Mike,

You know, I knew the pixel values depended on the gear, but somehow it
didn't
cross my mind to to convert those to arc seconds. Writing that at almost
5am might have had something to do with it :)

The 2.5 pixels FWHM (or 2.275" arc seconds) was unbinned at f/10. Only the
camera itself was attached, no filter wheel or anything. The values were
for
a 2 or 3 second exposure (forgot which now) on a dim star. I usually start
dimmer
for the final adjustments. In binned mode, the FWHM was somewhere around
1.2.

What I was really trying to ask was if my equipment is performing well. It
sounds like it is, which is great. I've heard enough about varying quality
from scope to scope I was concerned.

Now my main problem seems to be temperature. There sure is a hell of a lot
more thermal noise at -5C than -15C. :)


Darron Black

>Date: Sun, 01 Jul 2001 00:31:46 -0500
>Subject: Re: [M]: FWHM values, backlash correction, and the 201 autoguider
>
>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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