Near Infrared Telescope or how I said no to Cal-Tech

by Ted Frimet

Ok, so maybe it isn’t so fair of me to say, I said “no” to Cal-Tech. What really happened was that I stumbled upon their article, on infrared astronomy. And right out of the gate they designated infrared astronomy, as no-go territory for any amateur. And that got my attention, and I hope I got yours.

As my article on how to build a near infrared telescope is really very brief, I’d like to take more time, and invest you in my history of how I arrived at this point. Thank you. I must apologize, up front; I compose most of my writing, and dialogue in stream of consciousness. This may appear to be word salad to some, or a very appetizing main course for a few. You decide. What is really very cool, is that AAAP has provided a place, where some, or all of this text becomes web searchable, and may become the jumping off point for some young minds. So, with that in mind, I thank you, once again.

It was over three years ago when I stumbled upon micro-controllers, specifically the Arduino and its derivatives. I was wowed and amazed that during my lifetime, I could interact with a technology that was not only understandable, but also very affordable. Step by step, I learned how to program an Arduino and to control real world devices. Ok, not so, “real world”, however LED’s, LCD’s as output devices, and temperature and barometric pressures sensors, as input devices, count as “real”, don’t they ?

By the end of November 2015, I had cobbled together a poor mans LIDAR prototype. It was the result of a failure, actually. The story goes, that one day, as I was pondering the output of a BMP180 (barometric/pressure /temperature /altitude etc. sensor) I looked up. Yup, at the sky. And thought that the pressure reading I was observing was the direct consequence of pressure – right over my house. I then, thought, rather naively, that there were pressure waves that were forming clouds, directly overhead. And I started to think of where those pressure waves were coming from. And of course, silly me started to think of photonics pressure, from outside of our atmosphere shaping the clouds. A year later, I would be viewing sun spots (as I learned – at the lowest level of possible observation during the current cycle) and thinking of cloud formations as a result of those same solar observations. You could shoot holes through the aforementioned hypothesis. But you got to agree, at least it got me thinking. But I digress.

The failure wasn’t in trying to measure barometric’s. I was going to image the underside of cloud formations using a small grid of light sensitive resistors. The array soldering process was troublesome and painstakingly difficult, and before I could finish the array – A friend of mind quietly pointed out that the sensors I was employing could not possibly work in the fashion that I had designed. Too wide a viewing angle for any one sensor, it appears, is a failure for this application. However, during the project, I had decided to measure cloud height. That was a success. It was a success borne out of failure. I couldn’t be more pleased.

I built a prototype that measures cloud height. You can search YouTube for mrtfrimet, or Arduino Cloud Height Sensor, and find the three videos that discuss this in detail. Here is a brief: I utilized a Melexis infrared sensor, two barometric and temperature sensors (to average out any errors), and some insight from a children’s web page sponsored by NASA to better understand dew point. Also, I stood on the shoulders of giants that had previously programmed the libraries of my chosen devices – which cut down on development time, immensely. The result was spot on. And I have published the data on the web. It is public domain on Frizting. The breadboard PDF looks dandy, but the wiring is a mess. Not for the weak of heart, I suppose.

Back to the failure…When I came to terms that I could not image the underside of cloud cover, with my current build, I though of repurposing my prototype. And turned to infrared astronomy. It seemed particularly important to me, at the time, to think that since I worked with an IR sensor, that I could do IR astronomy. Clearly I had a lot to learn.
I figured on 12 to 18 months of learning how to make back yard observations, as a precursor to any prototype building. And here I am, today, with you, AAAP, learning during our public night viewings, StarQuest, and UACNJ dark sky at Jenny Jump. I learn more and more with every view, and am incredibly honored that those with decades of experience share their wisdom, and intellect with me, without so much as a second thought.

So, here I am, on the forefront of having acquired some amateur astronomer knowledge, that by February 2017, I should have enough experience to see the night sky, and appreciate the results in near infrared.

The poor man’s lidar, although a success in cloud height measurement, will not work in measuring near infrared – not in the way I thought of, a year ago. Not without modification, and possible destruction. And the sensors that can do the job, require cooling to 77K. Working with liquid nitrogen and dewer containers are not immediately within the grasp of my abilities. I can dream of doing that, some day. However, today is not that day. So I take a pass, for now, on low temperature sensors requiring the aforementioned -321 degree Farenheit cooling apparatus.

After searching the web for eyepiece and sensor information on CaF2, NaCL, ZnFe, KRS-5, AgCl, KBr, diamond, and lnSb, I stumbled across the more pedestrian and commonplace (to me anyway) germanium. It seems that Texas Instruments makes the OPT 101 in a plastic dual inline the is sensitive to 700nm thru 1100nm. And it does its job at room temperature.

Now before you get up and leave, let me shout out, something, here, on the nature of the bandwidth that I will be studying. It is a pretty clear fact that our atmosphere interferes with the electromagnetic spectrum, and infrared isn’t any exception. However, I’d like you to know how excited I am to find a sensor that can detect near infrared, and not require cooling. The OPT 101 was used in a medical prototype, for measuring some metrics concerning blood flow. So clearly, no liquid Nitrogen, and safe enough to be around sentient life forms. And it does have a response rate to a narrow band that corresponds to near infrared.

The other problem. My first scope, a 4.5 inch Newtonian acquired thru goodwill, has silvered mirrors. Not necessarily the best choice to reflect near infrared. A visitor and participant at our AAAP Star Quest, who is employed by a manufacturer of eye pieces, enlightened me that our reflectors can be bandwidth tuned. He even opted for dielectric coatings on his own telescope. Needless to say, he got my full attention. My mirrors could be recoated and made specific to reflect the narrow bandwidth that I’ve chosen to study. I think that this is called bandwidth tuning. But I like the poor mans approach. Ebay 24K gold leaf for a couple of dollars, and apply it to the primary and secondary mirrors. Gold reflects infrared, nicely. Wouldn’t you agree? However, the mechanics of removing a mirror, applying gold leaf, reinstallation, collimation, OH MY!! I am out of my league, not ever having removed a mirror or collimated in my life. I trust that you all have my back, to lend a hand, to collimate what will, no doubt, be the undoing of my first telescope. I will do a trust fall, right here and now. Be ready to breathe life back into my telescope!

Collimation. I suppose that collimating is easiest when you can see full spectrum light, but now I’ve coated my mirrors with gold. That makes it downright difficult. So I made it a thought experiment, and my hypothesis is that gold will reflect yellow and red light. And we should be able to “see” the “eye” to make adjustments. Fine tuning will have to wait to position a star, in mid field, and gauge the output of the prototype. No doubt of that.
And the next problem, yet. Infrared energy doesn’t play nice with optics. I started to research, CaF2, NaCl, ZnFe, KRS-5, ya$a ya$a ya$a, and finally stumbled across a fact that good old optical glass plays nicely with the little bandwidth that I am going to study.

The latest problem that raised an objection was that the OPT101 sensor has a response band on both the visible and near infrared. Ouch. I was going to get a lot of false readings, and didn’t want to fool myself into thinking that the harvested data was a success. How to block visible light, and keep near infrared out on a budget, like mine ? After a quick Google search, Dichroic scrap bubbled to the surface. So did a UK infrared camera in Hawaii. They both started to look like resources that I could tap into.

I started to see how much of such and such I needed, and sent off an email to the UK handlers of the infrared camera and see if they could part with 9mm x 7mm of filter. I sent the email too soon, as I quickly realized that I could manufacture my own. Not of dichroic glass, mind you, or of what must have been a very expensive bandpass filter that they employ. A simple imposition of polarizing film would be what I would experiment with, first. If it doesn’t work as planned, I’ll call out the Cavalry! Two sheets at 90 degree angles should, in theory, block all visible light, and allow infrared light to pass thru. I sent a second email to the UK folks, and said, “never mind”. And sent up my order, thru eBay, once again; this time for display screen polarizing film.

I think I have everything moving in the right direction. I have cleaned up my attic, and set aside a good deal of space to relearn micro-controllers, and programming. I have spent many hours with telescope time, learning how not to get lost in the night sky. Have ordered gold leaf, polarizing film, and a couple of OPT101’s (one to destroy – always happens, one to prototype with, and one to make permanent), and will repurpose my old telescope.

To insure a future initiative, I have enrolled in a free Microsoft Visual Basic suite, to relearn programming (this time with a Windows 10 application in mind) – with the intent of the follow on project controlling a one-axis right ascension controller, with a Windows 10 phone, via text messages from my Mac, from a remote location. The same phone, will be suspended with a clear view of the site scope optics and provide a visual feed, as well as the near infrared live data and sky temperature. My current drive controller is old school and is crystal based. It isn’t properly tuned for my mount and is mostly useless. So I ditch the store bought controller, and marry up the single axis drive, as is, with an Arduino properly programmed for precise control. Sounds promising. And should add a few months of what is a personally rewarding hobby.

I am planning on having the near infrared telescope prototype ready for February, 2017. Reality may bite, and that may change to February 2018 !! However, the plan, all the same, is to position the scope, in a “fixed” position to observe a slice of the night sky, as it passes overhead. And as the world turns, so goes the right ascension, as near infrared light energy will converge to be recorded, on my laptop, one locus, at time. I am also going to repurpose the poor man’s lidar to monitor sky temperature. As a minimum, if a bird were to fly overhead, I would have rather high “sky temperature” to invalidate any near IR readings from the telescope.

February may not give me excellent results, and life may throw a few hurdles my way. And I am, or will be patiently waiting for next November 2017 for better skies, or the following year. It does not matter if the prototype works, or not. It may very well lead to an amateur discovery that wasn’t even expected, or planned for. What is of particular importance to me, is that I get to share something of myself, with all of you, right now. Thank you, all, once again, for the opportunity to write to you, on Sidereal Times, the official publication of the Amateur Astronomers Association of Princeton. And special thanks to both our President, Rex Parker, who threw out the challenge to all of us, to write for our publication, and to our editors that will try to make sense out of my Steinbeck school of writing.

Part II

Sunday, November 27th.

I finally got a chance to coat the Jason Model No. 327 Equatorial Reflector Telescope mirror in gold. It wasn’t too difficult – however the results are far from professional.

If I run into zero chances of success, later in the game – I will start from scratch and re-coat. It’s only a few dollars waste, so no worries on budget. For now, like any other endeavor, let’s move on with momentum, and steer clear of inertia. And work with what I’ve got, so far…

It turns out that that there were three screws that secure the entire mirror assembly. After their removal, the entire assembly came out, without much of a fuss. And yes, I did confuse at least one screw, upon reassembly – so I may have moved the mirror out of collimation, somewhat. I backed off my mistake – and checked visually. It seems OK. You old astronomer experts must be chuckling, by now – remembering your first miss-steps. You might agree that I am having all the fun, right now!

Coating the mirror with a penny’s worth of gold wasn’t very fun, though. However I managed to get most of the mirror coated. If I had to guess, I’d say 95% or more success in a very short time. As an aside, please note that I whetted the mirror with lens cleaner (I think that the solution is mostly isopropyl alcohol). The fluid assisted in adhering the gold leaf to the mirrored surface. A lightly sprayed surface, is all it took to assist this application.

After reassembling the mirror back into the telescope – I did a quick “eye” peek thru the rack and pinion focusing tube (without any eyepiece) and saw quite the mess. Not a pretty sight! All that wrinkly, square pieces of gold foil, overlapping each other. Not for the feint of heart. What kept me sane, was remembering that this was cheaper than a real commercial coating. So far…

I installed an SR 4MM, and pointed the business end of the 4.5 inch Jason towards the sky (not sunward !!) and was amazed to see that most of the light appeared white – to an “off-white with a bluish hue”. And despite moving the rack and pinion knob thru the full range of focus -and out of focus- the poorly applied gold leaf; with all its tears and aberrations, simply did not show up at 4MM. Truly, I wasn’t certain what I would see, other than say, “gold” color.

Perhaps there is a good amount of visible light transmission thru the gold leaf, and back from the mirror. Not too much of a concern, as I will be installing polarizing film later in the prototype.

Either the smaller areas that were left uncoated are reflecting a ton of light (not likely) or the gold leaf is reflecting light (probable). I must also contribute, that the light that I see – may be incidental and not coming from the mirror at all. Sigh. Light bouncing off the insides of the OTA.

Judging from what I see in the sky, this early afternoon – cumulus clouds abound – is a pretty good sign for a mostly clear sky, tonight. AAAP Clear Sky Chart shows minimal cloud cover, and above average seeing, too, for tonight (November 27, 2016). So, perhaps I do not have to wait too long to see the “visual” results (sans the sensor)!

The idea behind the gold leaf was to improve any infrared reflectance – so that test will have to wait until I can hook up the Near IR sensors, next weekend. I’ve been hunting the web for ideas on installing the sensor and it appears that as long as I follow the manufacturers data sheet – I should get some valid results.

I expect a tight range of voltages and to convert and expand them into a number range that I can plot in Excel. As for the remainder of today’s prototype setup – to see any star light tonight – will be a fun “seeing” thru a thin gold coating.

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