Astronomy with a cellphone — new life for old ‘scopes: Part 1
Aiming a telescope.
Introduction:
The first part of this article is for those interested in using telescopes but may not know the historical methods for aiming at objects to observe. The second part focuses on the SkEye Cam app to provide old scopes with full featured pointing capabilities. The last part looks at how this was used on two older scopes, a Meade ETX-90 and a 25+ year old 10" dob that was created from a tube originally use on an equatorial mount.
This article is part of the Astronomy with a cellphone series, if you haven’t looked at some of those earlier articles you may want to take a peek there for more background.
The basic optical principles for creating a telescope for casual observing have been established for hundreds of years. There has been significant progress in the materials — glass, mirrors, mechanical systems — to increase light gathering by making larger telescopes more accurate and affordable, but the basics of refractor and reflector telescopes remains the same.
20 year old telescopes, if well maintained, can provide optical performance as good as new telescopes.
What has changed with telescopes and their mounts in the past few decades is the incorporation of electro-mechanical systems to make them easier to use, either for aiming them or for tracking stars for taking pictures.
This has made a lot of older telescopes available as owners traded up to get the newer capabilities. For people looking to get in to observing they’re a great way to get a lot of optical performance at a good price. Except there is the problem of learning how to use them.
This article conflates telescope and mount to just the word telescope to keep things simpler as well as naming conventions are not consistent; most people refer to a particular type of telescope as a dobsonian telescope when in fact it’s a newtonian reflector telescope on a dobsonian mount.
Telescopes enhance what our eye can see by both increasing the brightness of far away objects and by magnifying them. As a result a telescope looks at only a very small portion of the sky at a time, typically no more than the size of your pinky’s fingernail at arm’s length. Finding the object you wish to view is the first thing anyone using a telescope has to learn how to do.
When getting started the biggest frustration is learning how to find things. Bright objects, like the moon or the planets Jupiter and Saturn are easy to find as they are bright and very distinctive. They’re great for beginners, but may not always be visible as well as there are thousands of other things to aim your telescope at.
For people that have had a telescope for years there will always be some deep space object that they read about and would like to see what it looks like in their telescope.
The common traditional way to find objects is to use a technique called star-hopping. You need a star chart that shows the area of the sky that contains the object that you are looking for, and some brighter stars in close proximity to the object. Using your telescope’s finder scope you start at a nearby bright star and hop across other stars you can recognize in the finder. Typically this might be looking for groupings that make little triangles or boxes. While telescope finder scopes give a wider view than your telescope, it’s usually not big enough to put the bright star you started with and the object you are looking for in the same field of view. Some finder scopes invert and flip the image, further adding to the confusion when trying to match up a star chart as well as the apparent movement of stars in the finder scope in the opposite direction than the physical movement of the telescope.
Sometimes the desired object can be imagined to lie at the intersection of virtualized lines that can be extended from pairs of bright stars, somewhat eliminating the need to hop across stars. This tends to work better using what is sometimes erroneously called a zero power finder, though in real life that means 1x magnification, i.e nothing other than using your eye and some sort of sighting device versus a finder scope which provides magnification.
The galaxy M82 is not located near any bright stars but if you can imagine lines as shown in the above figure you could aim your telescope at the indicated area and most likely have M82 appear in the finder scope, i.e. be within a few degrees of the correct location. This technique can be easier to do when using “zero power” sighting devices to aim the telescope, and then use the finder to get the desired object centered so it can be seem in the much smaller field of view of the main telescope.
The author knows of one local astronomy club member that uses this type of technique to find M82 in a few seconds.
For the novice telescope observer this method can be just as daunting as trying to star hop. From light polluted locations the brighter stars can be washed out. Memorizing patterns for a dozen objects takes considerable practice. Depending on the season and time of night orientations can be rotated. Many times the object being sought after will not be bright enough to see in the finder — light pollution makes this true more often that not. Getting to the general area means you’re still faced with scanning tens of square degrees of sky for the desired object.
Starting in the 1980s and increasing in popularity over the subsequent 20 years small handheld special purpose computers became available for connecting to a telescope that had devices call encoders attached to each of the telescope’s mount two rotational axes. Usually called Digital Setting Circles (DSC), they typically had a two line LED display and some buttons. The encoders provide an electronic signal to the DSC computer that provides the angle of the telescope from some reference point. The DSC would then compute the azimuth and elevation of the telescope and compare that with a built in object database, and then show arrows for which way to aim the telescope. The trick to using these was in the calibration — you would have to first place the telescope in a known position, and then locate some stars to give the device the correct telescope orientation. This was a very error prone process with these early devices; the author remembers plenty of cursing using one.
By the late 1990s telescopes started including motors that the microprocessor could control, so instead of PushTo to move the telescope to an object it became a GoTo and moved under its own power. However the microprocessors used in these devices were limited and the software was not always robust in situations where the alignment wasn’t accurate or the telescope was not mechanically stable. Performing the alignment also required knowledge of being able to locate suitable stars for alignment.
These enhanced go-to telescopes benefited experienced telescope users that understood how to align and calibrate them before each use, as well as they could quickly realize when they were wrong. For beginners these devices added another layer of complication and increased frustration as new users would not have the experience to know when they were pointing in the wrong direction or how to revert back to manual methods when the battery gave out.
By the 2010s the computational ability of the microprocessors had increased, and people started having easy access to powerful laptops, tablets, and cell phones. The latter allowed replacing the somewhat cryptic two line displays and multitude of buttons with a more familiar user interface. The software improved and would better detect set up error. Later models added GPS to provide time and location information, and some even added a specialized camera to identify stars to provide a mostly hands-free alignment process.
All of these systems added expense and were built in to the specific telescope. Retrofitting older telescopes is possible but requires some mechanical and electrical skills as well as it doesn’t operate as seamlessly as a telescope with everything built in at the factory.
By 2020 the capability of cell phone devices had rapidly changed. In the battle for one-upping the competition cell phones have become more like a camera with phone attached. Except for the lowest end models these newer cell phones have cameras that can take pictures in very low light — even just star light without needing an excessively long exposure.
This enhanced cellphone camera capability opens up a totally new way to aim a telescope that doesn’t require complex retrofitting for older telescopes nor a steep learning curve for novice observers.
Before looking in detail at using your cellphone to aim your telescope there’s one more piece of background that will help understand what the system is doing so that you can get maximum utility out of the software.
What makes your cellphone able to accurately aim your telescope is running a piece of software that performs an astronomical plate solving algorithm. Originally astronomers used special photographic glass plates (see this PBS article for more history) to take astronomy pictures with as they were durable and not subject to mechanical distortion like film. Stability was a factor if you were using these pictures to measure star positions. The first plate solvers were women employed by Harvard University to manually record information off of photographic plates of stars taken through the best telescopes of the late 19th century. These woman were called computers and that term has stuck today, though now with a much different meaning.
Sophisticated software can now analyze a digital astrophotograph and extract information from it. One of the key capabilities is software that can determine which stars the picture represents. This is a difficult problem as the orientation, brightness, and area covered (magnification) may not be known. Software that determines which stars are in the picture is known as plate solving software.
The Astrometry.net website can be used online to analyze uploaded pictures. For those interested in the technical details the developers of that site have a paper available.
The typical method of plate solving creates a database from a star catalog which has the stars and their known positions. Arbitrary sets of 4 stars (sometimes 3 are used) are chosen and the relationship of their relative locations is used to create what’s called a hash value, which can be thought of as a unique identification number. It’s important to use relative measurements as that eliminates the problem of the image scale and orientation, i.e. no matter how big or small the magnification, nor the rotation of the image, the use of measurements of the stars relative to each other ensures the same hash value.
When an unknown image is provided, the software picks a set of 4 stars at random and calculates their hash value, and then searches the database of the reference stars with the same hash value. Once a match is found the software looks for other near by sets to confirm the match. It might take hundreds of thousands of attempts of picking random sets of 4 stars and computing their hash before a match is found. On a typical laptop this can be achieved in a few seconds to a few minutes. In addition to the Astrometry.net paper, the hnsky.org software that also performs this plate solving has a page that details the process further.
When executed on a cell phone mounted to a telescope the plate solving can make some simplifications. From the phone’s GPS it will know where it is and what is visible in the sky. The software also knows the general properties of cell phone cameras in terms of field of view and brightness range of stars that it can record with short exposures. These types of simplifications make it possible to do real time plate solving, which means as you move the telescope and the phone can calculate what it is pointing at in real time.
Part 2 looks in detail at the SkEye Cam software. Part 3 shows the setups for how this was used with two older telescopes to provide aiming capability for PushTo operation.
