Automation on a Budget - Part 1: Hardware

There are many possible equipment configurations for astronomical imaging, and many ways to automate the process. This article describes the hardware I’ve assembled; in the next article I will go over the software I created to make all this work. My third article will cover the operational procedures and target scripts.

With my setup, I’m imaging practically every hour of every clear night at a focal length over 2000mm — and sleeping through it. I provide my system with a list of targets to image, and have the option to let it decide on its own how to go about doing this. In the morning, I have dozens of focused images taken from dusk to dawn, along with calibration frames that are ready for processing. While this entire process was relatively inexpensive, I did invest a fair amount of time writing the software.

For those of us living under light pollution, it is still possible to image deep sky objects by stacking LOTS of individual exposures. Automation has helped me accumulate the multiple hours of exposures necessary to successfully image even faint targets. Although my results will never be as good as someone imaging under dark skies, the point is that I can image where I live and get what I consider to be respectable results. Plus, it’s a huge kick to just sit back with the toughest part deciding what I want to image next.

Quick Overview of My Setup
I use the following: Celestron C9.25 optical tube, mounted on a Losmandy G-11/Gemini mount, AstroTech 66mm f/6 refractor piggyback for guiding; imaging camera SBIG ST-402 with built-in filter wheel, images at f/10 on the C9.25, guiding camera Atik 16ic on the AT66; external focuser (Starlight Express “Feathertouch”) automated with Robofocus; and MaxIm DL software to control the cameras and for guiding, FocusMax software for automated focusing, and Pinpoint software to analyze images and determine their exact location in the sky. In addition, I use lots of custom software to automate all this via ASCOM standards interface to the other software components (don’t worry — there are commercial packages that can do this if you aren’t into writing your own).

I usually image at f/10, 2350mm focal length. This means 0.8 arcsec per pixel and an image size of 10 x 7 arcminutes with my camera. I work at such a long focal length because the majority of my targets are small and I want to see detail on them.

Figure 1

My automation approach depends on a couple of key concepts, the main one being the use of an astrometry program like Pinpoint from DC3-Dreams to analyze the images as they are taken and determine absolute coordinates in the sky. This lets my system automatically correct any pointing problems. Since my camera is small and I use a very long focal length, Pinpoint is not able to consistently “plate solve” with such a small field of view (“FOV”). I work around this issue by using my guide scope and its shorter focal length to provide a large FOV image for an initial plate solve to get in the right area, and then my narrow imager field can solve for precise placement.

In addition, this guider setup practically guarantees that any location in the sky will have a usable guide star somewhere in the guider’s FOV. This dual use of the guide scope lets me image without any manual intervention. (Note that a larger imaging camera, or shorter focal length scope, would also solve the Pinpoint issue, but the need for guide stars remains.) Figure 1 is a block diagram of the components of my system.

Mount
I have a Losmandy G-11 mount with a Gemini controller. While this is not a high end mount, I can consistently guide better than one arcsecond RMS error, and often better than 0.3 arcsec RMS. The G-11 can require some tuning to achieve its best performance, and there is much helpful discussion about this on the Yahoo Losmandy forum. One enhancement I found very helpful is an upgraded worm assembly from the French firm Ovision (ovision.com).

Figure 2

If you plan to only image at a shorter focal length such as 500mm, less expensive mounts would be just fine. However, to consistently image successfully at over 2000mm, I’m not aware of any less expensive options than the Losmandy G-11.

My recommendation, which is commonly repeated by many imagers, is to buy the best mount you can afford. All of your imaging efforts will ultimately be limited by your mount. If one of the more expensive mounts is in your price range, I highly recommend choosing it.

Price: G-11 w/ Gemini: approx $3200; Ovision worm upgrade: approx $500.

Telescope

Figure 3

My main imaging scope is a Celestron C9.25. I image at f/10, 2350 mm focal length so I can go after many smaller targets, especially galaxies such as the Arp catalog. It can be challenging to consistently image at such a long focal length, but I’m able to do it. Some people report poor imaging results using a scope larger than the C9.25 on a Losmandy G-11 mount, so I didn’t try to use a C11. Additionally, there are several arguments about why the C9.25 optical design has some inherent advantages, including a primary mirror with a longer f/ ratio that can be manufactured more consistently. I can say that I’ve been very happy with my C9.25.

Price: C9.25 OTA: approx $1000.

Guiding
At the focal length I use for imaging, any exposure more than a few dozen seconds requires guiding. While it’s possible to stack many unguided subexposures, this won’t elicit the best results; guided exposures are definitely preferred. I use a separate guide scope mounted on the C9.25 to provide a wide FOV of almost a half degree for my guide camera. At this scale, any field will have a star bright enough to serve as a guide. The guaranteed presence of a guide star for any target, without manual selection required, is very important for automated imaging.

Figure 4

For anyone familiar with guide scopes, the main challenge is often differential flexure, which refers to when the main and guide scopes shift relative to each other during imaging. I’ve heard from many people suffering from this problem, so I tried to increase the odds of my success by choosing a small guide scope to keep it as light as possible. I use an AstroTech 66mm f/6 refractor, secured to the C9.25 with a Losmandy dovetail bar and mounting rings. I also eliminated flexure between the guide camera and AT66 focuser using T-adapter threaded extension tubes to form a solid connection between the AT66 focusing tube and the camera body.

The guide camera is an Atik 16ic, which is relatively inexpensive and includes (unregulated) cooling of the imaging chip. I previously used a Meade DSI-C as a guide camera and it worked sufficiently in cold weather, but on warm evenings I could not always find a guide star because the thermal noise was too high.

MaxIm DL software controls the actual guiding. The guiding commands are sent to the mount’s guide port using an interface from Shoestring Astronomy (shoestringastronomy.com) called GPUSB.

Price: AT66: approx $400; mounting hardware: approx $200; Atik 16ic: $600; GPUSB: $71.

Imaging Camera
I use an SBIG ST-402 camera, which includes a built-in filter wheel under computer control so I have the option of color imaging. Plus, most importantly, it can cool the CCD chip to around 30 degrees C below ambient. This is currently the smallest stand-alone camera manufactured by SBIG, and it is popular with many people as a guide camera. Until recently it was also the only SBIG camera using the faster USB 2.0 interface, so full frame downloads are under a second.

Since this is a small camera, my FOV imaging at 2350mm is only 10 x 7 arcminutes. There are many targets in the sky smaller than this, but the small FOV makes it hard to locate the target, which is why my guider assists with this.

Price: SBIG ST-402 w/ filters: $1700.

Focusing
Temperature changes will shift the focus during the course of a night, especially for a long focal length scope. It is not critical for the guider to be exactly at focus, but if the imager is not correctly focused, your images will not reach their full potential. Periodic refocusing is important.
SCT telescopes usually focus by moving the primary mirror, but this is not ideal for imaging and can be very challenging or impossible to do consistently. I use the approach of locking the SCT focuser in place (using a Hutech focuser lock) and placing an external Crayford-style focuser on the end of the OTA, which in my case is the “Feathertouch” focuser from Starlight Express.

On the Feathertouch focuser, I installed a Robofocus unit to give me full computer control of the focuser, including an absolute digital readout of its location. Robofocus includes a temperature sensor and an option for automatic refocusing when the temperature changes, but I don’t use this feature. In my experience, the focus changes in my scope are not linear with the temperature change early in the evening when it drops fast; using the Robofocus temperature feature produced worse results for me.

To ensure that the optics stay focused, I instruct my software to periodically run FocusMax on a suitable star. Some people refocus after every image, but with my skies and temperature changes, I find it sufficient to refocus every one to two hours during the night.

Price: Feathertouch focuser: $350; Robofocus: $450.

Dew Control
Dew is generally a problem for telescope optics used for a long period at night, especially for SCTs due to the exposed corrector plate. I use a Kendrick Dew Control system with heater strips on the C9.25 at the corrector, and also around the objective of the AT66 guide refractor. Lately I’ve always been running the dew controller at full power. Some nights in the Midwest require this, and I haven’t seen any disadvantage to keeping it at full power on other nights as well.

I also use a dew shield around the C9.25 corrector. Some nights are so humid that even full power on the Kendrick isn’t enough, so I added the dew shield. I had some concern that the dew shield would hurt guiding quality on nights with some wind, but surprisingly this has not happened. The Losmandy mount keeps the scope steady even with the dew shield attached.

Price: Kendrick controller: $100; dew strap for refractor: $50; Astrozap flexible dew shield for C9.25: $50.

Power Supply and Electronics

Figure 5

As I “grew” my system, at one point I started having intermittent stalls of the Losmandy mount during my imaging sessions. I spent weeks investigating this, including fully dismantling the mount and re-lubricating all the parts. In the end, I found the problem to be insufficient electrical power when I added the dew strip on the AT66, increasing the overall power requirement. To ensure that this doesn’t happen again, I now have separate power supplies for the dew heater and the Losmandy/Gemini mount.

I also have several other devices that need power, such as the powered USB hubs and, of course, the cameras. I added a third power supply dedicated to all these other devices so that any transients caused during slewing the scope or adjusting the dew power level won’t affect the other electronics. I probably don’t need three power supplies, but after losing 6 weeks of imaging last year because the mount stalled during imaging, I don’t want to take a chance.

Figure 6

I use an extension cord to power the overall setup. My equipment is close to my house so this isn’t a problem. I feel it makes my setup simpler if I don’t have to worry about keeping batteries charged before each imaging session.

Several of the devices I use, such as Robofocus and the SBIG camera, have their own power supplies. To manage all the cables and provide enough outlets for all the plug-in transformers (sometimes affectionately called “wall warts”), I built a box to contain as much of the electronics and wiring as possible. It made sense to put the USB hubs inside this box because of the other equipment, but I still wanted to be able to easily see the status lights on the hubs and other equipment to diagnose any connection problems. To do so, I used a sheet of Plexiglas as the door to this box.

A design note about my box: I placed several strips of wood across the center of the box vertically to mount components on, allowing the excess wires and the power strip cords to be placed in the back half of the box. I decided not to place a solid wall there and just drill holes because I was concerned about wildlife possibly taking up residence in the back where I couldn’t easily see.

In my photos on the next page, some extra equipment is visible; there is a video switch (upper right) and USB video capture device (white object in upper left) used for video finders. I have a monochrome low-light camera with a 12mm lens for a wide field view of the sky, and a StellaCam II with 100mm lens for fine pointing adjustment. I use these for the initial sync of the mount to the sky when I power up. Inside the bottom of the box is a power strip and some nightlights that I thought I might need to keep the electronics warm in the Winter. I’ve never needed to do this even with the air temperature going below -10 degrees F.

Price: 12v 5A “Pyramid” power supplies: 3 x $30; enclosure box: approx $30; USB hubs and active USB extension cables: approx $100.

Observatory

Figure 7

I do not have an observatory. What I do have is a semi-permanent setup where I can start imaging each evening and close up in the morning with less than 5 minutes of effort. Imaging can obviously be done without something like this, but I find it extremely convenient. This way I can more easily take advantage of any clear nights and gather more images without much effort.

I call it my "telescope shelter." I started with an 8’ x 8’ canopy tent to cover my equipment during the day. I couldn’t move it by myself (attempts to drag it across the grass resulted in bent support poles), so I built a wooden frame and put wheels under the frame to make it easy to move.

Figure 8

It did partially collapse from a heavy snowfall the first winter after I built it, but I reinforced the tent poles and it has survived over two years since then without any problems. The shelter also has the advantage of not looking like anything important, so security hasn’t been an issue for me.

I made a few optional enhancements such as installing shelves on the frame for storage, and wrapping the sides in tarps for better weather protection. After a few months the wheels started to dig ruts in the grass, so I laid a couple of 10” decking planks for it to roll across and added strips of wood on the sides to keep the wheels from falling off.

Figure 9

I would like to install a pier at some point, but for now my mount sits on a Losmandy tripod on the ground. The only time this is a problem is when the ground transitions between frozen and thawed and moves the mount out of polar alignment. I can usually go weeks or months without having to adjust the alignment. I’ve learned to do a quick check on alignment before I start my automation script by doing GOTOs of objects on opposite sides of the sky. If these aren’t close to the desired targets, it means my tripod has shifted. If this is the case, I readjust the mount using a polar scope, and then spend half an hour rebuilding the Gemini pointing model.
Price: Canopy $80; miscellaneous lumber, hardware, and tarps: approx $150.

Figure 10

In rough numbers, the hardware investment is approximately $9000. I acquired the equipment over a period of about 3 years, with the single largest expense being the Losmandy mount. Additional images of my equipment are available online at .

In my next article, I’ll describe the software I’ve developed to automate my imaging, and my final article will cover operation and target selection.