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Old 16-02-2019, 08:56 PM
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Shiraz (Ray)
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Shiraz is offline
Join Date: Apr 2010
Location: ardrossan south australia
Posts: 4,784
Star test Skywatcher 250 f4 with RCC1 coma corrector


Decided to go back to square one with my CF Skywatcher + RCC1 coma corrector to make sure that it really was working as well as possible.

Found that it was not being properly collimated using just a laser - the laser (aimed through the CC) appears to do well enough to get the CC axis aligned with the centre of the primary and to get the primary roughly aligned, but a final star test is still required to harmonise the axes (and offsets?) of the main mirror and the CC. The process is a bit tedious, since the star test only applies near the centre of the fofv - so it requires: 1. move a primary adjuster, 2. recentre star and see what effect the primary movement had on the concentricity of the star structure, 3+. successively readjust primary and recentre star to converge on the most concentric out-of-focus star images on each side of focus (about 0.7 mm offset each side is OK for testing at f4). If the alignment is a bit off, out-of-focus stars across the field appear eccentric - presumably introduced by the corrector.

The attached images show star tests at 10 and 5 waves of defocus on either side of focus, with the system adjusted as well as I could get it (actual images on the bottom row). From Harold Suiter's book on star testing, these images correspond reasonably well to a 30% obstructed system with 1/4 wave SA. Cor Berrevoet's beaut "Aberrator" package was used to generate the ideal images (1/4 wave SA) for comparison with the measured star shapes. To my eye, the general agreement is good enough for the turbulent measuring conditions - this is more than acceptable performance for an astrograph. The star test is pointless off the centre of the field, but the properly collimated RCC1 + Skywatcher system seems to do OK at getting round stars over the medium-sized 1600 chip, so hopefully the optical system is (now) working well enough for seeing-limited galaxy imaging (at least with my seeing conditions).

In summary, the revisit was worth the effort. My previous collimation technique was not as good as I thought and the system requires primary mirror collimation using star testing to get the best possible alignment of the mirror and coma corrector. The RCC1 does not damage the central resolution - the properly aligned setup is ~diffraction limited in the field centre. Hope this info is of some use to anyone contemplating such a system.

Cheers Ray
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Last edited by Shiraz; 18-02-2019 at 08:59 AM.
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