Image Link and Sync
Telescope Initialization
Here is how to use Image Link
to accurately synchronize the scope, map points, or center objects
on the CCD detector. Image Link is very accurate - limited by your
scale in arcseconds/pixel.
In fact I argue that binning
say two by two in many cases (or even 3 by 3) will not compromise
the mapping or syncing in anyway. Binning will yield faster image
links because of the increased sensitivity of the CCD allowing
shorter exposures to be taken. Yes, that is to say your pointing
will not suffer when binning say 2-by-2 when mapping. Unless of
course your scale is on the order of 5 to 10 arcseconds BEFORE
binning that is. In these cases do not bin just use longer
exposures.
Another option is to
crop a smaller portion of the CCD image when mapping. If you have a
very large scale say 1 degree or larger often times having hundreds
of stars in the field can cause false matches. Best to crop down the
size of the image first before trying to map. This has two
significant advantages. A) the download/digitize time is a lot
faster, the images take up less space, and you will end up with
fewer false matches! Even if you have missed links (failure to link
caused by the smaller field) this out weighs the problem of getting
false links.
Image
Link and Sync is simply the best
technique to use when performing the initial telescope
synchronization when first starting out. To begin, the telescope must be somewhat close to
the target; however, the object to be synced on need not be
perfectly centered on the CCD detector! If the object to be synced
on
is anywhere on the detector, Image Link
takes care of the rest. After taking the initial CCD image of
say 5 to 8 seconds in length, paste it into TheSky and have Image Link
tell you where the telescope is actually
pointing.
The following describes this
procedure in more detail.
The
Image Link and Sync Procedure
First, take a
short CCD exposure in CCDSoft (say 5-10 seconds), then Edit | Paste
the image directly into TheSky where it is to be Image Linked. After analyzing the CCD image and finding a match to the
stars in TheSky Image Linked will then set
the scale in TheSky , matching your CCD camera and telescope, very
accurately. That is because the scale is computed based on the
actual image not by computing a theoretical scale. Small
discrepancies in manufacturing are then accounted for.
So the best way to synchronize
the telescope is not necessarily on a star in the field but rather
sync on the exact center of the CCD image (regardless of where you
have slewed). This is done using the Image Link
Information as found in the Object Information dialog box after
performing a successful link.
Successful Image
Link
Click "Accept Results" on
a Successful Link
When an Image Link
is successful you will see the above.
Next you will be
asked
Choose Yes if you are sure!
Now just left mouse
click anywhere on TheSky display, no need to be accurate, to bring up the Object Information
dialog.
Left Mouse Click on
TheSky Display anywhere
Use the Down Arrow on the right of
Object:
highlighted above
After a successful Image
Link has been performed simply left mouse click anywhere on TheSky
display. From the Object Information
dialog box you need to get to the entry for the Image Link
Information as shown. Image Link will NOT be the first
entry in the list when you click on an object or anywhere else in TheSky
display.
Get to
the Image Link Information
Use the Down Arrow
to the right of the Object
line
Now you will use the
Telescope tab to Sync the scope on the Image Link
coordinates. This is the actual center of the CCD image,
regardless of what you slewed to. Also, no re-centering, or jogging
of the telescope is required. Very handy.
Of course you can even use this
method to continually refine your telescope pointing all night
long. However, because it is very inefficient and prone to many
variables the preferred method for accurate telescope pointing is
to use telescope modeling software, i.e.
TPoint for
Windows.
But how can I put the telescope
back to the same position on successive nights?
Easy. Click here for a synopsis.
Example 2:
Here is an example of how to
center an object after a slew. Suppose you have slewed the telescope
to galaxy NGC3992 (M109). The telescope comes to a stop and the
on-screen cross hair indicates that the telescope is now pointing
accurately on the object.
Why doesn't it
in the first place?
However, because no telescope
points perfectly without modeling the object (M109) is normally not actually
centered in the field and maybe off by several arcminutes or more! (For an
extensive definition of problems related to telescope pointing
please refer to the following paper Patrick
Wallace, author of TPoint.)
Slew to an
object then use Image
Link
The
Image Link Information is accurate
not where the scope reports it
is!
If the telescope always
slewed exactly where you wanted it to TPoint
would not be needed, nor would objects need to be re-centered after
slewing. Of course this is far from reality.
Want to read about
it? <--Click here for a paper written by the world's foremost
authority on telescope pointing Mr. Patrick
Wallace.
Using the Image
Link Information to
Sync the scope is an extremely accurate method and does not
require any re-centering of the object, which requires acquiring one
or more images. When you Sync the scope on the actual
center of the image, the cross hairs shift from where they are to
the telescope's actual coordinates.
Note error in pointing
common
for all goto mounts
Telescope not where you wanted it!
NOTE: The telescope cross hair now reflects
the actual position of the telescope,
synchronized to
the nearest arcsecond or two depending on your
scale AND resolution of the telescope control system!
This is important to note. There are
many optical encoder systems or other control systems that have a
limited amount of accuracy. This can make the cross hairs fall
significantly off the target even though you have just synched on
the object. When TheSky queries the telescope for its' coordinates
these may be as far off as an arcminute or more limited not by
TheSky but rather how accurately your control system can report or
encoder system can report the position!
Why does this
matter?
If you were to zoom all the way in on
an object (1 arcminute field) at that point each pixel on the screen
is on the order of a fraction of an arcsecond. Simply take the field
of view in arcminutes
divided by your screen resolution in pixels and you have the
resolution in arc seconds/pixel. For example, a screen
resolution of 800 by 600 yields a resolution of 0.075 arc
seconds/pixel!
You would be lucky to
find a control system and/or a set of encoders that can actually
update to a fraction of an arcsecond or even an arcsecond. Meaning
that there will be a small error when comparing the scope's position
and the objects displayed position.
Solution: If you have
poor encoder resolution say less than about 1 to several million to
1 resolution do not zoom in! In doing so the cross hair will exhibit
erratic behavior because of the limitation. Again this has nothing
to do with TheSky but rather the telescope's ability to accurately
say "where it is" and that is 100% control system/protocol
dependent.
Older model LX-200 users
heed my warning! There are models that do not employ accurate
floating point numbers for the coordinates. These are the pre-High
Precision Models which
do not employ the long format mode! In these cases the
resolution of the cross hair plotting is limited to arcminute level
accuracy! This is also true of the common optical encoder systems
which are non goto where the encoder resolution is more often than
not less the several million to one tics (again arcminute level not
arcsecond!)
After accurately synching the telescope
using the Image Link
Information, try slewing to the object M109 again. The object
will now be centered on the CCD detector! However, this
assumes that the
object's
coordinates are accurate in the first place! That can be a BIG
assumption.
Note: This technique only
works well for a very small area of sky. The preferred method of
telescope modeling is by using TPoint
which
corrects for all repeatable pointing errors across the entire night
sky. TPoint is
more efficient and versatile then having to take multiple CCD images
for every slew. TPoint -
accept no substitute.
HINT! When you know
what your scale is in arcseconds/pixel you can actually have
Image Link find a match for you even if the object slewed to is no
where in the field! This works great for the initial sync and
often times saves you from having to hunt down the first star for
synching.
Tools | Image Link
Setup
Enter Known scale!
And enter a larger Search
Radius
The search radius is X times your field
of view. If your field of view is only 15 arcminutes in size then 10
times that is searching a full 2.5 degrees of sky for a match!
Even the worst pointing telescopes shouldn't have a problem meeting
that.
Using this technique with the Paramount ME
(assuming a reasonable polar alignment) the above allows you to Home
the Paramount and find exactly where it is pointing BEFORE ever
synching the Paramount or using any telescope modeling. Learn it
love it live it!
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