Does the William Optics Cat 91 require a separate field flattener?
No, it does not. The William Optics Cat 91 uses a 5-element Petzval optical design, which has a flattener built directly into the optical train. This provides a flat field across its entire 55mm image circle without any additional accessories.
What is the correct back spacing for my camera with the Cat 91?
The required back spacing for the Cat 91 depends on the adapter you use. With the included M48 adapter, the required distance from the adapter's threads to your camera sensor is 60.8mm. With the M54 adapter, the required distance is 65.8mm.
How does the Cat 91's f/4.9 ratio affect imaging the Andromeda Galaxy (M31)?
The fast f/4.9 focal ratio allows the Cat 91 to collect light very efficiently. When imaging a large, relatively bright target like the Andromeda Galaxy (M31), this means you can capture significant detail in its spiral arms and faint dust lanes with shorter individual sub-exposures compared to a slower f/6 or f/7 refractor.
Is the William Optics Cat 91 compatible with my ZWO EAF?
Yes, the WIFD (William Optics Internal Focus Design) focuser on the Cat 91 is fully compatible with ZWO EAF and other popular electronic autofocuser systems. This allows for precise, automated focusing for astrophotography.
Can I use a full-frame DSLR camera with the Cat 91?
Absolutely. The Cat 91's 55mm image circle is designed to fully illuminate full-frame sensors (typically 43mm diagonal) with no vignetting. You will need the appropriate T-mount adapter for your camera brand (e.g., Canon EF, Nikon F), which connects to the included M48 or M54 camera adapters.
What kind of mount is needed for the 4.72 kg Cat 91 OTA?
With an OTA weight of 4.72 kg (10.41 lbs), and a total weight with accessories around 6.3 kg, the Cat 91 works well on mid-range equatorial mounts. For imaging, a mount with a payload capacity of at least 25-30 lbs (11-14 kg) is recommended to ensure stability once you add a guide scope, cameras, and other accessories.
