Why are my C8 images so bad?

[mldee (Mike)]

I cannot get an acceptably "tight" image using my C8. Last night I thought it might be due to the 6.3 FR, so tonight tried it without, still using Eta C as the target, as it has a good selection of stars in the area as well.

Still seeing same distorted stars pointing in various directions, so it's not the FR. I rechecked the collimation on Canopus, and a defocused star donut was quite symmetrical each side of focus. I am inexperienced in close up "airy disk" collimation. I noticed with bright Canopus that as I focused it, there was a large halo around it, and a bigger one further out. This is using Neb. Is this the infamous airy disk or just some sort of halo? The reason I ask is that the focused star was not centred on the halo, but about 10% or so offset. This was just after my earlier collimation effort. Of course I tried to 'diddle it' and achieved nothing, as Canopus decided to not cooperate by going behind a tree. No other bright stars nearby, and it got cold. I gave up, so here I am.

My mount with Newt gives tight stars, so at this stage the EQ6 is not the suspect, and I still get the eggies with guiding turned off. Eta exposures are only 2 mins anyway.

It seems to do with collimation or focussing of the C8. Is it that crtical?
All suggestions welcomed.

[riklaunim]

Some miscollimation may cause shadow images on targets, and unfocused can make it much bigger.

[mithrandir (Andrew)]

Originally Posted by mldee

... I noticed with bright Canopus that as I focused it, there was a large halo around it, and a bigger one further out. This is using Neb. Is this the infamous airy disk or just some sort of halo? The reason I ask is that the focused star was not centred on the halo, but about 10% or so offset.

You sure it isn't a reflection off the CCD chip onto the last glass element and back onto the chip? If the glass and CCD are not both perfectly square to the light path you'll get an offset halo like that on bright stars.

[bmitchell82 (Brendan)]

mmmm Use a Defocused star for a start, you need to make sure that your Secondary is pointing directly at the middle of your focuser. without question. Then collimate the Primary back to the secondary. Don't use a really bright star as you can get reflections/distortions ect ect. go for a 3 mag star and use your camera to do the star test.

Center the defocused star in your FOV anywhere else is worthless for this task. now start tweeking move it till it is absolutely centered to the best of your eyes ability, being sure to adjust, then recenter, adjust, recenter.

then start bringing in the focus while the camera is taking shots. you will see the airy diffraction rings appear, and a decent exposure, eg 2-5 seconds should make it very apparent and easy to see.

Im not 100% positive, but make sure you are aware of Mirror flop, as SCT generally have 1 direction they are collimated to, eg, on the Infocus or Outfocus. I have collimated a RC and Cassigrain scope in this fashion and produced excellent results.

Hope that helps mike

Brendan

[Kal (Andrew)]

Originally Posted by mldee

Still seeing same distorted stars pointing in various directions, so it's not the FR.

By various directions do you mean distorted away from the centre? If so it would be field curvature.

I noticed with bright Canopus that as I focused it, there was a large halo around it, and a bigger one further out.

This sounds like a reflection, perhaps off the FR you were using at the time.