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

uniMap - A new plate solver+ application in progress

A few days back when looking for plate solving applications I ran across a work in progress called uniMap. It looks like a promising piece of software. Even in its "infancy" (version 0.0.3 pre-alpha) it does plate solving well. The nice thing about the software is that it seems to work even on not-so-good images and use it really simple compared to some others I've tried. The reason I included the "+" in the heading, is that it is not only a plate solver, but promises to be much more.

I did some testing with uniMap on photos published on this blog previously. No additional processing was done and the images used were in jpg format. So the quality of the images were not that great.

Plate solving
uniMap has various catalogs to choose from including the SAO, Hipparchos and Tycho-2 catalogs. I used these three for testing. The SAO and Hipparchos catalogs contain a few hundred thousand stars but the Tycho-2 contains ~2.5 million.

- The SAO catalog seemed to work quite well in most cases
- Hipparchos did not produce any results on any of the plates
- Tycho-2 worked the fastest

The plate solving can be made faster by providing "hints" to the program. These hints can be given as common star names, Messier catalog items, NGC catalog items or by RA and DEC. The hint system is quite flexible so it is really easy to provide the approximate location.

The first thing after opening the image a detection needs to be performed. This is simple, just click "detect" in the image menu after that provide the "hint" and then "match". Before doing match you need to tell uniMap which database to use for the solving process.

These images show the extent of the Tycho-2 catalog. As you can see the quality of the images is not that good, but uniMap manages to solve the plate. For these I used M57  and M13 as the hints to narrow down the search.

DSOs are displayed by blue colored ovals and their respective catalog numbers on the images.

One thing I would like to see implemented are the cardinal directions marked on the results.

An equatorial grid can be added to the image if needed. All labels and overlays (or at least most) can be modified to suit your needs.

Image analysis and information
After solving the plate there are varius things you can do with the image. By double clicking on a star you can get information related to that specific star or target.
Below is an image of the information windows which opens. In additiona to providing lots of catalog IDs there are also references to literature, information analysed from the image, histogram and radial plot of the target in question. 

I did a quick test on my photos checking the magnitude values the program gives for various targets and this is what I found. NOTE: My photos have not been properly calibrated for photometric measurements so these results are not representative of the capability of the software.

Even though the image calibration was not done according to the needs for photometric analysis the results for the magnitude measurements were quite surprising. The magnitudes the software reported were actually quite close to the values reported by Stellarium (not the best reference, I know). In the image below depicts the values reported by uniMap on one occasion. The information for the same star can be seen in the Stellarium info and the uniMap info screen. UniMap even gave the magnitude for M57 correctly (again according to Stellarium) from the image shown above.

Conclusion
All in all uniMap looks like a very promising piece of software for platesolving and image analysis, due to its ease of use and functionalities. In addition to features not covered here, it includes, among other functions, calibration and stacking of images, weather information (with seeing predictions), 3D analysis of the image data and some post processing functions.

I suggest you give uniMap a try and see for yourself.

The developer is currently looking for help with the project, so people with needed skills could help him out. I for one unfortunately do not have the necessary skills to help out, so I need to leave that to other people.

Minding the Heavens - book review

In the future I will try to post book reviews on books I've read. At least during the summer time when the nights are so light that observing is pretty much out of the question. The first review is about "Minding the Heavens" by Leila Belkora.

The book tells the story about the individuals who have made an impact on our understanding of the Milkyway. The story is told as a timeline of increasing awareness of our place in the universe through the lives and discoveries of Thomas Wright, William Herschel, Wilhelm Struve, William Huggins, Jacobus Kapteyn, Harlow Shapley and Edwin Hubble. I might even say that the book is composed of short biographies, but interestingly the lives of the people mentioned in the book are always somehow intertwined.

The book is well written and interesting. It does not contain techno babble or anything like that, but is told in "layman" terms in an easy to follow way.

I can recommend this book for those interested in the history of astronomy.

The book is available through Amazon.com in paperback.

Lyra Targets M57 Ring Nebula and Stephenson 1 Open Cluster

The nights are getting quite light at my latitude so this getting to be pretty much the last chance to get any DS photography done before the "summer break".
My first idea was to go for a retake of the Hercules cluster, but since Lyra was rising and the seeing was good I changed my plans and decided to try M57 "The Ring Nebula" and an open cluster with the designation Stephenson 1 around Delta Lyrae. I spent so time to get the polar alignment and tracking speed as close as possible to allow for at least 30s exposures for the Ring Nebula. It turned out that I could have used even longer exposures than 30s to get the SNR better, but I stuck with 30s to stay on the safe side.

I started with the Ring Nebula taking a few test photos and to my surprise I managed to get the nebula in the center of the field on the first try. The ISO400 30s subs look like this.

I took 30 frames out of which, 22 were usable in DeepSkyStacker (<80% score). I also took 10 darks out of which only 6 were saved on the memorycard (I guess some bit was stuck in a corner somewhere). After stacking the images there was some serious come apparent in the photos. Also the coma free area was not in the center of the image field. I suspect thet the focuser is not at a proper angle with regards to the light path. I will return to that during the summer observing break. Anyway the result of the first round of stacking and processing yielded a result like this.

This image is a 11min stack cropped 50% showing some of the surrounding area. Below is a 100% resolution image of just the nebula.

The image clearly needs more exposure, but even at these exposure lenths the central star is visible, which was a nice surprise.

The next target was the "not so famous" Stephenson 1 Open Cluster. Anybody can find this cluster easily, just point your binoculars towards Delta Lyrae.See the image below for guidance if unsure where to look.

 The Stephenson 1 cluster's main eye cather is the visual binary of Delta 2 and Delta 1 Lyrae. In reality these two star are a long way from each other. Visually and in photographs this is a stunning pair. Otherwise the cluster is quite nondescript. Below is aphoto taken of the cluster (ISO400, 4min, Canon 1000D).

RA Tracking motor modification

To run or not to run. That's the question.
That's what my original AstroMaster motor drive decided to ponder during one freezing evening a few weeks back.
I was trying to get the tracking runing smoothly to take some new test photos. While adjusting the tracking speed I noted a fluctuation in the drive speed (easily apparent by the sound it made). That turned the anticipated photo session in to trying to figure out what was wrong with the drive. I could not get very far with my investigations in the dark, so,  in with the scope.
The next day, after the equipment had thoroughly thawed out I tried a run inside the house. Quite soon after starting the motor I could smell the burning electronics. After a few moments of jumping up and down, and cursing the manufacturer of the drive down to the deepest pits of hell I removed the drive from the scope and took it to my desk. Out came the circuit board, which was actually quite easy. Remove the solder from the motor leads and gently pry the hot glued motor from the other side.
Taking a closer look at the circuit board it contains a lot more electronics that is really necessary for a simple DC- motor speed control. The burnt resistor is a 10 ohm shunt resistor in the circuit. The circuit card also contains an op-amp and a transistor as the main components for controlling the motor speed (of course in addition to the variable resistor). Probably the main reason for the shunt resistor burning was a problem with the transistor. I didn't want to redo or fix the original circuit board. There are better and easier ways to control a DC- motor rotation speed, mainly a PWM (Pulse Width Modulation) controller. The good thing about this way of controlling the motor is that it CAN handle the extra current required during the winter time use. The necessary main components are easy to find; LM555 oscillator IC, variable resistor and a MOSFET. Of course you will need a switch, resistor and a couple of capacitors. The beauty about this circuit is that you can basically copy it and make a controller for an electronic focuser.
I did a quick test of the board after finishing the the soldering on the vero board I used. Everything seems to work fine and the running speed is constant. An other good thing about this modification is that the original AstroMaster drive is quite high and makes contact with the mechanical parts of the mount at some angles. With the removal of the circuitry and cover from the top of the motor.  I'll make a final version of the board some time later, but now it waits for proper tests.
I finally managed to put the tracking circuitry in to proper action. I used EQAlign to verify my polar alignment and tracking speed. After a short (approx. 5min) testing of the tracking speed I was ready to take afew test photos. I installed the camera on the scope and linked it to the computer. LiveView on I used a bahtinov to get the focus correct. A few test shots of Arcturus .

Everything seemed OK.  The wind and a full moon posed some minor problems, but nothing to dissuade me from the long awaited photo session. I originally planned on photographing the Pinwheel Galaxy (M101), but decided to go with a an easier target for the first try. So I went for the Hercules cluster (M13).
After getting the cluster in the finder cross hairs a few test shots again with various exposure lengths. I decided to go with 30s sub frames.

This was enough to see if the tracking was performing as it should. I ended up with 11 30s exposures like the one on the left. I also took 4 30s darkframes. After dragging everything inside some crunching with DSS (Deep Sky Stacker) to see if the images were really usable. I ended up using 6 of sub frames based on the score given by DSS to the individual sub frames. The final image with some minor image processing is shown below.I also manage to catch some unsuspecting visitor on one of the sub frames (a satellite most likely) and you can also see the galaxy designated NGC 6207 on the bottom left (which roughly towards the celestial north).

It seems that the new circuit is working well and is considerably steadier than the original. I still need to get the tracking speed set as precisely as possible. That means running the EQAlign for alonger period of time to see the drift.
Until next time.

First Light EOS 1000D part II

Yesterday the sky cleared again and I had chance to continue with the testing of the EOS 1000D. With the moon as a waxing crescent I had a chance to try out photographing the moon.
I took pictures with various exposure lengths with the purpose of combining them later into a composite containing earthshine and the illuminated part. I also took a few photographs of Jupiter with the sole purpose of "just-for-fun" since it was quite nicely located near the crescent moon. Again all the photos are crops from the original jpg's without any post processing.

The first two photographs are of the crescent moon. The leftmost image is taken with ISO400 and 1/100s exposure. The exposure could have been a little shorter still to get a little better dynamic range on the illuminated part. The second image displays the earthshine and is taken at the same ISO speed with 1s exposure.
The next image was taken of Jupiter (ISO400, 4s) with the sole intention of capturing the Galilean moons.

After the Moon and Jupiter I returned to my previous target M42. I took a little bit more time to get the alignment and tracking speed properly setup. Eventually it turned out that propably because of the cold weather the tracking speed fluctuated so much that only a few images were good. I need to take a look at the mechanics of the drive. Anyway here is probably the best image from the batch ISO800 1 x 30s (again no post processing done and a crop of the original jpg).

First Light for EOS 1000D

"Santa" was so kind as to bring me a new camera. So, bye, bye to the Canon A530 and welcome EOS 1000D.
The time has gone while waiting for decent weather for testing the new toy and trying to familiarize myself with it. I did some testing indoors out through the window and noticed that the stock focuser does not work with the camera + adapter setup. The focus does not move enough inward. So some "small" changes needed to be done before going outside. The focuser and focuser tube were shortened in order to get the focus to infinity with the camera.
Finally on January 30 2011 the weather took a turn for better, well, enough to take the equiment out and do some quick testing. My intention was to test the camera mounting and see how things might go on a proper night of photography.
I chose M42 (The Orion Nebula) as my first target (surprise, surprise). The target was not located in the best possible direction from my observation site, it was just above the roof of the house. The good thing is that the roof is covered in snow, so there weren't any major updrafts of warm air. However the streetlights behind the house did manifest themselves in the longer exposures. I set up the rig and did a quick polar alignment, no drift alignment this time, because the temperature was dropping rapidly and I was oput only for a quick test.
First I took a few pictures with the camera optics only.  The following image was taken at 55mm focal length, ISO400, 30s exposure, f/4,9.

 The image is a cropped jpeg saved by the camera. No processing has been done on it. The Orion Nebula is already in this shot showing some nebulosity and color (i was a little surprised by this). The glow of the streetlamps behind the house can be seen coloring the bottom right corner yellowish extending almost to the top left corner. Also the tracking is not spot on, but sufficient for the tests I intended.
For the next images I installed the camera on the telescope. Focusing was done by taking short 2-5s exposures and adjusting in between. The focus is not as good as it could have been, but again enough for this session. I tried different ISO speeds and exposure times.
The next image is taken at ISO1600, 5s exposure. The graininess due to the high ISO -speed is clearly evident even though the exposure time is not that long. Also the shape and colors are quite distinct.

In the last image an exposuretime of 15s was used with ISO1600.  The background glow is getting quite obvious and distracting. The image is losing contrast and of course the tracking error is also having its toll on the image. There is however a noticeable difference in the contrast between the different areas of the nebula.

All things considered, I consider the test successful. I managed to get my first images of a deepsky object with just a quick setup. Of course without guiding I need to pay a lot of attention on the tracking and polar alignment of the scope. Due to the lack of guiding I need to also keep the exposure times short, that means more processing later and larger stacks of images.
While I'm writing this the sky is clearing up again, so maybe I get another session going today.

AstroMaster 130 Spidervane Modification

The spidervane modification has its roots in the results of the startests I performed on the scope. In the diffraction patterns the original 4-vane sipder produced very distinct diffractions.
This is of course partly due to the 4-vane configuration, which produces 4 strong diffraction spikes around bright stars. This is in some cases visually very pleasing and is some what expected of astrophotos. The original spidervanes also had an other issue which is visible in the startest image below.

The vanes produce a somewhat "blocky" star image most likely due to the 3mm thickness of the individual vanes. It is understandable that on a "budget" scope some compromises are made to lower the production costs.
An other issue I wanted to adress with the modification was to minimize the central obstruction size. In the original configuration the central obstruction is actually slightly more than the secondary mirror size requires. Secondly as I managed to chip the original secondary while trying to remove it from the mount, I purchased a slightly smaller secondary that still works on the scope without larger modifications to the tube . The original central obstruction is 44mm in diameter, which amounts to ~34% obstruction. The original secondary minor axis is 40mm. If mounted differently would already reduce the obstruction to ~31%. The new secondary has a minor axis of 37mm, which, if mounted correctly would give an obstruction of ~28%.  I'm hoping to get a little bit more contrast. Visually probably not notable, but when photographing though the scope then the situation might might be different.

I wanted to go for a rather simple design without any "gnarly" parts and easily upgradeable. Basically I want to be able to take the whole thing apart and make changes when necessary. The design basically consists of three parts; the central hub, detachable vanes and the secondary mount.
Unfortunately all the texts are in Finnish, but I think you get idea. As you can see I went for the 3-vane design in order to get rid of the 4 very bright diffraction spikes.
The main problem with the making of the vanes was to find thin and stiff enough material to do the job. It turned out that a metal ruler was more or less a perfect solution for it. Being only 1mm thick and very torsion and bending resistant, especially in the short lengths needed for this.
The central hub is made of POM (Polyoxymethylene) or simply a type of plastic which works well for precision parts and can handle temperatures down to -40C.  The secondary is mounted with three drops of standard silicone adhesive.

The final spider before painting matt black and installation of the secondary. As you can see the difference between the original and the modified vane is quite distinct. Of course the whole original plastic assembly needed to come off before the new vanes could be installed. The good thing is that the plastic tube end was attached to the tube with three screws with 120 degree separation, so no drilling was needed.
Below is the painted spider with the secondary installed (and the original for comparison) and the spider installed in to the scope.

Ever since the modification was completed the weather has been quite miserable, so no star tests have been performed with the new sipder installed. But when the skies clear and the temperatures rise to a manageable level I will perform the tests.

Endeavours with comet 103/P Hartley 2

In the fall of last year northern hemisphere observers had a nice opportunity to observe a bright comet, which was discernible even with the naked eye. Hartley was the first comet I actually observed and tried to image through the telescope.

Anyway it was quite a thriller to try and get the comet in to the field of view of the scope (which I never did manage), the issue being the red-dot-finder of the AstroMaster, which is really not worth anything. I how ever managed to get a decent set of photos with only the camera lens, which I then combined in to the photo in the gallery.

The image in this post is just to show the location of the comet. I will try to get back to the raw images sometime later to see if I can get a better result with the stacking and processing.
The image in the gallery is a composite of images taken Octber 13 2010 with a Canon A530 running CHDK (ISO800,  6x64s).

Star Testing an AstroMaster 130

It all started on a beautiful September evening in 2010. While trying afocal photography with the AstroMaster telescope and a Canon A530 camera, the results seemed somewhat strange.
Eventhough the tracking was not perfect and the focus was not spot-on, it was possible to discern some aberrations in the photographs that made me take a better look at the optical train of the scope.

The next step was to install a standard webcam at primefocus of the scope and start recording video. I chose two stars as reference, Polaris and Vega. What I saw in the videos made me a bit anxious at first, but after getting over the initial shock of the aberrations, the "what can do?", state of mind set in.

Initial captures from the first video (inside focus on the left and outside focus on the right)

It is apparent from the out-of-focus images that there is clearly something protruding in to the light path, at approx. 8 o'clock in the inside focus images and the reverse on the outside focus images). The in-focus image is quite dramatic. In the capture image strong deformation can be seen caused by pinched optics.

After some investigation the spike seen on the extra focal patterns was caused by an extra retainer for the secondary mirror and the pinched pattern was caused by the mounting of the secondary mirror. After removing the extra retainer and fixing the secondary mounting I conducted new tests.

Captures after clearing the lightpath and fixing the secondary mounting

The results of the test were quite promising, at least I knew I was going in the correct direction with the fixes. There seems to be some spherical aberration caused by an under corrected primary mirror, but nothing major. Also the in-focus image of the star is slightly square, probably due to the thick spidervanes supporting the secondary mirror. I already have redone the spidervanes and secondary support, but more about that in the DIY page.

Greetings

This is my futile attempt to document my endeavours in amateur astronomy. In this blog I can hopefully provide helpful information to others starting out in this hobby. I have noticed that it can and will most likely drag you in deeper and deeper, but the thing is to take things at your own pace and try to understand why certain things are and happen. One important thing is to read and then read some more. Also there are many useful websites and forums, which provide multitude of information from theory to practical implementations.

• Stargazers Lounge
• Cloudy Nights
• Tähdet ja Avaruus (for the Finnish speaking readers)

I will mostly concentrate on equipment and astrophotography on a budget with DIY flavour.

I hope you will enjoy your stay and maybe take some ideas with you when you leave.