Collimation issue or something else?

[E_ri_k (Erik)]

It was the best way I could describe them in that first image! I will have a look at the drawtube in the morning. Without the risk of sounding stupid, but is there a good way to check this during that day, other than looking for slop or sag in the focuser? Maybe take some flats and examine them?

Erik

[brian nordstrom (As avatar)]

If you have a clear blue sky , just point the scope to the zenith , with your longest focal length eyepiece , 32-50mm , what ever you have ( the longer the better ) and look at the blue image , it should be evenly blue right across the field of view ..
look carefully at the edges for a darker blue you have vignetting and if the darkness is uneven side to side you have something in the light path .. banana city makes me laugh .
Brian.

Originally Posted by E_ri_k

It was the best way I could describe them in that first image! I will have a look at the drawtube in the morning. Without the risk of sounding stupid, but is there a good way to check this during that day, other than looking for slop or sag in the focuser? Maybe take some flats and examine them?

Erik

[E_ri_k (Erik)]

You'll never believe it, it cleared up for half an hour......

So here are 3 images of the unfocused star. I am guessing collimation is out by that centre spot. It shows up ALOT more clearly when imaged rather than visually.

Another interesting thing. I took a unguided 30 second exposure into a random patch of sky, WITHOUT the OAG in place. Round stars except for that top left corner where they are only slightly oval. Pretty happy with that image, not as bad as the other night which was a guided image. Maybe there was a guiding issue which exaggerated something to make that corner look really bad??

Erik

[Barrykgerdes]

Hi Erik

The first picture looks reasonable. The diffraction spikes on that star show you are close to it but you will need to do a full star test collimation to look at the diffraction rings at high magnification. Then tackle the other problems. The vignetting could be due to the long path through the OAG and focuser.

Trevor had a similar trouble with elongation in a part of the photo and it was found to be a slightly tilted focal focal plane as I said earlier.

Barry

[E_ri_k (Erik)]

So I got Al's collimation aid and used it on one of my images. That centre spot is obviously off centre. Is this going to be a secondary adjustment or primary?

I thought if I adjust the secondary it will mess up the "Doughnut" shape? Or am I wrong?

Erik

[Barrykgerdes]

Hi Erik

That is only the first step in collimation. The real part is done when you make fine adjustments to get a nice symetrical pattern of airie rings.

I have attached to zip files containing Thierry Le Gaults directions for collimation. I think this is the best procedure I have seen.

The originals are available from his web site.

Barry