I have a VC200L, which is an RC with a built in field flattener. Collimation can be a long and tedious task. But I have never seen an oval star test pattern.
Look into the tube from the front. You will see the secondary reflected in the primary. Putting your head in the same position left, right, above and below the view should look the same. If it looks significantly different, then the primary is off and not aiming at the center of the secondary.
Is your secondary tight. If it is not snug, it can move as the scope cools and the metal shrinks. This happened to me.
Is the secondary centered? Using the Cheshire, you should be able to look through it at the secondary. It should be centered and you should be able to see your eye in the middle. Many scopes use cast aluminum for the secondary spider, so there is no centering adjustment possible. Because the secondary is on a stalk, it will appear off center if it is out of collimation.
It could also be that the focuser is way off and not pointing at the secondary. Has the scope been dropped?
As you can see, all the components are linked. For the above, you are looking for rough alignment. Even a roughly collimated scope should show a round star test pattern but the central shadow will be well off center.
If mechanically everything is centered and nothing is bent or broken, try bringing the star through focus. Does your oval star test pattern change from vertical to horizontal as you pass through focus? If the scope is collimated and this happens, then you have astigmatism.
Does this happen visually, or only with the camera? With a diagonal? With different eyepieces?
I'm thinking that something is seriously out of alignment and light is being cut off. See if you can identify which, if any, component is out of alignment. Don't be afraid to mess with the collimation. It takes time and patience. Let us know what you find out.
Edited by Paul Sweeney, 31 October 2021 - 02:58 PM.