William Optics GuideStar 61 APO guide scope, red (William Optics, SKU: M-GS61-RD). The GuideStar 61 is a refractor with a 61 mm ED objective and a 360 mm focal length, intended for guiding astrophotography setups. Its two-element objective uses FPL-53 glass. Coarse focusing is achieved by sliding the tube, while fine adjustment is made by rotating the objective assembly. It can also be used for imaging with a corrector of suitable optical length and a configuration that allows focus to be reached.
• doublet with ED FPL-53 glass
• focal length of 360 mm for guiding
• manual focusing by sliding the tube and rotating the objective
• body made from CNC-machined aluminium
• finder attachment for Vixen / Synta-style saddles
• optical tube William Optics GuideStar 61, red
• mounting ring machined by CNC with handle
• plate for a finder saddle
• adapter M54 / 2"
• soft carrying case
• optical design: ED apochromatic doublet
• glass in the ED element: FPL-53
• objective diameter: 61 mm
• focal length: 360 mm
• focal ratio: f/5.9
• tube length: 340 mm
• weight: 1.44 kg
• mounting plate length: 120 mm
• plate standard: Vixen / Synta finder type, not a full-size Vixen mount dovetail
• thread in the plate: 1/4"
• colour: red
• warranty: 24 months
The GuideStar 61 combines ED optics with a construction designed for attachment to a guiding setup.
The material below also includes a user’s TSred279 corrector test report; its results apply to that particular setup and are not manufacturer specifications.
User’s note on correctors
Author: M. Wu, University Erlangen–Nuremberg, Germany. This report concerns the tested unit and a particular configuration; it does not guarantee the same results with other setups. The catalogue suggested guiding telescopes of approximately 700–1200 mm focal length as an example use for the GS61, rather than an absolute compatibility limit. The catalogue also mentioned an optional FLAT61A corrector in a special low-profile configuration; the standard version should not be assumed to be compatible.
The author reported that the tested WO FLAT61R was too long for the GS61 to reach infinity focus; a shorter configuration was needed. The author then tested the TSred279 reducer/corrector, TS-Optics REFRACTOR 0.79× 2" ED Reducer Corrector, for approximately 4 hours over two nights between clouds.
In the tested setup, a spacing of 73.2 mm, including approximately 0.7 mm optical-path compensation for the 1.85 mm-thick L-eXtreme filter, allowed infinity focus. The author judged the result very good for Micro 4/3 and acceptable up to full frame. An ASI2600MC Pro with a filter drawer was used, and operation with an OAG was also checked. At 73.2 mm spacing, the front lens assembly was extended to approximately the 2 mm mark.
The example single exposure of the NGC 7000 region was 3 min long. It was automatically stretched in ASIAIR and saved as a JPG. During the test, the author obtained guiding better than 1 arcsecond.
Stars in all four corners showed symmetrical distortions described by the author as coma. This was interpreted as an absence of significant tilt; previously, a loose EvoGuide 50ED helical focuser had caused difficulties. Plate solving gave a focal length of 287 mm, corresponding to a reduction factor of 0.797×, slightly higher than the nominal 0.79×.
The author suggested that approximately 77 mm spacing might improve the corners, but this did not permit infinity focus. The remaining aberrations could also originate in the corrector design, and increasing spacing from 70 to 73 mm brought little improvement. The star shapes were not regarded as conclusive evidence of an incorrect working distance.
In the author’s weight comparison, the FLAT61R weighed approximately 500 g, or 325 g with its rotator removed, while the TSred279 weighed 195 g. This mattered when balancing the portable setup.
Additional examples: full frame, 55 mm spacing, no filter — shown as an unfavourable configuration. Automatic stretching exaggerates the vignetting; the author noted that flat-field calibration could address it.
Second additional example: full frame with 71.5 mm spacing.
