Focus Faq
A perfectly focused star image appears as a small disk in the eyepiece, called an airy disk. This is as good as it gets -- and it gets better only with larger aperture. However, in the real world there are effects that preclude seeing a perfect airy disk, some you can control, some you can’t.
What affects focus that I can’t control?
You can’t control the twinkling of stars which causes wander and blurring of the star in the eyepiece. Wander is the star image drifting/jumping around in the eyepiece by about 3-5 arcseconds. In a long exposure astrophotograph wander blurs the airy disk out to a 3-5 arcsecond image in the photo. Twinkling also causes the airy disk to blur out and sharpens up -- the “seeing” changes as the atmosphere twinkles. At moments of crisp seeing the airy disk is visible, at instances of bad seeing the star is a cottonball. You can’t control the atmosphere but you can minimize the effects of twinkling by observing stars nearer overhead, where there’s less atmosphere to look through.
What affects focus that I can control?
Two effects you can do something about are collimation and focus. our Aline collimation tool will help you with collimation. Fortunately collimation is something you only have to check occasionally. Focus, however, changes with every change of eyepiece or observer.
Just how sensitive is focus?
For a perfect airy disk star image that’s out-of-focus (defocused) you’ll see an image of the telescope aperture (entrance pupil, secondary obstructions and all) not an airy disk. The table below gives the diameter of airy disk for a perfectly focused star (in arcseconds) for three different apertures (4, 8, 10 inches, the larger the diameter of the aperture, the smaller/sharper/better the airy disk) and f-ratios (4, 10, 4.5 respectively) and the diameter of the defocused image of the telescope aperture (in arcseconds) for different amounts of defocus (1, 0.1, 0.01 mm). Two things to note in the table; (1) the shorter the f-ratio the more critical focus is, (2) You have to get the focus within a few 100ths of a mm to have the airy disk limits the sharpness of the star image, not the blur from defocus!
f ratio |
4 |
f ratio |
10 |
f ratio |
4.5 |
|||
diam |
100 |
mm |
diam |
203.2 |
mm |
diam |
254 |
mm |
f length |
400 |
mm |
f length |
2032 |
mm |
f length |
1143 |
mm |
airy disk |
2.5 |
arcsec |
airy disk |
1.2 |
arcsec |
airy disk |
1.0 |
arcsec |
defocus |
1 |
mm |
defocus |
1 |
mm |
defocus |
1 |
mm |
image |
131.4 |
arcsec |
image |
11.4 |
arcsec |
image |
41.1 |
arcsec |
defocus |
0.1 |
mm |
defocus |
0.1 |
mm |
defocus |
0.1 |
mm |
image |
15.4 |
arcsec |
image |
2.3 |
arcsec |
image |
5.0 |
arcsec |
defocus |
0.01 |
mm |
defocus |
0.01 |
mm |
defocus |
0.01 |
mm |
image |
3.8 |
arcsec |
image |
1.3 |
arcsec |
image |
1.4 |
arcsec |
So how little do I have to turn the focus knob to move the focus by a few 100ths of a mm?
For a Crayford focuser, a 1° of rotation of the focuser knob will change the focus by about 0.03 mm. All you have to do is be able to turn the focus knob by 1° or less! But to turn the focus knob by any amount, you have to get it started turning which takes a lot of torque. Once it starts turning you have to reduce the torque so it doesn’t turn too fast. All while you’re looking in the eyepiece to check the star’s image which is vibrating and blurring because your fingers on the focus knob are shaking the whole telescope. And when you think the focus is good, just before you crank it out of focus, stop turning the focus knob in less than a fraction of a degree. Sound familiar? :-)
Yuck! How can I make it easier?
For manual focus, the best thing is a speed reducer – the small knob on a 2-speed focuser that takes 6-10 turns to turn the big focuser knob (and focuser shaft) one turn. With a speed reducer, the 1° is magnified to comfortable 6°-10° rotation and the torque required to turn the focuser shaft is 6-10 times less. But you’re still manually turning the focus knob, so the telescope is still shaking and the star’s image is vibrating and blurring. You’re still stopping every so often to let the vibrations damp out so you can check the star image, then turn the focus knob hoping that you’ve turned it the right way and by the right amount, stop and check again, try to figure out if it’s better or worse, go the other way to check and so on. Sound familiar? :-)
Ick! How can I make it even easier?
Motorize your focuser with nFOCUS and a DC motor for visual observing and occassional astrophotography, or Seletek and a stepper motor for professional quality astrophotography!
Why nFOCUS and a DC motor for visual observing?
A motorized focuser won’t shake the telescope while your focusing, and the motor can make very small rotations of the focus knob more reliably than you can by hand even with a speed reducer, but not just any DC motor and any DC focus motor controller.
Most other DC motor controllers control the motor speed by with a potentiometer you have to turn to reduce the voltage to the motor, which also reduces the torque, often to the point where the motor will not start turning the focuser knob. So you have to use both hands, one to operate the button, one to adjust the potentiometer to high speed to get the motor started, then reduce it back down so it doesn’t turn too fast. And of course the motor continues to coast after you've lifted your thumb off of the button, so by the time you've reacted to what you see in the eyepiece, it's too late.
nFOCUS provides pulse-width-modulation speed control. Which means the length of the pulse of current to the motor can be adjusted shorter or longer, to produce from a 1° to 3° rotation of the focuser knob per step. And it’s always a high-torque 12 volt pulse that provides plenty of oomph to reliably start the focuser shaft turning. Step by step, degree by degree. And at the end of each pulse, nFOCUS electronically brakes the motor to a quick stop.
nFOCUS also lets you select the length of the pauses between pulses. With nFOCUS the focuser steps then pauses while you check the discrete changes in the focus before nFOCUS steps to the next focus position. The pauses also give you time to lift your thumb off of the button so you don’t miss perfect focus. As one of our customer's says;
"I recently got a new high performance scope and I have been working with it over the past week photographing the Pleiades cluster (M45). I realize now, even more then before, how critical your motor focus systems are to my success in photography. My new scope is a LZOS 105mm triplet f/6.2 with a reducer/flattener attached. Because the reducer/flattener reduces the effective focal length from 650mm to 520mm and the f-stop speed has been increased from f/6.2 to f/5, focus is even more critical because the increased f-stop speed has made the scope even more difficult to focus. And triplets are difficult enough to focus at f/7.
Using the nFOCUS DC motor control on a single pulse setting, I was able to fine tune the Starlight Feathertouch focus by a few millionths of an inch (linear travel) with one pulse. I found that even as the temperature changed during the night I needed to make very small adjustments to the focus. But by simply touching the "in" or "out" button I could make extremely small, repeatable, adjustment to the focus. It is the tiny adjustments, made reliably each time, that makes to Rigel systems motor focuser a critical part of my astrophotography It is excellent for the budget minded astrophotographer!
Before I got the motor focus control I routinely took 30 minutes to focus my scope. With the Rigel controller I can usually get to perfect focus in less than 5 minutes! There were nights I simply did not get the scope out because of the focus time I knew I would need to go through to get good photos. Thank you for a very fine product, at a good price, and thank you for improving my scope!
Steve"
nFOCUS is excellent for the budget minded astrophotographer, but for professional quality astrophotography we recommend Seletek and a stepper motor.
Why Seletek and a Stepper motor for professional quality astrophotography?
DC motors cannot be commanded to the exact same focus position every time -- stepper motors can. Professional astrophotographers autofocus their CCD cameras by taking a series of short exposure astrophotographs of a bright star at multiple, exact, repeatable, focus positions on either side of perfect focus. Each will out of focus by a different amount, as measured by their ASCOM compliant autofocus software (e.g., focusmax) which calculates the exact focal position of perfect focus and commands the Seletek controller to step the stepper to that exact perfect focus position.
Seletek has an internal and an optional external temperature sensor to support temperature compensation of the focus position. We’re talking 100ths of a mm here! As the night outside cools, the telescope contracts, and the focus changes.
With Seletek, once you have everything initialized with your autofocus software, you can execute a refocus in 30-40 seconds to adapt to filter changes, temperature changes or telescope flexure as you aim your telescope to different part of the sky. In 30-40 seconds your autofocus software will take a pre-programmed set of images to recalculate the perfect focus setting without your intervention.
And if you are automating your observatory, Seletek will not only control your focuser or two focusers, but you can have it control a focuser and a filter wheel or with the optional relay box, other observatory devices. And if you really do prefer manual control of focusing, you can add a manual focus hand box to Seletek.