By Bill Worley
I should start this by
telling you a little about my imaging experience. I started
imaging in 1999 with a Meade Starfinder 10 inch Dob! I had a
webcam hooked up to it and used it to image the moon and planets.
After playing around with that setup for a while I bought a
Meade LX200 10" f/6.3 in 2000. Again, I used my
bought a laptop to use with my new scope. Here is one of my
first decent Saturn
images. After that I decide I wanted more that a webcam could
do. So, I bought a Pictor 208XT for $499 and a 3.3 focal
reducer. My first image from that imager was extremely pleasing
to me at the time. It was M82.
After several months of playing with the 208XT I bought a SBIG ST-237A
in January 2002. Shortly after that I bought a wedge. The
first image shot with the 237A and wedge was M42
and that got my blood pumping... :) I used that setup until
November of 2004. The next imager, which is my current imager,
was a SBIG ST-2000XM with the ST-237A guiding ccd in one unit. To
say on-board guiding is great would have been an understatement.
My last and final purchase was a SBIG AO7 unit in June of 2006.
Now to my primer. This primer will hopefully give you some
direction in your imaging and get you to level where you are satisfied
with your images quicker. Any comments are welcome!
The LX200 can be cranky at times for no apparent reason for those
unfamilar with the LX200. I've put the most important issues to
solve at the top. The first thing I strongly recommend is buy a
wedge for your LX200. Without a wedge you are limited to 15-45
seconds on your exposure length depending on your focal length.
Everything below assumes you are using a wedge.
POLAR ALIGNMENT - This is a
very important aspect of imaging. Get a good polar alignment
using the drift method. Once you've done it a few time it will
become second nature. Take notes the first few times out on what
way make what adjustment. I do not recommend using the iterative
method if you want to image. Use a recitle eyepiece at around
200x. You will be able to see your polar mis-alignment alot
easier at higher powers. But, don't crank it up to 400x as
your probably be adjusting for seeing. I ended up putting in a
permanent pier because I was tired of 'wasting' time polar
aligning. But, a good polar alignment can be done in 45
minutes using the drift method. If you plan on guiding don't
think guiding will solve a poor polar alignment because it
won't. One last thing... do NOT use the bubble level
on the wedge. It is extremely inaccurate and will affect your
Dark Frames - will
help you minimize the noise produced by your imager. Subtracting
dark frames from your images will improve your results alot. If
your camera doesn't have a shutter you will have to cover the end of
the LX200. My Meade Pictor 208XT did not have a shutter
but every camera I've own since has had a shutter. A dark frame
is basically a image with your shutter closed. This produces a
repeatable noise in the image from your ccd imager which can be
cancelled out by
subtracting it from each image you have taken. Dark frames have
to be take at the same temp as the light images.
FOCAL REDUCERS -
In the beginning, I'd recommend buying a focal reducer. The
shorter focal length will make a less that good polar alignment produce
better result because the wider field of view is more forgiving
to errors. With smaller ccd chips (700 x 700) you can use a 3.3
focal reducer. If you have a ccd chip much bigger it will show
sever vingetting and coma. Vingetting can be fixed with flat
field images but the coma can not be corrected. There are also
6.3 focal reducers available which would be forgiving because you're
shooting at a shorter focal length than at your full focal length.
- or PPEC or SmartDrive. Whatever you want to call it.
Training your PEC (periodic error correction) of your LX200 can reduce
the spikes in errors of your RA gear to provide better images.
Every RA gear no matter how well made has inperfections. PEC on a
LX200 helps minimize those errors to (hopefully) a more manageable
level. It won't get rid of the errors but will make the
larger errors alot less. This helps your auto-guider work less
and provides better tracking. PEC training can be done by either
using a recitle eyepiece or a ccd imager. Each tooth of a LX200
equates to 2.4 seconds. A complete revolution of the RA gear
takes 8 minutes. If you use a imager an exposure of approximately
1.5 seconds should be enough to train the drive properly. With an
exposure less than that you will probably be following the seeing.
GUIDING - To
guide or not to guide??? When you first start imaging I'd
recommend not guiding. Only because that is one more piece in the
puzzle that can go wrong. Start with getting good images with a
3.3 focal reducer then move to longer focal ratios which are less
forgive that don't hide the LX200 error as easily because of the pixel
scale of your image. After you've started getting decent images
with a focal reducer buy an auto-guider and give it a try. I
highly recommend SBIG's ST series of imagers with a guiding ccd
intergraded into the ccd camera. With the intergrated guide
ccd chip you don't have to worry about a guidescope. You can also
guide with a seperate imager through a ccd imager and a guidescope
piggybacked ontop of your LX200. I did not like that setup.
Too many issues with flexure, etc. I also only run my guiding
aggressiveness at 3 or 4.
BALANCE - Once you
start guiding with your LX200. You'll soon realize how very
important a good balance is especially in DEC. If your balance is
off in your LX200, your DEC correction in one direction will be
almost non-existant if not entirely non-existant. If you have
trouble calibrating your guider your balance is probably off
some. To get good performance in guiding make sure the LX200 is
balanced! Setting your backlash on your LX200 up will help
tremendously in your dec corrections will guiding. Experiment
with the backlash setting to see what setting your LX200 works best at.
Collimation is yet another important aspect in getting the most out
your scope. A scope that is out of collimation will produce less
than stellar results. Here is an image of NGC891
with my scope out of collimation. Notice how the stars are in
focus on the right but look strange on the left? That's the
collimation of my LX200 being out. I usually use a recitle
eyepiece to help me know where the center of my field of view is
which is important in collimating a SCT. I get a decent
collimation with the eyepiece on BOTH sides of focus then attach my
imager. Using my imager I use the focus mode and take a 1 to 5
second exposure of a slight out of focus star. Then I adjust my
collimation to make the out of focus star pefectly circular the whole
way across my imager's field of view. Make sure you adjust
collimation on both sides of focus. Pay attention to the four
corners of your focusing images
Flat Field Images
- are basically correcting for any vingetting (unevenness in light)
over the field of view and any dust on your imager or scope that
may show up in your images of your LX200. Typically, there is a
brightness towards the middle that gets dimmer as you move away from
the center of the scope. A flat field is obtained by taking a
short image of a bright uniformly lit light source. I made a
lightbox for less than $20 that I put on the end of my scope. The
effort is well worth it.
- Adaptive Optics 7 has been a god send for those owning a LX200 and
wishing to do any higher resolution imaging. It has been a
welcome addition to anyone into imaging no matter what the quality of
their mount is. AO7 is one of the reasons I highly recommend the
SBIG ST series of imagers. The SBIG AO7 unit corrects by using
tilt/tip mirrors that are able to do 10 milliseconds corrections for
any mount errors. The rate of corrections is alot faster than any
mount could ever do. Bar None! I've done more than 8
corrections per second using my AO7 in the short time I have used it.
- Most of us find that we prefer different software for post processing
our images. I
prefer CCDSoft for dark and flat field subtraction, alignment,
and combining frames. for combining of my LRGB and DDP (if I use
it) I prefer AstroArt. Adobe Photoshop is used for finally
adjustments. Your taste may be different.
Length - When I was imaging in alt/az mode I shot anywhere for
15 to 30 seconds at f/3.2 or f/2. When I bought my wedge my
exposures were usually 45-60 seconds. Since I started guiding I
prefer to shoot 5 minute exposures. 5 minutes is a good balance
for minimizing noise but not losing too much if something goes wrong
during the exposure. Sky pollution can affect how long you can
expose a image before it gets light gradients or just over saturates
the image. In your area, 3 minute exposures might be the
maximum. Experiment to see what is best in your area. I
have seen decent images from 30 second subs. I've seen some
horrible 5 minute exposures. Don't get too wrapped up in going to
a longer exposure before you're ready.
thoughts on imaging using a Meade LX200... I'd highly recommend
Bennion's Image Acquisition Best Practices Video if you have a
broadband internet connection. It is excellent and gives alot of
insight into understanding the 'smaller' issues that can really affect
resolution of your images such as collimation and focus.
Here's a couple of links to my imaging progress over the years.
If someone would have given me more direction the learning curve would
have been alot shorter. Hopefully, you find direction in
First Pictor 208XT Image - M82.
First SBIG ST-237A Image - M42.
Last SBIG ST-237A Image - M27.
One of my First SBIG ST-2000XM Images - Horsehead
Latest Image from my SBIG ST-2000XM Imager - NGC6820/3.
Please E-Mail me
with any comments, corrections or suggestions are welcome.