the pixel size in microns, and divide by your focal length in mm.

For your focal length (8" f/10 = 2032mm) and the Pictor 416 in unbinned
mode (9 um),
your resolution is 0.91"/pixel.

If your FWHM values are 2.5, then this suggests that the diameter of the
1/2 falloff point
of the star (which is essentially the airy disc) is equal to 2.275". This
suggests "2 arc-second seeing."
Not bad. How short are your focus exposures? Much of the seeing can be
eliminated by using a
sufficiently short exposure, and then the numbers decrease.

Now - if you were operating in binned mode, your pixel size would be 18um,
not 9, and the resolution
would be 1.82"/pixel. Not good if your values are still 2.5. :-)

The best I've ever gotten, and I live in San Antonio, Tx at ~ 1000 ft above
sea level (seeing is usually
rough here) is 1.1 for an FWHM. My average is about 1.3-1.5. This is my 12"
Lx-200, with an f/6.3 reducer
and other optical train elements such that my resolution, unbinned, is
1.13"/pixel. When operating at my
native f/10 (fl ~ 3048mm), my best is around 2.3. In "high resolution"
mode, which is how I do planetary
imaging, I use a TeleVue 2.5x PowerMate that increases the ratio to f/22.75
(~ 7000mm) and my resolution
becomes 254mas/px. I can't get an FWHM better than 4.5 under those
circumstances.

Hopefully all this info has helped you judge your particular situation.
There are no "right" values, exactly, for
FWHM. The key is that they should be as small as possible. If you're
unsure, calculate the estimated "seeing"
from your resolution and FWHM, and see if it's reasonable. If you've gone
back and forth in and out of focus a couple
of times and believe you've hit the best, you probably have.

A couple of final caveats: Make sure your collimation is really good. Also
make sure that you are checking the
FWHM values of a star that is centered on the chip - not off to one side.
This goes double if you're using a
focal reducer. I get noticeable coma at the edge of the chip on my 12"
Lx-200 if I'm using the f/6.3 FR/FF. I
always focus with a centered star.

Hope this helps...

Mike F
http://home.satx.rr.com/mfastro

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