Guiding Equipment

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AO-7 Adaptive Optics

Autoguiding an LX200 or RCX400 can produce good results (round sharp stars) at lower focal lengths (e.g. 1000mm or below).  However, the accuracy of the mount on these telescopes even after PEC training does not yield very good results a the full focal length of the scope (2000mm or above).  This is because the mount position can only be corrected every few seconds at best and any error in the tracking of the mount during the correction periods shows up in the images taken at longer focal lengths as large of elongated stars.  It is not just the stars that are affected though.  The fine detail in the object being imaged is also lost as it is smeared out over the CCD chip during the exposure.  A very good solution to this problem is to use the AO-7 adaptive optics system from SBIG.  This unit attaches between the CCD camera and the telescope.  It has a mirror which reflects the light from the telescope onto the CCD chip.  Very short exposures of a guide star (up to 40 images per second) are taken using the guide chip in the CCD camera and the AO-7 tilts its mirror between each exposure to compensate for any movement caused by poor tracking or atmospheric disturbance.  If the guide star drifts too far over time for the mirror tilt to compensate, then the AO-7 sends pulses to the mount to adjust the guide star position back towards the centre of the mirror.  The frequent and very accurate correction of the tracking that is possible with the AO-7 mirror means that inaccuracies in the tracking of mount are corrected as well as some effects atmospheric disturbance.  You can see the difference between guiding with the AO-7 and the RCX400 in these images of M1.  In 2008 I replaced with AO-7 system with the new AO-8.  This has the advantage of straight through optical design.  This leads to a shorter backfocus enabling me to use the adaptive optics on the FS-60C which has a much shorter backfocus than the RCX400.  The straight through design also allows the the camera to be mounted in line with the optical axis of the telescope instead of at right angles.

Celestron Radial Off-axis guider

oag.JPG (98433 bytes)An off-axis guider (OAG) contains a small prism which diverts some of the light around the edge of the telescope's field of view (FOV) through 90 degrees and into an eyepiece.   The prism is small enough that it will not interfere with the FOV of a camera attached behind the OAG. This allows the observer to see a small part of the sky just outside the FOV that is being photographed.  This is useful for tracking the motion of the telescope when taking an image and making small corrections so that telescope drive or polar alignment errors do not lead to pictures with star trails. 

The technique for using the OAG for astrophotography is as follows.  The best eyepiece for use with an OAG is one that has illuminated cross hairs (the reason for this will become clear).  This type of eyepiece is often referred to as a reticle eyepiece.  Having centered the object to be photographed in the FOV of the camera, the observer looks through the reticle eyepiece and hopefully sees a star.  Assuming a star is there, then for the duration of the photograph the technique is to make adjustments to the telescopes RA and DEC drive so that the star always remains in the same position relative to the reticle's cross hairs.  This will ensure that the object being photographed always stays in the same position in the camera FOV.  Two things immediately leap out at you from this technique.  One is, what do I do if I can't see the star?  The second is, wouldn't it be nice if I could move the image of the star into the centre of the cross hairs in the reticle eyepiece. The Celestron Radial Off-axis Guider has solutions for both of these.

The Celestron OAG allows you to rotate the reticle eyepiece approximately 145 degrees around the outside of the telescope's field of view without changing the position of the camera or the focus of the telescope.  This allows a much better chance of finding a guide star without having to rotate the complete OAG camera assembly. 

The angle of the Celestron OAG prism is adjustable which enables you to move the image of the guide star in the reticle eyepiece FOV.  Combine this with the ability to rotate the reticle eyepiece in the eyepiece holder and any star can quickly be centred in the cross hairs.

Celestron Illuminated Microguide Reticle Eyepiece

celreticle.JPG (88969 bytes)The Celestron Illuminated Microguide Reticle Eyepiece is ideal for use with the Celestron Radial Off-axis guider.  It has four separate scales etched on the reticle which allows its use for guiding, measuring the angular position of double stars, determining the periodic error of your telescope's drive or measuring the position angle of a comet's tail.

The illumination is provided by a battery powered red LED.  The illumination power is adjustable, but even on the lowest setting it can drown out some faint guide stars that are visible with the LED switched off.  It is possible to overcome this problem by painting the LED with several coats of red nail varnish.  This reduces the illumination to an acceptable level.

I have found this to be a very useful and accurate piece of equipment.

Meade 9mm Illuminated Reticle Eyepiece

medreticle.JPG (95097 bytes)The Meade 9mm Illuminated Reticle Eyepiece is ideal for use with the Celestron Radial Off-Axis Guider.  Unlike the Celestron reticle eyepiece which can be be used to measure angular positions and distances as well as guide, the Meade reticle eyepiece is designed only for guiding.  It has cross hairs and a bullseye etched on the reticle.  However, unlike the Celestron eyepiece there are two screws on the Meade eyepiece that allow you to adjust the position of the bulls eye in the FOV.   When combined with capability of adjusting the prism on the Celestron OAG this makes it very easy to centre a guide star in the bullseye.  The LED in the Meade reticle eyepiece is attached to a cord which plugs into the control panel of the LX200.   The LX200 provides power to the LED but you can also use the LX200 handcontroller to adjust its brightness and pulse it on and off.  Using the LX200 handset I have found that I can set the LED brightness much lower than the lowest level attainable on the Celestron eyepiece.  That allows you to see fainter guide stars.  Also the pulse function should allow for even fainter guide stars still to be used.  The Meade eyepiece does have a smaller FOV than the Celestron eyepiece as it has a shorter focal length but I have not encountered any problems with this as I can move guide stars into the smaller FOV using the adjustable prism in the Celestron OAG.

LX10 Hand Controller

The LX10 hand controller allows you to make RA corrections when guiding. It can either stop the RA drive for Eastward correction or run the drive at 2x sidereal speed for Westward correction.  If you add the optional DEC motor to the LX10, the hand controller can also be used make 2x corrections in DEC.  The hand controller and DEC motor are essential items for anyone considering using the LX10 for astrophotography.

The hand controller can also be used to make small adjustments to the telescope position for centering an object on a CCD chip or in a reticle eyepiece.  I have found it useful to be able to operate the hand controller while sat at my PC looking at the images coming from the CCD camera as this allows me to position the an object correctly on the CCD chip more easily.  This would be easy if the PC is right next to the telescope but mine is several feet away.  To extend the handset cable so that it would reach the PC, I purchased a length of 6 wire telephone cable with RJ11 connectors on each end and a connector block to join it to the standard handset cable.  It is important to check that the cable and connector block are 'straight through'. That is, the RJ11 socket on the connector block that you plug the handset into must have each pin directly connected to the same pin on the LX10 drive base.  Some cables or connector blocks swap the wires over so that, for example, the left hand pin on one side of the connector block is connected to the middle pin on the other side.  If you do not observe this rule the hand controller will not operate correctly, or worse, your equipment may be damaged.