Sunday, August 17, 2014

Mounting QSI-540ws to Canon Lens

This post describes an experiment in connecting my QSI-540ws CCD to my Canon EF lenses. I started this experiment after last month's imaging of Rho Ophiuci. Even with my best attempt at a Peltier cooler box, the image noise was such that I began longing for my CCD camera.

EOS Adapter - My attempts in finding a suitable adapter were frustrating. I ended up purchasing the EOS adapter sold specifically for my QSI-500 series camera. It is designed with the correct spacing from the focal plane but was nearly $200. The adapter is shown in the center of the photo below. Note that this accessory has the opposite polarity of the easily obtained adapter used to attach a DSLR to a telescope.

With all of my Canon DSLR cameras, the bayonet EOS fitting is never as tight as I would like. This one was no different and allowed noticeable rotational play. In order to minimize the chances of this happening while imaging, I cut a strip of craft foam about 3/4" wide and long enough to wrap around the lens barrel. Tightening this down with a Velcro strap, which I glued to the foam with contact cement, provided enough friction across the joint to keep it from twisting. The final assembly is shown below.

Imaging Deck - In an earlier project, I created an imaging deck for a stand-alone, guided DSLR. That version used a ball-mount for the camera, inadequate for the telephoto lens and heavy CCD camera I am trying to use now. In this project, I started by screwing a 4" Vixen dovetail bar to the tripod ring on the lens, as shown above. I mounted an ADM Vixen saddle to the end of a 7" Losmandy dovetail bar. I had to drill and thread two extra 1/4"-20 holes in the saddle for this. I also use a scrap piece of 1/4" thick plastic as a spacer to elevate the saddle enough to clear the large silver knob on the Losmandy-style saddle that came stock on my GM-8. You can see this in the image below. Previously, I had used the ADM dual Vixen/Losmandy saddle with my mount and "machined" a DEC axis spacer-puck to allow it to clear the RA motor assembly. However, after I upgraded the worm gears to the OPWB kit and installed the new motor mounts, the saddle no longer cleared the motor housings. As you can see in the photos, I have kept the silver puck but reverted back to the stock saddle. With this configuration, my new imaging deck clears easily. In fact, I am able to safely slew the RA axis over 45 degrees past the meridian on either side of the mount, effectively ending the need to do meridian flips half way through an imaging series.

Balancing - Without the additional weight of a telescope, the GM-8 is not able to balance correctly with the 7 lb counterweight fitted to the shaft, even if positioned as far up as possible. In the photos, you can see that I have added three 1 pound weights to the imaging deck. These are end-shaft counterweights for an Orion Atlas. I believe they have 6mm threaded posts. I bolted these from below with the posts facing up. On the top-most two weights, I removed the posts and re-threaded the upper half of the hole to 1/4"-20. I then attached a pair of Orion/Synta finder shoes (Scopestuff #RDPQ), each with a single centered 1/4"-20 bolt. I mounted a red-dot finder on the inner shoe and an Orion Magnificent Mini AutoGuider on the outer shoe. Use of these centered bolts makes it easy to adjust the pointing direction of the finder and guider.

Cabling - The camera and guider require a total of four cables, plus an additional two if I add the dew heater straps. To keep the cables under control, I enclosed these in a 1/2" Nylon expandable braided sleeve from the local electronics store. Strapping this to the Vixen saddle knob with a Velcro strap and pointing it off to the right side ensured that it did not snag on anything as I slewed across the sky. This cable bundle is shown in the photo below. It works both with the camera rotated on its side, as shown, or with the cable ports pointing up. I am not able to rotate the camera with the ports facing down with this system.

Communications - The cable bundle is routed down and strapped to the RA/DEC cable bundle as shown below. The two USB cables are connected to a 4-port powered hub. The RJ-25 cable from the auto-guider plugs directly into the Gemini board. The AC power adapter for the QSI is strapped to the tripod leg below the hub. In addition to the two camera connections, an FTDI USB-Serial adapter is plugged into the hub to connect the laptop to Gemini control board. As well, a TEMPerHum PC temperature and humidity sensor is plugged into the hub allowing me to monitor ambient temperature and dew-point to get an idea of when I might need to refocus.

Computer and Software - My Windows 7 laptop rests on a ledge made from a large kitchen cutting board and attached to the tripod leg of the GM-8 mount. I run the following software during and after imaging sessions:
  • PHD Guider - Auto-guiding with the Orion Starshoot.
  • Astro Photography Tool - Temperature monitoring, camera focusing and control, image framing, image acquisition.
  • SkyTools 3 - Session planning, star atlas, target selection, logging.
  • ASCOM Gemini Driver and POTH Hub - Simultaneous connection of APT and SkyTools to the GM-8.
  • DeepSkyStacker - Calibrating and stacking images.
  • Photoshop CS5 - Post-processing

Imaging Result - The image below of the Butterfly Nebula and Crescent Nebula was composed of two 30 minute exposures taken through an Astronomiks 12nm H-alpha filter. These were calibrated with three dark and bias frames. I am pleased with the results. The Canon EF 70-200mm f/4L lens at the long focal length produces great stars across the field of view. As expected, the stars are somewhat under-sampled, even at 200mm.

I am not seeing the pinched shape on bright stars which I have observed before with this lens wide open. I wonder if the effect is not with the lens but with the DSLR. I did see a pronounced reflection halo around the bright star Sadr. Below is a enlarged view of the effect. On the left is an enlargement of Sadr from the picture above. On the right is an enlargement of Alniyat from the image of Rho Ophiuci in the previous post. I have adjusted the relative sizes to show equivalent image-scales. The DSLR at 4.3um has much finer pitch than the QSI at 7.4um. In the QSI image, the center of the frame is to the upper right. In the DSRL image, the center of the frame is slightly down and to the right. The dark pinched region is always oriented circumferentially.

Focusing and Stopping - With this CCD adapter, it is not possible to close the iris of an EF lens as this requires the electronics in the DSLR camera. Until I rig some sort of external stop, I have to image with the lens wide open. Fortunately, this particular zoom lens has good wide open performance, except for this pinching effect on the brightest stars.

The lack of electronic focus control is a much bigger issue. Like most EF lenses, manually focusing on stars is feels more like squeezing the focusing ring than twisting it. Nonetheless, using a Bahtinov mask and focusing on Sadr, I was able to get the diffraction pattern centered within a few minutes. Using 3 second exposures on a small region-of-interest, a feature recently added to Astro Photography Tool, made this task tractable.

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