Astrophotography Skills – Focusing

Introduction

Surprise: Focusing is hard

The topic of focusing is one of many surprises for beginners to astrophotography. It’s hard work: it takes practice and proper equipment, and it will take you quite some time and effort to become good at it.

If you have a regular camera, you probably don’t think much about focusing. On your cell phone, focusing is automatic, and you may not even realize it is happening. If you have a DSLR, you probably have auto-focus lenses that work really well, although you may have, on rare occasions, been disappointed by a picture where the camera auto-focused on the wrong thing. Many cameras also have a completely manual focus option, and if you photograph landscapes or certain other subjects, you might have used it.

But, overall, you’ve probably not thought of focusing as something hard. Unless you’ve tried focusing your DSLR in very dim light, like at night in the dark. That’s why many cameras, or accessory flash units, have some kind of focus-assist light, illuminating the subject enough for the autofocus circuitry to work. In astrophotography, of course, that’s not an option; targets are dim and too far away to illuminate with an artificial source.

Why is it hard?

On a DSLR or mirrorless camera, astronomy subjects are simply too dim to focus easily with your eyes. If your camera has a viewfinder (although most modern cameras do not) you will find you can’t see much, or anything at all, in the viewfinder when it is connected to a telescope. And, you may not be able to see much on the camera’s rear view screen, since it is not designed to frame and display very dim subjects.

On a dedicated CCD camera, there is no viewfinder or viewscreen at all. Taking an exposure and viewing the result on your computer is the only way to see what you are pointed at, and if it is in focus.

In addition to the challenge of achieving focus, you will likely need to re-focus periodically. You will need to re-focus when you change filters, and possibly at regular intervals during the night as the outdoor temperature changes.

The focus mechanism on your telescope may also contribute to making focusing difficult. Inexpensive focusers using rack-and-pinion gears will have significant backlash, which makes fine adjustment difficult manually, and makes automated focusing almost impossible. Worse, some SCTs focus by moving the main mirror, which means focusing shifts the image slightly, usually enough to throw your subject right out of the frame as you adjust focus.

Finally, the optical design of your telescope affects focusing. “Fast“ optical designs are more desirable for imaging, because they reduce exposure times, but fast optics (like most refractors, at about f/6) have a shallow depth of field which means the distances you are moving while focusing are very small and precise. Slow optics (like most SCTs, at f/10) have deep depth of field, which spreads the area of “pretty good” focus out over quite a large range, making it harder to find that specific spot of perfect focus.

How to focus

So, if focusing is so difficult, how can we do it well?

There are a variety of techniques and devices that can help you achieve good focus. Let’s review several accessories and techniques, below, to help you select what may work for you.

Improved focuser

On a moderately-priced telescope, it is often a good investment to upgrade to an after-market, high-quality, focuser. (I restrict this advice to mid-level telescopes because a high-end telescope probably already has a high-quality focuser, while a low-end telescope probably does not warrant the investment.)

Adding an improved focuser will give you fine slow-motion adjustment, and greatly reduced backlash. And, on an SCT, it will allow you to focus without moving the mirror, and, therefore, without shifting your target out of your field of view.

The standard for upgraded focusers is called the Crayford focuser. These are available in multiple brands and for practically every make and design of telescope. They focus by moving an internal tube back and forth against a bearing, with negligible backlash. And, they almost always come with two-speed controls, for fine focus.

Focusing with an eyepiece

One simple solution to the difficulty in focusing a camera connected to a telescope is to avoid the problem, by focusing with an eyepiece, then swapping the eyepiece for your camera.

The problem is that the focus settings will be slightly different for an eyepiece than for your connected camera. There are several ways to address this.

Parfocal ring

The simplest is a simple accessory called a parfocal ring. This is a thin ring of metal that slips over the mounting tube of an eyepiece, and can be fixed in position with a small set screw. You use this as a “depth stop”, helping you slide an eyepiece into the eyepiece holder only far enough to precisely reproduce the focus position of your camera when it replaces the eyepiece. To use this device,

• Set aside a moderate focal-length eyepiece that you will use for focusing and for nothing else. It doesn’t have to be high-quality; something like a decent mid-range Plossl will do fine.
• Mount your camera on your telescope, and carefully focus it on a star using repeated exposures or some other method.
• Lock the focus setting of your telescope in this position if possible. (Many focusers have a knurled setscrew that can be tightened to lock the focus position.)
• Carefully remove the camera from the telescope without changing the telescope’s focus position.
• Slide the parfocal ring all the way onto the tube of your eyepiece.
• Carefully insert the eyepiece into the telescope. Tighten the eyepiece-retaining screw just enough that the eyepiece stays where you put it, but loosely enough that you can slide the eyepiece in and out with a bit of effort – possibly twisting it slightly.
• Focus the view in your eyepiece by sliding the eyepiece in and out of the telescope, not by adjusting the focuser.
• When you find the position at which the view is in focus, tighten the eyepiece retaining screw to hold the eyepiece in that position, then slide the parfocal ring down so that it sits flush against the telescope, and lock it in this position with its setscrew.
• Remember to unlock the focuser if you locked it before.

Now, in future, you can use this eyepiece, inserted into the telescope far enough for the parfocal ring to contact the base, to focus the telescope, knowing that the camera will be in focus when it replaces the eyepiece.

The following images show a parfocal ring being installed. Sorry, I didn’t want to remove my camera from the telescope to take these photos, so please imagine the diagonal on the table is the eyepiece holder of your telescope.

Parfocal ring and eyepiece

Parfocal ring on eyepiece

Focusing by sliding eyepiece

Ring locked down against base

Eyepiece with ring, ready for focusing

Flip-mirror

Another way to focus your telescope with an eyepiece, then to switch to a camera, is with a flip-mirror. This is a small optical box on which you can mount both your camera and an eyepiece, and can switch between them by flipping a lever that moves an internal mirror into or out of the path of the light. (Be sure to set it up so the the mirror-diverted light goes to the eyepiece, and the un-diverted light goes to the camera. You want as little glass as possible interfering with the photons you collect in your camera.)

As with the parfocal ring, a flip-mirror needs to be calibrated one time. Then, in theory, you can easily switch between an eyepiece view for focusing, and the camera for imaging.

Personally, I have never had much luck with flip-mirrors. The box adds several centimeters to the length of the optical path and, on my telescopes, made the path too long to achieve focus when combined with other accessories that I needed. However, with the right combination of equipment, I’m sure they are very convenient.

Because of the uncertainty whether they will work with your combination of gear, this is an accessory where “try before you buy” is highly recommended. Arrange to borrow a flip-mirror from someone, and try it on your own gear, before you make such a purchase.

Focusing using your camera

The above tricks for focusing with an eyepiece are convenient, and might be a suitable way for a beginner to start. However, nothing beats focusing with your actual camera. After all, that’s really the point, isn’t it; that the camera is in focus, not any other combination of accessories?

Consider the parfocal ring or flip-mirror a tool for getting close to correct focus, but plan to fine-tune with actual test images.

There are a variety of techniques for focusing using your actual camera.

DSLR viewfinder magnifier

This accessory is almost obsolete, since most modern cameras no longer have optical viewfinders. With older film SLR cameras, and DSLRs that had viewfinders, one could mount a small magnifier on the viewfinder’s eye port. It was easier to focus on the magnified image, since stars would be both larger and brighter in the view. Such magnifiers also sometimes turned the light path 90°, allowing a more comfortable viewing posture.

DSLR screen

If you are imaging with a DSLR or mirrorless camera, you might be able to focus on the camera’s rear viewing screen. It’s unlikely you will be able to focus on dim targets such as nebulae, but if there is a bright star near your target, you can temporarily point at that for focusing purposes.

On many modern cameras, there is a setting somewhere to control the sensitivity of the rear viewing screen. If your camera has such a setting, you may be able to crank up the sensitivity of the screen enough to be able to see stars for focusing. However, the “auto brightness” setting for your camera’s viewscreen will probably not work for framing and focusing astrophotographs, because the camera is looking for a scene of average brightness, not for tiny points of light on a black background. Some cameras also have the ability to magnify the image displayed on the rear screen. If you have this feature and can magnify a reasonably bright star on the screen, you may be able to focus directly by watching the image on the screen.

Focusing by repeated imaging

The only way to approach perfect focus is to slowly adjust your focus settings while repeatedly taking images with your camera. This will give you better results than the view screen on your DSLR, and is the only way to focus a dedicated CCD camera.

Focusing by repeated exposures can be quite tedious if you are simply staring at tiny dots on your computer screen. You will quickly lose track of whether the points of light in the image you are presently inspecting are slightly larger or slightly smaller than the points of light were in the previous image.

You can improve this situation by adding a device that temporarily modifies the light path in your telescope, making it much more obvious when the image is in or out of focus.

Hartmann mask

The simplest such device is called a Hartmann mask. This is simply an opaque disk, mounted at the objective (front) end of the telescope, and containing several small holes – usually two or three – that are distributed around the center of the mask.

The Hartmann mask is slipped over the end of the telescope, which is pointed at a bright star near your target. Optical interference effects cause the bright star to appear multiple times in the view, with the distance between the multiple images increasing as the image moves out of focus, and decreasing as the image approaches perfect focus.

With a Hartmann mask in place, you set your camera to take repeated exposures as rapidly as possible-usually every few seconds-and adjust the focus until the multiple star images merge into a single image. When you see a single image through the Hartmann mask, you are in perfect focus. Then, you carefully slip the Hartmann mask off the end of the telescope (don’t forget this step), and proceed with your imaging.

You can buy Hartmann masks at most astronomy stores, or online; but don’t. They are trivially easy to make yourself. All you need is some lightweight cardboard, some tape, a pair of scissors, and a spare half-hour. I’ll eventually write some instructions to make your own, but it should be fairly obvious from these photos – cardboard, duct tape, and an exacto knife will do the job. It doesn’t have to be fancy.

Hartmann masks work so well, and are so easy to make, that I highly recommend you use one for focusing your camera manually. The only practical solution that is better is automated focusing, which we will describe next.

Automated focusing

The ultimate in focusing is automated focusing using a motorized focuser. Automated, motorized focusing has several advantages:

• It’s fast and convenient;
• It’s high-tech and cool, impressing and amusing you and your friends;
• Most important, it is very accurate. Automated focusing will consistently achieve focus that is better than you can do manually.

There are some additional benefits, too, that might not immediately occur to you:

• Because you don’t have to physically touch your focuser, it facilitates remote operation of your telescope.
• It also facilitates’s periodic re-focusing when the temperature changes or when you change filters.

Automated focusing requires that you have a motor-driven focuser that can be controlled from your computer. After-market digitally-controlled motors can be added to many hi-end Crayford focusers, or purpose-built motorized focusers can be purchased.

Automated focusing also requires special “focus-control software”, that can both direct your camera to take images and direct your motorized focuser to move to a specific point.

Conceptually, automated focusing is quite simple:

• The focus-control software directs your camera to take a test image of a suitable star;
• This image is evaluated for the quality of its focus, and compared to the previous tests image. (“Quality of focus” is calculated by measuring the diameter of the disk of the out-of-focus star. Specifically, a measure called Full-Width Half-Maximum, or FWHM, is used to find the diameter at which the luminosity of the star falls to one-half of the maximum value.)
• The focuser is then adjusted by a small amount, and this sequence repeats.

You might think that the focus-control software simply evaluates image after image searching for the one with the smallest pinpoint stars – the same thing that you do when focusing manually. In fact, most focus-control software does something more sophisticated (and more accurate). By measuring and quantifying the quality of focus at several points on either side of focus, the software builds an internal model that it uses to calculate the point at which perfect focus should be obtained, then moves the focuser there.

Some focus-control software calculates a new model every time it is run. Other packages require that you run a calibration phase in which they pre-calculate a focusing model, then use the pre-calculated model for focusing.

As an example, here are three of the more popular focus-control software packages:

FocusMax

Formerly shareware, this application was recently acquired by CCDWare (the same company that makes the excellent PEMPro and CCDStack applications). FocusMax requires a calibration phase in which it pre-calculates a focus model. While this pre-calculation takes some time, it doesn’t have to be repeated unless you change your telescope or camera, and actual focusing is then very rapid because the model is already available.

With a good saved V-Curve precalibration set, FocusMax achieves perfect focus very quickly.

MaximDL’s Focus Feature

MaximDL has a simple auto-focus feature built in. It calculates a model in real time, so does not require pre-calibration.

TheSkyX’s @focus2 and @focus3 Features

TheSkyX (professional), with the Camera Control plug-in enabled, contains focusing features called “@focus2” and “@focus3”. These are similar, with @focus3 being newer and faster. They calculate a model in real time, and do not require pre-calibration. This is what I use most of the time, because it is integrated with the control software I use most.

Note that, with all automated focusing systems, you need to be “near” focus before you invoke the auto-focus system. You can use any of the other techniques described on this page to get approximately in focus before starting the auto-focus routine. It doesn’t have to be highly accurate – just “pretty close”.

Technique

Once you have established the focusing mechanisms that you will use, the basic procedure for focusing is as follows:

1. Choose a star, near your target, to focus on.
   • Use a star, not a nebula, galaxy, cluster, or planet. Your focusing will depend on bringing a star to a pinpoint of light.
   • Pick a star that is not too bright, and not too dim. If you are using automated focusing software, the software may help you select a suitable star. Otherwise, you will have to develop your own judgment. You want a star that is bright enough to be easily distinguished from background noise, but not so bright that it over-exposes and makes diameter measurement difficult.
   • The apparent position in the sky of your focus star should be “not too far” from your imaging target; close enough in the sky that it is unlikely components of your telescope and camera will shift position when you move from the focus star to the target.
   • Don’t worry about the actual distance to the star or target. (It might occur to you that your focus star should be about the same distance away as your actual target, but this is unimportant. Both are so far away that they are effectively at infinity.)
2. Do an initial rough focusing with any convenient method.
   • An eyepiece and parfocal ring or flip-mirror;
   • Setting a motorized focuser to a pre-recorded position number;
   • Or, simply moving to a line pre-marked on the focuser tube with a felt pen.
3. Now, set your image-capture software to take repeated short exposures, and slowly adjust the focus until it is perfect. Use a Hartmann mask to help you evaluate the focus. Or,
4. Use automated focusing software with a motorized focuser.

When to re-focus

Perfect focus will not remain constant throughout an evening. Several things can affect the focus of your system:

• Minor shift of components in the optical system;
• Changing to a different filter during filtered monochrome imaging;
• Significant temperature changes outdoors, which will cause optical components to shrink or expand, changing your focus.

So, you will need to re-focus when any of these things happen:

• After a long slew to a target in a substantially different area of the sky;
• After a long sequence of exposures causes the scope to “cross the Meridian”, changing the weight of your components to the other side;
• After changing filters (unless you have either expensive parfocal filters or an automated focusing a setup that knows how much to compensate for each filter);
• If the temperature changes significantly.

With some software, you can automate re-focusing, to occur at a set time interval or when the temperature changes by a given amount. (Some focusers can automatically adjust themselves when the temperature changes, while on some systems the focuser or some other device reports the ambient temperature to your control software, allowing it to decide when to re-focus.)

Conclusion

Focusing is considerably harder than you probably thought – much harder than you are used to from your household camera or cellphone. You will need to develop the skill of approaching perfect focus while inspecting repeated images taken with your camera. You can “throw money at the problem” by investing in a motorized focuser and automation software, but you can also make a substantial improvement in your focusing results with a simple homemade “Hartmann mask”.

Now that we’re in focus, we can calibrate and start up our AutoGuiding system, if we’re using it, then move on to actually taking image exposures.