Telescope Reviews: Help with late model C8se

The reason that Celestron used to touch of the seconary mirrors was to null out astigmatism.

I think most likely your scope just needs fine collimation, but lets pretend that this is not the case.

I can't comment on whether they do this any longer and it isn't relevent to your post (whether they do or not), but that is why they used to do it.

If there is a bit of astigmatism in the primary, rather than take it out by polishing, it was much easier for a skilled optical worker to simply introduce a bit of opposite sign astigmatism in the secondary to cancel out the astigmatism in the primary.

What this means is that if the secondary was retouched, then for it to work properly, it has to be in the correct alighment to the primary.

There is such good news for you though. If the secondary was re-touched to balance out astigmatism, the effect of a secondary that is not aligned will be that there will be astigmatism in the image and this is very easy to check for.

I would advise the use of an artificial star for this test because seeing is rarely good enough to test unless the astigmatism is quite pronounced.

You can start with a real star tough.

Rack your mirror out of focus by turning the knob clockwise a few turns.

When you see the Fresnel pattern, start turning the knob slowly in. As the rings collapse, you are watching that they remain perfectly round, especiallly just before the bright ring collapses into the Airy Disk.

Perfectly Round.

As you continue to turn counterclocwise though best focus, you should see the rings emarge perfectly round. You want to see this as close to the point that you emerge from best focus as possible.

If the rings change to either ellipictal or a cross shape, then there is astigmatism.

This is only good for rough testing.

For fine testing, it is best to use an in-focus star (artificial pin-hole would be best for this). What you are looking for is that when a star (or your pinhole source)
is at best focus, the ring is evenly illuminated.

If it has "Nodes" where there are bright and dim parts in the ring, this indicates that there is some kind of astigmations. NOt if the ring is like a "C". That is collimation. More like an even ring with one part bit brighter or dimmer. This may indicate a rotational error.

I think your problem is fine collimation though. Using the defcused image to collimate only gives you a rough collimation. To get really good collimation, you should use an in-focus star, or if seeing is not so good, use an artifical source.

Not saying that you may not have astigmatism, but I think fine collimation may be more the issue.

But when you do fine colliation, you will see if there is any astigmatism there, so it is the best place to start.