AstronomyShed Tutorials Autoguiding - Part 3 - Using you autoguiding setup This is the third of 3 parts detailing my experience of auto guiding for astrophotography. Prerequisites In writing this article I have assumed that you have read and followed the previous 2 articles in this series. These articles covered hardware and software setup issues.

This last part will detail how I use my autoguiding setup on a night by night basis.

Polar alignment

Polar alignment is essential for good autoguiding. Poor polar alignment will cause the autoguiding to struggle. Follow one of the polar alignment tutorials on The Shed and make life easier for yourself.

Balance

Many people take great pains to 'perfectly' balance their scope on their mount without realising this will actually cause problems for their autoguiding, and infact their tracking in general.

It is better to slightly unbalance the mount in both right ascension (RA) and declination (DEC) so that there is always a load on the teeth of the gears in the mounts drive train. The gear teeth are therefore always meshed together. This helps to keep backlash at bay.

Some people say that the mount should be biased counter weight and camera end heavy, I've not actually noticed a difference which way the weight is biased, but biased it should be.

I usually balance and then move the counter weight an extra 10mm down the counter weight bar. I also balance without the camera on the telescope and then let the cameras weight supply the bias after it is attached.

Cables

The mount will usually end up with a number of cables wrapped around its legs. Even when arranged neatly these cables can snag and generally drag on the ground. The amount that they can interfere with the smooth motion of the mount is surprising and can cause large jumps in you guiding.

Starting out

I polar align with out the telescope or weights on the mount. This makes alignment easier.

Next I fit the telescope and weights to the mount and balance the setup.

Now I turn on my PC and the mount. On the mount I enter time, date, location, etc as prompted and then make a 2 star alignment.

Next I slew to a bright star like Capella.

Now I run PHD.

Next I select my camera and my mount using the camera and telescope icon bottom left. If the HC Buttons window has come up with the main program window over them I move them to one side on the desk top.

Next I start the camera looping using the Loop icon. Using a Bahtinov mask I focus the guide camera on the guide scope using my bright star. Bahtinov masks are an essential for good focusing, even when using my 9x50 finderguider I have made a small mask.

Now I slew the mount to my intended target for the night using the mount Hand Controller.

At this point I usually go back in the house.

Now I run my imaging camera software. Depending on which camera I am using for imaging I may need to adjust my alignment slightly to get good framing. I do this framing adjustment using PHD's on screen HC Buttons menu.

Once the image is framed and the mount is tracking I look for a guide star.

Using the mouse I click on a bright star near to the centre of the guiding image. You can also get PHD to auto select a star for you using the Tools pull down menu. Be sure not to select a hot pixel by mistake!

Also, don't select a large bloated star. These tend to have saturated the camera and result in a profile with a flat top. In this case PHD cannot accurately find the middle of the star profile.

It is possible to view the star profile using the Tools pull down menu.

Once selected a green box appears around the guide star.

Now press the Brain icon, PHD will start to calibrate itself, moving the mount East, then West, then North and then finally South. At each stage PHD is learning how far it has to move the mount to get a resulting movement of the guide star in the image. When guiding PHD uses this knowledge in reverse to move the mount the correct amount to compensate for movement of the guide star. This movement is caused by the imperfections in the gears and motors in the mount.

The cross hairs indicate that calibration is taking place and help to show how it is progressing. In my case the green square represents +/-25 pixels in the North/South and East/West directions.

The figures at the bottom of the window show that the mount has taken 11 steps to move the mount 24.8 pixels in the x axis. I adjust the Maximum Step size parameter in the Brain menu so that calibration in the West and East directions takes about 10 to 20 steps. If it takes more then 60 steps then calibration fails. More often than not this is caused by mistaking a hot pixel for a star!

Note that during these calibration steps the image has only moved 0.3 pixels in the y axis. This is because I always try to align my guide camera at right angles in the guide scope, either to the RA or the DEC axis.

When East and West calibration is complete the software automatically clears any backlash, then it starts North/South calibration.

North and South calibration usually take about 10 steps on my mounts, less than for East/West calibration.

Following a successful calibration PHD automatically starts guiding. I usually leave the system for a minute or 2 to settle.

I usually use the Tools pull down menu to Show Graph, which gives a graphical representation of how your guiding is progressing.

Basically you are wanting a blue RA and a Red DEC trace that show a low level, smooth oscillation about the mid line of the graph.

Many people go to great pains to get the graph as flat and as smooth as is possible. I follow a more practical approach of only adjusting anything if the stars in my images start to look elongated. The excursions in the graph can be alarmingly high before this practical limit is reached.

In the graph below you can see excursions caused by gusts of wind, these had no visible effect on the shape of the stars in my guided images.

This is a graph taken whilst imaging M82, you can just see its outline in the guide camera image. This is an almost classic graph, low excursions, with an underlying low frequency sine wave oscillation.

In this graph, taken whilst imaging M51 you can see the effect of snagging where

my camera cables were dragging along the ground as the mount moved.

On nights of variable or poor seeing it can be helpful to increase the Min Motion and camera integration time. This helps stop the system chasing the seeing rather than real errors in guiding.

High cloud can cause the intensity of the guide star to vary greatly. PHD calculates a parameter called star mass and warns if this is varying. If guiding is lost the software gives an audible warning. Guiding can be lost for several seconds with little guide star motion detected when it returns.