Astrophotography is becoming more accessible as equipment choices increase while prices drop. Advances in camera technology have led the way. In the early 2000s, a mono CCD camera and filter wheel set was out of reach for many. Now, imagers have a broad selection of mono cameras to choose from, some costing no more than a low-end DSLR camera.
More cameras being purchased means more demand for filter wheels. I tested the Pegasus Astro Indigo 2-inch Filter Wheel and report on the results here.
The Indigo arrived in a compact box with a number of adapters, screws, and a USB 2.0 cable. The design and construction are well executed in all metal with some CNC parts. The anodized aluminum components keep the weight reasonable at about 1000 grams (2.2lbs), including adapters. There isn’t any machining away of unnecessary material to further reduce the weight, though, as some other brands do.
The outside casing is smooth, with minimal protrusions that might snag cables.
Filter Size Compatibility
At the time of writing, Pegasus only offered an Indigo version with seven positions for either 2-inch threaded filters or 50mm round unmounted filters.
That’s good for full frame sensor users, with round 50mm unmounted filters providing more clear aperture than 2-inch filters. Full frame sensor users with fast optical systems below f/4.0 might prefer 50x50mm square filters, though, and they aren’t compatible with this filter wheel.
For small sensors like APS-C, I recommend 36mm filters for most imaging setups. My tests with 36mm filters and the APS-C QHYCCD 268m showed that larger filters weren’t necessary, so larger filters and filter wheels add unnecessary expense and weight.
The filter wheel is relatively slim at 19mm thick, with mounting adapters adding 0.6mm, which is helpful when back focus is limited. The wheel is quite large, though, with an outer diameter of 234mm (9.3 inches), substantially larger than similar filter wheels.
Adapters and Connections
Pegasus has an interesting system for attaching cameras and telescopes. The threaded adapters are inserted under the casing, and the attachment points are slotted so the orientation can be adjusted somewhat.
This could prevent having the filter wheel in an awkward orientation, as sometimes happens with traditional threaded connections. It’s also useful for aligning an off-axis guider to match the angle of the filter wheel and camera.
There are a number of adapters with M54 and M48 threads provided, along with two spacers.
QHYCCD Camera Connection
Cameras with the popular 62mm, six-hole pattern, such as those from QHYCCD, can be bolted directly to the filter wheel.
Unlike QHYCCD filter wheels with a built-in camera-side recess, however, the camera side of the Indigo is flat, so QHYCCD cameras sit 3mm farther from the filters, using some back focus. The stepped lip on QHYCCD cameras also ensures there are no light leaks on the QHYCCD camera when attached within the recess of the QHYCCD filter wheel.
The downside of these bolted connections is that the internal carousel must be disassembled to gain access to the camera mounting screws.
Installing 2-inch threaded filters into the Indigo is straightforward, with female threaded slots on the filter wheel accepting the male filter threads.
50mm round unmounted filters are held in place with three small screws and soft washers. This is not my preferred method as the outer edges of the glass are left uncovered, potentially allowing stray light or reflections. Most importantly, overtightening the mounting screws could crack the filters, even with the felt washers under them.
The included screws are chrome-plated and the washers are bright red. I prefer having everything close to the light path as dark as possible.
I recommend adding black 3D printed masks. These aren’t provided with the Indigo but can be made or purchased from third-party vendors.
The drive mechanism can be accessed for cleaning or maintenance by opening the filter wheel cover.
Pegasus uses a stepper motor with a friction drive gear that mates to the grooved outer edge of the wheel. There is no spring loading to provide pressure, but a slotted, sliding screw appears to make pressure adjustment possible.
The electronics board houses an infrared sensor for determining the wheel’s position.
Software & Connection
The filter wheel is conveniently powered from a USB 2.0 cable, with no 12V power.
Using the Indigo filter wheel requires installing the Pegasus Unity platform. While the platform works fine, it uses 280 MB of storage on my onboard telescope PC. I would have preferred a standalone ASCOM driver.
The Indigo filter wheel rotates either clockwise or counterclockwise, depending on the closest path to the target filter position. Indexing is good, repeatedly finding filter positions with a high level of accuracy.
A significant problem, though, was carousel positions being off-center. I tried a number of troubleshooting steps and ensured the firmware was up to date. The requested slot positions were off-center by about 1.4mm.
This isn’t a problem if you use a smaller APS-C sensor but with a more demanding full-frame sensor like the one used for this testing. With its 43mm chip combined with a fairly fast f/4.7 telescope the light cone is quite wide and requires perfect centering. The Indigo’s carousel positioning error resulted in uneven illumination as seen below. This image has been stretched to increase the contrast and make the vignetting easier to see.
Below are the results with my f/4.7 optical system and full frame camera using 50mm unmounted filters (after flat field calibration).
I contacted Pegasus Astro and they thought the problem might be fixed with a firmware update. After several weeks without a solution, though, I opted to return the filter wheel to the vendor before the refund period ended.
The Pegasus filter wheel may appeal to some. The slim design and flexibility of both camera and telescope connections are useful, and the single USB 2.0 connection is convenient.
The body’s diameter is quite large for these filter sizes, though, and the lack of smaller alternatives means it’s not ideal for smaller sized sensors (APS-C and smaller) that don’t really require 2-inch filters.
In my case, with a demanding f/4.7 and a full frame sensor, the filter wheel’s inability to center perfectly caused unacceptably uneven illumination in my images.
Camera and telescope connection options
Single USB 2.0 for connectively and power
Unable to perfectly center filters
Not available in other sizes
Large physical size