Quote:
Originally Posted by stephend
This would explain why stars in my scope mostly display tails, which no amount of collimation, even with a laser collimator, will fix.
Has anyone any experience of this, and for the future, when I build my 24" reflector, what focal length should I be grinding to avoid this?
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Most of what you are seeing is astigmatism with some coma and field curvature thrown in. The stars look like open umbrellas with their handles pointing to th edge of the field.
If you use a Nagler eypiece it will flatten the field and correct the astigmatism. then you actually see the coma, which you'll see is a much more minor aberration. If you team up a Televue Paracor with a Televue Nagler eyepiece you'll generally excellent tight round star images to the edge of the field even down to F 3.5 focal ratio.
if you are planning a 24" scope the cost of a Paracor is minor. You would be best to make it around F4 to F4.5 , which keeps the wind profile down and try and minimise the number of steps to climb. It is possible these days to get mirrors down to F3.7 these days.
If you have not ground a mirror and you are planning to make a good 24" then you'd better get cracking , you have a long journey ahead of you
