Tracking down my tilt.....arrgghhh

Imme (Jon) · #1 17-05-2018, 02:19 PM

OK guru's......gimme some love here people.

I've been chasing a tilt issue for months.

First up - the rig.

GSO F5 reflector
GSO coma corrector (at 78mm spacing which I believe is in the perfect window)
ASI 1600 OSC -15C
EQ6 guided
ZWO tilt adjuster installed (but moving it anything from flat cause more issues...remove it completely and the issue remains....so take it out the equation)

What I know -

Guiding is good
Regardless of sub length I get elongated stars so not a guiding issue
Colimation is as good as I can get it using a laser collimation tool which was then followed up with a star test with artificial star at approx. 50m distance.
Tilt across the image is the same
Appears to me that the image is in the same focus across the image so sensor tilt out?
I doubt it is a scope issue as my old reflector (at F4) did the same thing....although it was a little worse on that scope (longer trailing)
Don't think it's the camera either......my old camera did the same
Coma corrector.....nope, don't think so.....I've tried some shots with it not in the image train and I still had trailing

The second pic is a 1600x zoom of a star.....notice the chromatic spectrum displayed Red, Orange, Yellow, Green , Blue, Indigo, Violet......same order as a rainbow.........to me that says it is some form of optical/focus/splayed spectrum type issue that causes the spectrum to stretch. I could work with this if I was using a refractor, they have lens that do strange things.......but this is a reflector!

So.........throw me some ideas please everyone.

What do you think I should be checking next to try and fix this?

The images below are from a 1 hour stack....but it doesn't matter, a single image would basically be the same...at 1 second or 5 minutes exposure length!

raymo · #2 17-05-2018, 04:29 PM

O.K., back to basics. Process of elimination; Eliminate the whole imaging
train. Pop a high power EP into the focuser, find a reasonably bright star,
and move it around the the field of view, defocussed a little if necessary, and if it starts out round and remains round, and the amount of coma is the same in all directions, then the focuser is perpendicular to the tube. If the star is elongated, you will know where you have to shim the focuser by the direction of elongation. When the focuser is deemed to be fine, add some weight to the procedure by using your camera [without the coma corrector], and with the R.A. drive running take some snaps, say 10 secs, and if the stars are round, then the focuser's draw tube is not sagging enough to cause a problem. If the stars are not round, then either the draw tube is sagging, or the camera is not perpendicular to the focuser.[as can happen with a worn or very cheap T ring when using a DSLR]. If the
stars are round, put the coma corrector back in place and snap again; if
they are now elongated again, you know the coma corrector is not perpendicular to the light path.
The chromatic effects are coming from the optics used to magnify the star, not the scope. My 24mm Koenig was unusable for projection
use because of its awful CA.
raymo

Imme (Jon) · #3 17-05-2018, 04:52 PM

Cheers Raymo.....excellent advice.
I’ve done an artificial star test at 200x mag and it appeared ok to me so I will start from basics and add bit by bit to the setup until I experience the elongated stars again.
It’s great advice and obviosuly a very intelligent way of doing things which I should have thought of myself......guess I couldnt see the forest through the trees as they say