RC8 collimation or focus problem?

[BeardFace]

Hey all,

So I just tested my new (second hand) RC8 out for the first time last night and the stars are looking a little wonky:

http://www.iceinspace.com.au/forum/attachment.php?attachmentid=260459&stc=1&d=1591702935

So I'm wondering if this is a collimation issue or a focus issue? I got the focus as close as I could. As for collimation the secondary mirror and the focus tube are both in line, the only thing I think might be out is the primary?

Any help would be greatly appreciated.

Cheers

[multiweb]

That still looks like it's out of focus and also a tad collimation but not much.

[luka]

It could also be field curvature.
What is the sensor size in your camera? Can you post the whole image?

[doppler]

Hi Nial, be careful when mucking around with your RC collimation, they are very touchy, very small adjustments only required.
Have a look at this guide it might help you, Have a read of part 7 this might explain your wonky stars. https://www.dropbox.com/sh/1349fp34gqpbalm/AADPeMNonKZNk-1AkKUZ1dL4a?dl=0
Rick

[BeardFace]

Thanks for the help all. My camera is a Canon EOS 5D mk3, so 36 × 24 mm full-frame cmos sensor. I certainly haven't touched the primary mirror yet and I hope I don't have to :shrug:

Here's the full image: https://drive.google.com/open?id=1C25n2G5_QaQckXfYm2BNbsGejZ PxGf-g

This was ISO 800 and 2 minute sub

[doppler]

Just had a quick look at the full sized image, focus is out a bit, note the double spikes on the lower bright stars. The stars to the left of the image look worse but wont be as bad with better focus. There might be a bit of tilt on your camera, possibly the camera is not sitting square in the focuser tube, but it could still be a collimation thing. I have an RC6" but have only ever had to tweak the secondary a very slight bit. Work on the focus and go from there.
Rick

[BeardFace]

Awesome, thanks a heap. I think you're right about the camera not sitting straight, the focuser doesn't seem like the best. I'll give it a shot a bit later :)

[multiweb]

Your on-axis stars are still out of focus which makes the edges only worse. Regardless of tilt I still think your collimation is slightly out. In order I'd do:

1_ collimate on a star in the center of the field.
2_ focus accurately on a star on the center of the field (use a bahtinov mask)
3_ check any remaining tilt if any
4_ sort out any field curvature left if any.

[BeardFace]

Funny you should mention that, I was *just* looking up bahtinov masks. Thanks for all the tips :)

[luka]

Without a flattener an RC8 with a full frame sensor will show strong effects of field curvature in the corners of the images. A good example showing this is here (https://www.reddit.com/r/astrophotography/comments/3qxxtg/gso_8rc_with_and_without_field_flat tener_example/). With a tilt the things get even more complicated.

Unless you have proper tools (Tak scope) I would not touch the collimation until you can get rid of the tilt and get the focus right.

Pick a bright star in the middle of the image and adjust focus until the double diffraction pattern is gone.
Take an image of a bright globular cluster (Omega Cen is a good one at this time of the year). You should be able to see lots of stars all the way to the end of the image which can help diagnose the tilt.