Northern Lights 2013-03-17

Aurora Borealis - Northern Lights ... A couple of days, or nights actually, ago we had a great opportunity to witness one of natures' more beautiful spectacles. I had received notifications from my astronomy club for a couple of days that the conditions were such that aurorae were to be expected. But as always, life rolls on and I happened to completely forget the notifications. But then on the evening of March 17 at around 20:30 I went for a walk with our dogs and as soon as I got away from under the street lamps I remembered the notifications. Pretty much the whole northern hemisphere of the sky was ablaze with aurorae. After a brisk walk in the -12C air I managed to get back home. At that time the activity was showing signs weakening, however there a constant steady glow in the sky that promised that there was more to come. So I gathered my gear and suited up, a bit more than for the walk, and set up my camera in the backyard. On hindsight it would have better to go to the edge of a nearby field to get a better view of the northern horizon, but I just wanted to get ready as soon as possible.

After setting up the camera and taking a couple of test photos of the moon, it was time to wait. Fortunately the wait wasn't long before things started happening again. Below are a few photos from that session.

Venus Transit 6.6.2012

Been a while since the last post ... a couple of drafts are almost ready to publish regarding Dark Frames and couple of DIY posts. I hope I'll get them done sometime soon.

Sunspots and Venus 2012-06-06 07:30:59

First the mandatory warning:

Never, ever look at the sun without proper eye protection.

Anyway June 5.-6. 2012 was a memorable day for most amateur astronomers because of the last transit of Venus for more than a hundred years. At least here in Finland the days prior to the event were spent mostly by getting ready and staring at the weather forecasts ... and biting nails. The forecasts were not encouraging first in the southern parts and then in the northern parts then all that was left to do was, pick a spot and hope for at least a quick peek of the event. I decided to try my luck from my backyard. That meant missing the first half of the transit due to the rather southern latitude and natural obstructions towards the sunrise.

I woke up at 0300 and set everything up for when the sun would clear the obstructions (trees and such) at about 6:15 so I had plenty of time to contemplate the weather (mist, thick low clouds and thin high clouds), was not looking good ... At about 0500 small tears started to appear in the lower clouds. A small spark of hope was flitting its way in to my head. More finger nail biting and waiting. The tears were getting larger by the minute, but the high clouds were still there.

The moment of truth was quickly approaching ... it looked like there was going to be some "clear" moments. I fired some test shots while the sun was still behind the trees. The exposure times varied quite a bit due to the cloud cover.

There was nothing more I could do but fire away at each "clear" moment and try to adjust the exposure times on the fly (which actually worked out quite well). I managed to get 150 shots out of which approximately 50 or so (the processing is not completely done) are somewhat satisfactory. The main problem was the blurring due to the high clouds even during the clearest moments. Anyway here are some shots from the session. The shots are the cropped jpgs with some levels adjustment (the raw photos are still in the works). All times on the photos are local time (GMT +2). These photos were taken with the following setup:

Celestron AstroMaster 130 f/5

Canon EOS 1000D at prime focus

Baader Astrosolar ND 5 filter

full aperture used

Venus_transit_2012-06-06 06:26:23

Venus_transit_2012-06-06 06:43:15

Venus_transit_2012-06-06 07:25:55

Venus_transit_2012-06-06 07:41:40

Dark Frames - Who? What? Where?

Everyone aspiring to take some astrophotographs sooner or later will run in to preprocessing and dark frames. There is lots of information available around the internet and various books by noteworthy astrophotographers. After reading through many and more websites and books and trying to figure out what actually happens during dark frame subtraction of light frames I decided to try and do some testing by myself. After all, just blindly following instructions given by someone else on different equipment, does not necessarily yield the best results. Also in some references dark frames are stated as unnecessary. This can easily lead to confusion when deciding on your astrophotography session.

In this post I'll be using as an example my shots of M33 (Triangulum Galaxy "Pinwheel Galaxy"). Please note that I myself am a novice at astrophotography and this also serves as my log of my ideas and thoughts during processing. So, do not take everything as a fact. Try things out by yourself to get to know your equipment and how it functions.

My equipment for this session was a modded AstroMaster 130 and Canon EOS 1000D. All images and dark frames are 45s exposures @ISO1600 (that's why they are quite noisy)

Theory (the boring bit ...)

The main purpose of the dark frame is to negate the effects of dark current your imager. Dark current can exhibit itself as fixed pattern noise or tempral noise in the images. Fixed pattern noise is always the same, only the intensity levels change with respect to the exposure length. Temporal noise is basically random noise, which is always different. Dark frames which are taken at the shortest possible exposure length are called bias frames typically bias frames are only used when scaling different length dark frames. I will not discuss bias frames in this post. Dark frames are subtracted from the light frames, but more of that later on in the post.

In order for dark frame subtraction to be effective, the dark frames need to be taken at the same imager settings as the light frames with the same exposure time. In the perfect world the dark frames would be black regardless of exposure time. This unfortunately is not the case, at least with my imager. Perfectly black dark frames would mean that the light frames do not contain any electronics induced signal, therefore the light frames would contain only the signal captured from the target (wouldn't that be wonderful!). Even in a semi-perfect world the dark frames would all be identical other than for the intensity of the fixed pattern noise. The main problem arises from temporal noise which is always different from image to image. If we didn't have temporal noise one dark frame would be enough to get rid of the unwanted signal from our precious light frames. Each pixel on the sensor receives photons or believes to receive photons in the case of dark frames. 

The photons are stored in the pixels (I will leave out the technical stuff). The stored amount of the photons in each pixel is then read out of the sensor and a value depending on the stored amount of photons is given for each pixel. The value for each individual pixel depends on the available bit depth of the conversion. This available bit depth gives us our maximum number of values each pixel can have. 1 bit conversion would give each pixel only black or white. 8 bit conversion would give 256 levels of grey for each pixel, and so forth. Grey? What do you mean grey? I have a color DSLR.

Since you presumably are working with a DSLR each pixel in the finished image is a combination of 4 pixels, which are all actually greyscale pixels with small colored filters. So when we are working with dark frames we want to handle each pixel separately in order to correct the possible deviations in the actual pixels. During the conversion of the raw image to a color image the value for each pixel is calculated from the values of the pixels surrounding it. There are various algorithms for doing this from simple to really complex. In order for the dark frame subtraction to work later on, the dark frames should be kept in raw format until they are applied to the light frames.

Basically the dark frames act as a noise map for your light frames defining the location and strength of the non-signal data accumulated for each pixel on the CMOS/CCD array of the imager. Some image processing software allow you to generate a bad pixel map to get rid of the "hot" and "cold" pixels in the image. Hot meaning that the said pixel is saturated (or close to) and cold pixels are completely dark. Whether to use use bad pixel mapping or stacking dark frames is up to you (but don't do both).

I suggest using an image with various colors to adjust your software's bayer conversion algorithm. At least I noticed that for some mysterious reason, my software mapped the raw pixels to wrong colors.

Combining

The same is true for dark frames as for lights, combination rules the day. In order to get a smooth outcome dark frames should be combined. There are a bunch different combination styles available in dedicated software. Typically dark frames are averaged. Since the noise in each image is different, not counting dead or hot pixels, averaging gives a good result. Combination is used to avoid introducing additional noise in to the light image, but even one dark frame is usually better than none at all. Combination should be performed without any alignment. I've also tried standard deviation stacking for dark frames, but the result has not been as good as with average combining. In the image below are depicted two separate dark frames and the result of average combining 30 dark frames. Even though dark frames are expected to black (no light reaches the sensor) it can easily be seen that the noise induces a quite constant signal level throughout the image. This same level, as an average, is also injected in to your light frames, that's why we need to get rid of that signal in order to increase contrast in the images.

Applying

The combined "Master Dark Frame" is subtracted from each light frame before debayering the light frames. The image series below depicts the difference between the images after subtracting the dark frames from the light frames. The top row is an area from the right top of a single image (hence the extreme coma, we won't worry about that for now) without any stretching (as the image looks originally). The bottom row is strongly stretched to bring out the noise in the images. The stretching has been done to the same values for each image in order to bring out the difference. As you can see, even the subtraction of one dark frame already brings the background sky level closer to what we expect the sky background to be. The addition of additional dark frames in to the Master Dark Frame evens out the noise even more and gives a more even background and better contrast.

Summary

• Take the dark frames in conjunction with the light frames in order to have equal (or close to) temperature of the sensor
• Prevent light from reaching the sensor
• Use the same ISO -speed as for the lights
• Use the same exposure time as for the lights
• Preferably take more than one dark frame 
• Do not debayer the frames (dark or light)
• Average combine the dark frames without aligning --> Master Dark Frame
• Subtract the combined Master Dark Frame from each light frame

Scope Alignment

Aligning a traditional GEM without any self alignment capabilities has got to be one of the most difficult things to do properly. Of couse now a days there are GOTO systems that do just about everything for you, but what to do when the mount standing in the livingroom corner is not one of those. There are a few ways of aligning the mount RA to the North Celestial Pole (NCP). Which way works for you depends on what your intetions for the observin session are. Below are described a few ways of doing the alignment.

You can just simply use the finder and the telescope to do the alignment to the NCP . It is actually easy to align the scope for visual observation this way. However, there is a catch, as always, if your mount RA and Dec axis are not perfectly perpendicular and your OTA mounted parallel to the to the RA axis, your RA axis is NOT pointing where it should. Only your OTA is. When this happens your target will eventually drift from view even with RA tracking.

You might have a mount with a Polar Alignment Scope fitted to the end of the RA axis. Using the PA Scope is easy and you get good alignment for visual work . Using this method you don't have to worry about the orientation of the OTA at all. The Polar Alignment scope views differ slightly for different manufacturers, but the basic idea is to adjust the Altitude and Azimuth adjustments so that Polaris, and possibly some other reference star, are located in marked positions in the PA scope field of view.

 The third way of aligning is called drift alignment. Drift alignment is usually used to align permanently fixed mounts and movable mounts for astrophotography. There are different variations of drift aligning, but here I will concentrate on the traditional way and a simplified way that can be used to perform the alignment using a scope mounted DSLR.

Traditional drift align
The traditional dift align method can be performed either visually or using a webcam. When using a webcam, there are some software tools available to help with the procedure. One such piece of software is EQAlign, which is also free. The basic idea behind the drift align process is to adjust the Azimuth or Altitude while tracking a star at the local Meridian or due East. If you are doing the alignment visually you need an eyepiece with crosshairs (preferably double).

1. The first thing to do is to level your mount and then perform a rough polar alignment of the mount by using a compass and setting the altitude to your latitude or by some other means at your disposal.
2. Next you should point the scope at a star due South (local meridian) and stop the tracking motor if running. Note the direction of the motion of the star and rotate the eyepiece so that the star moves along one of the crosshairs. The direction of travel is West.
3. Recenter the star on the crosshairs and start tracking. Monitor the drift of the star in the eyepiece. If the star drifts North, move Azimuth towards East and if it drifts South, move the Azimuth towards West. The longer you monitor the star, the better the accuracy.
4. Next you need to find a star due east and perform the same routine there except you are adjusting the Altitude. If the star drifts North, adjust the altitude down and if the star drifts South adjust the altitude up.
5. Repeat steps 2-4 until there is no more drift.

If you are using a webcam and a piece of software like EQAlign, the software guides you through the process. Also due to the smaller field of view offered by the webcam the drifting of the reference star is more obvious.

DSLR drift align
A variation of the drift align procedure can be performed with a DSLR connected to a computer. You will need software to control and view the images taken with the camera. There are various software tools that enable you to do this; Nebulosity, BackyardEOS and Astro Photography Tool to name a few (and MaximDL of course).

• Anyway, the basic idea is the same as with the traditional drift align method, but the DSRL align method utilizes the long exposure capability of the camera to get a good visual indication of the alignment error.The first thing to do is to level your mount and then perform a rough polar alignment of the mount by using a compass and setting the altitude to your latitude or by some other means at your disposal.
• Next you need to point the scope due South (local meridian) and keep the tracking running. You don't necessarily need bright stars since you will be using the camera to capture the photons so even faint stars are fine. You should set your camera to take at least 2 min exposures and use ISO setting of 1600 or more (these don't need to be pretty)
• Start the exposure: 

1. Let the mount track for the first 15s (this will give you a starting point)
2. After 15s stop tracking or set tracking speed to lowest possible
3. After 1 min of exposure set tracking to fastest possible

•  As a result of the exposure you will get a star trail image that most likely looks like the letter "V".  The orientation depends on your camera angle, but it doesn't matter. The purpose is to get the "V" in to a single line. You need to adjust the Azimuth either East or West depending on which way the "V" opens. 

In this scenario my star trails drifted North, so I need to adjust my Azimuth East. (If you are wondering about the orientation of the trails; it's due to my camera angle)

•  After adjusting the Azimuth redo the exposure. If the "V" spreads you adjusted in the wrong direction. If it closes you adjusted in the correct direction. Continue repeating the exposures until you get a single line as a result. 

After two adjustments of the Azimuth, the star trail is a single line (don't worry you'll soon figure out how much to move axis).

•  Next target either East or West and repeat steps 3-5 except this time adjust the Altitude.  

Pointing East. My star trail has drifted North, so I need to adjust the altitude down.

In the end the star trails are again lined up.

• You should retry the Southern orientation to make sure that the Azimuth setting is still correct. If you have not leveled the mount with a bubble level or some other means, you should repeat the South and East/West adjustments as many times as needed to get the startrails to stay in line.

Try to target stars as close to to 0 degrees declination as possible. If you want more accurate alignment you can extend the exposure time as much as you wish.

At least for me this method is easier and faster to do than the traditional way and I don't need to bring the webcam along. It can be performed with the imaging setup.

Clear Skies

Focuser Project

It's been some time since I last posted and even today the topic is an on going focuser update project. The purpose is to make a crayford style focuser to replace the rack and pinion focuser of the AstroMaster. The reason for this focuser update is that the stock focuser does not have enough backfocus to allow for DSLR primefocus photography as it is. In order to take the photos I've posted in my blog, I had to do a quick and dirty modification of the stock focuser and it has left me with some slack between the drawtube and the focuser frame.

In order for prime focus photography to work, the telescope needs to have enough back focus to allow for the DSLR imaging sensor to move to the focal plane. The amount of back focus needed depends on your setup (camera+adapter+acessories). Quite often with the "budget" newtonians this is not possible without doing some modifications to the telescope. There are a few possibilities to get the focal plane to hit the sensor of the camera with a newtonian:

1. Move the main mirror inwards in the tube. This often means shortening the telescope tube and also there is the possibility that the secondary does not catch all the light coming from the main mirror, depending on the size of the secondary.
2. Get the focuser to move inward the necessary amount. This can be achieved by getting a lower profile focuser or modifying the original.
3. Move the focal plane with the use of a barlow lens or other optical means. This however means that there are more optical elements in the light path, amounting to additional light loss and possibly more coma due to extended focal length.

In my case I started off with modifying the original focuser. The DSLR I'm using (EOS 1000D) has the sensor 44mm from the front mounting flange + additional 15mm for the T-adapter and the adapter for the 1 1/4 inch eyepiece holder, for a total of 59mm backfocus. After doing some measurements I needed some 30mm of additional backfocus for the camera to achieve focus. What I did was first of all remove the top portion of the drawtube frame. The top of the drawtube frame is what keeps the drawtube from wobbling, so when I removedit, I had to improvise with some small screws and teflon strips. That got me some 15 mm and the other 15mm comes from attaching the camera directly in to the drawtube, without the eyepiece holder, using a self made adaptor.

This setup has gotten me this far, but I started thinking about a rugged re-usable focuser. Meaning that if I switch scopes I can use the same focuser  with small changes. The basic idea is to have a straight base and then have an adapter plate to fit to the shape of the telescope tube. Also I want to have the possibility to adjust the tilt of the whole focuser.

Rummaging through the metal scrap at work, I found a nice piece of aluminium to act as the starting point for the focuser frame. A 40mm diameter hole was drilled in the center (to be finetuned later).

I machined the rough dimensions at work with a larger milling machine, the fine tuning and smaller work will be performed with a smaller milling machine.

After some hours (and lots of aluminium chips) later the main components were finished. From left to right the components are;

• The drawtube
• Main frame of the focuser (one piece)
• Focuser axel
• Holder for the focuser axel
• Eyepiece holder

The pros at work were nice enough to make me the draw tube and eyepiece holder with the lathe there. The height of the focuser frame is 50 mm and together with the drawtube+eyepiece holder is a total of 70mm. Height is 10mm less than the current, quick and dirty, modified stock focuser on my AstroMaster.

Currently there are no bearings in the main frame of the focuser for the drawtube. The mechanism seems to work fine since the sides polished to some degree. The white part in the focuser axel holder is teflon, which is used as a bearing for the focuser axel. The screws (one visible in the photo) protruding from the backside are used for setting the correct pressure to the axel-drawtube contact.

The drawtube axel is made from acidproof steel, which makes for a "sticky" contact between it and the drawtube. No additional friction providing material is needed.

The four large hex screws are used for adjusting the tilt of the focuser frame (they are at a staright agnle to the base even though in the photo they don't seem to be). The smaller screw on the right side of the frame is used for locking the drawtube if needed. The frame also has some spare to acommodate for a larger drawtube, for 2 inch accessories, if necessary.

All in all the focuser turned out fine. Now what is needed is some black coating on the inside of the drawtube and possibly on the outside (it's a bit too shiny) and the actual knobs for the focuser axel.


Clear Skies

Monthly Constellation - Ursae Minoris

During the light summer months here in the north there is not much observing going on, so I thought I'd start a Monthly Constellation series concentrating on the constellations visible from latitude 60 degrees North. The idea is to provide general information about the constellation and possible targets for amateur astronomers. An other reason is to familiarize my self with the constelleations, so this is also a learning process for me. The info and targets are under no circumstances complete. The reader can find lots of additional information by searching the Web.

Ursa Minor (Little Bear)

Ursa Minor is a constellation everybody is more or less familiar with, beacuse it contains Polaris (the North Star). Some people might not be aware that the constellation itself looks like a smaller version of the Big Dipper asterism. From urban skies the constellation can be hard discern, because it is mostly comprised of stars of magnitude ~2 - 5. However from areas with darker skies the Little Bear is easy to spot circling around the north celestial pole.

Ursae Minoris

Polaris is not exactly at the north celestial pole, but little less than 1 degree removed. Polaris was once used as the standard candle for determining stellar magnitudes, but then it was discovered that it is infact a variable star and therefore not a very good standard candle. In addition to being a variable star Polaris is also a binary star.

Targets
Even though being rather poor in obervational targets, there are a few that you can take a look at.

Binary Stars
Polaris (Alpha (α) Ursae Minoris): A combination of magnitude 2 and 9 stars, with a separation of 19 arcseconds. Even though usually designated as a binary star Polaris is infact a multiple star system composed of 4 stars in addition to Polaris itself. The components of the system are Alpha Urase Minoris A, Ab, B, C and D.

Polaris

Epsilon (ε) Ursae Minoris: A triple star system composed of Epsilon Ursae Minoris A, which is a spectroscopic binary and component B located 77 arcseconds away. Being a spectroscopic binary, component A can not be resolved with optical means only via spectral analysis of the dopplershifts of the components. This triple is the third star on the handle of the Little Dipper starting from Polaris.

Galaxies
NGC 6217: A face-on barred spiral of which the Hubble Space Telescope has taken a grand image. This galaxy can be difficult to find visually having a low surface brightness of magnitude 11.2 and angular size of 3,3 arcminutes compared to the Andromeda Galaxy's 3.5 and almost 3 degrees. Due to the low brightness it is mainly a photographic target (unless you happen to own a really big light bucket). If you have a equatorial mount without GOTO the galaxy can be found by first finding Zeta Ursae Minoris, the star joining the bowl and handle of the dipper, then rotating West in RA 48 minutes. This should land the galaxy in the field of view. The difference in declination is only 24'.

NGC 6217 location

That's about it for Ursa Minor.

Clear Skies