CCD Picture Techniques, Part 2

I haven't written anything down lately because I've spent the last couple of months working out various details in my workflow for taking pictures of tiny dim objects trillions of miles away from the Earth. Astrophotography is perhaps the most purely technical of all of the possible photographic disciplines. Everywhere you turn you are up against a lot of problems.

The first thing we learned on this web site is that a really good equatorial mount solves a huge number of problems. The mount will point your telescope with ease and precision, so you don't have to worry about hunting around in the sky for really small things. It will also track objects in the sky with great accuracy, so you can use a relatively simple camera and still take pictures of reasonable quality even at fairly long exposure times. You can do pretty well with this fairly simple setup, and even get some pictures that look pretty impressive. Like this:

This picture is hiding a lot of problems though. I've hidden them from you by burying them in the blacks. But in doing that I've also buried some of the detail. If you pull up the detail you can see what I mean:

Here we can see some of the same issues that I discussed in part 1. I've used the healing brush to substitute for flat frames, and it only sort of works. I've hidden a lot of background noise by burying the low values. And you can also see that as good as the mount is, at more than two minutes of exposure the stars are sort of oval shaped instead of nice and sharp.

After a few months of learning with the simple camera I decided to upgrade my goals. I wanted to be able to take longer exposures and I wanted to fine-tune my pre-processing.

For long exposures, I realized that I would finally have to come to terms with guiding. For pre-processing I decided I needed a more streamlined tool and also that I really needed to shoot flat frames which I had until now ignored. Flats are their own unique adventure, so let's cover guiding first.

Guiding is the act of correcting the tracking of the mount over time to compensate for unavoidable but (hopefully) small errors in the mechanics of the gear/motor train. First you point the camera at the object you want to take a picture of. Then you point a second optic at a star nearby and you keep that star in the exactly the same relative position to the target for however long you want to run the exposure. The key word here is exactly.

In the past the poor astrophotographer would have to sit out with his telescope staring into an eyepiece. If the guide star moved he would nudge the telescope this way and that to re-center it. These days we have computers to do this for us. So, you set up a second camera, point it at the guide star and run some software that repeatedly takes a picture of the star and makes sure that it sits in place. The software computes the star's position based on a short exposure image and then nudges the mount for you while you sit in your house and watch NFL football. How great is that?

The big choice you have in setting up a guider is to decide whether or not the guide camera will use the same optics as the main camera or not. Separate guide scopes are convenient because they provide a large field of view from which to choose a guide star. However, they can suffer from a wide range of problems that all boil down to this: if you use a second telescope to guide, the second telescope may move or not move in exactly the same way as the optics of the main telescope. When this happens the relative position of the guide star and the target will no longer be fixed and you will accumulate tracking errors. These errors may be too small to be noticeable, but if they are not they may prove to be very hard to find and remove.

To avoid problems with "differential flex" you can set up your guide camera to use the same optical path as your main camera. What you do is attach an "off axis guider" to the system. This device as a small prism in it that deflects a bit of the light coming to the main camera and shunts it to the guider, which sits off to the side. Assuming you can get a good star into this smaller off axis field of view you can then guide the main telescope without worrying about flex. The inconvenience is that you might not be able to find such a star and then you have to nudge the telescope or guide camera around until you find one. The other annoyance with these systems is that you have to make sure that the guider sits at exactly the same distance from the focal plane as the main camera. This can be tedious to set up, but you only have to do it once.

For the truly lazy the SBIG camera company developed a unique device in the early 90s. The SBIG camera uses two sensors to essentially incorporate an off-axis guider into the body of the main camera. The result is a single camera body with two sensors in it, one for imaging and one for guiding:

The light path from the telescope comes hits both chips at once without the need for a pickoff prism. In addition, both chips are automatically in focus at the same time. Thus, to guide, you point the camera to place the target on the main CCD and a guide star on the guiding CCD and start up the guiding software. Done and done.

The SBIG "self guided" cameras suffer from some of the same inconveniences as off-axis guiders. The field of the guide CCD is pretty small and sometimes you have to move the main camera a lot to get a good star. In addition, if you shoot through filters the filters sit in front of the guider which means that you have less light to guide with. This is especially difficult with filters that cut off most of the visible light coming from the sky (like H-alpha filters).

By now you know that I would not have spent all those words telling you how the camera worked if I hadn't decided to pick one up. SBIG has sold tons of these over the years, so they are easy to find used at good prices. So I found a nice ST-2000XM monochrome camera and got it set up.

At this point the main issue was software. I'd have liked to be able to use a piece of software called PhD for doing the guiding. This package is developed by the same guy who built Nebulosity, which I had been using for capture and pre-processing. Nebulosity is fairly robust and competent, so I'd have liked to stick with it. But, you can use Nebulosity and PhD with a single camera at the same time because the device only shows up on the USB bus once. Therefore, you have to find a program that can talk to both CCDs at once.

On the Mac there is only one such program and it's called Equinox Image. This is the companion to the Equinox planetarium program, and it's pretty good. I used "EI" to take learn the ins and outs of the camera. As promised, setting up the guider was straightforward and in no time I could take three to five minute exposures with perfect tracking every time:

The combination of the mount and guider is so smooth that you can't even see the image shift at all over a sequence of ten or fifteen frames. Truly amazing.

While Equinox Image was mostly satisfactory, I eventually started looking for something else for two reasons:

1. For whatever reason the native USB stack on the Mac is not super reliable. Or maybe the SBIG USB drivers are not great. In any case, my laptop would regularly lose contact with the camera requiring a restart of everything. Which was annoying.

2. After getting everything set up I found that Nebulosity's workflow for processing multiple sets of images with dark, bias and flat frames to be tedious and repetitive.

So I ended up downloading the demo to Maxim DL, which is the grand poo-bah of imaging software under Windows. Maxim is an old-school Windows-95 style application in the truest sense of phrase "old-school Windows-95 style application." The user interface is a mess of tabs inside windows inside windows that hide popup menus inside menus. But it does two things super well.

1. It has a streamlined engine for image pre-processing and stacking. You set it up once and hit one button and it goes. This is great.

2. It has a super plate solving utility that can figure out where your mount is pointed and automatically center things for you. This is great if you want to take pictures of the same target over multiple nights.

Maxim also has some nice utilities for taking a long series of pictures with multiple (RGB) filters. This makes it possible to set up a "run" for the night and then go inside and watch more NFL football. There are even people that do completely automatic capture with Maxim and a scripting program. In fact, automatic astrophotography may be the one place where people actually have taken effective advantage of COM scripting for something besides enterprise IT business logic.

Finally, for whatever reason, the USB communications between the camera and my VMWare virtual machine was rock solid. Much better than the native interface in MacOS. Go figure. After a few runs with Maxim I was hooked.

Next time I'll outline my current workflow with Maxim, describe my flat field adventures, and start a fun long term project involving objects that no one has any right to be able to take pictures of from a back yard. Good times.

CCD Picture Techniques Part 1

Here is an obvious fact that you learn when you try and take pictures of distant astronomical objects: distant astronomical objects are really really dim.

Consider the following photograph of a regular terrestrial scene (as they say in the astro-photo biz):

The following histogram gives you an idea of the distribution of different brightness levels in the above picture. To read the graph, you interpret values on the left as dark pixels and values to the right as bright pixels. Then the height of the plot is the number of pixels in the picture with that particular range of values in it.

Most photographs have a histogram that looks something like the one in this example. You have a small number of pixels that are super dark or super bright and you have a lot of pixels with all the values in between. This means that you picture is not clipped off and all the detail is visible.

Astrophotographs are not like this. Here is a typical frame out of a CCD camera:

This is a 3 minute exposure of a pretty bright galaxy called NGC2903. If you stare at the frame a bit you can sort of see the shape of the galaxy in there, but it's not very interesting to look at.

Here is the histogram:

What this histogram says is "that picture is really dim."

The goal in processing this image is to take all the bits that represent "signal" (that is, stuff you want to look at) and make them bright. The main problem is that there are all kinds of bits that we don't want to see ("noise") that will become visible when we make all the dim things bright.

To give you an idea of what I'm talking about, in the following frame what I've done is to push the levels of the picture to increase the brightness and contrast:

The result is to take what used to be in that tiny little sliver of a histogram and expand it all out to cover more of the range we want, like this:

Of course, this picture is not that nice to look at.

There are various problems:

1. There is a lot of background noise, which we have amplified by pushing the levels so hard.

2. There are various optical problems. You can see dust in the optical system.

3. There are hot pixels and other noise related to dark current.

4. You can't really see it in this example, but there is a gradient over the entire image related to light pollution in my back yard. Generally my sky is darker to the east and brighter to the south and west.

If I had to sum up astronomical image processing in one sentence it would be: "the art of making the signal bright, in a pleasing way, while hiding the noise without being obvious about it." That is, we want to use as many tricks as possible to make the galaxy bright and pretty while avoiding the trap of also showing you all of the problems in the image.

In my previous post I lamented that these and other issues were almost impossible to fix with the video camera. With a CCD still camera it's still hard, but it's much more doable.

There are several tools available to the CCD user to remove background noise from an image while retaining detail. These fall into three general strategies:

1. Any single exposure will probably be short enough to be noisy, so combine average many noisy exposures to smooth out the final result.

2. Use the CCD calibration tools that are available to you (see below).

3. Smart post production can make a big difference.

The first item speaks for itself. Take as many exposures as you can stomach. I tend to work in two modes. If I am just exploring new objects to see what they look like I'll take just a few exposures and live with noisy images. But if I decide to really go after a favorite object, then I'll take as many exposures as I can, possibly over several nights to try and minimize the final noise profile.

The second item takes more explanation. CCD "calibration" refers to post-processing your images to remove noise that is generated by either the CCD hardware itself or your optics. Recall from before that the main issues here are dark current and read noise.

Dark current adds noise to a picture by causing the CCD wells to register "signal" that did not come from light hitting the sensor. Luckily, there is an easy way to compensate. Say you are taking 3min exposures of your object. Then what you do is take a frame with the sensor covered up that is exactly 3min long with the CCD at the same temperature. On average this "dark frame" will contain just the noise generated by the dark current while you were shooting your 3min frame. So, you just subtract the dark frame from the original image and you are done. Right?

Actually, it's not quite that simple. CCD images also contain a lot of random noise (read noise, noise in the dark current, etc) that is different for every frame. So if you just took a single dark frame and subtracted it you would be adding this random noise to your picture, which isn't great. The solution is to shoot many dark frames and average them together. This smooths out the random noise and leaves a more consistent noise profile behind.

In addition to dark frames, one will also collect "bias" frames, which characterize the minimum signal level, or offset in each frame that you shoot with the camera. A bias frame is basically a zero length dark. Again, you take a couple of dozen of these and average them together to minimize read noise and such. If you take very short flat frames (see below) you can use bias frames to effectively do "dark subtraction" on them, since the dark current will not be significant. You can also bias-subtract your darks which allows you to scale the darks to different exposure times. I personally have not tried to do this.

Darks and bias frames let you minimize the noise introduced into your pictures by the CCD itself. We take many such frames and average them together to smooth out the parts of the noise that we can't capture directly. The read noise is a good example of this sort of noise. Read noise will be in every shot you take, you can't get rid of it because you can't capture it. Even doing dark subtraction just adds the read noise that you couldn't get rid of in the dark frame into your lights. This turns out to be why the CCD people take so many exposures (72 hours on the Horsehead nebula!!). The more you take the more you can minimize the bad parts of the noise, leaving your signal behind.

What's left to deal with are defects generated by the telescope itself. In our example these are easy to see:

1. Uneven illumination caused by light falloff in the optical train.

2. Shadows caused by dust.

There are various techniques for automatically removing these problems using "flats". The idea is that you point your telescope and camera at a perfectly uniform light source and take an exposure that exactly hits a mid-tone on the sensor. Then you shoot dark frames at the same exposure ("dark flats", or "flat darks?"). Then you divide the resulting data out of your exposure frames.

Personally, I take a different approach to this. First,I am too lazy to shoot flats. Second, since I'm shooting from my yard, I have a lot of gradients caused by light pollution, so I need software tools to deal with these. Such tools will generally also deal with gradients caused by uneven illumination. So I just do that. There is a piece of magic software called Pixinsight that does a very good job modelling and removing gradients and other background noise. I've gotten by just using a software solution for now. It has generally worked OK. But I may break down and actually shoot flats at some point.

As for dust … I've had reasonable success just cloning it out in Photoshop. I don't have that many dust shadows. The bigger ones are harder to remove, and if I had more of them I'd probably learn to shoot flats.

So, here is the workflow for your basic black and white CCD image.

1. Shoot as many "light" frames as you can stand. Averaging many frames reduces the noise inherent in the image itself.

2. Cover up the telescope (or get a CCD camera with a shutter) and shoot as many dark frames as you can stand. 10-15 is usually enough. This will minimize issues with dark current noise, hot pixels, and read noise which are all caused by the CCD sensor.

3. Shoot flats if you want to. This will help minimize defects caused by your optics.

Now load all this up into your favorite imaging software (Nebulosity, Maxim) and tell it to calibrate your frames. When you are done, you'll have nice clean single frames. Now use the same software to register and "stack" these frames. The result will be a single combined image that you can then stretch out to bring up the detail. The amount of noise you have left will depend entirely on how long your exposures were. This, in the end, will determine how much detail you can pull out.

Here's the final version of the example object that we started with. The blacks here are actually clipped because I was not that good at this yet.

Here is a better image where I didn't have to clip the blacks to hide the noise:

These image are all limited by a couple of things:

1. I can't expose more than around two minutes at once because even the awesome mount I bought can't go much longer without noticeable tracking error.

2. I was not that good at the post-processing tools yet.

Next time we'll see how one can progress past these issues mostly by spending more money.

C, C and D

It all started with the hot pixels. The Mallincam video camera that I had bought provided wondrous views of the skies above, but these views were always accompanied by a collection of colorful extras: pixels activated by the heat of the camera would glow red, green or blue next to the object that I was looking at. I would try to ignore them, but they sat there staring me in the face.

M82 was still a sight to behold:

But I constantly thought "there must be something I can do about this noise."

First, where does the noise come from? To know this you have to know a bit about how CCD cameras work. A CCD is an integrated circuit that is made up of an array of "wells." Each of these wells makes a single photo-sensitive site. The hardware takes advantage of an effect from quantum physics called the photoelectric effect. In the late 1800s Heinrich Hertz observed that if you shine a light on some materials, they will give off electricity. CCDs use this effect to turn photon hits into electric charge. Conceptually, as light hits the chip electrons collect at the bottom of the wells. When the exposure is done, the charge at each site is read off one by one and turned into a small binary value. This collection of binary values can then be made into an image.

In the Mallincam the digital image is turned into an analog video signal by hardware in the camera itself. This signal goes over a composite or S-video cable to your computer where you turn it back into digital video with a capture card. All of this saves you the bother of processing the digital image yourself. But, it also prevents you from processing the digital image itself.

The reason you might want to process the digital image yourself is to manage noise. In a perfect world, the only thing making electrons at the bottom of those wells would be actual photons from the heavens. In addition, a perfect world would have circuits that can read the charge perfectly and without any error. Of course, this perfect world does not exist. All sorts of factors conspire to put false data into our digital images and this false data is what we call "noise."

If you spend any time at all looking at Mallincam pictures you can see a lot of different kinds of noise. There are the hot pixels which are caused by heat and dark current (see below). There is the greenish amp glow in the corner from the video amplifier. There is noise caused by the conversion to and from analog video. Finally, if you aren't careful, there is electrical noise caused by your power supply transmitting RF into the video cables.

The noise that I'm interested in tonight is called "dark current." Dark current is extra electrical noise created by the sensor hardware itself. This causes electrons to collect in wells even though no photons were captured. If the dark current is strong enough the pixel will register as having data when in fact it captured none. So you get false, or "hot" pixels.

The amount of dark current that you see depends on the temperature of the sensor. So, the first thing fancy CCD cameras do to combat dark current is to use sophisticated cooling hardware to make the chip as cold as possible.

The second thing you can do to manage dark current noise is to take dark frames. Here you cover the CCD and shoot a picture with exactly the same exposure time and at the same temperature as you used for the picture you took. Then, you take the resulting data and subtract it from the data for the actual exposure (also called the "light" frame). If you are lucky, this will exactly cancel out any false data in the light frame.

This is what I wanted to do with my Mallincam pictures. But I couldn't. With the Mallincam you can't actually capture the light frame and dark frame data. All you can do is grab an image file that has bits in it that are the result of image processing in the camera, then a digital to analog conversion and then an analog to digital conversion. All of this processing destroys the integrity of the signal and makes it impossible to do dark subtraction. At least in my experience.

So I went out and got a small CCD camera to play with. I picked a Starlight Xpress SXV-M7 monochrome camera. Why?

1. It's about the same size and weight as the Mallincam, so I'm used to handling it.

2. It uses a similar sensor to the Mallincam, except that it's black and white only. These Sony sensors are pretty sensitive and low noise.

3. I got black and white so I would not have to worry about color. Color in the Mallincam was always the best and worst thing about it. Plus, it was galaxy season anyway.

I found the SX camera pretty much as easy to use as the Mallincam. The nicest thing was that I only needed a single USB cable to run the whole thing. The most complicated thingwas that I had to learn how to process the digital data myself. We'll get into that next time. But after a bit of practice, I did finally manage to get rid of those hot pixels:

It turns out that a few months with this camera taught that there is a lot more to noise in CCD imaging than I had considered at first. Next time we'll see that to a first approximation processing astronomical pictures is just one huge exercise in hiding the noise so that you can exaggerate the signal.

Mount Lessons

I've had my Astro-Physics Mach1GTO equatorial mount for about two months now. The mount has continued to perform better than I could have expected, and I expected a lot. So while it may seem like I have already gushed uncontrollably about it I have just a few more things to say before I just go back to happily using the thing.

Setup and Alignment

I have my setup routine pretty much down. Even though it takes more trips, it's actually faster than setting up the CG-5 for use with the camera. For visual use it would only be a bit slower. Here is what I do:

1. Put the portable pier down on my patio. I have a fixed spot for it.

2. Roll the mount out in its rolling box. Put it in the pier. Attach the three knobs that hold the mount on the pier.

3. Attach the counterweight shaft and the two weights.

4. Go in the house and get the telescope. Put it in the saddle.

5. Balance the telescope. I found this tricky at first because the axis bearings on the Mach1 are pretty stiff. You can't really balance the telescope by finding the point at which the telescope does not move when you undo the clutches. The axis tension tends to hold it in place. Instead you need to find the point where the motion along the axis is most smooth and uniform. It takes a bit of practice.

6. Set the mount in the "Park 1" position, where the RA axis is horizontal and the DEC axis has the telescope pointing north. The telescope tube and the counterweight shaft should be level.

7. Use the polar scope to align the mount on Polaris.

8. Plug everything in, Resume from Park 1 on the hand controller.

9. Now I run one pass of the "quick drift" polar alignment scheme which is outlined in this pdf file. While the description in the documentation is somewhat involved, the scheme is actually very simple and mostly involves pointing the telescope at two stars that you can pick. I'll provide details in a later section.

10. Now hook up the camera and laptop. Find a star near where you want to look at things. Point the mount there. Center it in the camera view. Focus. Hit "recalibrate" on the hand pad. Done.

Here is what I like about this setup routine:

1. I only need two stars (the two used for polar alignment). It's easy to find two stars to use. It could sometimes be a pain to find six for the Celestron.

2. No complicated and fragile pointing model. The multi-point pointing models that the Celestron and Meade mounts use are all derived from the software used to make the alt-az fork mounts point and track objects for visual observation. To "align" these mounts you point them at various stars and the computer builds a little model to translate celestial coordinate to alt-az and the poor user does not have to mess about with "complications" like polar alignment. This is great but also annoying in some ways. If for whatever reason you accidentally shifted the telescope after building the model, your only real hope is to rebuild the model from scratch. With the Celestron GEM mounts this manifests itself as needing to redo pointing alignment after you have used the pointing model to find tune polar alignment. Which is unfortunate.

Here is my epiphany about alignment: if your equatorial mount is well built and precise, the only alignment you need to do is polar alignment. The Astro-Physics literature actually says this, but I did not believe it. I was wrong.

The various multi-star alignment routines that have been built to make life "easier" for users has actually worked to hide this fundamental fact from them and thus made the world ultimately more confusing. The latest high end mount from Meade (the LX-800) takes this even further. Check out this thread about it to see what I mean. In addition to multi-star "alignment" you now also have to deal with guider setup and plate solving before you can look at things in your telescope. Is this really easier to use?

Stability and Consistency

In the last six weeks I've continued to make progress on my projects to make rudimentary video images of all the Messier and Herschel 400 objects. This has gone very well, and I've added images of a few dozen new objects to my little collection. Two nights of this work stand out. They might be the same night, but since I do not keep good notes, I have forgotten. On April 18th I captured what was for me a record number of images in a relatively short amount of time. You can see all 29 of them here. Being able to work that fast with the mount and the camera brings up the intriguing possibility of doing an entire Messier Marathon with the camera. But I'm not really interested in that. Anyway, the consistency of the mount allows you do work this way. Once aligned and calibrated the mount just never seems to go "off". Object after object can hit the small chip of the video camera.

In addition, the mount seems impervious to wind. Wind is the enemy of stability and good tracking, yet one of the best nights I had in the last six weeks was on a night where the gusts were up to 20-25mph in my back yard. Remarkable.

Meridian Games

The meridian is a something that hangs over every German equatorial mount. In this design the telescope sits on top of the declination axis and is counterweighted on the other side. Here is a poor schematic:

In this design the telescope and the counterweights are always on opposite sides of the pier. When the mount is properly aligned, the RA axis will point straight north, which means the pier will be right on top of an imaginary line that runs North/South right down the middle of the sky which we call the meridian. Another bad picture:

The issue with German mounts is that as you track closer to the meridian you can end up in one of two bad situations:

1. The weights or scope can hit the pier.

2. The weights can end up above the scope, which is bad for balance.

So avoid these things, the controller forces you to follow some rules. The main rule is that if the scope is pointing East its body should be on the West side of the mount and vice versa. This keeps things from hitting the pier and keeps the weights below the scope.

Which brings us back to the meridian. What the above rule means is that if we want to go from looking at things in the East to looking at things in the West we need to flip the scope over the mount so it's on the opposite side:

This is a rather drastic and potentially destructive move. Things in your telescope can get out of alignment. You can catch wires on knobs. And, it can throw off your pointing. So, users of German mounts tend to try and avoid flipping at all costs. But, this is annoying since the exact time when you want to look at most objects is when they cross the meridian, because that’s when they are highest.

The Mach1 mount does a few things to make this better. First, the geometry of the mount allows it to track far past the meridian if you want it to. So you can pick up an object on the East side and follow it across its highest position in the sky without needing to flip anything around. Second, the build quality of the mount assures that when you do flip over, all you need to do to restore good pointing is to calibrate with the position of one bright star on the new side. I ended up always needing to do this with the Celestron mount anyway, even with all the fancy alignment software.

Finally, the control software in the mount allows you to shift where the mount thinks the meridian is, and thus delay or force flips when you want to. So, if you want to pick up an object that is 30min East of the meridian and track it until it is low in the West without flipping, you can tell the mount that the meridian is actually one hour further East than it really is. It will then dutifully flip the scope over as if the object is already in the West and then track it for hours without flipping. Just make sure nothing hits the tripod when you start this maneuver. If you are really a mount dork, you can read more about this here or in the AP documentation at their web site.

Astro-Physics even has a clever scheme that uses flips to make sure your polar axis is aligned. This is the basis of the "quick drift" alignment scheme I mentioned before. Here is how it works. First, set up a finder scope on your main telescope that has a crosshair that you can rotate so that one direction corresponds to East/West and the other to North/South. Orion makes a good one that can do this.

Now, pick a star near the meridian and near zenith and have the mount point your finder scope at it. Center the star and hit "recalibrate". Now shift the meridian either East or West by one hour depending on which side of the meridian you are on to make the mount flip over. If you are well aligned, the star will still be centered in the finder. Any shift East/West is the finder scope not being quite aligned with the mount. Any shift North/South is the mount not being quite aligned in altitude. Use the altitude adjustment to get rid of half the North/South error. Use the adjustment on your finder scope to get rid of half of the East/West error. Use the keypad to center the star the rest of the way. Flip the mount again. Iterate this process until the star stays centered.

Now pick a second star that is at also near the meridian and at least 30 degrees away of the first star. Point the telescope there. If you are polar aligned, the star will again be centered in your finder. Any shift is a result of azimuth error, so with the azimuth adjuster to remove it. Now point back at the zenith star. If it is off center, center it with the keypad, hit recalibrate, and slew back to the second star. Adjust the azimuth again. Repeat this until the star stays centered as you slew back and forth. You are done.

There are a few things to like about this scheme:

1. It's easy. In particular, once the finder scope is set up well it usually only takes one or two iterations to get aligned.

2. It's stateless. You can redo the procedure any time you want without worrying about confusing any software. All you are doing is adjusting the mount's geometry relative to the sky. The control software is not keeping any sort of score.

3. It uses a nice ergonomic finder on top of the telescope. No fussing with polar scopes that are hard to look through. I use the polar scope to get a rough alignment, but always do this scheme for finer adjustments.

Small Details

Finally, here are some small things that make the mount a pleasure to use.

1. The cables are great. The power cable has a locking mechanism so it won't work its way loose without you knowing it. The motor and keypad cables are all these cool XLR connectors like for pro audio equipment. It all just works.

2. I like the LED keypad display. It works better than the LCDs I've used before.

3. The documentation and support are great.

There isn't too much more to say. If you are in the market for a really really good telescope mount and you have the funds, I can't imagine doing better than the Astro-Physics product. It is a well-built no nonsense straightforward piece of equipment that just works and is gloriously free of useless bells and whistles. It's a reminder that it's still possible to achieve great things by building in small numbers and paying careful attention to detail.

Value for Money

I’m about a year into using my astronomical video camera to view deep-sky objects from the city and capture small and simple pictures of what I see. All things considered the experience has been tremendous, but there was an obvious weak link: the mount that I bought does not track well.

This is not to say that the mount did not perform up to my expectations. In fact, given the relatively small amount of money that I paid the mount has been excellent. Good telescope mounts are hard to build well, and even harder to build cheaply.

That said, you can do better, and about six months in I decided that I would be doing this enough to consider how to do better. After my first investigations, I wrote this about the relationship between cost and quality in a mount:

In general you will find that in terms of mechanics you get what you pay for. There is a direct and linear relationship between how much money you spend and how well the mount will hold weight and smoothly track the sky with the least amount of fuss. When you spend more money you get more reliable machinery that is built to a higher standard of precision. Those gears in the motor drive will be asymptotically closer theoretical perfection. More importantly, by building in small numbers the premium manufacturers can maintain a tight hold on testing and quality control.

What this means to me is that you should start cheap, decide if you are serious, and if you are serious then go and buy your last mount ever. Mounts are like tripods in photography. If you are really serious about photography it’s well known that you will eventually spend $1500 on a carbon fiber tripod and a really good ballhead. The only question is whether you spend $3000 on inferior tripods before finally upgrading to the one you should have bought anyway.

Having started cheap and decided I was serious, the only question now was: which mount is the last one I want to buy. The choices before me were:

1. A more expensive Chinese mount. Here you have the CGEM from Celestron, and the various Synta mounts like the Orion Atlas. All of these have the same basic mechanics and manufacturing quality as the CG-5 that I had bought, but are built larger and heavier so they are more stable. I could not find a convincing case that the performance of this hardware is consistently good enough that I would not want to throw it away in a year. The Atlas and the CGEM also weigh almost 40 pounds. More on this later. iOptron in Taiwan also makes a mount in this class, but which is lighter.

2. Losmandy G-11. By all accounts this is an excellent piece of hardware. Unfortunately they’ve been in a two year death-march updating their control software, which does not fill me with confidence. The G-11 is also relatively large and heavy given the load that it can carry. Finally, there is a lot of Internet literature on do it yourself tweaks to this mount, which brings it down in my mind. But I’m not really being fair.

3. Takahashi EM-11 or EM-200. Here we are finally getting into the “just works and works forever” category. Beautiful fit and finish. Apparently great mechanics. Best polar alignment scheme in the business (more on this later). But, it requires a laptop for doing automatic pointing. And, the main documentation on the entire mount is a 10 page pamphlet poorly translated from Japanese into English. You can also only get service on this mount from a single company that is far away from me.

4. Astro-Physics Mach1. You saw this coming. Since this is the one I picked, I will now ramble on about it at length.

My pre-purchase justification for going with the Mach1 was as follows:

1. Weighs 30% less than the Chinese mounts but carries at least twice the weight.

2. Weighs about the same as the EM-200 but is more stable.

3. Extensive and competently produced documentation and support materials available on their web site.

4. Extensive and competently produced software support on the PC side if I want to go that way.

5. A few clever control features in the mount itself (more on this later).

6. Nearly 100% positive evaluation of the mount on the Internets.

7. Excellent Internet support. The owner of the company can be found doing remote diagnosis of issues on the mailing list. Impressive.

My main pre-purchase worries about the mount were:

1. Complicated to set up and polar align compared to the Tak, where you just point the polar scope and go.

2. Expensive.

3. Maybe a bit too large. The EM-11 is more my size, and would retain my “bring the mount and tripod out in one step” workflow.

4. Expensive.

Still, I had nothing to lose by putting my name on the waiting list (Astro-Physics makes two production runs of the Mach1 per year, one in the spring and one in the fall. So you put your name on a list and wait until the next run) while I deliberated further. By the time my name came up on the list six months later I had decided that $6000 on a mount is expensive, but not as expensive as spending $4000 now and then $6000 later when the first one didn’t work. Two weeks later four large boxes appeared at my house.

Here is what you can say about Astro-Physics, their excellent product photography does not do their products justice. At this point I should include a picture of the mount head, or the polar alignment adjuster knobs, or the keypad, or the wires that go from the control box to the motors or even the knobs that hold everything together. But, I lack the talent and technical ability to create an adequate picture. Every one of these items is built and finished to a degree of polish that I can only describe as good enough to make Apple jealous. I spent five minutes just turning the knobs on the Vixen style saddle. I had never seen machined knobs that were that good. Not even on my Really-Right-Stuff ballheads which until now were my standard for gratuitously expensive pieces of machined aluminum.

I spent the first night with the hardware just putting it together. There were lots of parts and knobs and whatnot. By the time I had it set up it was time to go to bed, even though the sky was clear. This is a hard situation to be in.

Happily over the next two weeks we had enough good weather in Pittsburgh for me to get the mount set up at night. Some impressions from the first setup:

1. Polar alignment was not bad. The polar scope is easy to use, and the clever “quick star drift” scheme outlined in the documentation works well. You can get set up in about 15 or 20min, which is the same time it used to take me with the CG-5′s computer. The adjustment knobs on the mount for altitude and azimuth are incredible. This is still more complicated than the Takahashi mounts with their superbly integrated polar scopes. On those you run a computer program that tells you where in the polar finder to put polaris and you just do it. You are done in five minutes.

2. The mechanics of the mount, in use, are practically perfect and absolutely predictable. With good polar alignment you can calibrate on a single star and afterwards every single GOTO goes exactly where you want. The CG-5′s alignment scheme, while clever, was never consistent. Some nights pointing would be perfect and other nights it would drift around unpredictably. You also have to memorize where six or seven bright stars are in your sky on any given night to make it work. The Mach1 only needs you to find one star.

3. When moving the mount with the keypad there is no apparent backlash in any direction. This is unlike the CG-5 where at slow speeds you’d have to wait a second or two for the gears to wind up before the mount would move.

4. The mount remembers the date and time. Hallelujah.

5. My one and only gripe so far: the clutch knobs can be hard to use because they are so close to the motor boxes. Oh, and the portable pier/tripod is not the easiest thing to fold up or adjust. That’s all I can think of.

I also got to set up with my camera. Here’s a single shot to give you an idea of how well the mount performed:

This is NGC2903 and the picture is a stack of 12 frames shot at 2 minutes exposure for each frame. The telescope was being used at 1000mm focal length. Every one of the 12 frames was perfect enough to use. The object barely moved on the screen after each exposure.

Just last month I shot this object with the old mount using my small refractor at around a 300mm focal length:

This is a stack of 1 minute frames at less than half the focal length as I used above. And the tracking is visibly worse.

“But you should be using a guider” you might say. And you’d be right. I should be using a guider to smooth out the raw tracking error in any mount. But guiders are notoriously finicky devices, and their ease of use, or lack thereof, is pretty much linearly related to the underlying performance of the mount they are guiding. So the way I see it, I’ve finally bought a mount that will be worth guiding, because the guiding will just work.

In my mind, this gets to the core value of this product. This is a product that defines the phrase “just works.” The behavior and performance of the mount is absolutely predictable. And the value of that is hard to measure.

“Value” is one of those buzz-words that makes the rounds of business meetings and marketing materials. I think in most cases it refers to something that a company would like you do believe you are buying, but in general it’s not the case. Most of the time when you spend more money you have just spent more money. It’s not often that you spend more money for something and when that something arrives you can stare at it and see the exact linear relationship between the money that you have spent and the value that you have gained. All I can say about the Astro-Physics Mach1 is that you can see every penny of it. It cost ten times more, and really is ten times better than the cheap mount that it replaced. You can’t say that about too many other products.