Need hlep with weird star shape

Hi.

I had this many times before on my C8 Edge and it was fixed with collimation (I had some other issues with mine, such as a rotated corrector, but even before fixing that, collimation fixed these misshapen stars). Even a little bit out can give the sorts of results you are seeing.

I also have an ASI2600MM camera. I’d be checking collimation before anything else, such as tilt.

Others may have different/better input, but what worked for me is the following.

• Pick a bright star (e.g. Sirius)

• Use a Bahtinov mask to focus as best you can manually.

• Use a tri-Bahtinov mask to do the collimation. I have found this to be far easier and more accurate than the donought method.

A very handy piece of software to pair with the tri-Bahtinov (which you can also use for the donought method) is the SkyCal Focus and Collimation Tool, which is discussed in the following link, and where you can also find links to download: https://www.cloudynights.com/forums/topic/987005-skycal-focus-and-collimation-software-v40-release/

It is a brilliant tool and makes what is often a very frustrating process so much easier (and more accurate).

Also, collimating with a screw driver in the dark is a real nuisance (and risks scratching your corrector if you drop it). Replace them with some knobs. There are Bob’s knobs, but I found the perfect thing more readily (and cheaply) at my local hardware store.

I hope that is useful.

Cheers.

Paul

P.S. I think I’d be doing this without the Barlow first so that you are looking at an image through the scopes optics alone, without anything else interfering. Barlow can come later.

Tony Gondola · Jan 27, 2026, 04:14 PM

I would re-collimate as perfectly as you can without the reducer or barlow in place. Keep the donut centered and progressively get closer and closer to focus, adjusting all the way in. Once done you’ll know that isn’t the issue

Thanks Tony. Will give another attempt to collimating it. There is always a fear or wariness of something new going wrong in this hobby! Only last week I self-serviced the EAF to disassemble it - realized there was a piece of dirt sticking to the gears. It is now fixed and functional. I havent done collimation of the C8 ever yet, and when I had received it stock, I was glad to see ‘decent’ donut. Looks like it is time to learn again!

What Paul said….

This is one of my favorite sources for reworking an SCT from the bottom up: Aligning not just the secondary, but also the baffle tube and primary, and making sure that the secondary mirror and corrector plate are properly centered over the primary.

https://www.wilmslowastro.com/tips/c14_optics_alignment.html

I’d be careful approaching this if you don’t have some degree of mechanical savoir faire, and not attempt it at all if aligning the secondary gives you satisfactory results. A lot of the older used SCTs made pre-2000 or so were really slapped together in a helter skelter manner. The Meade that I bought had the corrector jammed all the way to one side of the tube and the secondary glued close to one edge of it’s mount to compensate for the fact that the baffle tube (and thus the focuser) was several degrees out of alignment with the optical tube.