[Paul Haese]

Lee,

I have just been travelling down this same road. I have learnt that a good many things I took for granted in imaging are very different with an f4 Newtonian. Last night was the first night I had actually obtained round stars in every corner with nice tight focus.

This is what I did and it may help you with what you are going through.

I bought a Moonlite focuser and am using a GPU coma corrector (CC). I installed it thinking that a longer draw tube would be better. This is in fact a mistake. I found that having the draw tube extended too far created greater slop and made star shapes worse. I suggest using a 1.25" of 2" flange which goes on the bottom of the focusor. Doing this reduced star shape deformation remarkably.

Next I stiffened the secondary assembly. The secondary assembly on the GSO truss Newtonian is at present pretty average and not suitable for carry a good focuser and QSI683WSG-8. Spiezy was the first to put extra poles in the cage and I did likewise, though I think it needs diagonal bracing to reduce racking. Being a tube you are less likely to have this problem but you might have some hoop flex and stiffening via more support is a good thing as you have done.

Next I looked at every bolt and fixing on the scope and I mean every one of them. Tighten up everything, check your primary support, check your primary side slop (not an issue with a conical mirror on my scope but worth checking) and pack out if needed. Also check the secondary assembly itself. Make sure it does not have any movement in the stalk.

Now this I think is important and might pertain to your scope. Check the actual secondary fixing to the stalk. Mine was glued down with a very wide narrow smear of silastic. That caused astigmatism and I think I can see some in your image. The way to test this is to take an image intra and one extra from focus. If star shapes near focus on both side have an oval on both sides and each oval is 90 degrees to each other then you have some astigmatism. It's worth a check to eliminate this issue. I reglued mine down with 3 small blobs of aquarium grade silicone and supported the stalk with match sticks to keep them up high enough so as not to smear. This eliminated astigmatism.

As to collimation, I use the Glatter laser to check but essentially use the Cats eye collimation system for collimation. I cannot stress enough how finicky you have to be with this. The hotspot or triangle has to be perfectly inside the black cat white circle. Just a tiny bit out makes a huge difference. Then use the Infinity Cheshire very accurately to adjust the secondary. Then importantly repeat the process until you have both ends perfect. I finally use the laser to check both the centre spot alignment and use the Tublug to check the primary return. If every thing looks aligned you are now highly collimated. I found I had to be particularly fussy to get good collimation and again I think I can see a little collimation error in some of your star shapes.

Now check you CC back focus distance. CC's are particularly fussy about the correct back focus. Most on the market have a 55mm back focus but some vary a little to 50mm. The GPU allows some slack of about 1mm. The Baader one was a little more tolerant and allowed 5-6mm. Make sure you have that correct distance. This is probably one of the major problems you have here.

Screw thread every thing. Don't use compression rings and these can slightly miss align the CC and that can create tilt. Testing as suggested will tell you where the error lies. I bought adapters to screw thread and it made a huge difference.

Adjust your Moonlite for the correct lifting capacity. I did mine up too much and it caused flexing in the tube. Make sure it holds the weight under gravity but only just. That seems to work best. Though I still seem to have a tiny amount of slop in the focuser but I think that is part of how gravity is working against the focuser.

If your Moonlite has a rotator facility with the tilt adjuster, check that the tilt adjustment is actually adjusted correctly. Mine was way out and causing a focuser mis-alightment. To check this I put the laser into the focuser and very lightly clamped that down in the focuser and then rotated the entire focuser. The laser should not travel around in a circle; the laser should stay in the one spot.

Finally, and I think this is the main problem you have here; is tilt. Once you have the correct spacing of the CC and established you have tilt in the camera, then pack out with shims. I used the metal out of a coke can which is thin and it works well. This should go under the camera adapter behind the CC. It takes more than you might think.

I hope this has helped. It has taken me a while and I got some good advice from several people, which I have relayed to you. Best of luck.