High Resolution CCD Imaging

by Roland Christen

 

When I first started out imaging with my new HX516 CCD camera, I
tried to capture the planets at the fastest possible exposure to
minimize the image degradation caused by atmospheric "seeing". By
seeing, I mean the rapid distortion and blurring of the image that
occurs when the atmosphere is not steady. Where I image in Northern
Illinois, the atmosphere is rarely stable because we happen to be
under the jet stream most of the year. On those rare occasions when
the stars do not twinkle excessively, it is possible to see some of
the subtler details on the Moon and Planets at high powers.

Capturing this detail with a CCD camera is a whole different
ballgame. I have over 4000 images of the the Moon and Planets, most
of them hopelessly blurred due to seeing degradation. Out of these, I
have maybe 100 good to great images. In order to maximize my quality,
I did some experiments with different focal lengths, binning of the
pixels, exposure times and stacking. The results surprised me.

The instrument I used for most of my images is a 10" F14.6
Mak-Cassegrain which can be converted to F9 with a telecompressor,
and F30 with a 2" Barlow. The instrument has a theoretical resolving
power of 0.45 arc seconds. The HX516 CCD camera has a 7.4 micron
pixel size. Download time is 3 seconds in 2x2 binned mode, and 20
seconds in the unbinned mode. When used at the various focal ratios,
the pixel resolution is as follows:


unbinned at F9 = 0.67 arc sec per pixel
2x2 binning at F9 = 1.34 arc sec per pixel

unbinned at F14.6 = 0.41 arc sec per pixel
2x2 binning at F14.6 = 0.82 arc sec per pixel

unbinned at F30 = 0.2 arc sec per pixel
2x2 binning at F30 = 0.4 arc sec per pixel


I found the F9 focal ratio almost useless for recording any but the
most rudimentary  detail on the Moon and planets. I did all my
original imaging at F14.6 in the 2x2 binned mode and stacked as many
of the better images as possible. On Jupiter, due to rotation of the
planet, I was limited to perhaps 15 images, out of which maybe 7 or 8
were useable for stacking. In the unbinned mode, perhaps 3 images
were useable. The results were not too bad, but did not even come
close to what was being published by several of the top amateurs.

On  suggestion by Maurizio DiScullo, I did a series of Jupiter and
Saturn images at F30. because of our local seeing, I did these in the
2x2 binned mode (0.4 arc sec per pixel), and immediately recorded
much more sub-arc second detail than before. At F30, I recorded the
Enke minima on Saturn, lots of festoons and white ovals on Jupiter,
several craterlets in Plato and the rille in the Lunar valley. These
images were better for sure, but still not world quality.


A few weeks ago, the Moon was placed high in the sky at first
quarter, and the seeing was not too bad, certainly not Florida skies,
but good enough where I could easily see quite a few craterlets in
Plato, and the rille in the Lunar valley. The craterlets have very
low contrast, and are on the order of several arc seconds in
diameter. I imaged these at F9, F14.6 and F30, in the unbinned mode.
I was amazed that there were no craterlets in any of my F9 or F14.6
images, but they were sharp and clear at F30. The seeing was really
no different at the different focal ratios, but the ability to record
low contrast detail was very much affected by the arc-sec per pixel
ratio.


At F9 (0.67 arc sec per pixel) no craterlets and no
Lunar Valley rille was recorded in any of the images. Even
after stacking a dozen or more of the sharpest images, the floor
of Plato remained smooth as a baby's bottom.

At F14.6 (0.41 arc sec per pixel) the images recorded much
more detail, but there is only a hint of the central craterlet
in Plato, and the Lunar Vally rille just barely recorded as
a thin dimensionless line. This is after stacking more than a
dozen images also.


At F30 (0.2 arc sec per pixel) I had to mosaic several
images to get both Plato and the Lunar valley in one
picture. 3 or 4 of the individual images showed the central
craterlets of Plato clearly, and after stacking these, several more
appeared. There is even a hint of a short crater chain at the left
edge of Plato. The rille in the Lunar Valley shows much more clearly,
unfortunately by the time I took these exposures, the Moon had moved
lower in the sky, and thin clouds lowered the contrast. Also the
seeing had begun to degrade somewhat. Nevertheless, this portion of
the image still shows much clearer detail than the shorter focal
ratios did.

I believe most amateurs are missing a bet by trying to image at too
short a focal length, or using too large pixels. The usual rule of
thumb is to use 0.8 arc sec per pixel on the planets, and 2 arc sec
per pixel on deep sky. Even on deep sky images, I believe that scopes
can achieve much higher resolution by using as long a focal length as
possible. Most of my deep sky imaging is being done at 0.82 arc sec
per pixel, and I am now experimenting with 0.38 arc sec per pixel on
deep sky using an ST10E camera. This may sound like overkill in skies
that typically yield 2-3 arc second stellar images (some would call
this gross oversampling). However, by limiting the exposure to 15-20
seconds, co-adding many images and throwing out a few of the really
poor ones, you can build up a very high resolution final image even
in crummy seeing.

CCD cameras are quite capable of recording very faint detail even at
the traditional "long" focal ratios of planetary Cassegrains. In my
skies, a 5 > minute total exposure at F14.6 is just about the limit
before the sky totally washes out all detail. Even a 1 minute
exposure reaches 16th magnitude with the HX516 camera.


Roland Christen  ASTRO-PHYSICS 
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