Wednesday, April 29, 2020

TPM Telrad Pattern Matching

Another writeup on the subject which is so trivial that it deserves a detailed explanation! How to use the Telrad properly? Many folks using it just as a glorified red dot finder. Which is sad to see knowing its potential rivaling even most advanced GoTo systems... But let's start from afar with the most frequent question a beginner or mature amateur astronomer is constantly concerned about: How to point a telescope to the invisible for naked eyes deep-sky target? I mean, in a reliable way? [the write up is still under development]

Photo by Guillermo Ferla on Unsplash

I bet you have heard dozens of answers to that already. After asking that question for over 40 years myself and trying what others doing to solve that problem I have finally got the definite answer: the TPM. Which stands for "Telrad Pattern Matching".

Back to our question. What is the formalized goal of all celestial target pointing methods? Surely is to get the object you want to observe into the field of view of your telescope eyepiece (into the magnified and enhanced circular portion of the sky you see when looking into the telescope).

How to make that task so trivial that there is no question of how to do that even for a little kid? Think about the Moon, for example. Obviously, as soon as you can see its shiny disc or sickle in the sky you can raise your hand and point at it with your finger immediately. What could be more trivial than that? That's because the "eyepiece" FOV (the Field Of View) is equal to your eye's, you can clearly see your target, there are no other similar objects in the sky visible, and the "telescope" you are pointing is just a finger on your hand.

But is it at all possible to point a telescope to, let's say, the Horse Head Nebula, which is hard to see even through a decent telescope, with the same ease and triviality? Yes, it is! By using the Telrad Pattern matching (TPM) method.

OK. So what's the heck is that TPM after all? Almost exactly the same thing as pointing with your finger! But:
Instead of the easy to recognize Moon limb, you are pointing at multiple naked eye stars near your invisible target. You can call it an Asterism if you wish. But the difference is that it's a dynamic thing. You can add or remove stars (or even bright DSOs) to/from it as needed or desired.

Instead of the finger's contour (which you move in the sky towards the Moon limb), you are pointing with the special reticle "projected" in the sky. The primary difference from the fingertip is that it's much larger and has more features to catch your attention when deciding if you have your finger pointing at the moon already or need to move it a bit more in a certain direction.

As your memory is not yet familiar with your new "finger" and your new "Moon" let's add exact virtual copies of them (the Telrad reticle and the Asterism respectively) on the digital star chart computer screen showing you the live example how these two pieces should look like perfectly aligned in the sky when your telescope is pointing exactly at your desired but yet invisible target and centered in the Telrad reticle ("Moon" on the tip of your "finger") in a guaranteed way. Which is frankly to enhance your brain capacity at the task of precise pointing in an unusual environment.

The TPM could also be conceived as a way of the deep sky target enhancement (target was invisible, now it's clearly visible), pointing finger enhancement (it was small, dark and featureless, now it's large, lit, with many features to align, and even semi-transparent), and the human brain enhancement (with the easy to see and comprehend cheat-sheet right in front of your nose and side by side with the "Moon" and the "finger" clearly visible again) all the way to making our ultimate task as easy as finger-pointing at the Moon now.

That's all.

Or, thinking in reverse: the TPM is as natural as pointing with your finger at the Moon because in your brain the procedure of finger-pointing at the Moon means moving your stretched out arm until the contour of your index finger is touching the Moon limb's edge at the lowest point. TPM means the exactly same thing in terms of your brain and body work required. Looks complex? But that's just a weird way the "human's finger-pointing process" could be described to an alien. For the typical human brain it's totally natural! Got a tough target to see? Just enhance what you are naturally using for pointing to a certain level and allow the brain to do the work you are perfecting from the time you have opened your eyes for the first time: recognize and match visual patterns around you to survive by  solving problems moving your body accordingly! Weird, yeah.

A typical question: Why can't we point a telescope just by a "small finger" like a red dot or a laser beam's end? Because the target is invisible to you, naked-eye stars around it are usually far apart, thus any single dot positioning will be mostly just guessing the invisible object's location between them, not pointing at it. The guesswork is reducing the chance your target will be close to the center of the FOV due to a plethora of natural factors (physiological, psychological, optical,  perceptual, atmospheric, etc) as well as artificial factors (the chart orientation, chart projection, RDF mount angle, RDF aberrations, etc), so if it's not a bright and familiarly shaped object obvious in the FOV you may spend hours hunting for your target around that guessed spot again and again.

Due to the large size and multiple alignment points in the Telrad reticle, instead of guessing you are aligning these points with multiple visible stars and thus automatically triangulating the target's position from most reliable matching points close to each other or aligned with each other in a certain way which helps with the precise matching of views a lot even just from your visual memory. Each parameter your brain is taking into the account automatically while matching these views feature by feature is adding to the accuracy of pointing. And that's not by tedious measurements from multiple points (a well known geometric method of using Telrad or RDF), you are matching the entire pattern unconsciously, with the goal of making it look exactly the same as on the chart's picture. That "the same" is based on zillions of factors catching in your neurons, not just a single dot far away from any visual anchors as with the RDF.

There are some caveats though, which many who heard about the Telrad and even about the TPM method might be overlooking when trying to understand or practice it, so lets polish your understanding of the TPM a little before getting to the actual step by step flow example:


  1. A telescope you can move manually (even if by using an electronic remote control device or technology).
  2. Telrad or QuInsight collimating pointing device. The Red Dot, Green laser, optical finders with more than 1x magnification wouldn't work. Its mount must hold the alignment with the telescope optical axis while you are using it.
  3. Digital star chart capable of displaying a precise model of Telrad or QuInsight rings pattern on the star chart as a live overlay image. The only digital star chart in existence which natively supports both Telrad and Quinsight reticles' precise model is DSO Planner. As of 2020, other apps have a rudimentary support for Telrad only (see "Before you start" #4 below why), but some apps have a feature which allows to show multiple eyepieces FOV rings, which can be hacked together to mimic Telrad or QuInsight rings in a usable way (see User Note 6 below).
  4. Either a good knowledge of constellation stars or the digital star chart app feature showing on the screen the chart of the sky region behind it when you hold the screen up to the sky (often called Digital Compass). Such a feature will help you to distinguish bright stars in the sky until you memorize them well enough. The DSO Planner app is one step ahead by providing that feature along with the additional Digital Horizon feature, which in is also leveling the horizon line on the chart with the real thing. So you can distinguish constellations easier from a possibly awkward posture behind Telrad or if you have the smartphone mounted on the OTA.
  5. Enough stars, which you can see by your naked eye. If you are in an urban zone where you can barely see 2-3 stars, even after prolonged darkness adaptation (which is another huge and controversial subject I'm leaving for a separate writeup in the future), you are in trouble. Though, with the experience using TPM and having the QuInsight device (vs Telrad) it might become possible though. As you master the method you could do well with just a SINGLE star visible close enough to the rings pattern.

Before you start

  1. While you are a beginner, make sure you are using the widest FOV eyepiece available for your telescope, as initially your target might end up within 0.5-1 degrees from the FOV center after using the TPM.
  2. Make sure the Telrad is precisely aligned with the center of the eyepiece FOV (you may want to temporarily install a higher zoom eyepiece to double-check for that or use a "crosshair" eyepiece of sorts).
  3. Using your star chart app, find an abundant with stars region of the sky at a convenient for Telrad viewing altitude (but no lower than 45 deg). Center one star in the Telrad rings. Center that same star on your star chart with Telrad overlay On. Observe stars behind Telrad rings and compare the view with the chart (you can move the OTA around the location to have stars touching the rings, just adjust the star chart accordingly too). If you see that in the sky either:
    1. A ring's size (diameter) doesn't match the rings on the chart noticeably.
    2. The Angle of gaps markers in the rings doesn't match the chart markers for them.
    3. The width of the rings overlay doesn't match rings on the chart.
  4. Then go to the app settings and change parameters used to draw Telrad rings on the chart so everything you see in the sky on a test star pattern is as precise match to the star chart's image as possible. You might need to revisit settings multiple times. Including as you gain the TPM experience, as you will notice more and more tiny mismatches to correct. That's the key to the ultimate success of the TPM in any practical situation from lack of the naked eye stars to horizon features blocking the open view of a pattern. And that's what all other apps on the market are lacking badly.
    Keep in mind that you will need to re-calibrate your Telrad (like described above) after the reticle refocusing again (Telrad reticle must be refocused after a while as it's often hiking out of focus after multiple axis alignments unconsciously performed in the same direction; the QuInsight free from that). So better to refocus it first if already needed.
  5. Make sure the app is showing the star chart for your actual local time and geographic location, and that its real-time clock is ticking (so the chart is updated to the real sky view expected at least every 30 sec). 

The flow

  1. On the star chart select the object you want to see in the telescope and make it centered in the simulated Telrad rings overlay on the chart (some apps may require special measures turned on to keep the object always centered on the chart as the stars are moving with time so the app might move them away from Telrad's center eventually).
  2. Using a low star chart zoom, figure what constellation or/and bright stars are surrounding the target.
  3. Find these constellations/stars in the sky (use the apps digital compass for assistance with the general direction).
  4. Point the telescope in that direction roughly.
  5. Prepare to work with Telrad (e.g. move your chair, drop your knee pad, don eyeglasses).
  6. Zoom the chart to have Telrad rings filling as much space on the screen as needed to show at least 2 bright stars which you can clearly distinguish in the sky AND on the chart in a favorable pattern inside and/or close around the rings overlay. Ideally, you want stars touching one of the rings, or nearly precisely in between two of them. Later, as you get a certain "spatial feeling" in the sky you would do very well with almost any pattern.
  7. The final step is to look through Telrad and move the telescope until the real Telrad rings and corresponding real stars you see in the sky are creating the exact same pattern which you see on the star chart.
As soon as the pattern is matched, you can be 100% sure your target is in the field of view of your eyepiece. Even if you cannot see it there from the first glance! As many truly serious objects require the field identification which allows you to find the exact spot where the object or its contours sits between faint stars. And for that, you need a really good digital star chart again, which is capable of showing you ALL stars which you could possibly see in your telescope, so you could match it with that eyepiece pattern as well but now with the frame of your (usually) higher magnification observing eyepiece FOV displayed on the star chart instead of Telrad's. Try that with a paper star atlas!

An avid TPM user's notes

  1. Ideally, as a beginner, you want to compare the sky view and the chart side by side. So you can get used to the way the star chart app is abstracting the reality. There will be never an exact match of the screen and the sky, obviously, so you need to adjust your "pattern matching brain algorithm" to the same level you have it adjusted for matching let's say an animal which you see on a painting, for example. For that you need the Telrad (or QuInsight) mounted in a way making that easy. For over a decade I've been using my Telrad on the side of the 12" Dobsonian. Yes, under the eyepiece, not above it as everyone else do. So the pointing looked like targeting with a shoulder-mounted bazooka! Not for everyone's knees, I understand, but extremely convenient for side by side comparison and study of the handheld screen and the Telrad views.
  2. To compare by just memorizing the pattern use gaps in Telrad rings. I have my Telrad oriented in a way so one of the gaps is clearly on the top of the view, that's considered a 12 o'clock orientation. You've got the idea already? Now, I can remember the pattern as "the top star is within ring 3 and 2 at 1:30 o'clock, the lower star is at 7:45 outside of the ring 3 for a bit less than a degree". That's enough for a better than 0.5 degrees accuracy of pointing, which is all I need to go straight to my main eyepiece at 90x magnification after sweeping my Dob in a fluid wide arch with a brief kneeling at the end of it (in like 5 seconds). The only app supporting the Telrad gaps angle adjustment is the DSO Planner again.
  3. Ideally, you want that "pair of stars to match" landing on opposite sides from the target (to "counter-cancel" various hard to avoid errors of the entire setup). But as your experience grows, stars on one side and even a single star away from the rings could work in a pinch. That allows to point in urban environment with a lot of the sky obscured or hiding in the light pollution.
  4. Remember about atmospheric extinction. It's distorting star patterns relative to the unchanged Telrad rings drawn by the app as ideal circles. The error is not very significant but might create a confusion. Especially when you have your rings calibrated in the app conveniently pointing at some stars close to the horizon on a warm nigh (high refraction). 
  5. All other pointing methods available for amateurs I have ever tried or know about (except for the ancient Star Hopping and the 2020es high tech Celestron Star Sense Explorer) can't match the TPM in simplicity, reliability, and speed as all of them are either relative to some initially calibrated point thus prone to the mechanical failures or deficiencies, or incapable of compensating for ever changing conditions and user errors. Others are simply too slow either in the preparation or in the application (I'm leaving the side by side comparison to a later write up maybe).
  6. If you've got a star chart app that doesn't have adjustable Telrad rings see if it has a possibility to show multiple eyepiece FOV rings instead. In that case, you can try modelling Telrad rings one by one by adding special "ultra-wide" fake eyepieces tweaking their parameters until the EP FOV ring matches one of the Telrad rings you see in the sky. Match by the same edge of the ring each time (i.e. by inside contour only to avoid introducing a ~0.1 deg error) as the width of these fake rings is usually not adjustable. Sadly, you can't adjust the gaps angle that way either, so you will lose quite a bit in the pointing accuracy,  speed, and convenience due to inability to leverage 8-16 gaps markers.
  7. The nearly-religious care of your eyes darkness adaptation is the key for any astronomy observations of deep sky objects success, the same is true for the TPM success as even though you are starting from really bright stars, the view background full of faint stars makes the procedure more accurate, intuitive, and quick as it becomes a natural habit. Study proper techniques for darkness adaptation, its preservation, regaining, and maintenance. They are especially crucial in a light polluted observing location. Improve your observatory equipment and location to help with that as much as possible too.
In addition to simple and reliable target pointing benefits, the TPM allows you to focus on the sky more than on pointing aids. For a newbie, it's a great way to learn the sky, so at some point, all you need to point at some faint goodies is the Telrad and a couple of patterns from your good times under the starry sky memories.

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