When imaging deep-sky objects with my long exposure modified cameras I often have to discard frames because the stars are not nicely round, but have become streaks.
This is caused by the imperfection of the gears of my LX200 mount: the Periodic Error.
The problem is that I can only assess the quality of an image AFTER the whole exposure time has elapsed and then I am too late, because the streaks are already there :-\
The now following part describes how to use K3CCDTools to image with one webcam and autoguide with another webcam.
And here I describe how I image with K3CCDTools and my ATK16IC camera and Autoguide with Guidedog and a modified webcam.
The solution for this is AUTOGUIDING, whereby TWO webcams are used and automatically guiding corrections are made 'on the fly' during the long exposure imaging by the imaging webcam.
So: one webcam does the actual long exposure IMAGING, while the second GUIDING webcam is aimed at a bright star with shorter exposure times [but it is very nice if this Guiding webcam is also modified for long exposure, so that you can use e.g. exposure times of 1 second which makes it MUCH easier to find a good guide star].
The Long Exposure of my webcams is triggered via the parallel port of my laptop computer, and as I have one one parallel port to service two webcams I needed a Splitter Box
The computer software assesses the incoming image information from the guiding webcam and determines the shift in pixels and gives a message to the RA and DEC motors of the LX200 in terms of "a little faster/slower please".
This all happens while the long exposure imaging is going on.
Here are my own experiences with my LX200; whereby I image via my LX200 telescope and guide with a piggybacked 700mm refractor: my "Lidl" scope
One more thing: it is very nice that your Polar Alignment is as perfect as you can get it.
And lastly: make sure that your setup is properly balanced
February 24, 2006.
LX200 with piggy-backed Lidl scope
For Autoguiding I used the following:
- One USB port for the Imaging Webcam
- One USB port for the Guiding webcam
- The parallel port - with Splitter Box - for Long Exposure control
- The serial port to communicate - via a cable - with the RS-232 port of the LX200
- Two webcams; one for imaging and one for guiding
- Piggy back rail for the guiding refractor
- "Lidl" Skylux refractor which in this tutorial I use as Guide Scope
- Ideally the Guide Scope should have a focal length equal to or larger than the Imaging Scope. If you cannot manage that try for the next best and Guide with the longest focal length available.
In my case this is the 700mm refractor.
- I noticed that dew shields are required for both the LX200 and the Guide Scope.
- K3CCDTools with LX200 driver
2. Preparing for Autoguiding
Note: I assume you already know how to image without guiding; should you not know this then first go to my
Tutorials page, e.g. Beginners or DSO Tutorial
- Connect all the hardware, including both webcams.
[On this website you can read how to launch 2 instances of K3CCDTools, each with its own webcam]
- Make sure your scope is well balanced
- Open the first instance and select the Imaging Webcam (the Brightness Meter should show changing values)
- Now open the second instance and activate the Guiding instance
Here are side-by-side the settings I use: I always use the instance 1 as Imaging Instance and instance 2 for Guiding.
I have marked my long exposure cables also 1 and 2, so that I do not mix up things.
|Imaging Instance 
||Guiding Instance 
Note the difference in bit pattern (the green zeros and ones) !!
Note the difference in bit pattern (the green zeros and ones) !!
As I have an LX200 10" F/10 Classic telescope I selected the LX200 interface, which works fine.
3. Setting up the Imaging instance
Note: this Tutorial assumes you use a modified webcam for imaging, because that is my situation.
But of course you can use any other imaging equipment, such as DSI, Digital Camera etc. or even a film based photo camera.
For all imaging equipment the same issue applies: "we want no streaky but pin-pointed stars".
4. Setting up the Guiding instance
- Select the imaging instance
Make sure you have set the frame rate at 5 frames per second:
- Aim the scope at a bright star, focus very carefully and tweak the camera settings till it is nice and bright on your preview screen: now you will be able to find your DSO, because your setup is perfectly focused.
- Next make sure your imaging camera is perfectly East/West orientated: activate the K3CCDTools and use the N/S buttons to put the star on the horizontal line.
Next use the E/W buttons to move the star East/West: ideally the star should walk nicely on the line; rotate the camera to achieve this.
- Now find the DSO you wanted to image and tweak the imaging camera settings just as you would do when imaging without guiding
5. Automatic PEC Training of an LX200 Classic with K3CCDTools
- Select the Guiding instance
- Connect the Serial port of your computer with the RS232 port of the LX200 with the serial cable (see your LX200 manual)
- Via "Settings" and the "Guide" tab define your main environment
- Enter the details of your guiding scope and camera via Options|Telescope and CCD camera
Make sure you have set the frame rate at 5 frames per second:
- Aim the Guiding scope at a bright star, focus very carefully and tweak the camera settings till it is nice and bright on your preview screen.
This step is required so that later you will be able to find a - probably much weaker - guide star.
- Now make sure your guiding camera is perfectly East/West orientated and use K3CCDTools to determine the exact camera angle as follows:
- Put the star you want to guide on in the centre of the screen and turn on the reticle so you have the cross hairs on your screen.
- Move your mount in the RA (=East/West) direction only (this should be left or right on the screen)
- Adjust the camera until this motion is as close to as horizontal as possible (K3CCDTools will measure the angle and compensate but the closer the camera is correctly mounted the better)
- Put the star back in the centre and switch the LX200 off for just a second to verify that the star moves to the right (if it does not, rotate the camera 180 degrees; you can do this before step 3 otherwise you will need to repeat step 3 again).
- Now put the star all the way on the left side of the screen.
- Start the K3CCDTools Drift Explorer via this button:
- Place the tracking box around the star
- Press the angle button on the drift explorer
- Now press the EAST button on your hand controller to move the star almost to the right hand side of the screen.
You should do this as fast as the program can track the star to reduce the effects of poor polar alignment. If you go the wrong way do not worry, just go the opposite direction.
The program only measures the angle between the start and stop points.
- Press the angle button again. You should now have an angle. Depending on how accurately you did step 3 the angle should be very close to 0.0.
- Now the actual guiding can be initiated
- Note: after having imaged my first object I put the cap on the OTA and continue capturing: this way I get the dark frames.
While this is going on I slew the LX200 to the next object (experience has taught me that I am on that object then, although I cannot see it as the cap is on my OTA) I swap to the guiding instance and look for a guide star.
Sometimes I am lucky and a nice guide star is already waiting for me!
Other times I have to search for a guide star which I do very pragmatically: I loosen the 3 piggy back ring adjustment bolts that are closest to me and look through the (very tiny) finder of the Lidl scope.
While holding the OTA of the guide scope in my hand I manoeuvre the star to the centre of the cross hairs; at the same time I look at the preview screen. When I see the star I quickly fasten the piggy back ring adjustment bolts and I am ready.
This usually takes only a few minutes and when the guide star is nicely on screen I also have enough dark frames so I can close the imaging AVI, take the cap off the OTA and bingo: there is my new object on screen!
- Currently I use these settings of the Drift Explorer:
(Note: you can use my settings as a start, but after that you have to find out yourself what setting to use in your particular situation.
I too frequently change my settings, as the seeing is an important factor, so an imaging session usually starts with some time experimenting to find out the best settings)
- Please allow me to explain the parameters in my own words here (but you should read the K3CCDTools manual too of course!)
Incorrect values might cause your guiding to become too 'nervous' and thus overcorrect or too 'lazy' causing incorrect tracking
Dead zone parameter
Because of the turbulence the guide star might more or less 'dance' on your screen. Now, you would not want to do the guiding based on a dancing star, would you?
The Dead Zone parameter takes care of that and ignores the turbulence. More turbulence = increase dead zone value.
You have to experiment to select the best value for your setup and actual turbulence.
Hint (from Tom How): defocusing the guide star VERY SLIGHTLY can often help in hiding the effects of seeing from the guide camera.
If you notice a lot of drift with your mount then - in case of manual guiding with a hand controller - you would press the appropriate button longer than when you experience less drift.
This is exactly what the K parameter does: it amplifies the normal adjustment.
Again you have to experiment
Q is the time you would 'normally' press the button when doing the guiding manually.
For your convenience I have copied the following text from the K3CCDTools manual:
The length of the autoguide pulse (in milliseconds) is calculated according to an equation:
t = K * delta + Q
- where delta is the measured drift
- K & Q are the coefficients
This means, that Q defines the minimal pulse length when the autoguiding intervention is made, while K defines the amplification of the measured drift.
Here you specify how frequently the system should 'look' at the guide star.
The value depends on your imaging exposure time: for example it would be silly to 'look' at the guide star only every 15 seconds when you are doing a 60 second imaging exposure, as you might be too late and the streaks/elongated stars are already there. But also: when the frequency is too high your guiding will be nervous.
Experience will tell what value to use, but take it easy.
- Now click on the top left button of the Drift Explorer panel and select the bright guide star on the preview screen
- Enable RA and DEC Control and checkmark the Guiding box; optionally also checkmark the Guide Log box
- Presto: the Left/Right/Up/Down Arrow buttons start to blink, indicating that the your setup is already autoguiding!
- To verify that the guiding commands actually reach the LX200 and are also correctly interpreted you can deliberately fool the LX200 by pressing the RA and DEC buttons on the Menu bar of the Drift Explorer.
As this reverses the guiding operation, the star on the imaging instance should race off screen, because the guiding now has an opposite effect and the error is magnified.
You can use this 'feature' to ensure that everything is indeed working.
- Note: the arrow button at bottom right is to resetting all pressed buttons and also reset to the normal sidereal speed.
Did you know that you can use your Guiding instance for automatic PEC Training?
[Note: PEC training is only applicable for Polar mode mounted LX200's and not for Alt/Az; see your LX200 manual.]
It works like this:
- Prepare the Guiding instance as explained in the previous chapter
- Make sure that you have selected the LX200 as guide scope with the correct focal length [2500mm] and the correct camera via Options|Telescope and CCD camera
- Click on the top left button of the Drift Explorer panel and select the bright guide star on the preview screen
- Enable RA and DEC Control and checkmark the Guiding box; optionally also checkmark the Guide Log box: you are guiding now
- Use the LX200 Keypad Hand Controller to select the SMART function
Choose 'LEARN' if you want to start the PEC training from scratch or 'UPDATE' if you want to refine the PEC training
- When the LX200 is ready to start the PEC training it will start beeping and will continue to do so untill the full cycle (480 seconds) has been completed. You do absolutely nothing, just wait till the beeping stops. That is all.
6. Examples/results of my Autoguiding setup
Nice flat graphs !
54 autoguided exposures of 40 seconds:
This screen print nicely shows my autoguiding accuracy: round stars!
Imaging: William Optics Zenithstar 80 ED II APO, f/6.8 [focal length 545 mm]; exposure time: 300 seconds; camera ATK16IC.
Guiding: LX200 10-inch f/10 [focal length 2500mm]; camera SC3 modified webcam.
Autoguiding with GuideDog
I am always ready to try something new so I tried out to Autoguide my WO ZS 80 ED II APO and ATK16IC camera with my LX200 and Vesta SC3 camera with K3CCDTools as imaging software and GuideDog as Autoguiding software.
I used instance 1 of K3CCDTools as imaging instance [as usual], but instead of firing up the 2nd instance of K3CCDTools for autoguiding I now launched GuideDog instead.
That turned out rather well, but intially I of course had to learn how to operate GuideDog.
After a while I settled for the following settings [which might need fine tuining but nevertheless I record them here for your and my own reference]:
[For Relay Box and Logging I used the default settings].