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CCD Imaging Primer |
| Imaging Made Easy and Fun! |
| Focus |
Focus should be done on a bright star close (within a few degrees) to the object you want to image (for SCTs anyway). See the discussion of Pointing earlier for more on going to stars near your object.
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The Focus Window
The Subframe may be chosen by simply drawing a box on the image around the star you want to focus on. Once this is done, subsequent focus images will be much faster
since only part of the frame needs to be downloaded from the camera. The Delay gives you time to tweak focus between exposures.
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| I have found that FWHM in x and y of about 2.5 and 2.0 respectively is sometimes about as good as I can get (with the MX5), although I usually try for around 2¼ or less in x. Any bigger and the images lose detail and have rather bloated stars. An example of this is shown in the two Dumbbell Nebula insets at right. The top one shows the bloated star images typical of bad focus while the bottom one has good focus and reasonably tight star images. The FWHM numbers do fluctuate significantly over just a few seconds due to seeing, and this sometimes makes focusing a little tricky. A few nights' experience with your own setup will familiarize you with what kind of numbers you should be aiming for (since FWHM numbers depend on seeing and pixel scale). |  |
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FWHM x and y numbers of 2.5 and 2.0 or less are what you should aim for. Use these numbers, along with the "Max" number and the wireframe diagram to achieve good focus.
The Maxim 2.x Inspect window introduced the "Large Display" button. The big display itself is shown below, and is handy for reading the computer screen at a distance! In version 3, only one FWHM number is given, and a new quantity "½ FD" (half flux density) is added.
Rough Focusing
The simplest technique for focusing is to manually turn the focus knob on your scope. I start by turning it 30 times (roughly quarter turns) to the right (clockwise). Then I turn it in the same fashion to the left 15 times (counterclockwise). Now I take a 3 second exposure. There should be a huge doughnut on the screen (bigger than the frame). Putting the focus into continuous mode with a 10-second delay, I make further adjustments to focus until the star is a small blob on screen.
Fine Focusing
With the focus close, I switch the focus exposure to 0.01 second continous, magnify a subframe around the star by 400% and use the "Large Display" feature for easy inspection of the FWHM numbers. Making small tweaks of the focus knob, I try to minimize the FWHM numbers. If I can get them down to around 2¼ in x, I'm happy.
Note that when fine focusing, passing through the focus and then reversing the direction that you are turning the focus knob is not an option! Doing so will almost certainly guarantee the mirror will shift during the course of your deep sky image sequence because you loosened the focusing mechanism. It was the racking all the way out (30 "turns") and then back in again (during focusing) that tightened things up and this helps ward off mirror shift.
Focusing Tips & Tricks
I like to find a star that has a brightness of about 4000 (the "Max" number in the Inspect window) with a 2-second exposure. Max numbers of less than 4000 aren't much good for focusing. You can use a really bright star for focus with a 0.01 second exposure, but the focus won't be as good.
You will see quite a bit of variation in the numbers even when you have not made any adjustments to the focus. This is due to seeing (as mentioned earlier). If you are focused at say 2.3 by 2.0, then those numbers should come up say ¼ of the time with higher numbers the rest of the time. It is important to estimate the effect seeing is having on your focus numbers and to guess where you really are so that you don't pass through focus! The final determination of focus is made by taking a full frame and looking at the stars to see how "tight" they are (something that gets easier with experience).
Sometimes a "Qwik Focus" type of device helps in getting good focus. In keeping with simplicity and cheapness, mine is currently a piece of cardboard with two 3" holes cut in it (one to the left, one to the right of the secondary obstruction) which I duct tape to the end of my dewcap. Not elegant, but it works. With this device in place, stars will appear double. As you move towards focus, the images will merge. At this point you are very close to focus. Now remove the "Qwik Focus" type device and continue focusing, paying attention to the FWHM x & y numbers as well as the "Max" number in the Inspect window to fine tune the focus.
Electric Focuser Advantages
A final note on focusing. With SCTs, you have to be vigilant for a shift in focus if you are spending more than a few minutes imaging an object. I have found by experience that if the focus knob is "tight" (more difficult to turn) the chances of the mirror shifting during imaging an object are small. If the focus knob is "loose" (easy to turn) then the mirror will almost certainly shift within the next 30 minutes! The way to make the focus knob "tight" is to turn many turns in one direction and then turn it the other way to focus. During the first few turns of the focus knob after a change of direction, you should feel the knob "load up". If you pass through the focus point and have to start turning the knob in the opposite direction you are toast! This is where "in-out" type motorized focusers like the Optec TCF or JMI NGF have an advantage -- they allow you to pass through focus and then come back (there is almost perfect repeatability!). They also allow finer adjustments in focus than turning the knob by hand. Additionally, electric focusers allow for remote control which allows you to do things like image from inside your house (especially nice in the winter)!
Additionally, the Optec TCF focuser will automatically compensate for temperature-induced changes in focus. The serial version of this focuser (the TCF-S) will also communicate with your computer via the serial port and this allows for programs like MaxIm DL/CCD 3.x to automatically focus your images for you! These automatic temperature compensation and "autofocus" capabilities are the current cutting edge of focusing technology!
