AP Corner June 2022; Finding Your Focus Part 2

sw.jpgBy Don Selle

In Part 1 of this article, I described several techniques you could use to manually focus your smartphone or your DSLR used either stand alone or attached to your telescope. Adding an inexpensive Bhatinov mask to the front of your telescope or DSLR lens (yes you can get them that size) makes achieving critical focus using the camera’s live view focusing very doable. This technique can also be used for a dedicated astro-imaging camera on your telescope if you view the focus images as they are downloaded to the computer controlling the camera.

The Bhatinov mask works great, but let’s face it, manual focusing can be a tedious and time-consuming task. When I first started astro-imaging, manual focusing was the norm for me and others like me starting out because the motorized focusers were few, and they were rather expensive. In addition, there was not much software to automate the focusing process and there were no real interface standards to allow computer control of the telescope focuser. So, we learned to focus by hand and by eye. 

Achieving a good focus by watching graphs generated by the imaging software or the spikes from a focusing mask then making adjustments using a focus knob, even one with fine focus, is still a bit hit and miss. It seemed like I would pass over the focus point several times before I hit it just right. Spending 15 to 20 minutes getting focused was not uncommon. When using a monochrome camera, this process needed to be done for each filter used during an imaging session

I would say without a doubt that adding autofocus to my imaging rig was the best upgrade I made (though getting a high-quality mount is a close second). The improvement in your finished images when you are using well focused sub-frames is very evident, and the time saved over manual focusing by itself is worth the expense and work. If you are serious about getting into astro-imaging, I would say that adding autofocus to your imaging rig is essential.

Adding autofocus. 

crayford.gifThese days, adding autofocus to your system is fairly straight forward, though it will take some mechanical ability to add a new focuser and or focus motor, and perhaps some computer and software ability to get it all working with your imaging software and camera. That said, you will need to make a few decisions up front.

Using a stock focuser.

If the stock focuser on your OTA is well built, adding a third-party focus motor and controller should work well for you. If your stock focuser is loose or sags, you might be better off upgrading it.

The quality of the stock focusers that come with new model “imaging” grade OTA’s has improved quite a bit in the last several years. These are generally Crayford focusers and usually work well, especially if you are using a lightweight imaging camera. Typically, they also come with a fine focus knob. 

A Crayford focuser uses a smooth spindle held in contact with a flat section of the drawtube to move the drawtube to achieve focus. A small diameter spindle is used to increase the resolution of the focuser (more rotations of the spindle per unit of drawtube travel). The smaller the diameter though, the higher the contact pressure must be to achieve sufficient friction to “lift” the camera equipment. Most of these focusers have a tension adjustment on them which increases the contact pressure. When correctly tightened, this virtually eliminates any backlash in the focuser (see below). Be careful that it is not too tight, or you could mechanically lock the focuser.

You will need to check to see that the focuser tube is tight and well supported by the spindle and bearings. The drawtube should not shift or droop as it is extended and retracted. The sturdier the focuser the fewer problems you should have. If it is solid and smooth operating adding a focus motor should work just fine for you.

There are several OTAs which come with rack and pinion focusers. A well-made R&P focuser is very robust and can be motorized like a Crayford focuser. Just like a Crayford focuser, you must be sure that the drawtube is well supported and does not droop. The R&P focuser will “lift” more weight than a Crayford focuser, but due to the mechanics of the rack and pinion, unless it well manufactured, it may be more prone to exhibit backlash.

Adding an aftermarket focuser.

Adding an aftermarket focuser can be a good option as they are very well built, precise and very repeatable.  If your budget allows, adding an aftermarket focuser and the focus motor designed for it give you better results and less difficulty than adding a motor to a stock focuser.

There are several manufacturers of these focusers such as Moonlite, Starlight Instruments and Optec. All of these are very well built and have a wide range of adapters to allow you to install them onto your OTA. They also have very well designed and well established autofocus systems, comprised of an add-on focus motor and a separate controller. These work very well and the drivers are available in most imaging programs. They also have ACOM drivers which allow you to use your focus motor with almost any astro-imaging or observatory automation software out there.

Focus Motor and Controller.

By now I hope I have convinced you that upgrading to autofocus is really worth the time effort and money. If you are thinking of upgrading a stock focuser, you are doing so at a good time. Focus motor technology has improved considerably. My recommendation would be that you research and purchase an “all-in-one” focus motor with the controller built into it. 

This all-in-one advancement eliminates the need for a separate focus motor controller. With no separate controller, the cost of upgrading is reduced. This configuration was inspired by the new trend in astro-imaging equipment whereby the computer controlling your system is carried on the mount itself and controlled via Wifi. Even if you use an outboard computer, all-in-one focus motors still simplifies the wiring you need to do to connect your imaging electronics to the outboard control computer. 

This is the configuration I use on my observatory rig, with a single power and USB cable threaded through the mount directly to the scope where I have a power distribution box and a USB hub. Either way, with the computer on the mount or outboard, this configuration solves the problem of cable drag, and reduces the risk of cable pull-out.

If you already have a Moonlite or Starlight Instruments focuser, but not the focus motor and controller, don’t worry. You can retrofit these focusers with the manufacturer's motors and controllers. If you have had the focuser for a few years you should contact the manufacturer to see if the current focus motors will work with yours, as the focuser designs do change. Make sure that you provide them with information about your focuser

It is also possible to add an all-in one focus motor like the ZWO EAF to both Starlight and Moonlite focusers (I have direct experience with the Moonlite) and have a very good, motorized focuser for considerably less costs.

 

measure.jpgSetting up your focuser

Regardless of whether you have a motorized focuser from the manufacturer, or have installed the focus motor yourself, you will need to set up your focuser if you wish it to perform at its best. This involves three steps, setting the zero point for your focuser, measuring the movement per “tic” or count, and setting the backlash adjustment.

Initial setup of your focuser will help ensure that you get the most out of it. Repeatability is the key (see below) and taking these steps will ensure that within its mechanical limitations, your focuser will operate in a predictable and repeatable manner.

  • Set the zero point – your focuser does not include an absolute encoder to ensure that the controller “knows” where the drawtube is. The controller “knows” where the drawtube is in a relative sense by tracking the direction and number of electrical impulses sent to the motor. If by some chance, the focuser stalls, the controller will happily count motor tics while the camera never moves, and the focus motor heats up. To ensure that this does not happen, and we know where the drawtube is, we must set the counter to zero at a known point. To set the zero point:
    • Connect the focuser to your control computer and ensure the focus controller shows as a “connected device”.
    • Using the manual control for the focus motor, run the focuser until the drawtube is all the way in to where the drawtube is prevented from moving any further.
    • Either in the standalone focus control program, or in the driver for the focuser there should be a Set Zero control. Set the count to Zero – you have now set the focuser zero point
    • When your system is portable, you should check the zero point every time you set it up to take images. If your system is in an observatory, I would suggest that you check the zero point of your focuser on a regular basis, as it can drift slightly.
  •   Measure the movement per “tic” – This can be done fairly simply using either the measurements on your focuser drawtube or using an inexpensive digital caliper. Personally, I prefer using the caliper.  From the zero point, run the focuser out a specific number of tics. I would use a fairly large number (in the thousands). Measure how far the drawtube moved and record.

Repeat the measurement from the zero point several times, take the average of the distance measurements and divide by the number of tics moved. This is normally expressed in microns (millionths of a meter) per tic, so if you did not measure in microns, make sure you do the conversion.

  • Set the backlash adjustment – This is an important step if you want your focuser to perform in a repeatable way (see below) and increase its precision. Almost all of the focus motors available use stepper motors with reduction gears to make the rotation angle per tic very small. Backlash is the mechanical “slop” or imprecision in the gears that results when the direction of the drawtube is reversed. When reversing, a finite number of tics is required before the draw tube itself reverses direction. To set the backlash adjustment:
    • The movement of the drawtube can change very slightly with an increase in load. For the most conservative measurement of backlash, you should arrange the focuser, with your imaging system in it, pointing up as close to vertical as possible. This is best done with your OTA and imaging system mounted on a GEM (German Equatorial Mount)
    • Starting with the drawtube at an arbitrary point out from the zero point, measure how far out the drawtube is. Next, move the focuser in a set number of tics (but not as far as zero). Measure the drawtube position, record the difference between the two measurements.
    • Repeat the sequence only this time with the drawtube moving out. Record the difference between the starting measurement and the ending measurement. You should note that the distance traveled outward is slightly less than the distance traveling inward. Record the difference. This difference is due to “backlash”.
    • Repeat this cycle several times, each time recording the backlash distance. Average and then divide by the movement per tic you previously measured. Be sure that your units are in microns, and if you have done this, you now have the number of “tics” necessary to adjust for the backlash in your focuser.
    • Open the focuser standalone control program, or the focus motor/controller driver. You should find a means to input an adjustment for backlash in one or the other. 

Repeatability

Why should you go to all of the trouble to set up your focuser, when on any given night (assuming your backlash is small enough) you can get your camera focused quite nicely (thank you!). Its all about ensuring that the operation of your focuser can be repeated, and the camera can be moved to a focus point that is within the Critical Focus Zone of your OTA and imager. 

With repeatable focusing it is possible to make the most of your imaging time. For example, after setting up your equipment, you can use the focus settings you previously recorded to take your dusk flats. Additionally, some imaging programs keep track of the focuser position each time you refocus, along with the filter you are using, and the temperature reported by your focus controller. Over time enough data is acquired so that offsets in focuser position for each filter can be calculated, and the impact of the changing temperature on the focus of your system can be assessed. 

Using Filter Offsets- If you use different filters in your system, either with a monochrome camera or if you use narrow band filters with an OSC camera, each filter will cause a slight change in the focus of the camera. If these filter offsets are known, this can eliminate the need to spend the time to refocus for each filter, giving you more time to acquire photons.

With a monochrome camera, using filter offsets can allow taking sequences of images with different filters, such as luminance, red, green and blue without focusing between each filter, rather than 1 hour of luminance, refocus 1 hour of red refocus etc. This ensures that each imaging session will result in usable data. 

I can’t tell you how many partial data sets I have collected over the years because the clouds rolled in before I was able to complete my exposure plan. Some I was able to finally complete over multiple nights, though more often than not, they still remain to be finished.

Critical Focus Zone (CFZ) and the need to refocus

There are several different approached to determining by calculation the critical focus zone for your OTA camera combination. Suffice it to say that the results are in the range of 10’s to 100’s of microns. It also should be noted that CFZ is directly proportional to the focal ratio (f) of your OTA. Here is a calculator for CFZ you can use to check out your system. 

https://www.wilmslowastro.com/software/formulae.htm#CFZ

This one assumes that we want to sample a star with a minimum of 2 x 2 pixels in your camera and does not include any impact of atmospheric seeing so it will be conservative.

Due to the fact that the CFZ is so small, changing temperatures while you are imaging can impact your focus due to contraction of materials. Your CFZ may be less than 100 microns, and your focuser has settled somewhere in this 100 micron band. Given that that a typical aluminum refractor OTA will expand or contract 20 to 25 microns for every 2 degrees Fahrenheit change in temperature, it should be apparent that you should refocus your telescope periodically during your imaging session.

There are no hard and fast guidelines but refocusing every 1 to 2 hours of imaging is typical. Obviously, you will need to refocus more often if temperature drops are extreme especially if you are imaging with a low focal ratio OTA, since your CFZ will be smaller when using such an OTA.

Auto Focus Methods

Once you have added the autofocus equipment, you will need to obtain and learn the software that actually does the focusing. While there are stand-alone autofocus programs (Focus Max is the granddaddy of them all), most good imaging programs will have one or more focusing routines built in. These routines break down into two categories – single star algorithms, and multiple star algorithms. 

Both algorithms start with your rough focus, then change the position of the camera to several points inside and outside of your rough focus. The size, brightness and shape of the stars are evaluated at each point and an interpolation is done to determine where the best focus would be based on the measurements.

Single star algorithms should work well if your image field is flat (i.e. in focus to the edges and corners of the camera field of view). Multiple star algorithms evaluate several stars in the field and then determine the focal position where they are all reasonably in focus. Try them both and see which works best for you.

You should also remember that the atmospheric seeing (how steady the air is) will also have an apparent effect the focus of your image, alternately inflating or minimizing the size of the stars. Due to this effect, you should consider taking several focus images at each focus point so that the algorithm can average out the effect of seeing. How many you should take depends on how fast your camera downloads and your computer processes each focus image. I typically use 3 or 5 samples depending on how I think the seeing is, as more than 5 samples does not provide a large improvement in the results.

Conclusion – So I hope by now that I have convinced you that adding autofocus to your imaging system is a very good thing to do, and with a little bit of background knowledge and a little bit of perseverance, you can do this. 

If you have recently upgraded to autofocusing let me know how you managed!

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