Author Archives: Stephen McIntyre

Barnard’s Loop – 2018-02-14

Barnard’s Loop (Sh 2-276)
Acquisition 2018-02-14, Denholm Observatory

This image captures about half of the structure known as “Barnard’s Loop”. Barnard’s Loop is a huge semi-circular shaped emission nebula in the constellation Orion and part of the even larger molecular hydrogen cloud that covers much of the constellation. The remainder of the “loop” extends down and to the right.

Barnards Loop 2018-02-14

Barnards Loop 2018-02-14

The distance to Barnard’s loop is estimated somewhere between 500ly and 1500ly,  so at 10° in angular size, that makes the actual size somewhere between 100ly and 300ly. The apparent magnitude is listed as 5, but with that light being spread out over the total area, it barely stands out against the background sky – even in a photograph. This long exposure photograph required careful “stretching” to exaggerate the emission nebula.

Also in the field of view is the Orion Nebula, Horse Head Nebula and the Flame Nebula.

Image Details:
Canon T2i with Astrodon IR/UV filter inside
Astronomik UHC EOS Clip Filter
Canon 70-200L lens at 100mm f/2.8
26 x 480s at ISO1600 (total integration 3hrs 28m)
Skywatcher EQ5 Pro
Guided with Short Tube 80, Point Grey Chameleon, Metaguide
Acquisition with Backyard EOS
Processing with PixInsight (Mask creation with Photoshop CS5)

FLO Panorama 2018-02-10

I did a quick panorama for FLO to get a better idea of the skyline and obstructions. The view is from the mound just from in front of the telescope room door and facing south.  I have included two panorama images below – one as an annotated JPG and the other as a PNG with a transparent sky for use in chart applications . A few notes first.

I didn’t have my compass with me while taking the pictures, so i assumed the buildings were oriented North-South and used the “south” facing view as the location for 180° in the reference image. However, when i checked Google Maps it appears the buildings are lined up a little west of south.

FLO Google Maps Aerial 2018-02-10

FLO Google Maps Aerial 2018-02-10

The aerial picture for FLO in Google Maps is low resolution so it was a bit of guess to get a precise reading – something between 7° and 18° west of south. I went with 18° (azimuth 198°) and adjusted my reference image accordingly. Also, aligning the camera to the buildings was a bit of guess, so there is some additional error there.

Next time i am at FLO i will recheck the orientation with my compass and then update the panorama images accordingly.

The tree line directly south appears to be about altitude 22°. The trees to the south east are a little lower at 18°. The large tree to the south west is altitude 24° at azimuth 253° (assuming the i have south in the right place).

The full sized images are 7200 x 1132 px with a scale of 20 pixels / degree. South (az = 180°) is in the image centre at x = 3600 px and the horizon (alt = 0°) is at y = 832px (0,0 being at the left-top). It is of course a full 360° panorama. Skyline altitude and azimuth to other points can be calculated from the pixel location and image scale.

You can click on the images below to view and download the full sized images.

The annotated panorama:

FLO Panorama 2018-02-10 Annotated

FLO Panorama 2018-02-10 Annotated

The PNG file should be suitable as a horizon panorama for some chart applications (e.g. Starry Night Pro). Although meta data is usually required by the application to describe the image scale and orientation (see below). (Note that some browsers display PNG transparent areas as white. But a downloaded PNG file has an alpha channel and with the right viewer, the sky will appear transparent.)

FLO panorama 2018-02-10

FLO panorama 2018-02-10

The PC version of Starry Night Pro 7 horizon meta data file describing the above PNG file is FLO-panorama-2018-02-10. SNP also includes a thumbnail version of the panorama. The program may create this the first time the horizon is used. If not, just create a 256x64px version of the panorama PNG file.

Super, Blue, Blood Moon 2018-01-31

The second full moon of the month is now popularly called a “Blue Moon” (never mind that it got the name through an error – that’s what its now called). On January 1st 2018 we had a full moon (below) and another full moon on January 31st.

Super Moon 2018-01-01

Super Moon 2018-01-01

On January 31st, the moon was again full, marking the second full moon of the month. The full moon actually occurred at about 8:28am EST on the morning of January 31st as it was setting in the western sky. So perhaps a few people missed it by going out later that evening to watch the waning full moon rise in the east. The photo below was taken on January 31st at 00:49 EDT when the moon was transiting and about 7hrs before full. Note that the rotation of the January 1st full moon image is different than the January 31st moon. This is because the photographs were taken using an alt-azimuth mount so the camera is always parallel to the horizon.

Blue Moon 2018-01-31 00:49 EST

A Blue Moon 2018-01-31 00:49 EST

The full moon was also considered a “Super Moon” because it was near perihelion at that time. The angular size was 33.7arc-sec  with the largest angular diameter being 34.1″.

Later that morning as the moon was setting in Ottawa, the moon was eclipsed by the Earth. The penumbra started at about 5:52am EST and umbra started at about 6:50am EST. Unfortunately, the moon also set in Ottawa at about 7:26am EST so by the time the umbra appeared, the moon was only 4deg above the horizon. The western sky also had a layer of clouds that obscured the view so only the penumbra was more or less visible. The photo below was taken at 6:42am EST – about 20 min into the penumbra and about 10min before the umbra shadow would be seen.

Penumbra Lunar Eclipse 2018-01-31 06:42am EST

Penumbra Lunar Eclipse 2018-01-31 06:42am EST

The moon sank into the clouds as the umbra started to cross the moon. So 9 minutes into the full eclipse the moon was hiding behind the hazy clouds on the western horizon.

Penumbra Lunar Eclipse 2018-01-31 06:59am EST

Penumbra Lunar Eclipse 2018-01-31 06:59am EST

The full eclipse would occur at full moon – 8:28am EST – well after the moon set in Ottawa.

Heart and Soul Nebula 2018-01-14

I imaged the Heart and Soul Nebula on 2018-01-14. It was quite cold at -18°C. I haven’t imaged anything in awhile and the cold temperatures usually makes things more difficult. I was concerned that some piece of the work flow wouldn’t work.

Heart and Soul Nebula 2018-01-14

Heart and Soul Nebula 2018-01-14

I did have a lot of trouble framing the field of view. The Heart and Soul nebula aren’t visible so it required a reasonably precise telescope pointing to get things started. Assuming the scope is pointing in the general area, then a series of pictures is taken that hopefully show this nebula and can be used to frame the image.

I had changed out the HD11 for the AT106 scope and piggy backed the camera with the Canon 70-200mm lens on a standard camera tripod head. I did a quite alignment and then did a precise goto to aim the scope. However, there aren’t any bright stars in the FoV so it took awhile to frame the object and get the guide camera/scope pointed at the guide star. With a straight through arrangement for the quide camera, it was physically challenging to get under the scope to see.

I almost gave up when the computer decided to stop working just after i got everything framed. At -18c my hands were frozen as was the rest of me. I rebooted the PC and went in side for a bit to warm up. Fortunately round 2 was a much easier setup and i refound the quide star and got things running pretty quickly.

I started with 180s exposures and retreated into the house where i can monitor things using a remote desktop. After a few exposures, i decided that 180s wasn’t long enough to capture the nebula, even though the stars were almost saturated. I did a few exposures at 360s. However, the platesolve2 app that FlexRx uses could not solve the image. This would be ok, but it leaves the meteguide shiftrate at an abnormally high value and i lost about a 1/2 hr of data due to bad tracking (high shift rate).

The remote desktop allows me to control BackyardEOS remotely. So after several unsuccessful attempts to get platesolve to work, i shut down flexRx and restarted the image run. I also decided to go with 240s exposures for the remainder of the capture.

After about 2hr, i woke up (yes, i sleep while the camera is working) to check things out. More bad luck! The clouds had rolled in which were not predicted by the CSC. So i shut down for the night and closed up the observatory.

I use PixInsight for image processing. This includes all calibration steps, alignment, stacking and post processing. I had a fair amount of difficulty aligning the subs. Red and Blue channels were fine. But there were many green channel subs that would not align. It took several passes using different settings and different reference images to get everything aligned. (I calibrate in raw, bayer format. Then debayer. Then split the channels into RGB and align all channels, all subs in one go. Then stack each channel separately. And finally combine the channel stacks into an RBG image for post processing.)

There are a lot of stars in the FoV of the Heart and Soul Nebula and the nebulae are faint in comparison to the star field. A standard processing would result in a dense star field obscuring the nebulae. I tried various techniques to actually remove (or reduce) the stars but none were successful. Generally this involves building a perfect star mask and then using a Morphological Transformation to reduce the stars. The MT attempts to shrink the stars and back fill with the surrounding colours. (Sort of like content aware fill in PhotoShop, but with shrinking star rather than a selection.)

I did minimize the stars along the way by choosing more moderate star masks and stretching the nebula while holding the stars more or less constant. The trick is in keeping a nice profile to the stars so they don’t look clipped or bright stars don’t look flat or washed out.

The purest view is the stars are there and an accurate representation of the FoV should include them. I think i managed a nice balance between stars and nebula.

M31 – Andromeda with Hubble’s Cepheid Variable V1 – 2017-01-15

The image below is a closeup of the Andromeda galaxy core. The area photographed includes the Cepheid variable star – M31_V1 – that Hubble used to calculate the first reliable distance to the “Andromeda Nebula”. His result proved conclusively that “nebula” were distant objects that were not part of the Milky Way.

M31 - Andromeda with Hubble V1 - 2017-01-15

M31 – Andromeda with Hubble V1 – 2017-01-15

The wide field view of Andromeda below shows the section of the galaxy captured in the close up image as well as locators for M31_V1.

M31 - Hubble V1 - Locator

M31 – Hubble V1 – Locator

Hubble used the recently commissioned 100″ Hooker telescope on top of Mount Wilson to capture a 1hr photographic plate of M31 on October 5/6, 1923. A crop from the original glass plate H335H (Hooker 335 Hubble) is shown below along with the same area from my image. Perhaps the plate has faded over the years or the technicians had keen eyes, but M31_V1 is barely visible.

V1 Comparison - H335H and My M31 (negative)

V1 Comparison – H335H and My M31 (negative)

Hubble had originally identified the star as a Super Nova and marked it “N”. Nova had been seen on plates before, so it was reasonable to assume that a star that had not been seen before was a super nova. However he later decided it was a variable star and marked it “VAR!” (presumably by examining other plates and finding a star of different brightness in the same location). This was significant because Henrietta Leavitt had earlier discovered a method for accurately calculating distances to Cepheid variable stars. Hubble’s discovery of M31_V1 was the first time a variable star had been observed in a nebula.

Cepheid variables are a class of variable stars that can be used as a “standard candle”. That is, their absolute (actual) brightness can be calculated, so their apparent brightness is then a measure of their distance (the dimmer the star, the further away it is). Today, Cepheid variables are still an important tool for determining distances to galaxies.

In 1908 Henrietta Swan Leavitt discovered a Period-Luminosity relationship for Cepheid variables that allows their absolute luminosity (brightness) to be calculated from their period – the time the star takes to vary from minimum to maximum intensity. At the time, she had been working for the Harvard College Observatory as a “computer” and assigned the task of identifying variable stars from photographic plates. Although the women computers were low paid and not even considered astronomers, they still made significant contributions to astronomy. She writes in her 1912 article defining the P-L relationship: “A remarkable relation between the brightness of these variables and the length of their periods will be noticed.

Prior to Leavitt’s ground breaking discovery, various techniques had been tried to determine the size of the Milky Way and the distance to M31 in particular, but none were conclusive. The prevailing view prior to Hubble’s measurement was that our Milky Way galaxy was about 30,000 light years in diameter (current estimates range from 100,000 to 180,000ly). And the Milky Way was the entire extent of the universe. The fuzzy objects visible in telescopes were described as “nebula” and thought to be within the boundaries of the Milky Way. There were some suggestions that these nebula lie on the periphery and could be as far away as 300,000ly, but the evidence was unconvincing.

Hubble’s H335H image was the first time a variable star had been identify in a “spiral nebula”, so this was the first opportunity to conclusively state the distance to M31. Hubble then took a series of plates to characterize the period and light curve of M31_V1 and confirmed it as a Cepheid variable. Using Leavitt’s P-L relationship rule and work Shapley had done to calibrate the rule to absolute distances, Hubble was able to calculate the distance to M31. He announced the results in a January 1925 paper presented to the American Astronomical Society stating definitively that M31 was 1,000,000ly away and clearly not contained within the Milky Way. (Current estimates put M31 at a distance of 2.5m light years.)

Hubble would then go on to locate variable stars in other nebula at even greater distances. Again he used Leaveitt’s P-L relationship to calculate the distances as well as the Doppler red-shift affect to determine their relative speeds. His observations showed that the further away a galaxy was, the faster it was receding from us. His now famous article in the 1929 Proceedings of the National Academy of Sciences declared for the first time that the universe was expanding!

And if it was expanding into the future, it must have been smaller in the past. This would eventually lead to the idea of the “Big Bang!”

Moon in Haze – 2016-10-16

I took this picture of the moon on a hazy, slightly foggy evening in October when the moon was one day past full.

Moon - 15-8days - In Haze - 2016-10-16

Moon – 15-8days – In Haze – 2016-10-16 (v2)

Taking a picture of the moon on a hazy evening proved to be quite challenging. To the naked eye, the moon looked much as it does on a clear night, albeit with a little less contrast. The halo surrounding the moon was bright but not overwhelming and had a tinge of colour. The pictures however were in stark contrast to what i had seen. A short exposure captured the contrast and detail on the moon’s disk but almost no halo. An exposure long enough to record the halo ended up with an overexposed moon.

Exposure Range - Normal Processing

Exposure Range – Normal Processing

I figured the problem was the dynamic range – the difference between the dark and bright areas was too large for the camera to capture. So i so decided to make a High Dynamic Range (HDR) composite from a range of exposures. But that didn’t really solve the problem either. The HDR image had enough range to capture both the halo and the bright moon without saturating or underexposing anything. But in order to get the halo bright enough to match what i saw, the moon details ended up looking washed out with almost no contrast.

I think the visual impression of being able to see both the halo and the moon details at the same time is just that – an impression. We might first take in the halo and then shift our focus to the moon’s disk to see the light and dark contrast on the surface. In our mind’s eye we integrate the two views which leaves us with the impression of seeing both the halo and the disk at the same time.

I ended up using masks to artificially darkening the moon’s surface in the HDR image in order to show both the halo and the moon details. The HDR image had all the resolution and range to show as much moon surface detail as i wanted, but when overdone, it looked like i just pasted a copy of some other moon shot on top of the halo. So i had to limit this technique in order to keep things looking natural. Which left the moon’s surface still looking a little washed out and the halo not quite as bright as it appeared to the naked eye.

Processing Details

Of the many different exposures, 10 images were selected for the HDR composite with a range of about 2-1/2 stops and a relatively smooth increment in exposure value (EV) between images.

Non-linear vs Linear

The normal processing of the raw sensor data applies a strong non-linear transformation to the otherwise linear sensor pixel values – sometimes referred to as the Digital Development Process (DDP). This reflects how our eyes perceive brightness and results in a “normal” looking image. The camera does this when creating a JPG and also what the Canon DPP tool does by default when displaying a RAW file or creating a JPG or TIFF file. For an image with a wide dynamic range to start with, this non-linear transformation tends to blow out the brighter areas of the scene or under-expose endarker areas. The darker areas are less affected due to the nature of the non-linear transformation.

The screen shot below is for an exposure in the middle range of the 10 selected for the HDR composite. It shows the default (non-linear) processing that DDP uses for a raw CR2 file. With the normal processing, all but the darkest area of the moon’s surface appears saturated.

Normal Processing - 1x50s, f/5.0

Normal Processing – 1x50s, f/5.0

The next screen shot is the same image as above, but with the “Linear” option selected in DPP. All of the darker areas of the moon’s disk are now in range and visible. There is still some saturation of the brighter areas but this is expected since this is the mid-range EV of the set. The HDR integration process will use pixel data from the shorter, less saturated images to fill in areas of the brighter images.

Linear Processing, 1x50s. f/5.0

Linear Processing, 1x50s. f/5.0

Note that although the halo isn’t visible in the linear image. It’s still the same data as the saturated DPP image and therefore still there. So the levels can be adjusted later to bring back this detail just as the non-linear (DDP) stretch brought out the halo. But now this can be done selectively.

I figure the linear version recovers about 2 stops of range over the non-linear version. So the 2-1/2 stops in normal processing becomes 4-1/2 stops in linear mode.

HDR Process

The 10 CR2 files were converted to linear mode 16bit TIFF files and loaded into Photoshop CS5 as layers in a single PSD file.  The PS auto-align feature wouldn’t align the images, so each layer was aligned manually.

Aligning lunar images is actually quite easy to do with PS. The layer to align is placed above the reference frame. Then the upper layer opacity is set to 50% and inverted to make it obvious which layer is which. Using the selection tool (arrow) the layer can be nudged into place. When the layers are exactly aligned, the two layers more of less cancel each other out and turn neutral grey. Correcting for rotation errors is a little more tedious but doable. Manual alignment doesn’t do sub-pixel adjustments but it’s ok for this type of work.

The image below shows the 10 images used for the HDR composite aligned and sorted by exposure values (slightly stretched to better show the range and increments):

HDR Exposure Value Set

HDR Exposure Value Set

PS CS5 actually has 2 HDR tools – neither of which produced anything close to being useful. PixInsight also has an HDR Integration tool and it produced a reasonable result. But the tool could not be coerced into including a significant halo from the lonager exposures. And the HDR integration looked very artificial and was difficult to corrected post merge.

So the only option left was a manual merge using PS layers and masks. The layers were stacked in increasing EV.

HDR Layers with Masks

HDR Layers with Masks

A mask was applied to each layer using a range mask to exclude the saturated areas. Then each mask was adjust using levels to increase the contrast. Initially the opacity of each layer was set to give each layer equal weight. (The strange but true formula for determining the percentage for each layer is : opacity = 100 / layerNumber. So setting the 2nd layer opacity to 50% and the 3rd layer to 33% gives the layers equal weight for blending.)

It turned out that equal weights didn’t allow enough of the brighter pixels from the longer exposures to contribute to the blended image. So the opacity was set to 100% for all layers and the blending left mostly to the masks.

With the HDR blending done, levels and curves were used to brighten the image. Later, a mask was used to allow the halo to be brightened while leaving the moon’s disk untouched.

Cygnus-Sadr Region – 2016-09-26

Friends of mine had a star registered with the name “MadVic” as a gift for their 60th wedding anniversary. So i decided to followup with an image of the region that showed the actual star.

Cygnus-Sadr Region - 2016-09-26

Cygnus-Sadr Region – 2016-09-26

The region around the centre of Cygnus is known as the “Gamma Cygni Nebula” (IC 1318); named after the bright star Gamma Cygni – Sadr. The nebula is a  large region filled with ionized hydrogen which shows up as the red background glow in the long exposure image above. The image is just a small section of the nebula. The bright stars also overwhelm the fainter background. The star Sadr in the lower right is particularly bright and it’s glow obscures the hydrogen cloud behind it.

Visually the region looks more like the image below. The star “MadVic” is marked with the green bars.

Cygnus-Sadr Region as it would look visual

Cygnus-Sadr Region as it would look visual

The star name was registered with the “International Star Registry” (ISR) as “MadVic”. While naming a star with the ISR isn’t quite the same thing as having the star name officially recognized by the “International Astronomical Union” (IAU), it is still fun.

ISR MadVic corresponds with official star catalog designations GSC 3160:00031 (Hubble Guide Star Catalog – GSC V1.2) and also USNO J2021000+412939 (the United States Naval Observatory – USNO-B1) . It is a magnitude 12.75 star in the constellation Cygnus at coordinates 20h 20m 59.95s D 41° 29′ 39.27″ (J2000). That’s about 1deg NW of the bright star Sadr at the centre of the cross in Cygnus.

The Constellation Cygnus

The Constellation Cygnus

While binoculars are great for finding constellations and large star clusters, MadVic is too faint to be seen even with binoculars. You can get a pretty good idea of where it is in the sky though.

MadVic Location in a 7x50 Binocular FoV

MadVic Location in a 7×50 Binocular FoV

A magnitude 12.75 star is just at the visual limit of a 4″ refractor even from a dark site.  The image below shows the view using a 4″ refractor with an 8mm eyepiece which translates to a magnification of 86x. The line of three stars just below “MadVic” will show up nicely and provide a guide to locating MadVic.

MadViv View using a 4" refractor and 8mm Eyepeice

MadViv View using a 4″ Refractor and 8mm Eyepeice

An 11′ scope would be better and then the star could be seen even from a moderately dark location. With a 8mm EP, the three “locator” stars are still in the field of view but much more obvious. MadVic is also easily identified as the corner star of a right angle triangle formed by three stars of similar magnitude.

MadVic using an 11" SCT and 8mm EyePiece

MadVic using an 11″ SCT and 8mm EyePiece

From a site with a limiting magnitude a little under 5, the 11″ SCT with 8mm eyepiece showed more or less the same stars indicated in the finder image above. The magnitude 12 stars were very faint though and at the limit of being visible. A darker site would make finding MadVic much easier.

MadVic sketch from the Eyepeice (redrawn to scale and flipped)

MadVic sketch from the Eyepiece (redrawn to scale and flipped)

The next two images show the precise location of MadVic. (Sorry, no fancy mouse overs.) Click on the next image to get the full sized version and then zoom in to see MadVic as photographed. The image at the bottom is a diagram highlighting the main objects in the camera field.

MadVic Locator in Image

MadVic Locator in Image

Cygnus-Sadr 2016-09-26 Annotations

Cygnus-Sadr 2016-09-26 Annotations

 

Barn Door Tracker with Variable Speed Motor

My Bard Door Tracker

I decided to build a “Barn Door Tracker” so that i could have a portable mount for taking astrophotos with a wide angle lens. Something that i could setup quickly and was compact and light.

Barn Door Tracker version 2 - Straight Rod

My Barn Door – Tracker Version 2 – Straight Rod

But the main purpose was a “make fun” project that involved some mechanical design and an opportunity to experiment with a hobby oriented micro-controller – the Adruino.

This design uses a straight rod driven by a stepper motor. The motor is controlled by an EasyDriver stepper controller and an Adruino Nano micro-controller. The micro-controller manages the speed of the stepper so it can vary the speed of the motor as the door opens, compensating for the geometry of the straight rod, single arm barn door. The variable speed motor therefore maintains a constant tracking rate for the full swing of the door arm from the closed starting position to the fully open, maximum travel position.

What is a Barn Door Tracker

For astrophotography, anything more than a quick snapshot requires some way to account for the earth’s rotation. With a camera on a fixed tripod, an exposure of more than about 20-30 seconds will result in stars that look anywhere from little ovals when using a wide angle lens to long streaks for a telephoto lens. The reason for this is the stars move across the sky as the earth turns, just at the sun moves across the sky in the daytime. And even though the motion isn’t obvious, like an hour hand on a clock, the stars do move and that motion shows up in a photograph.

The sophisticated solution is a motorized astronomy “mount” that is programmed to track the sky at the same rate as the earth turns. These mounts tend to be somewhat bulky but more to the point, expensive.

Enter the “Barn Door Tracker” – a simple and inexpensive mechanism to turn the camera in time with the earth’s rotation. According to Wikipedia the barn door tracker was introduced in 1975 by George Haig with plans published in the April ’75 edition of Sky & Telescope. The barn door tracker is also known as a “Haig mount” and also a “Scotch Mount”. (I don’t know where the latter term comes from.)

Original Barn Door Tracker Design

Example of a Barn Door Tracker with hand crank

Example of a Barn Door Barn Door Tracker with hand crank (un-credited)

The original design consisted of two pieces of wood connected at one end with a simple door hinge. At the other end was a threaded rod and a hand crank.

Turning the crank caused the “door” to open. With some math, a watch and a little practice, the crack could be turned to move the door at the same rate the stars move across the sky. The tracker was mounted on a tripod and positioned so the axis of the hinge was aligned to the celestial North Pole. The camera was then mounted on the top board using a simple ball and socket or like mechanism. So with this setup, turning the crack at the correct rate moved the camera so it was always pointed at the object being photographed.

Motorized Barn Door Tracker

Example of a Barn Door Barn Door Tracker with synchronous motor

Example of a Barn Door Tracker with synchronous motor (un-credited)

The next innovation – for obvious reasons – was to replace the hand crank with a motor. In the 70s, electric wall clocks were generally driven by 60hz synchronous motors. This made them easy to find or salvage from a discarded clock.

A synchronous motor has the advantage of turning at a very precise rate – as precise as the 60hz supply voltage. By carefully measuring the length of the barn door so that combined with the thread pitch of the drive rod, it was possible to use the standard 60hz synchronous motor to accurately track the sky. One disadvantage was the need for 60hz power (standard household current in NA).

Multi-Arm Barn Door Tracker

The more subtle disadvantage of the original tracker is that a straight rod driving the door doesn’t translate into a constant tracking rate. As the door opens wider, the constant rate at the drive rod results in an ever increasing tracking speed. At start up the door tracks the sky just fine, but after an hour or so the tracking is slightly fast. After a couple of hours it’s noticeable too fast.

Example of a Barn Door Multi-arm Barn Door Tracker

Example of a Multi-arm Barn Door Tracker (un-credited)

So the next innovations were various configurations of the “double arm” tracker. The multi-arm geometry allowed the constant drive speed of the straight rod to produce a more or less constant tracking for a longer period of time. But at the cost of complexity and mechanical stability.

My Curved Rod Barn Door Tracker

The root of the problem is that a straight rod driving across the chord of an arch does not produce a constant motion along the circumference. So a more direct solution is to replace the straight rod with a curved rod – one with a curve that matches the arch of the barn door.
This solves the math such that the constant drive speed along the curved rod produces a constant tracking rate through all angles of the door.

My Barn Door Tracker - V1 - Curved Rod

My Barn Door Tracker – Version 1 – Curved Rod

At right is the first version of my barn door tracker which had a curved rod. I decided to make it out of aluminum rather than wood mainly  because i liked the look of it. I also thought that the door arms could be made more compact than with wood and still retain the necessary stiffness.

Aluminum is actually quite easy to work with – basic carpentry tools and a good file are sufficient to produce acceptable results. I also have a lightweight tabletop lathe which is adequate for turning small bearings and connectors.

The motor is a stepper driven by an “EasyDriver” card which in turn is controlled by an Arduino Nano micro-controller.

But the mechanical design is also more complicated than the simple one arm, straight rod tracker.

The basic straight rod design drives a threaded rod through a drive nut mounted in the upper arm. The other end of the rod simple presses against the lower arm and is free to slide as the door opens. The motor enhancement is just an extension of the rod and therefore drives the rod directly. Very simple and easy to build.

Barn Door Tracker - V1 - Transfer Gears

Barn Door Tracker – V1 – Transfer Gears

A curved rod cannot be attached directly to the motor, so gears are required to transfer the motor rotation to a drive nut on the curved rod.

Barn Door Tracker - V1 - Floating Rod Support

Barn Door Tracker – V1 – Floating Rod Support

Curving the rod is also a bit of a challenge. In terms of tracking accuracy, the radius just needs to be close. But the mechanical design requires the curve to be very close the radius of the door arm. Otherwise the gears will bind as the door opens. To allow for the radius to be slightly off, the upper support for the curved rod was made to float, but only in one direction. If the support was simply loose, then the curved rod flopped to the side – which also created binding. So a pin through the rod created a pivot point which allowed the rod to move in the required direction but kept it from flopping sideways.

Barn Door Tracker V1 - Tracking

Barn Door Tracker V1 – Tracking

And gears are bad! Ok, gears aren’t bad, but getting a gear which is perfectly round and mounted precisely centred on it’s axis is a challenge when using basic carpentry tools. When dealing with arc-second tracking and um pixels, it’s not good enough to just get a good fit. Things have to be very accurate!

The tracking measurements showed a significant error with a peak-to-peak error of 76″ (21″ rms). I could see the guide star wobbling back and forth in the guide app (Metaguide) so i didn’t bother with a test photograph with the DSLR.

I thought the tracking was bad enough to to warrant a redesign!

I tried to implement a Periodic Error Correction (PEC) using the micro controller. I thought it would be a straight forward task to include a PEC table that determined how much the motor needed to speed up or slow down to compensate for the PE. I used PECPrep to filter the PE for just the dominant period which corresponded to the period of the large gear on the drive rod. Then created an excel spreadsheet to convert the tracking deviation into tracking speed. However, i could not make any significant improvement. Either my math or programming was off or my understanding of how to implement PEC.

My Straight Door Barn Tracker with Variable Drive Speed

Version 2 goes back to the mechanically simpler straight rod design but uses the micro-controller to adjust the drive speed as the barn door opens. I found the idea on the web, so i can’t take credit for that novelty.

Barn Door Tracker V2 - front and back

Barn Door Tracker V2 – front and back

The conversion to a straight rod required very little mechanical redesign. Since the micro-controller determines the rate the motor turns at, the arm length is not critical – it just needs to be accounted for in the math to determine the step rate.

A new mounting was required for the motor to allow it to tilt as the door opens. The pivot allows for about 60deg of opening which translates to 4hrs of tracking. The drive rod is connected to the motor shaft with a simple in-line coupler. The coupler is threaded but a set-screw is still required to hold the rod in place. I choose to have the rod push the door open which means the coupler tends to unscrew the drive rod. (This happened during a test when the set-screw was loose.)

Barn Door Tracker V2 - motor mount

Barn Door Tracker V2 – motor mount

Barn Door Tracker V2 - drive rod coupling

Barn Door Tracker V2 – drive rod coupling

 


Similarly, the drive nut on the lower arm needs to pivot as the door opens. The brass drive nut has pins that fit into the slots in the lower arm. This component was machined from a length of hexagonal brass stock i had lying around. The “keepers” overlap the pins and hold the nut in place. Gravity would probably have sufficed, but i wanted it to be secure. The wings are secured in place by fancy knurled knobs (purchased) that are easy to turn even with gloves on. This arrangement allows for the drive nut to be easily freed from the lower arm and quickly turned back to the starting position.

Barn Door Tracker V2 - drive nut pivot

Barn Door Tracker V2 – drive nut pivot

As well, it makes it easy to dismantle the tracker – meaning removing the drive rod. Allowing the drive rod to be easily removed makes the whole package more portable. I can toss the dismantled tracker in the trunk with the camping gear and not worry about something getting bent.

Another reason for making the tracker out of aluminum was i thought i could make a sturdier hinge than i could buy in the hardware store. I think i accomplished that. The pin for the hinge is a piece of 5mm diameter stainless steel i salvaged from a discarded printer. I used my tabletop lathe to bore a hole in the centre of a length 1/2″ aluminum square stock. A file was used to round the corners.

Barn Door Tracker V2 - hinge

Barn Door Tracker V2 – hinge

The last component is the controller. It consists of an Arduino Nano micro-controller and an EasyDriver driver specifically designed for small bipolar motors. These components are mounted on a standard perf-board and enclosed in a discard iPod packaging case.

Barn Door Tracker V2 - controller

Barn Door Tracker V2 – controller

The hand controller includes:

  • three buttons for reverse, run/stop, fast forward
  • power LED which shows the tracker status (stopped, tracking ffw/rev)
  • a second LED (mounted on the Nano) to show additional control info
  • an analog POT to adjust the tracking speed
  • on/off switch
  • 12v power supply standard jack

In order to compensate for the different tracking speeds vs drive speeds at different angles, the motor speed (step interval) needs to vary as the door opens. The Arduino atmeg-328 isn’t great for float arithmetic and it would be a time consuming calculation to do the trig functions. So again from the example on the web, the controller uses a table to look up the drive speed for different door angles. One thing great about stepper motors is the controller knows precisely how many steps it’s made (assuming nothing jams). So the easiest way to track the door angle is to count the number of steps and use the step count as the index into the drive speed table. An excel spreadsheet was used to calculate the door angle for each table entry (step count) and then from the door angle calculate the required drive speed for that angle. Changing drive speed about once ever 0.5deg or so maintains a sufficiently accurate tracking rate, even for a fully open door.

I wire wrapped the connections onto a standard perf-board. A wire wrap tool i found out is expensive, so i made one using the 5mm stainless steel rod from the printer. This is actually version 2 of the hand controller as well. I fried the first version completely destroying the EasyDriver and disabling one of the digital i/o pins on the Nano. Since i had soldered the connections on version 1 of the controller, it made it very difficult to salvage the still working components. So version 2 is wire wrapped.

Barn Door Tracker V2 - Tracking

Barn Door Tracker V2 – Tracking

I was a bit surprised when i measured the tracking that there was a significant periodic error of 37″ peak-to-peak (10″ rms). This is much better than the with the curved rod but still some concern. The dominate period is the motor period so the likely culprit is the drive rod. A p2p error of 37″ translates to about 0.002″ along the drive rod so it doesn’t take much of a mechanical error to show up in the tracking. Further investigation is required to isolate the problem.

Again i made an attempt at re-implementing PEC. And again the same disappointing results. So i moved on to a real test.

The first light test was done on 2016-02-22, a night with a full moon and wispy clouds. The first target was Sirius. It’s easy to find in the view finder and easy to focus on. I use Backyard EOS so focusing is actually pretty easy. Framing with the lisn’t weight ball and socket is a little unsteady. But since i was using a 28mm lens, i didn’t exactly have to be precise.

Sirius 120sec, 28mm, f4-0, ISO400, 2016-02-22

Sirius 120sec, 28mm, f4-0, ISO400, 2016-02-22

This is a single frame with a mild stretch applied and no calibration frames. It’s a tight crop of the centre of the field, but at the full resolution. M41, The Little Beehive, shows up nicely. Tracking is pretty good. I think the major culprit is polar alignment. I have yet to align the polar alignment scope. I can also fine tune the tracking rate but i’ll do this after more tests with a proper PA.

The next target was Orion with a 5min exposure.

Orion 300sec, 28mm, f6-3, ISO200 2016-02-22

Orion 300sec, 28mm, f6-3, ISO200 2016-02-22

Again the tracking is pretty good with less than adequate PA showing up with the north-south elongated stars. This is a single frame with a moderate crop of just Orion and again the full pixel resolution. The processing, as above, is a mild stretch, no darks or flats.

i doubt i’ll use this mount for anything more than 5min exposures. For longer exposures i’ll use my telescope mount and probably guide, even if imaging with a wide angle lens.

Summary

A barn door tracker used with a wide angle lens or even a mid-range telephoto lens is a viable alternative to an expensive and bulky telescope mount.

This project was a lot of fun to do. I purposely choose a complex implementation just for the fun of it. A useful barn door tracker can also be constructed from simple parts using basic carpentry skills, especially when considering a manual crank.

Vallis Alpes Comparison – 2013-04-21

I recently came across an image of Vallis Alpes on the moon (Alpine Valley) created by the Lunar Orbiter 4 space craft in 1967. It struck me as amusing that my own image taken in April of 2013 compares favourably with the Obiter’s image.

Moon - Vallis Alpes Comparison 2013-04-21

Moon – Vallis Alpes Comparison 2013-04-21

The Orbiter’s altitude varied from 2,100km to 6,000km so I nominally choose 5,000km as the distance of the Orbiter’s image. The mission cost in 1967 was $163m US. In today’s dollars (2015) that works out to about $1,2 billion.

My equipment is a little less expensive and i was about 371,000 km way when i took the image.

To be fair, there is a lot more detail in the orbiters image. For example, the rille (trench) running down the middle of the valley is between 700 and 1,200m across. This detail can just be seen on my image, while it’s quite clear on the Orbiter’s image and even shows the details of some small craters that impacted after the rill was formed.

My image of Vallis Alpes is a close crop to a much a larger image of the area including the crater Plato and the top mare Imbrium.

Moon Plato and Vallis Alpes 11.74days 2013-04-21 v1

Moon Plato and Vallis Alpes 11.74days 2013-04-21 v1

 

Finding the Andromeda Galaxy – 2015-11-05

M31_Andromeda_Galaxy_2010-11-08_v3_SJM
As night sky objects go, i think the Andromeda Galaxy has the most engaging story. And it’s a relatively modern story considering the ancient folklore associated with the constellations and the heavens in general.

In 1920 there was a raging disagreement among astronomers and scientist in general about the size of  universe. The generally accepted view at the time was that our Milky Way galaxy was somewhere between 7,000ly and 30,000 light years in diameter. And the Milky Way was the entire extent of the universe. The fuzzy dense objects visible in telescopes (that we now know are distant galaxies) were described as nebula – “a cloud of gas and dust in outer space, visible in the night sky either as an indistinct bright patch”. And these nebula were thought to be part of the Milky Way.

To jump forward a bit, the Milky Way is currently estimated to be between 100,000ly and 180,000ly in diameter and about 2,000ly thick. The “observable universe” is thought to be somewhere around 93 billion light years across.

The discussions about the size of the universe culminated in what has become to be known as “the great debate” between Harlow Shapley and Heber Curtis. Shapley argued that the Milky Way was the entire extent of the universe and that nebula such as Andromeda were contained within the Milky Way. Although he maintained that the main part of the Milky Way could not be more that 30,000ly across, he did propose that nebula such as Andromeda and globular clusters could form at the distant edges making the diameter 300,000ly. Therefore also fixing the upper size of the universe at 300,000ly. Curtis argued that the Andromeda nebula was in fact at a great distance and an “island universe” (a term coined by Immanual Kant). He accepted the thinking of the day that the Milky Way was no more than 30,000 ly in diameter but proposed that the Andromeda nebula was 500,000ly away and other such nebula as far as 100mly. (Current estimates put Andromeda at 2.5 million light years.)

Well before the debate reached this pinnacle, Henrietta Leavitt, a deaf astronomer working at the Harvard College Observatory, discovered a relationship between the absolute luminosity of a particular type of variable star and its period. In short, she found a way to measure, with certainty, stellar distances using the period of Cepheid Variables.

A variable star changes brightness over a predictable time frame – the period. In 1908, Leavitt noticed a relationship between Cepheid Variable periods and their absolute luminosity and published the results of her initial observations. Then in 1912 after analyzing more stars, she confirmed her observations. Essentially, she found that the actual or absolute brightness of a Cepheid Variable can be calculated from its period. And knowing the absolute brightness, its apparent brightness can be used as a measure of its distance – the further away it is, the dimmer it will appear.

As a side note, Shapley took over as director of the Harvard observatory in 1921 and promoted Leavitt to head of Stellar Photometry.

In 1919, the 100″ Hooker telescope – the largest telescope at the time – was installed on Mt Washington. At about the same time, Edwin Hubble started working at the observatory. Between 1922 and 1923, Hubble used the Hooker telescope to photograph the Andromeda “nebula” and was able to identify Cepheid variables. Aware of Leavitt’s observations, he was then able to calculate the distance to Andromeda. His initial estimate put it at 930,000ly – not quite the actual 2.5mly – but sufficient to declare that the Andromeda Nebula was indeed a separate galaxy. Shapley was still unconvinced as were many of the astronomers of the day. But by 1925 it was clear that the results were inescapable and the universe got a whole lot bigger.

Hubble went on to measure the distances to a number of other “nebula”; confirming that they too were separate galaxies. Then in 1929 Hubble published another ground breaking result – the universe was expanding! He discovered that the further a galaxy was away, the faster it was receding from us. (He used redshift to calculate relative velocity.) And if the universe was expanding, they it must have been smaller in the past. In 1931 Georges Lemaître, a Belgian cosmologist and Catholic priest proposed that the universe must have started out as a single point – later to be coined the Big Bang. As early as 1922, Alexander Friedmann had produced a solution to Einstein’s General Relativity field equations that showed space must be expanding.

So as late as 1920, the universe and the size of the Milky Way were thought to be in the range of 7,000 to 30,000 ly across. Then with a series of observations from 1908 (Leavitt) to 1924 (Hubble) the size of universe expanded to millions and billions of lights years.

The background is interesting, but what makes it even more engaging is that the Andromeda Galaxy can actually be seen unaided! At a distance of 2.5 millions light-years, it’s the furthest object that can be seen without a telescope or binoculars and only requires a moderately dark sky to see it.

By November it’s high in the sky facing south in the evening and appears as a faint, but distinct oval smudge. The darker the skies, the more obvious it is.

November_2013_AllSky_Chart

The chart below provides some tips on how to find it. Pegasus is due south in the November evening sky and is the large square (or diamond) shape as wide as your hand is stretched from thumb to baby finger. The Andromeda constellation is the “v” shaped arrangement of stars to the east and they share the star Alpheratz. Step 1: Start with Alpheratz in the upper left corner of Pegasus. Count three stars down to the left to Mirach (with Alpheratz being star No 1). Step 2: Then count three stars up (Mirach as No 1). The second set of 3 stars are faint so may not be obvious at first. Step 3: M31 will be 4° to the right of the 3rd star (about 2 finger widths) and elongated as shown.

finder_m31_star_hopping