RedCat 71 Optical Design

The William Optics RedCat 71 is the bigger brother to the highly-regarded RedCat 51, which has been raved about by astrophotographers since its release. Building on the RedCat 51's success, the RedCat 71 keeps the same f/4.9 focal ratio with a Petzval optical design. This optical design uses four lens elements instead of the three found in triplet refractors. By incorporating this extra element, the RedCat 71 is able to achieve a flat field without the need for an additional field flattener.

RedCat 71 Image Circle & Camera Compatibility

The flat field image circle on the WO RedCat 71 is over 45mm in diameter, meaning it can provide round stars to the corners on a full frame camera, not something many telescopes can claim in this price range. The RedCat 71 telescope comes with a 48mm thread on the back, meaning you can easily attach any dedicated astronomy camera with a standard M48x0.75 connection such as ZWO. For DSLR and mirrorless users, simply add an M48 t-ring for your camera type and you'll be ready to do astrophotography right out of the box.

RedCat 71 Helical Focuser & Autofocus

The premium quality of Anodizing finish and calibrated helical focuser with 2GT gear ring is much more precise than those on typical camera lenses, and it is lockable, making it easy to nail and maintain focus. (2GT standard gear ring, 153 teeth, Diameter 98mm, width 8mm). Astrophotographers looking to add an autofocus solution to their RedCat 71 may be interested in the ProAstroGear "Big" Black-CAT Mount, which allows users to attach the ZWO EAF to the RedCat 71's helical focuser.

 

RedCat 71 Back Focus

The lens assembly can be moved within the telescope body over 50mm by rotating the focusing collar. The distance from the base of the M48 male thread on the Redcat 71 to the chip plane is 60mm with the lens assembly completely retracted. With the lens assembly fully extended, the chip-to-thread distance must at least be 60 - 50 = 10mm. In other words, the thread-to-chip distance can be anywhere between 10mm and 60mm to reach focus. This is easily achieved with DSLRs with standard M48 T-rings. Many dedicated astronomy cameras will also reach focus with a standard 2" M48 threaded connection or with accessories such as filter wheels.

 

 

RedCat 71 Specifications:

• Focal Length: 348mm
• Diameter: 71mm
• Aperture: f/4.9
• Optical Design: Petzval (4 element)
• Focuser: Helical Focuser with 2GT Gear Ring
• Image Circle: >43mm
• Tube Length (with dew shield): 435mm
• Dew Shield Circumference: 323.4mm
• Dew Shield Diameter: 103mm
• Rear Camera Side Threads: M48x0.75
• Weight:
  • 6.2lbs / 2.8kg (OTA only)
  • 8.2lbs / 3.7kg (OTA with ring, dovetail and saddle)

Schematics