Auto guiding

[Neo_uk]

Hi,

can someone please explain what auto guiding is ???? and dont say something that guides automaticly lol.

regards chris.

[steppenwolf]

Any motorised mount will suffer from inaccuracies in it's tracking. These inaccuracies result in star trailing and blurring in astro-images. To overcome these errors, it was common practice to install a second telescope on top of or next to the imaging telescope and use a crosshair eyepiece to keep a 'guide star' centred in the field of view. If the guide star moved off the crosshair, the operator would make an adjustment to the pointing of the telescope to compensate for the movement of the guide star. This way, if the guide star remained in the right place, so too would the object being imaged because the telescope being use to image was attached to the same mount so experienced the same corrections - Phew!! Now.....

....... Instead of using a Mark I Eyeball, install a second camera on the guide 'scope and use a computer to determine the movement of the guide star images colleted by this second camera and use the same computer to control the mount to keep it pointing at the guide star and you have an AUTOGUIDER!

[Neo_uk]

Thanks steve

thats very informative and everything i needed to know......you got it all there in a nutshell.

regards chris.

[Beyond_Vision]

An equatorial mount if perfectly aligned with the North or South Celestial pole and which has perfect timing of it rotation in the RA axis, would result in the mount being able to take infinitely long image exposures.

Of course nothing is perfect in life, polar alignment will not be spot on, and the mounts motor will have an error in its rotational speed. With a good drift alignment (polar alignment method)and good mount drive calibration (called PEC and Backlash Training) it is possible to do exposure of around 5 mins or so before star trails may be seen in the image. This time will vary due to focal ratio of the scope as scopes with large focal lengths have a higher magnification which will exaggerate any errors. The quality of the mounts motors and control electronics will effect the image exposure times

This is where Auto guiding comes into play it allows the user to take images with longer exposures by accurately controlling the pointing of the mount. There are three main methods.

The first method uses a single scope and a single specialist CCD camera which effectively contains 'two CCD's' one CCD is for taking the main image and another that takes an image that the guiding control software uses. The exposure time of each CCD is independent with the Guide CCD being read out at a much higher rate. The guiding software on the PC that the CCD is attached to, will allow the user to select a star in the guide image. The software will then attempt to move the mount in RA and Dec axis so as to keep the star selected at the same pixel co-ordinates. This will then result in the imaging CCD remaining pointing accurately at the target object without any evidence of the target rotating in the field of view during a long exposure.

The second method uses a single scope and an off-axis guider and one guide CCD camera and either one imaging CCD or a DSLR. A small portion of the light is split off from the main part of the scope and is passed to the guide CCD. The remainder of the light is then passed on to the main imaging CCD. The guiding software will then control the mounts RA and DEC motors as in method 1.

Method three which is the most commonly used consists of two scopes either mounted side by side or piggy back on one another. One telescope will be fitted with a guide CCD camera and other will have either a imaging CCD or a DSLR fitted. The guide software will control the mount as per the other two methods.

Pros and Cons

Method 1 the CCD cameras with built in guiding can be expensive and may take a performance hit over separate CCDs. The advantage is the set up is simpler only one camera to interface to no problems with alignment and only one scope required. The disadvantage is that if there are no bright stars in the field of view of the scope it cannot be moved off target to find a guide star since the imaging CCD would not be centred on the target.

Method 2 has the advantage of only needing one scope but you will have to buy an off axis guider attachment. The off axis guider also robs some of the light that could have gone to the main imaging CCD therefore requiring a longer exposure. The guide CCD needs to be quite sensitive due to only a small proportion of the light being split off to it. The same problem with lack of guide stars in the view may be present on this system as per method 1.

Method three has the disadvantage of requiring 1 guiding CCD and one Imaging CCD or DSLR and two scopes plus a means of attaching two scopes to a mount. There is also an increase in weight which may over stress the mount if not balanced correctly. The advantage is that one scope can be moved slightly with respect to the other so that you can find a suitable guide star. All the light through the imaging scope will fall on the CCD nothing is split off as per method 2. All the light is hitting the imaging CCD unlike the 1st method where the guiding and imaging CCDs may be interleaved. It is important that the guidescopes magnification is the same or if not greater than the imaging scope in order to accurately guide.

Hopefully that is about it, anybody feel free to correct me or add any other known methods

Regards

Kevin

[Sam]

That's an extremely useful post Kevin - Thank you Chris for asking the question.

Sam