[AZ-Observing] Re: Flats and Bias
- From: Jeff Hopkins <phxjeff@xxxxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx
- Date: Sun, 15 Jul 2007 10:30:14 -0700
Hello Jimmy,
You should have bought a copy of "AutoStar CCD Photometry."
http://www.hposoft.com/ASCCD.html
For photometry, dark and flat fields are very important. If you use
dark frames, the bias is included. The dark frames with reduce and
pixel counts caused by heat and noise. The flat frames calibrate the
pixels to account for imperfections in the optical train and pixels.
If your optics and CCD are clean (and it takes great efforts to get
the dust to a minimum on the CCD chips), for just pretty picture
imaging you can sometimes get away from worrying about flat frames.
For photometry, they are important so I have done extensive
experimentation with them. Each technique has advantages and
disadvantages. You can make a light box, you can put a tee shirt over
the end of the telescope and illuminate it. You can aim at an
illuminated white card or sheet, you can take twilight flats (time
runs out fast, however, and you MUST turn off the drive as you will
pick up many stars even while the sky is still light. Some people
also take sky flats at night. This requires thousands of images to be
stacked. Naturally you can spend a great deal of time taking flats.
Once you have them you do not need to take more unless you disturb
the optical train (refocus, move camera (rotate or remove and
replace). This is why a fixed setup is important. Each night you just
turn on equipment, open the observatory and start imaging. Otherwise
you must take flats each time you set up.
What happens with the flats (as I understand it as it is handled by
software), is all the pixels are averaged and that value is then
divided into each pixel to produce a calibrated image.
Perhaps the most important part of taking flats is to get the CCD
uniformly illuminated. You must also expose to at least half well
depth. This means you must know the well depth and any amplification
so you know what the full well ADU (Analog to Digital Units) counts
are. It is also important to use a full spectrum light source (not
florescent or LED).
Lastly, while I have tried all the means of making flats,and the
resulting images vary greatly, I have found the end calibration is
not critical. In other words all the flats worked about the same.
Taking flats is an art and you will need to do a lot of experimenting.
Let me know if you have any questions. God luck.
Jeff
At 09:57 -0700 07/15/2007, Jimmy Ray wrote:
>To All,
>
>With everyone's input the issue of sharp focus and some of the exposure time
>issues appear to be resolved (thanks again to the community at large). The
>perplexing thing I'm trying to figure out now is that of "Flats" and "Bias"
>frames. I think that dark frames are fairly straight forward (cover the
>scope and shoot a frame using the same exposure, etc. and make at least
>three). But as for "Flats" what is the easiest / best method to do so? Do
>you make "Bias" frames at the same time or at the time your making dark
>frames? Anyone have a little insight or direction here?
>
>Jimmy Ray
--
Jeff Hopkins
HPO SOFT
Counting Photons
http://www.hposoft.com/Astro/astro.html
Hopkins Phoenix Observatory
7812 West Clayton Drive
Phoenix, Arizona 85033-2439 U.S.A.
(623)849-5889
(623) 247-1190 (Fax)
www.hposoft.com
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