Focusing the Telescope with CCDSoft

 

GENERAL: we will determine the best telescope focus by obtaining a sequence of short exposures of a bright star, stepping the focus slightly from one exposure to the next. CCDSoft has a handy tool which allows us to determine which focus setting gives the narrowest, roundest images, with the highest peak intensity, ie, the best focus. We then set the telescope focus to that value. You can read more about this procedure in the CCDSoft Manual.

 

1) point the telescope at a bright star ( V ~ 9.0) near the zenith. You might pick a star from this list of candidates.

2) set a starting focus. Below are some final focus values at different dome temparatures, to give you a first guess at where your final focus might be. Take the estimated final focus, and subtract 5 focus units, to get your starting focus. Drive the focus 10-15 units below this value, then approach the starting value from below. There may be some "backlash" in the focus screws, so always aproach focus from below. For example, it is 15C in the dome, from the list below, we expect a final focus around 2345, so our starting focus will be 5 lower, or 2340.

2) start a sequence of 12 sec focus exposures (flash camera icon). In the dialog box,

this will take one 12 sec exposure and write it to the screen, then pause.

3) find your bright star on the image. Place a box around it by putting the cursor to the lower-left of the star, holding down the mouse buttone, and dragging the cursor to the upper right. This defines a region inside which the focus tool in CCDSoft will look for a stellar image (be sure your bright star is the only star in the box). Hit continue and the CCD will take another 12 sec image and display its radial profile and peak counts in a special box.

4) increase the telescope focus by one unit (to 2341 in our example) and hit continue to take a new exposure at this focus setting. CCDSoft will add the new stellar profile and peak count value to its box. Carefully record the focus value associated with each image, for example on a sheet of scrap paper... its easy to forget which focus value corresponds to which profile on the focus plot!

5) repeat Step4. If all goes well, you will see that the first exposures in your sequence had low, broad profiles and few counts at peak, but as the focus increased, the images get taller and narrower. At some point, the images began to broaden and flatten again. This means that you passed through best focus, and it is time to stop your sequence. Look back and decide at which focus setting you got the tallest and narrowest profiles. This is your final focus value.

6) set the telescope focus to the final value, but drive the focus 10-15 units below this value and approach it from below to avoid "backlash" effects.

7) record the final focus value and dome temperature on the observing log.

 

NOTE: the telescope focus is probably a function of temparature, but we haven't yet determined the nature of the function. You may have to refocus during the night if the temperature changes much. We have also seen evidence that the focus changes when the telescope moves across the Meridian, probably due to the mirror shifting a tiny bit as its weight is redistributed. Be aware that you might need to refocus after a Meridian crossing, and please record clear events of this phenomenon so we can confirm, quantify, and attempt to correct it.

NOTE: the focus exposures should always be 10-15 sec each. Stellar scintillation (the variation of the star image in brightness and position with time) happens rapidly, as you know from watching stars twinkle. However, if you take a short exposure (a few sec or less), you are abt to "freeze" some of this motion, and get a deceptively good or bad estimate of the image shape and height. Observing the star for 10-15 sec enables these variations to average out to a characteristic value. Observing much longer than 20 sec is probably a waste of your time -- chose a brighter star to focus on.

 

Updated 1999 June 04