Awesome advice, thanks a heap!
Quote:
Originally Posted by mental4astro
Nial,
To my eye there are two things happening here:
1, your collimation is not quite tight enough. The image of Achernar is not symmetrical. However you do your collimation, be it with just a Cheshire eyepiece or auto collimator or Cheshire + laser (a laser alone will not do it all), you still need to do a final confirmation by star testing. You start with a large doughnut to see if there is any gross mis-collimation first. Then you do the final check with a small doughnut - this is because with a large one, if there is a wee mis-alignment still present, the error will be spread out and hence evened. This goes for not just Newts but all reflectors (SCT's, Maks, etc).
2, the primary mirror is being pinched in its cell. I recognise this by the uneven aberration that appears in the top right corner of the third and fifth photos. The little clips that hold the primary in place are wound in too tight, and this is enough to distort the figure of that big lump of glass. And it takes surprisingly little force to distort the figure. Remember, we are talking about wavelengths of light. The uneven flaring of the star in second photo is another tell-tale-sign of a pinched primary. You will need to remove the cell from the OTA and loosen the clips, then refasten them but to only finger firm pressure. Keep note of how tight those clips are when you loosen them. If there is significant force needed to loosen them then you know they were over tight. As the mirrors are shipped in place in their cells, all too often these clips are over tightened by the factory monkeys...
This problem of overtightened optic retainers is also very common with SCTs! The corrector plate retention ring is all too often WAY over tightened in factory (as happened to me) or unwittingly by owners who removed the corrector and then replaced it. The corrector is very thin, and it really does not take very much pressure from the screws holding the retention ring in place to distort the glass. And then you will end up with a wretched time trying to get a good image at any magnification over 150X. This can happen with Maks too even with their thicker correctors.
Alex.
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