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The standard use of
the LX90 and other fork-mounted scopes is Alt/Az mounting. This is fine
for visual use and for short exposure astrophotography (perhaps up to 30
or 40 seconds), but is not the most desirable way to capture images. The
problem with Alt/Az is that it cannot rotate the scope as it tracks the
image and therefore field-rotation will be evident. Field-rotation is
where the centred image physically rotates upon itself and other objects
near the edge of the FOV will appear to display arc trailing.
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Imaging
methods |
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In order to avoid
field rotation during astrophotography, you have three options:
1. Take short (eg. 30 second) exposures and stack them
I am not opposed to this method and it surely works. Indeed many of the
images on my website were obtained in this manner. However, using this
method is labour and computer resource intensive. I can demonstrate this
in the following table:
Processing software:
Images Plus v2.0
Computer: Dell Inspiron 2.0 GHz MP-IV 512 Mb RAM
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Scenario 1 - 100
x 30 seconds |
Scenario 2 - 10
x 300 seconds |
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Process |
Time (seconds) |
Hard disk space (Mb) |
Time (seconds) |
Hard disk space (Mb) |
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Acquisition |
100 x 30 + 100 x 10 = 4000 |
100 x 6.5 = 650 |
10 x 300 + 10 x 10 = 3100 |
10 x 6.5 = 65 |
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Convert to TIF |
100 x 15 = 1500 |
100 x 36 = 3600 |
10 x 15 = 150 |
10 x 36 = 360 |
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Calibration |
100 x 30 = 3000 |
100 x 36 = 3600 |
10 x 30 = 300 |
10 x 36 = 360 |
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De-rotation |
200 x 10 = 2000 |
100 x 36 = 3600 |
20 x 10 = 200 |
10 x 36 = 360 |
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Combination |
100 x 20 = 2000 |
1 x 36 = 36 |
20 x 20 = 400 |
1 x 36 = 36 |
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TOTAL |
8900 (148.3 minutes) |
11486 (11.468 Gb) |
4150 (69.2 minutes) |
1181 (1.181 Gb) |
2. Use a field de-rotator
A field de-rotator attaches to the visual back of the scope. This
electronic devise slowly rotates the camera to offset any rotation of
the image through the scope. However, a field de-rotator is
prohibitively expensive (much more than a standard wedge) and problems
have been reported with their use. That's a lot of money to pay for
something that might not quite do the job!
3. Autoguide your exposures using a second camera
You need more than an autoguiding camera to autoguide. You also need a
wedge so that you can
polar align your
scope, a guidescope to guide through, and a field computer that can run
guiding software.
After experimenting
for several months with the multiple short exposures method, I have now
begun experimenting with autoguiding. Here is a list of the equipment I
use in addition to my LX90:
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Device |
My Equipment |
Description |
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Guide scope |
Sky-Watcher 804 |
Short-tube 80 mm achromatic refractor
piggybacked on my Lx90 |
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Guide camera |
Philips ToUcam 740k Pro webcam |
This webcam is NOT modified for long
exposures |
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Computer |
DELL Inspiron 8500 laptop |
2.2 GHz, 512 Mb RAM. Autoguiding
occurs through the parallel port (see below) |
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Scope port |
APM (Accessory Port Module) |
This accessory for the LX90 accepts
guide commands of the generic ST4 type |
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Relay box |
Shoestring Astronomy
guide interface |
This device converts parallel signals
into guide port signals recognisable by the APM |
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Guide software |
GuideDog by
Steve Barkes |
This is a simple freeware application
for autoguiding (K3CCDTools also works) |
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Autoguiding software
such as GuideDog or K3CCDTools can correct for deviations in both RA and
DEC. The better your initial polar alignment, the less work the
autoguider has to do to counter DEC drift. Theoretically, if you have
perfect polar alignment as you
might attain through the
drift alignment method, you could switch off the DEC motor
altogether (assuming this was possible for the LX90) and select only for
corrections in RA. Moreover, if after
periodic error training, you also have perfect RA motion, you should
not need to autoguide. Unfortunately, however, we don't live is such a
perfect world. |
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My
autoguiding technique |
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With GuideDog (or K3CCDTools) and an unmodified webcam,
you can guide your scope via the serial port using the #505 cable that
plugs into the Autostar handbox OR you can guide from your parallel port
directly through the APM. The problem with the latter is that you need a
relay or interface box to convert the signals into something readable by
the APM autoguiding port. Of course, you need the APM also. (The APM
Autoguiding port is a standard ST4-type used by a number of
manufacturers.)
The parallel port and relay interface method is considerably better for
guiding. Dick Seymour has reported that the serial method updates the
scope position every second or so but only if resources aren't being
directed elsewhere at the time, whereas the APM autoguider method
updates the star position 10 or more times per second and is independent
of what the scope may be thinking at the time.
The Guide Port Interface
is available from
Shoestring
Astronomy in the USA. The version I bought also has a 'pass thru' (GPINT-PT)
that additionally permits a second device, typically a Long Exposure
(LX) modified webcam shutter control device, to utilise the same
parallel port. An image of the device is shown below. |
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The image below shows my first autoguided image. It is
a single 600 second (10 minute) exposure of M42 taken on
30 October, 2004. The intention was not to create a great
image but simply to check the autoguiding potential of this setup. A
full description of this image is further below. |
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This image is a 10 minute (600
second) autoguided exposure of M42 taken after
PEC training and
retraining. This was taken using
Canon's 300D DSLR at f/6.3. The image was captured using DSLR Focus as a
large/fine JPG image at ISO 800. Autoguiding was via a ToUcam Pro 740k
unmodified webcam through the parallel port to the ST4 (CCD) input on
the APM. The guidescope was a Sky-Watcher 80 mm Short Tube refractor
(f/5). Guiding corrections were made automatically using
GuideDog
software by Steve Barkes. The image size was reduced in Photoshop and
slightly darkened using Photoshop levels. |
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Autoguiding conclusions |
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Meade's LX90 SCT can autoguide very
effectively, particularly if the APM is used via a relay box . There is
no discernable affect of the worm gear cycle this image above (image is
10 minutes; worm gear cycle is about 8.5 minutes). The stars in this
image are nearly perfectly round (some vignetting is evident due to the
f/6.3 FR) and there is no reason why longer exposures of 30 or even 60
minutes couldn't achieve success should one desire to do so. My ultimate
plan is to take several 5 minute images of dim objects for later average
or median combination. |
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All information and images are copyright
© 2003-2004 by P B Langsford. Please ask if you wish to use them or link
to them |
