Learning Curve -Technical Issues
|The purpose of this page is to summarize some of the technical issues encountered in learning CCD image collection and processing. I am still a beginner, and have made many mistakes. It requires a great deal of patience, and discussion in the user groups to learn tips and avoid making mistakes. I hope some of this will be of help. |
Halos Around Stars in CCD Images
EEver see these donuts in your image? Ever see blue halos around
bright stars? I've done a bit of investigating and found some
surprising results. They come from various reflections between
the filters and camera, within the camera and even within the
CCD on certain cameras. Click on the image to see the report.
Click on image or here to see Halo Report
Light Box for Taking Flats
The following describes the light box recently built for my C11 SCT
telescope. The design goals were:
1. Light weight
2. Powered by 9V battery
3. Bright LEDs to take LRGB flats with ~1/3 CCD full-well
(~15,000 ADU) in ~10s
4. Uniform illumination
It was decided to use two 14" square, white, translucent plastic
sheets, 1/4" thick, available locally in plastic specialty stores. 1/4"
30x40" white poster (foamcore) boards were used for construction
of the light box. The design and electrical schematic are provided
in the following Adobe Acrobat file:
The white plastic sheets make up the base of the light box, separated
by about 5". Two foamcore sheets with a cutout for the C11 are
attached to the bottom. White duct tape is used to secure all pieces.
The bottom foamcore pieces are slightly larger than the plastic
pieces, and serve as a resting point for the top enclosure.
The top is made from foamcore and is about 15" tall. Triangles, 7"
on a side, were secured into each upper inside corner. They are
used as internal diffusers to even out the scattered light. Small 1"
boxes made of foamcore hold the LEDs about halfway up each
corner. They are wired in parallel and connected to a 5K Ohm
linear potentiometer using a 100 Ohm resistor to prevent over-
load. The shaft of the pot sticks out of the box and is secured
by a knurled knob. Also, leads extend to a 9V battery held on
the outside of the box near the knob. An on-off switch can be used.
If not used, then disconnect the battery leads when not in use
since the circuit will draw down the power of the 9V battery over
time. The top assembly slides down over the base.
All electrical parts were purchased from Radio Shack, and the
part numbers are identified in PDF document. Total cost was
estimated at about $40 USD. Each LED was about $5. Each of
the plastic sheets and the poster boards were also ~$5. You will
need a good razor blade knife and a soldering iron.
In retrospect, it may have been better to use black poster board. It
would be more star-party-friendly. The white box can be spray-
painted black, if necessary.
Base showing top plastic piece
and slightly larger bottom .
A 12" ruler is provided for
scale. The larger bottom
piece contains the cutout that fits
the outer diameter of the telescope,
in this case, an 11" Celestron SCT.
Bottom of base, showing bottom
plastic piece through cutout. Be
sure to tape completely around
the cutout to prevent foam particles
from being dislodged. They will
cling to the plastic by static charge
and will appear as dark spots in
the flat image.
Inside view the top unit. Note the
7" triangular inserts in each corner
that act as light diffusers. Each white
LED is contained in a 1" foamcore
box about halfway up the sides in
each corner. You can see the leads
here (longer lead is +). The pot is
on the left wall. Cover the wires with
white duct tape.
The bright white LEDs are manufactured by Nichia in Japan. I have
downloaded the technical literature on these NSPW500BS LEDs
here from the Nichia web site:
They combine a high quality phosphor with their recently
commercialized indium-gallium-nitride (InGaN) blue LED. They
found that when blue light from the InGaN die passes through a
thin phosphor coating, a portion of the blue light is down-converted
to yellow light. This yellow light mixes with the remaining blue
light to create a bright white light.
The emission spectrum of this bright white
LED is shown here from the PDF document.
It shows the blue emission from the InGaN
die at ~ 460 nm and the yellow (shifted light
from the phosphor) peaking near 560 nm.
There is little blue intensity below 450 nm,
whereas astronomical blue filters on CCD
cameras extend down to about 370 nm.
The yellow shifted light extends to about 750 nm, which
covers most red astronomical
The color balance of the LED is quite good, as shown in the
following chromaticity graph. For my C11, IDAS/Hutech Type III
RGB filters and the Kodak KAF3200ME CCD in my FLI camera,
my RGB Weight Calculator predicts weights of 0.9:1.0:1.6. These
weights, used for astromonical images, provide a white point adjust-
ment equivalent to using a sun-like G2V solar analog star. We see
sunlight as white. I found that I obtained about 17,000 ADU
in 5 s of exposure for the red and green filters using the light box
with these white LEDs. I needed 8 s to achieve 17,000 ADU for
the blue filter. Thus, I'm getting about the same R:G:B ratios with
the light box as I would for a white-point adjusted image of a
galaxy. This means that my light box is a suitable device for taking
flats for astronomical color imaging.
A flat image taken with this lightbox is shown below. With a new
battery, 5 seconds were required to obtain about 17,000 ADU with
the clear filter (unbinned on the CCD - CM10ME), about 5
seconds for red and green (Hutech/IDAS Type III -binned 2x2), and
about 8 seconds for blue (Hutech/IDAS Type III - binned 2x2).
At least 12 flats for each are taken to create a master flat. During
the time of image collection, the ADU count did not vary by more
than 50 or so. For me, attempting to do this with twilight flats
was stressful due to the limited time and concern about rapidly
changing light levels.
Also, the light box can slide over and cover the BORG 76ED APO
guide scope that I also use for wide field-of-view images. Never-
theless, I will likely build an inset for the bottom of the light box
that will allow it to fit the BORG directly.
Lastly, I have made one additional change by placing a connection
near the battery for 12V DC input from a modified cable coming
from the Gemini control unit for the MI-250 mount, or from
a 110VAC-to12VDC power converter. It is wired in parallel with
the leads going to the 9V battery.
This image show the light box on
its side at full intensity with a fresh
Although the clear filter used for this image had some dust, leading
to dust motes in the above image, there is reasonably uniform
illumination across the CCD. The darker area, lower center,
was later found to be a fingerprint smudge on the window
into the CCD. It was cleaned with methanol and is no longer present.
Don S. Goldman