Get Focussed: Feather Touch 10:1 Focuser


In a previous post we discussed depth of focus and the difficulties obtaining good focus with fast optical systems. In this post we’ll discuss a solution to the problem, the Feather Touch 10:1 focuser for the Celestron CPC 1100 GPS telescope.

We’ll briefly review the challenge of focusing fast telescopes and then describe how the Feather Touch 10:1 focuser will help. We’ll conclude with the installation of the focuser on a CPC 1100.


In a previous post we derived three equations for depth of focus and presented a table illustrating the depth of focus for various f-numbers. This table is included below for convenience.


The most important conclusion that can be drawn from this data, for the discussion at hand, is that fast telescopes, such as those operating at f/2, have very narrow depth of focus. For example, an f/10 system has a depth of focus of approximately 263 microns, while an f/2 system’s depth of focus is only about 10.5 microns. These figures are for seeing disks with a diameter equal to the diameter of the Airy disk. In great seeing conditions where the seeing disk might be 2″, these figures would increase to approximately 543 microns and 21 microns respectively.

Clearly, for fast telescopes, such as those operating at f/2, the depth of focus is very narrow. These tight tolerances may make focusing these systems a challenge.

My Application

As previously mentioned, at f/10 in ideal seeing, the depth of focus is 263 microns while at f/2 this is reduced by 25x, resulting in a narrow band of only 10.5 microns. However, with diminished viewing and larger seeing disks these numbers will grow. The application that spawned this investigation was astrophotography with a Celestron CPC 1100 GPS in its f/2 configuration. The question that started it all was, is the standard focuser on the CPC 1100 adequate to deal with these tight tolerances. We know how fine the focusing must be. In this section we’ll determine whether the standard focuser is up to the task or whether we need to look at other options.

Standard Focusing Mechanism

The focusing mechanism that comes standard with the Celestron CPC 1100 is illustrated to the right. As the focus knob is rotated, the focuser either pushes or pulls a threaded rod, that is connected to the primary mirror, in or out.

The rod and the focuser have a thread pitch of approximately 0.8 threads per millimeter. In other words, each rotation of the knob moves the primary mirror in or out 0.8mm, or 800 microns.

In ideal seeing conditions, with this telescope in the f/10 configuration, we have a depth of focus of 263 microns. So the entire depth of focus can be traversed with just under 1/3 of a turn of the focus knob. With degraded, but great, seeing conditions where the seeing disk is 2″ in diameter, the depth of focus increases to 542 microns, or 2/3 of a turn of the focus knob. It seems reasonable to expect, and experience has demonstrated, that achieving focus with these constraints is straightforward.

However, when this system is configured to operate at f/2 the depth of focus under ideal conditions is reduced to just 10.5 microns. Even with a seeing disk of 2″ the depth of focus is only 21 microns. The entire depth of focus is traversed with 1/78 and 1/38 of a turn of the focus knob respectively. These are very tight tolerances that would make achieving focus very difficult.

Feather Touch 10:1

After some investigation, I decided to try and solve the focussing problem, identified in the previous section, by acquiring and installing a Feather Touch 10:1 focuser for the CPC 1100. This focuser has dual speeds, a 1:1 ratio focusing knob, black in the image, and a brass colored 10:1 ratio knob.

Recall from the previous section that our challenge is to focus a system with a depth of focus of 10.5 microns in ideal conditions and 21 microns in the best likely conditions. At a 1:1 ratio the entire depth of focus is traversed in 1/78 and 1/38 of a turn of the focus knob respectively. With a 10:1 ratio these tolerances increase to 1/8 and 1/4 of a turn respectively. While these are still tight tolerances, I believe focus will be obtainable.

In the next section I’ll quickly walk through the installation of this focuser. It was fairly straightforward and required few tools.


In this section we’ll walk through the installation of the Feather Touch 10:1 focuser for the Celestron CPC 1100 GPS telescope. There were three items in the box: the focuser, a stop and a small washer. The installation requires the reuse of four screws associated with the standard Celestron focuser.  You’ll need a 1/2 inch open end wrench and a medium sized Phillips-head screwdriver.

The first task is to position the CPC 1100 in a horizontal position, altitude of 0°, so the primary mirror does not move during installation. Then we turn to removing the standard focuser. After pulling the rubber knob straight off and removing the three visible screws holding the focuser to the rear of the telescope, we have something that looks like the image to the right. Hang on to the three screws as they will be used to install the new focusing mechanism.

You must now rotate the brass focussing assembly clockwise until you see the head of the Phillips-head screw hidden inside. It takes quite a few turns to move the mirror back and the screw head never emerges from the hole, but does become easily accessible. When you’ve removed the screw, remove the small washer and replace it with the new washer that came with the Feather Touch focuser. Retain this screw as it will be reinstalled shortly.

Now the brass assembly needs to be removed. Rotate it in a counterclockwise direction until it is detached. It takes quite a few turns to free it. When it is removed, take the stop off of the new focuser and thread it onto the threaded shaft far enough that a few threads of the shaft are exposed. At this point it looks something like the image to the right.

Now reinstall the screw, with its new washer, into the end of the threaded rod. Screw the Feather Touch focuser onto the exposed threads of the rod until it reaches the stop. Hold the stop with a 1/2 inch open end wrench and tighten the focuser against the stop. You can grasp the focuser easily by the 1:1 ratio focus knob. This must be tight!

Now rotate the focuser down the rod until its mounting plate is flush against the back of the telescope. It may be necessary to push the mirror slightly forward to get the Feather Touch to seat properly. Reinstall the three screws retained earlier, and you’re done. The finished product is illustrated to the right.

Now you are ready to focus normally with the black knob and with precision with the 10:1 ratio brass knob.


In this document we presented the depth of focus for various f-numbers and determined that focusing fast optical systems is problematic. Finally we presented a solution and described its installation.

The August and Kevo Door Locks


I have been waiting for the right technology to enable me to rid myself of my traditional door locks. While I’m not completely satisfied with my current selection, I found two I was willing to try, the August (on the left) and the Kevo (on the right). In this post I’ll discuss both.


Let’s face it; Apple has set an amazingly high bar when it comes to product packaging. I love acquiring new Apple products because it is fun to open them and experience the unveiling. The August wins this battle hands down. While probably not a big deal in the end, it sets the initial impression upon which all the rest of the experience is built.


The Kevo is a Kwikset product and installs just like any other Kwikset deadbolt device. The August on the other hand is very simple and quick to install.

When you install the Kevo lock, you install a new outside key mechanism, a new deadbolt, and a new interior locking mechanism. The box contains all of the components, hardware, and even adapter pieces to adapt the system to various sorts of doors, doorknob holes, etc. It is very complete and only requires you provide a screwdriver and effort. If you are putting these locks on several doors of your home, it comes with the tools and instructions to rekey the exterior key system so a single physical key will open all of the doors.

The August system only replaces the interior portion of your existing deadbolt; this has pros and cons. This characteristic makes the installation a snap. You simply remove the interior portion of your existing lock, attach the August mounting bracket in its place, and slip August onto the bracket. In addition, if you rent or lease a property and the contract stipulates you can’t change the locks, this enables you to have a smart lock and still be in compliance. The downside of this simple interior replacement is that it can cost you. In my application not only was I interested in obtaining smart locks, I was eager to replace my failing existing lock systems. In my scenario I had to purchase August, a new deadbolt system, and then throw the interior portion of the brand new mechanism away. With August costing $250, roughly $60 more than Kevo, this additional expense is significant.


Physical Keys

Both systems enable you to gain access to the protected premises using traditional physical keys. As mentioned earlier the Kevo comes with new physical keys while the August uses the same exterior key that came with your original lock.

Virtual Keys

The biggest difference between the two locks, in terms of virtual keys, is that those for the Kevo cost $1.99 as an in app purchase, for any beyond the first two, while those for August are free. This is mitigated a bit by the fact that when you purchase a Kevo key and allocate it to a guest, family member, etc. you can reclaim it and reissue it without paying an additional fee. This is much like having additional keys made for your home and temporarily giving them to others.

Virtual keys for the Kevo system are distributed to others via email while those of August are distributed via text message. Shared keys can be adjusted to give guest access for a period of time, for set times and days of each week, or for anytime access. Keys can also be distributed that permit admin access to see entry logs, distribute keys to others, etc. Both systems are very similar in this respect.

Once a key is distributed in the Kevo system it will be active until you delete the individuals access rights. In the August system the key can be revoked or temporarily disabled. This feature might be convenient when you’re having your carpets cleaned, wood floors refinished, etc. and you don’t want others in your home, but you also don’t want to send the message, “we don’t love you anymore”.


To gain access to the premises with the Kevo system you approach the door with your iOS or Android device on your person, with the app installed, and reach out and touch the lock. The LEDs in the lock blink blue and a few seconds later the door is unlocked. When you exit the premises you simply touch the lock again and it locks. This feels very natural and sounds great, but it doesn’t always work. Most of the time the described scenario works, but just as you get in the habit of leaving, touching the lock, and proceeding on your way, you don’t hear the familiar locking sound and you have to return and repeat your effort. Unlocking the door sometimes requires two tries, not often, but sometimes.

There is an alternative scenario. Through a $50 / year subscription to Kevo Plus you can unlock and lock the door remotely as long as you have Internet connectivity. In addition, the Kevo does come with a FOB that enables those without a smart phone to access your home. However, since the system allows the use of a physical key, that is smaller than the FOB, the value of this feature is questionable; kids do love it!

There are two main ways to gain access using the August. The first is by pulling out your phone, opening the app, waiting a few seconds for the app to recognize the lock, and then pressing the unlock button. I wouldn’t mind pulling my phone out to unlock the door, but having to find the app, open it, wait for it to find the lock, and then finally unlocking the door drives me crazy. Thank goodness there is a second way.

In the August app you enable the auto unlock feature and set a radius from the lock on a map. When you leave your home, lock the door, and exit the predefined radius, the app takes note. The app interprets your reentering the predefined radius as an indication that you’re returning home and starts trying to acquire the lock via Bluetooth. When you get within Bluetooth range the lock unlocks. When you get to the door you simply enter. This feature makes the lock usable.

Your existing physical keys still allow you access to the premises; there is no FOB sort of device. For an additional one-time $50 purchase you can buy the August Connect device that connects the lock to your home WiFi and hence the Internet. This is useful because it enables you to lock and unlock the door from anywhere you have Internet connectivity. Instead of sharing keys with other random visitors you can simply open the door for them and lock it when they leave.

Physical Construction

The Kevo feels like any Kwikset deadbolt system, substantial and reasonable quality. The August feels less durable, more plastic, and for my test unit the battery cover periodically falls off. This does not affect the security of the premises, but does feel like you purchased an expensive, but cheap toy.

Family Picks

Thus far my family likes the August better than the Kevo. The Kevo fails them periodically and this does not instill trust; none of us want to carry a physical key for the time it refuses to open. With August Connect you acquire remote locking for a one-time $50 purchase while the same functionality for Kevo is $50 per year. The auto open mode on August makes it useable, without it August would be intolerable to use. While the August costs $60 more then the Kevo you get unlimited free virtual keys and August Connect as a one-time feature. After two years of use the Kevo would cost as much as the August and that cost would continue to increase.

Other Dissatisfactions

Neither of these devices has an open API that would allow them to be connected to other home systems. August supplies a closed API, for their trusted partners, through their August Connect device. I am unaware of an API for the Kevo system.

Both companies take the typical path of requiring the user to acquire an account, install a specialized app, and “caring” for the user’s data, keys, credentials, etc. These closed systems make it difficult to include these locks into the broader and more interesting smart home movement.