Saturday, November 14, 2015

Astrophotography: Solving Some Problems, Finding New Ones

I made something I like!

These are some of my changes:

  • Better polar alignment; not seeing field rotation. This is my longest exposure too, at 480 seconds, which I would think would show that sort of thing if it were off by much at all.
  • Neodymium filter for light pollution, it blocked quite a bit!
  • More spacers between the reducer/corrector and the camera to reduce the vignette problems to a minimum.
You can see I haven't fixed the primary tube reflection yet, still waiting on materials to be delivered.

I plate solved the image using Astrometry and came up with a 1.1 arc-seconds per pixel. The theoretical limit of my 6" scope is around 0.9 arc-seconds. Seeing conditions were much worse than that, and for where I am will probably never be better than about 2 arc-seconds.So I think we are in a good place.

I also calculated out an effective focal length of 926mm f/6.2, not far from the advertised 945mm f/6,3 the reducer/corrector is supposed to get. I didn't try plate solving previous images, but I can tell you with some certainty that the Celestron recommendation of 105mm between the corrector and focal plane, as well as the internet's prediction of 85mm are wrong. Total distance from the back of the corrector to the surface of my sensor is 155mm. If you have a Celestron C6 and the standard f/6.3 reducer this is probably about where you want to be, at least if you are using an APS-C sensor sized camera.

For pure resolution, however, our tracking is not quite perfect. Here's a single color channel from the image above, cropped to the center of the nebula where the trapezium stars are very close together:

And the same spot, but with a quick 1 second exposure:

You can clearly see all four stars as separate in the 1 second shot. This means that either my guiding isn't reacting fast enough, isn't predictable enough, or doesn't have enough resolution to keep things perfectly centered. I suspect the latter is my issue. I will probably need to get a small scope with a longer focal length to keep up with the main telescope's resolution. Seeing conditions are at play here too, but unless I was particularly lucky with that 1 second picture they should be effecting that image similarly.

For reference, the two stars closer together there are 8.7 arc-seconds apart:

Also, now that the field rotation is gone/minimized I can see we have some comatic aberration on stars at the outside edges of the frame:

Note that's coma, not chroma. This is the nature of the telescope. Stars in the center of the field focus to the same point from anywhere the light is gathered on the lens, but at the edges of the field those stars are focused at slightly different places depending on how close to the center of the lens the light was gathered. The reducer/corrector may be helping this or hurting it, depending on how much you believe I have moved the camera away from the "optimum" focal distance. There are additional correctors to help this, but this is so subtle I'm going to leave it alone for now.

Friday, November 13, 2015

Cheap light pollution filter

As soon as you say "filter" the public perception is that the images are photoshopped, or a little bit fake. I'm not one for heavily processing my images for other kinds of photography, and I'm more interested in being able to show what's up there in the night sky on the familiar terms that the general public is familiar with. There will always be limitations to this: "Is this what I would see?" doesn't exactly work in the world of nighttime photography in general.
  • Your eyes are more sensitive, but they can't accumulate light over time like a camera. So things can be much brighter in pictures.
  • Your eyes see vivid color in the day time, but the darker it gets the more your eyes rely on the "rods" of your retina, which are monochromatic; things appear a bit more blue than grey, but sensing reds, greens, and vivid blues is out.
  • You're looking through a telescope, which is a form of filtering on its own. Your viewing angle is cut down from maybe 170 degrees to 1-2 degrees. Your effective pupil size is also expanded from a few millimeters to the diameter of the telescope to gather more light.
But none of those things speak to real filters, which is what I'm adding to the system but going to try to keep the color "real" as much as possible, at least for now.

Astronomy filters can be very specific, and the cost of even the simpler ones is very high. I'm only a few miles outside of the city and live where the air is quite thick, which means street lamps add an orange glow to the sky and really get in the way of seeing what's up there. The goal is to take pictures of things outside of our atmosphere, not the atmosphere itself, right?

The orange comes from sodium, and the good news is that there is a cheap solution to this. "Red enhancing" or Didymium filters are made with neodymium. This happens to block that range of light without blocking much else. Amazon had the 52mm version for $23, which happens to be exactly what I need. I just had to remove it from the threaded lens mount so I could put it in to the telescope:

You can see by looking at the white cloth under the filter that it doesn't change the color very much.

I wedged it in to the T2-FX adapter and used some cardboard as a spacer. I will replace the cardboard with something better and less reflective soon, but this was more than enough to test with.

Thursday, November 12, 2015

Image artifacts: Astrophotography is touchy

Once I tied the camera to the back of my telescope and set up guiding I found plenty of new problems to solve:

I've boosted that image up a bit to make the flaws really obvious; this is a narrow view of the Pleiades. Surprisingly you can see some of the blue wisps of nebula around the stars (ignore the horseshoe shapes, those are artifacts)!

My scope setup is:
  • Celestron C6
  • Celestron f6.3 focal reducer/corrector
  • Standard Celestron SCT-T2 adapter
  • T2-FujiFX adapter
  • Fuji X-T1
Tracking is:
  • Really cheap Orion 9x50mm
  • Even cheaper Microsoft LifeCam with the filters and lenses removed
  • PHD2 giuding software
  • GPUSB-ST4 box
Everything that's wrong:
  • The orange-brown glow is my light polluted skies. We get nights that are clearer than that, but I think a filter would go a long way.
  • Stars in the center are nice and round, but the farther from center they are they are radially stretched. I believe this is a testament to how well autoguiding works. The system is locked on to the center star, but the alignment was off and so after a long exposure (300 seconds) the scope was not rotating exactly in tune with the skies. This makes the field seem to spin.
  • This thing that's going on:

    is vignetting; light is being blocked by the sides of the scope's center passthrough tube. This might mean the camera is the improper distance away from the focal reducer (I don't have this without a focal reducer, but then my field of view is crazy narrow). The outlet of the scope is also narrow, which will force me to put the camera at a not-so-optimal focal point.
  • This one is light reflecting off of the inside of the telescope:

    looks like a mix of the main body (very pale, I might not bother) and the primary mirror passthrough baffle. It's black but a bit glossy, and this is apparently a common complain for the C6. There's even a very light reflection inside the reflection going on for the brightest off-axis star, that's probably the T2 adapter tube.
The solutions are going to be a mix of things:
  • Better polar alignment when imaging
  • Light pollution filter of some kind. I'm guessing 90% of sources are sodium based.
  • Move the camera back from the scope until I don't have vignetting; this turns out to be pretty far back. I've heard the "correct" number quoted at either 85mm or 105mm, the first place I found it vignette free was close to 140mm between the corrector and image sensor plane.
  • Black out the primary tube with protostar stick-on flocking. This might never go away completely, apparently SCT style scopes often have this issue. Sadly I don't have access to that crazy science grade paint that blocks 99.99xx% of light. If you do give me a holler.
Overall I think this is going pretty well, but I'm sure I'll find new problems once I fix some of these...

Wednesday, November 11, 2015

"I accidentally a good equatorial mount."

That's the gist of it anyway. I bought a Celestron AVX mount for peanuts because it said it was defective. I figured that the mechanics of it were well liked on the internet, but that some of the early versions had electrical problems. My goal was either to fix the control boards (if I could find something fixable) or to replace it all with my own control system.

When it arrived on my doorstep I disassembled it completely and started looking over everything. Checked all the components that were check-able with a multi-meter and couldn't find a darn thing wrong. The control board was also the brand new version...something just didn't add up.

The mount came bare (no tripod, motor cables, power adapter, hand controller, weights, etc), so I had to get a few things before I could test it. Here are some mysteries I spent too much time researching, but solved:

  • The 8 pin, RJ45 connector between the DEC motor and port is, in fact, just a standard straight-through cable like those used for networking. I made this to size using some CAT-5 and connectors I had.
  • The Nextstar+ hand controllers are all the same, the two model numbers for EQ and ALT-AZ are basically just there to let you know which firmware was loaded on the board. I know this because the budget friendly used one I acquired turned out to be loaded for ALT-AZ and gave me all sorts of trouble before I reloaded it (twice, the update software defaults to what it has regardless of which mounts it detects). Note the "+" though, the non-plus ones use different things inside and probably won't run the newer firmware.
  • You can use a regular 12v power adapter and ignore the fancy thread-on-overpriced one Celestron sells. I recommend tying it off though, the clipping part of it is rather weak and you don't want it to get pulled out of the connector if you can help it. I will probably replace this connector with either a standard one or a waterproof one in the future.
Once I plugged everything in the darn thing fired right up. I pointed it around during the day until I was satisfied there was nothing wrong with it, and then later on that night I tried to align it. That's when I discovered the ALT-AZ vs EQ firmware thing. Even though "EQ North" was a tracking option in the menu, it has no idea how to deal with the mount.

I managed to get a few images out of it with nothing more than a "that looks about right" aiming of the scope at Polaris and then manually pointing it at things, but nothing to write home about.

Orion Nebula: Fuji X-T1. 30 second exposure, ISO 1000 @ 200mm F4.8
Things are forgiving at 200mm. It takes a lot more to get things right through a real telescope...

Monday, November 9, 2015

Minor Note...

Having just tossed that last post on here as a convenient dumping ground, I couldn't help but skim my old content. I haven't added anything in a little over three years and I can't get my head around how much has changed.

  • I'm still working on a lot of little projects, but in totally different ways.
  • I still love hiking and camping, but ghud ghawd I have learned a lot. These days I sleep outside just because I want a good night's sleep.
  • Which has helped me to travel this world a bit, nothing like the freedom of being able to get on a jet plane to anywhere with nothing but a backpack and a smile. I could, and probably should write books about how to do this.
  • I have definitely grown as a photographer. This has become a very important part of my life and I'm pleased to see this. I think drawing is a core of my approach, so I have worried that by not doing much drawing I wouldn't improve.
  • As far as I can tell I didn't write a thing in here about my physical activities, which are also central to my life at this point. I'm certain this started (restarted) in 2009, but apparently it was sometime after 2012 that it became apparent that this is essential to my self expression. Mind and body are not separate, at least for me.
I have no idea if anyone actually follows this blog after all this time, and I suppose don't care either way. Blogging for a day is good fun or narcissism, but blogging over years is something else entirely.

Data dump: Dew Heater Edition

I have a number of projects going on right now, which I will hopefully have more to say about later on. Right now I think it's important that I get a few pieces of information and findings out there on the internet both for my own benefit and for other people who might be working in similar things.

First: Here in the deep, deep, swampy south the dew point outside is typically one degree or less below air temperature. This means anything outside that has a view to the open sky will build up dew on it within a few minutes of exposure (the physics of radiative heat loss, maybe another post on another day).

In my case I'm concerned about my telescopes, which means I want heating but not enough to cause visual distortion: air of two different temperatures creates a lensing effect at their boundary, think ripples of air over hot asphalt on a summer day. Remember this?

That's colder/lighter gas distorting the air. Supposedly this happens significantly enough at 2 degrees F or less, but I don't have good data on that. Either way, the right answer is to keep a telescope as close to the air temperature around it as possible.

The plan is to make a nice microcontroller circuit that measures the air temp and scope temp, and if the scope temp is low turn on a heater. This doesn't need to be terribly quick or smart or use PID since there's a lot of thermal mass involved, and I've designed the heaters to be very mild.

I collected up most of the parts to do this, but a good test night rolled around first so I grabbed a cheap temperature controller I got from Ebay a while back just to get the heater kicking. Finding information on setting this up was difficult (it probably came with printed instructions, which don't last long in my possession). So here they are, if you have a cheap Chinese controller that looks like this:

Mine is labeled W2020 temperature controller, but let's assume it's China and there are 300 model numbers for the same product. Pressing "Set" once will let you use the arrow keys to change the desired temperature. Pressing and holding it for 5 seconds will enable a menu, which works like this:

CodeCode descriptionSetting rangeFactory setting
P1Hysteresis setting0.1-302.0
P2Highest setting upper limit120120
P3Lowest setting upper limit-55-55
P4Temperature correction+10~-10C0
P5Delay start time0-10minutes0
P6buzzer switchon/offoff
P7Value holdon/offoff
P8Restore factory settingson/offoff

Change P0 to H since we're heating an object, and P1 (how far above/below desired temp it kicks on) to something nice and low since we want to keep it pretty close to our set temperature (in fact the temperature will likely spike above our set a little bit because the heater wire has some thermal mass).

Oh, and the hookups on the back: NTC is the temperature probe, OUT is the heater (neither of these are polar since they are effectively resistors) and IN is power supply, with the little "o" denoting the positive side.

There are commercially available telescope products to do all of this, but being the telescope world they are stupid expensive and probably don't have a lot of active development going on with them. Sorry astronomer friends, but that's the world I can see from here: once something "works" the discussion is done, even if it means using parts that haven't been produced in the last decade. Also for the disciples of Carl Sagan the physics of things is not often considered too deeply.

My own complaining aside, the groundwork is solid. The internet tells me commercial heaters are about 0.75W/inch of circumference. This results in a heater that will just barely get warm to the touch, but seem like it isn't even working when stuck to a cold metal telescope. If you're lazy on math like I am and using a 12v power supply:

amps * ohms = volts
amps * volts = watts
desired resistance (ohms) = 0.75watts / inches
desired resistence @ 12v = 192 ohms/inch of telescope circumference
Adjust if you're in a place that is freezing cold and humid, or if you're insulating the thing to start with, or not using 12v or whatever. I'm only here to solve my own problems.

I grabbed some rolls of nichrome laying around the lab (doesn't everyone?) and came up with nice long wind of 18 gauge 80/20 wire. Pro tip: other websites will tell you to make a mess of parallel heaters or funky ways of staggering your wire to come up with enough length for a given resistance. Easier way: buy a few different thicknesses of wire, thicker stuff will end up with a longer length for the given resistance. If you have a small bodie'd thing like a spotting scope you probably want 26+ gauge, bigger scopes might be 16-20 gauge. Note that having too short of a run also means you will be putting all of that heat in to a smaller area, risking damage by making a miniature toaster coil for your precious equipment. Get the right wire for the job. It will make your life easier.

I also didn't like the idea that the commercial heaters only heated the very front of the scope: the whole scope gets soaked here, which I don't want it dripping on to other electronics, rusting, mildewing, or just generally being gross to carry back in to the house. There's also some general nonsense about trying to radiate heat back across the glass from in front, which sounds nice except for the body of the scope (and therefore the back of the glass) having a big thermal gradient as a result. Keep the body of the scope, inside and out, at air temperature and things will be much happier:

Kapton tape is cool stuff: extremely heat tolerant (we use this stuff on the heater part of our 3d printer, which spends hours at 260+C), durable and thin. Once I had figured out about how many winds of wire would go around the scope I marked out the appropriate distance between them and laid down a couple layers to electrically insulate the wire from the tube. I wrapped the coil and a top layer of kapton together leaving just a couple of ends exposed for connection. You can see that by having some 14 feet of wire to provide the appropriate wattage I had a lot more flexibility than "just the tip" heaters like astrozap, kendrick, dewbuster, etc. Plus my cost was $10 for the controller and 'it was collecting dust' for the wire, say $10-20 if you had to buy it (you will have a lot leftover if you do). A roll of kapton tape is only a few dollars, and you should have some just for the fun of it. They sent that stuff to the moon, you know.

The test was pretty simple: I set the scope up outside and let it cool down until dew formed on the front glass (less than 10 minutes). This was a rare night in which outside was colder than inside (everyone else talks about letting the scope cool down after being inside...I get that for a couple months each year here). I turned on the heater and 5 minutes later the controller shut off: the glass was clear. Yeehaw. I left it outside for a few hours and the heater would click on and off every few minutes, scope never showed a hint of dew again.

The upside is that it does the trick, the downside is that I have to manually set a temperature. Next step is making a real controller to automate an accurate temperature setting.