Thursday, February 11, 2016

The Spectre of the iPhone Spectroscope

UPDATE: Feb 27th 2016 I have got them calibrated now. The iPhone6 version calibration is still a bit rough. Lining up on the Helium Lines seems to produce the best results. The camera response is markedly different for the iPhone over the Cannon 550D. Now that I know what we are looking at here, next time I image I can make sure I get the image in a bit better focus. I have also found a way to reduce camera shake by delaying the shutter open until 3 secs after you press the shutter button. Whilst you can see visually the same lines, when extracted using Visual Spec the camera response seems a bit different.

UPDATE: Feb 22nd 2016 Well here is the comparison - again I am still very new at this and am still learning how to properly calibrate the specturm.

Colour, the final frontier. Of course if you want to be a "real astronomer" the visible spectrum is only a small part of the story. Much of our knowledge of astronomy comes from spectra and radio astronomy.

At the 2014 NACAA Conference I was drooling over Ken Harrison's amazing Spectroscopes after his talk and made the comment, it would seem so unfair to buy one off the shelf without going through the pain [Learning Experience] of trying to build one myself. So true to my word I had a go at the basic process of "getting colour".

I wasn't too concerned at this stage about quality, just some rational experimentation with the process to gain a deeper understanding. So I went off to the local hardware and bought a few bits and pieces and broke up a cd (carefully) and peeled the film back. At this stage it was unclear to me whether Barry Manilow CDs or Justin Beiber CDs would provide the best high or low resolution diffraction grating. Carefully pulling apart a three blade razor and using two of them to create a slit, challenged my dexterity. After tinkering away for a couple of sessions over a few weeks, I managed to come up with this monstrosity. Splits the light, can see colour, but probably absolutely useless for real science, but fun to play with and demonstrates the principle of a spectroscope well.

The first place to start this journey is with Ken Harrison's book Astronomical Spectroscopy for Amateurs. It is well written but looks quite daunting when you first pick it up. However if you work through it bit by bit, its a great book with everything you need to know.

Feeling a little more confident, I invested in the next logical step - a Star Analyser 100. This is the entry level standard for beginners, and produces fine results as we'll see.

After passing it around the family to have a look at the cool effects looking at the ceiling lights, I lost the plot and departed from Ken's careful, meticulously presented steps, with the outrageous thought - I wonder how this baby would go on an iPhone! I am constantly amazing at STEM events and star parties how the "younglings" immediately are so amazed by what they see through the eyepiece they want to whip out the smartphone and take an image home with them.

Surely it couldn't possibly work. After all the iPhone 6 sensor is only 4.8mm by 3.6mm, it has a focal length of 29mm and is f2.2. But its an 8M pixel camera (said my evil twin subconscious), your Fingerlakes PL11000M is only 11M pixels so its only 3000 less pixels, how bad could it be....hang-on whats the pixel size ...ah 1.5um versus 9um, interesting. So the sensor is 3264 x 2448, interesting ..... iPhones do take good photos......on a sunny day.....not in the night sky. This went on for a while!

So in the end there was nothing else to do but try it, and learn from your spectacular mistakes!

The next problem was how do I keep the shutter of the iPhone open for long enough to take star photos on a guided telescope - this is not the moon, a great iPhone target normally, but with the standard phone settings thats all the iPhone seemed good for. So channeling my teenagers I thought, there must be an app for that. To my amazement I could not only find one, there was a choice of several apps and 645 Pro could even do it with an Kodak Ektrachrome 64 film "feel to it". NOTE: to those born after the 90's, can you imagine only getting 36 images on one roll of film and not being able "to delete the bad ones" until after you had paid $25 and sent it off to the developers and had it returned to your letter box. In those days the lens ONLY pointed away from you - THE HORROR! Anyway I digress. 645 Pro basically turns your iPhone into a simulated DSLR and enters the workflow of the photography before any JPEG compression. You can set ISO and shutter speed, bracket exposures do all sorts of things that you can do on a DSLR.

With my trusty new app, my camera adapter, my star analyser, a 25mm eyepiece, I was ready for action.

I started on Canopus, after the bright star align was completed, but was more interested in Betelgeuse. After slewing and removing the eyepiece and inserting my newly built contraption, I was sure it was the world's first ultra-low resolution Spectroscope. A quick google search showed I was in fact two years behind the times. However to my amazement I had colour and with some detail, I got "lines" as well. I messed around trying different settings and moved over to Betelgeuse and took some more and put the 2 x barlow in front of the Cannon 550D and tried that for comparison purposes. (See image - top of page)

As an experienced "normal" (although I know you are wondering by now how "normal" that is) astrophotographer I was keen not to overexpose the image - Hmmmm - I have no idea what the well depth of an iPhone sensor is, lets just take as many as we can and see how we go.

I must say the results amazed me. Punching the air in victory, my evil sub-conscious dredged up an "I am invincible" [from the Bond film Golden Eye].

By now it was approaching midnight, and quietly tiptoeing around my back yard I packed up the telescope and headed inside. Damn ... I was so excited, I forgot to change the camera settings to save in dRAW/TIFF - back to the drawing board! Well at least I have some nice completely useless JPGs, but what an exercise that was, one I know I will be able to use again and again.

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