As an avid astrophotographer, improvement is always the word of the day (or night, as the case may be). As can be imagined, getting good focus is one of the most critical aspects of any type of photography, including astrophotography. An astrophotographer can have the best mount, the best camera, and the best telescope with perfect Polar alignment, but without good focus, it’s all for naught. Many telescopes tubes are made from aluminum. As most people understand, when objects get cold, they shrink. It turns out that this is one of the fundamental challenges in getting good focus on a telescope. Some background Before getting into the details of the subject of this article, A little background is in order. Deep sky astrophotography is a hobby or profession, where the faint, nebula, galaxies, clusters, or other similar targets, which lie outside our solar system. The vast majority of these are so dim that they are essentially invisible to the naked eye. There are some exceptions, but generally this is the case. Some examples are shown here:
NGC1499 – The California Nebula – 18 Hours of total exposure time 1.
M42 - The Great Orion Nebula – 11 hours total exposure time 1
In order to image these faint objects, long exposures are required. In theory this sounds simple, but since the earth is rotating, the stars (and deep sky objects) appear to move, with respect to the Earth. So the astrophotographer needs a way to track the stars, while imaging these targets. For imagers, this is most commonly done with an equatorial mount. This is a mount that is aligned with the polar axis of the Earth, and rotates at the exact same rate as the Earth. However, even the best mounts cannot be manufactured with 100 percent perfect accuracy. These minute imperfections can cause very small periodic errors, when tracking the stars. It may not be a problem for visual astronomy, but can sometimes affect the precision needed for long-exposure astrophotography. So most serious astrophotographers use an autoguider or guide camera. This is a separate camera that tracks the stars. Sometimes they are arranged with a separate telescope riding on top of the main imaging scope. Other times, an off-axis guider is used. This picks off a small amount of light in the main telescope (out of the field of view of the main imaging camera). Whichever method is used, the camera provides feedback to the mount as to which way to make adjustments to the mount, in order to keep a guide star perfectly centered. Once set, a good mount with an accurate guider can track a guide star across the sky, with sub-arc-second accuracy.
Below is a photo of the author’s system, which identifies the primary components of an astrophotography system. Not shown are the power supplies and computer, needed to control the system and acquire the images.
Labeled Deep Sky Imaging System
The image can now be tracked across the sky. The next aspect of astrophotography is to magnify and photograph it. That’s where the telescope and camera come into play. The telescope has optical elements, which magnify the target onto the eye (through an eyepiece) or to the film plane of a camera. Since many deep sky targets are very faint, it is required to take long exposures. It is also very desirable to use a monochrome camera and colored filters (for maximum resolution) and take multiple exposures of the same target. These are digitally stacked in post processing to increase the signal-to-noise ratio of the image. The author (from a light polluted location) will typically shoot multiple 10 minute exposures for Clear, Red, Green, and Blue filters and 30 minute exposures for narrow bandwidth filters (for emission nebula). These special filters record the particular emission lines of Hydrogen, Oxygen and Sulfur, which are common in emission-type astronomical targets. The author will usually spend a minimum of 3 hours of open shutter time, for each filter. More time is better, but can sometimes be difficult to do, because of weather. A completed image usually requires several nights to complete the imaging. Complex narrowband images typically require much more time. The author has spent as much as 7 nights imaging a single target.
The Problem At the start of the evening, imagers will complete their set-up and alignments and then, once the telescope and equipment has equilibrated to the ambient temperature, will focus the telescope. Since the imaging usually spans a good portion of the night, there is usually a shift in temperature, over the duration of the imaging session. This might only be 10F - 15F in the summer time. But living in the desert, the author regularly experiences more than 20F of temperature change, in the winter months. The effect that the changing temperature imparts to the telescope is not insignificant. As the temperature drops and the telescope follows, the distance between the optical elements and the focal plane of the camera shift, due to the thermal characteristics of the materials used to construct the telescope. This has the effect of changing the focus of the imaging system. One would think that this material change would be insignificant. But this is not the case. Many imagers use small focal ratio telescopes, which tend to compound the problem. The Critical Focus Zone (CFZ) is the distance in which the focuser can move, and the telescope still be in focus. The CFZ of the telescope is proportional to the f# of the telescope. The smaller the f# of the telescope, the smaller the CFZ. Below is a graphic explaining CFZ 2.
Critical Focus Zone Diagram
The author uses a Takahashi FSQ-106ED refractor. This refractor uses an f5 optical system. So looking at the equation below, we can calculate the critical focus zone. CFZ = 4.88 * Wavelength (in microns) * f/ratio^2
Looking at several center wavelengths in the visible spectrum, we can calculate the CFZ for this telescope: Reds ~ 650nm (0.65 micron) Greens ~ 550nm (0.55 micron) Blues ~ 450nm (0.45 micron) Red - CFZ = 4.88 * 0.65 * 25 = 79.3 microns Green - CFZ = 4.88 * 0.55 * 25 = 67.1 microns Blue - CFZ = 4.88 * 0.45 * 25 = 54.9 microns As the graphic above shows, The CFZ is how far the focal plane can be out of position and still be in focus. For this telescope, this amount is a mere 55µm (the imager must consider the smallest amount that can throw the telescope out of focus). Typically the focus is centered somewhere in this 55µm window. So a change of about half this amount is enough to change the focus enough to affect the image quality. Aluminum has a coefficient of thermal expansion (CTE) of about 24 µm/m-°C. The FSQ-106 has a focal length of about ½ meter. So using a ½ meter aluminum tube (as an approximation of the telescope tube), this equates to a thermally induced dimension change of about 12 µm/°C. If the tube experiences a 20 degree (F) temperature drop (~7 deg C), the tube will change dimension by more than 80µm! It’s clear now that temperature plays a huge roll in keeping the telescope in focus. In fact, the Author has experienced changes in focus in the time it takes to do a single 30 minute exposure. There are several focuser manufacturers that have options to change the focus, with respect to the temperature. There are even add-on kits to motorize the focusers. This typically is done with a stepper motor connected to the focus shaft. The focus shift is determined experimentally and calibrated to move the proper direction and amount, for the amount of temperature change. However, these can be expensive and complicated. They typically also require the telescope to be slewed to a brighter star, focused and then slewed back to the imaging target. This takes away from the precious imaging time and also requires either a lot of manual input or a complex array of software and plate solving techniques, to get back to the exact imaging position. Another alternative that some manufacturers implement is to use Carbon Fiber (CF) telescope tubes. Carbon fiber has a negligible CTE (in the frame of reference of telescope temperatures). However, there are also potential drawbacks to using CF, which are out of the scope of this article. The Solution
So what to do? The need for controlling the focus was apparent. After thinking about all the different solutions, the author came upon a revelation. Why chase the focus? Why add all the complexity of a motorized focuser? Why add the need to slew to bright focus stars? Why not just fix the root problem, the change the dimension, caused by thermal changes? So the search for a system was on. What was needed was a thermal system to control the temperature of the telescope tube, to a reasonable degree of accuracy. This would require 3 main items, a thermal wrap system, a means to measure the temperature, and a temperature controller. The author searched many different types of thermal pipe tape, tile floor heat systems, and many others. But what ended-up to be the solution that was implemented was 2 Walgreens heating pads. These were close to the dimension that was needed to cover the telescope tube and two units were more than enough to wrap the telescope tube. In addition, the heating pads had a soft covering blanket that helped to minimize the heat escaping and would prevent scratching the telescope’s paint (astronomers are very protective of their equipment).
Telescope Tube with Heating Pads Also implemented were an industrial temperature controller and a thermocouple. The thermocouple was inserted between the telescope tube and the heat pad and connected to the temperature controller. The system is powered by 120V, but requires very little power to maintain the temperature, once equilibrium has been reached.
Another benefit that was not initially considered was that this system helps to prevent dew and frost from building up on the telescope’s optics. The author uses a temperature that is only a few degrees above ambient, to prevent any thermal gradients in the air around or in front of the telescope. This would have negative effects on the image quality. However, this is usually enough to prevent the formation of dew or frost. Assembly The Telescope Thermal Focus Management System (TTFMS for short J) is made up of 3 main systems. One is the heater, which is wrapped around the telescope tube, next is the power and temperature control system, and third is the thermocouple for temperature feedback. The author wanted the system to be as compact as possible, since it would be used with many other systems, controlling the telescope, mount, filter wheel and CCD camera. So an enclosure was machined and assembled from 2 pieces of aluminum C-channel, which was in the shop. Custom machined plates were fabricated for both ends to accommodate the temperature controller, power switch, and power indicator, on the front. On the rear, is the power entry, power outlet (to the heaters), a fuse and t he thermocouple interface. The author used a thermocouple (T-Couple) connector, in order to have the ability to plug and un-plug the thermocouple. However, this is not necessary. The thermocouple can be connected directly to the temperature controller. One important aspect of thermocouples is that proper wiring and connectors be used. These work by measuring the small voltage generated at the junction when dissimilar metals are in contact. So if improper wiring is used, it may not provide accurate temperature readings. The author used J-type thermocouples, J-type connectors, and Jtype extension wire. The controller also contains an output light, which indicates when the Solid State Relay (SSR) is active, and power is being applied to the heater(s). This is not completely necessary, as most temperature controllers have an output indicator built into the display. However, this provides indication from a greater distance. The output of the temperature controller has a 120V grounded outlet. This too can be eliminated, if the builder wants to wire the heaters directly to the temperature controller. Here are a couple photos of the assembled system.
Front View of Temperature Controller
Rear View of Temperature Controller
View of the Internals of the Temperature Controller
Below is the schematic diagram for the controller. This is the basic outline. It can be simplified even more, or made more elaborate, to suit one’s application. The amperage of the fuse will be determined by the size of the heaters used. The author used a 10A SSR, so that’s the limiting factor in controlling larger heaters. But the power required to maintain the temperature of the telescope tube is very small, so high currents are not likely to be necessary, unless a very large telescope is used.
Schematic Diagram The next elements of the system are the heating pads. The author used heating pads from Walgreens. It was first thought that these heating pads might be able to be used with the controller that comes with the pads. However, the temperature settings are too broad and
the majority of them shut-off after a specified amount of time. Without a more accurate temperature setting, the system may not hold the telescope tube to a tight temperature range. Also, it’s important to set the temperature a few degrees above ambient, or above the dew/frost point. The stock controllers don’t allow this amount of flexibility. Here are the heating pads that the author used. They were on sale for $11 each, but many other options are available.
Heating Pad Box from Walgreens Once the heating pads were in-hand, the next step was to determine how the heaters were connected to the control box. The heaters typically have 3 or 4 wires going to them, two for heater connection, and the one or two for some type of thermal feedback. The power wires were located by looking at the PCB of the heater. Since the heater will draw more current than a temperature measuring circuit, the traces and wires will be heavier. In the case of the Walgreens units, the heater connections were designated by H1 and H2. The following photo shows the obvious connections for the heater.
Printed Circuit Board from Heating Pads, showing Heater Terminals (H1 & H2) Once located, the wires can be verified by measuring the resistance of the terminals. The resistance on these particular heating pads was about 200 Ohms. The Author used 2 pads in parallel, in order to cover the entire telescope tube. This gave a final resistance of 100 Ohms. Using Ohm’s Law, this means that the maximum current draw is about 1.2A, from the 2 heating pads wired in parallel. Although this is the instantaneous current, the average power is very low, once the set temperature has been reached. The final piece of the puzzle is the thermocouple or similar temperature feedback device. Most controllers will accept many types of temperature sensors. So the user is free to use whichever unit is best suited for the application. As mentioned above, the author used a J-
type thermocouple. This type was chosen for the compatible temperature span, but mostly because, some J-type parts were already available in the parts bin. This thermocouple runs from the temperature controller, to the heating pads, wrapped around the telescope tube.
Inserting the Thermocouple between the Telescope and Heating Pad In order to make the temperature readings more consistent and accurate, a small pouch was made on the inside face of one of the heating pads. This pocket was constructed by cutting a small 1.5 inch x 1.5 inch square of tape and covering it with a piece of cloth so 3 sides of the adhesive are exposed. This is stuck on the inside surface of one of the heating pads (in the plastic surface, not the cloth covering). This allows the thermocouple to be slid into the pocket and held close to the surface of the hearing pad. The diagram below outlines the design of the pocket.
Exposed Adhesive Thermocouple Pocket Operation Once the thermocouple is installed and the power and heating pad(s) are connected, the system can be operated. The system must come to equilibrium before focusing, or the focus will change slightly, as the heaters warm-up the telescope tube. Once warmed-up, focus the telescope and begin imaging. The author has performed over a year’s-worth of imaging using this system. There have been very few times that the system needed to be refocus, due to a temperature change. Temperature spans of more than 20F have been encountered, with virtually hands-off operation of the focus system. The system will easily hold the temperature to an accuracy of less than 1F. So instead of focusing every 30 minutes (between exposures), the author now focuses once, at the beginning of the imaging session. Each image is checked for focus, but rarely needs attention. References: 1. www.ericchesak.com 2. www.newastro.com/samples/c2a.pdf 3. www.astrodon.com/Orphan/parfocal_and_critical_focus_zone/