Quote:
Originally Posted by Jason D
Your collimation seems OK. You defocused the star little too much. When that happens, you will start viewing mechanical alignment. Proper star collimation is done at high mag with very few rings showing. If you see the spider vanes diffraction then Mechanical alignment will interfere with the reading. See attachment. I rotated one of the defocused photos by 180 degrees to undo what happens when your cross the focal plane. As you can see, all photos line up. You have a fast scope with a large secondary mirror, hence, the secondary mirror offset will be more prominent on both sides of focus in opposite directions.
However, I do see some astigmatism as you have already noted.
Jason
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Thanks Jason for the advice and images, it never occurred to me to check the reverse. Yes collimation should be checked slightly out of focus. With my RC and SCT I usually only go a ring or two. I had not considered that going too far out would affect the reading on this type of Newtonian. I have learnt yet another thing about fast Newtonians.
It's good to know that the secondary location in the images are simply a product of the offset if you go too far out. I did not know if this would be the case. Your wealth of experience is invaluable and much appreciated.
Could the astigmatism be induced from the Wynne (maybe not likely given I have rotated the Wynne several times and get the same direction and shape or is this possible with a Wynne?) or perhaps not properly cooled optics? I only cooled these for 30 minutes prior to this test. Not enough but I wanted to check. I know it's not the primary as I have rotated it 45 degrees and get the same result. I did change the silicone on the secondary but perhaps ran it too thin when I reglued it.
Quote:
Originally Posted by Shiraz
agree with Jason, it could just be the normal offset mechanical alignment. However, it is also possible that the light column may be intercepting the tube on one side. If this is the case, the most likely cause is that the secondary is too far in/out. This is nigh on impossible to get right by eye with a fast Newt - you can collimate it perfectly, but the light column may still not be exactly central.
FWIW, I ended up using back projection to get that aspect right. The linked image https://astrob.in/full/367081/0/ shows how to use a small led torch that is taped up to fit in a 1.25 adapter on the scope - doesn't need to be exact, because it is being used with a divergent beam. A bit of translucent tape increases the size of the light source, which helps a little. With a sheet of baking paper over the input aperture, back projection from the torch shows where the light column has to go (in the reverse direction) to get to the detector. A few minutes effort will quickly show up if you actually have a secondary positioning problem.
To illustrate what can go wrong, panel 3 shows a badly adjusted scope, with the shadow of the spider showing some distortion, the secondary shadow being clearly skewed and the primary light column being offset from the centre of the OTA due to poor collimation (by a lot). It is pretty easy to work out how to adjust the secondary skew, straighten the spider and then collimate the system to get back to something like panel 4 where the secondary shadow is now circular with a reasonable looking offset and the spider is straight.
However, the in/out of the secondary is still wrong as the light from the primary is still not centered. This is not adjusted by moving the primary, since that will screw up the collimation. instead the secondary is moved in/out and the the scope is recollimated to see what effect the movement has. Eventually gets to the result of panel 5 where the primary light column is reasonably close to central - not quite prefect, but this is as good as I can do with my old scope.
The primary rule for using this technique is that the results of moving the secondary can only be assessed after the system has been recollimated. ie move secondary - collimate - assess - move secondary - collimate - assess...etc.. If you don't recollimate after movement, the results are meaningless.
Hope this helps - cheers ray
edit: don't put the scope on the floor as shown in the illustration images - the collimation can be upset by the weight on the primary adjusters if they stick out - do it on the mount
also, the inherent asymmetry of a fast Newt means that the star skirts are not quite symetrical - FWIW I find that it is best to take all colour data on one side of the meridian to minimise multi-colour star skirts at high stretch |
Thanks Ray for you help too. I cannot move the spider arms but I can certainly check for illumination, rotation and in and out. That is a really great way of testing. Next time I am down I'll give it a go. I think I have rotation very near perfect but in and out is as you say near impossible to check with the oversized secondary. Thanks once again.
Fortunately, so far I have not found too much of a problem with the star skirts. Nothing moves on this scope once it is locked down, and star shapes remain consistent.