Network Nebula (false color)

Hello folks,
Here is another narrowband shot of another part of the Veil nebula complex. NGC 6992, the Network nebula.

Using:
SII(5 x 15 min subs) for red,
Ha(5 x 15 min subs) for green,
OIII(4 x 15 min subs) for blue.

Still trying to eliminate those oblong stars. I have gotten the imaging train as stiff as possible. Now I wonder if I am having issues with the collimation of the refractors optics or the focuser. Next clear night (tonight I hope) I will try the same imaging setup on my Televue 85 and see if the stars are still oblong! If they are, then I know it is the mount and not the telescope.

Praise and criticism are welcome.

Attached Image:

You scope has coma and you also have some tracking/guiding issues. You cannot normally elimiate coma in a refractor by simply adjusting the collimation, i.e. the tilt of the lens cell. That's because refractors are designed with zero inherent coma, therefore adjusting the tilt of the lens cell will not allow you to introduce opposite coma to compensate like you can in a Newtonian. All that will likely happen is that you will introduce astigmatism when you tilt a refractor lens. The one exception is a Petzval, which must have the front lens accurately squared on, otherwise there is massive coma.

Coma in an airspaced objective arises either with poor homogeneity in one or more of the elements, or improper spacer thicknesses around the perimeter. Therfore, to really fix it requires very careful adjustment of the three spacers. Some refractor designs allow you to move the rear element sideways to cancel out the coma, but this actually adds a small amount of lateral color to the image.

Rolando

Enlarge the four corners of the picture, and put them in the same screen so you can compare the direction of drift. You will see, I think, that not all stars "drift" the same way. The coma points slightly differently in the four corners. Therefore, it is not just mount problems.

Alex