Reverse vignetting?

[metastable]

A lengthy dew shield might help.

[Larry 1969]

A lengthy dew shield might help.

My focuser sits 6 inches inside the tube and I have a dew shield that extends 8 inches past that. You think I need more?

Larry

[metastable]

It depends… does any light from the moon (or other sources) illuminate the area of the main tube within the black cone of the diagram when you are using the current dew shield?

[Larry 1969]

You're referring to this area, correct? I just want to be sure I understand.
I'll have to check next time I set up and that doesn't look like anytime soon...

Thanks!

Larry

[yobbo89]

you can get an lop sided vignetting by having the secondary not properly alighned to your focuser ie depth dimension /the center bolt adjustment .

[Larry 1969]

you can get an lop sided vignetting by having the secondary not properly alighned to your focuser ie depth dimension /the center bolt adjustment .

I bought a tool for that. It's basically a Cheshire eyepiece with a clear filter attached that has concentric circles. Maybe I misused it???

[yobbo89]

you can get an lop sided vignetting by having the secondary not properly alighned to your focuser ie depth dimension /the center bolt adjustment .

I bought a tool for that. It's basically a Cheshire eyepiece with a clear filter attached that has concentric circles. Maybe I misused it???

if you have any error with the light cone illuminating center on the cmos/ccd then it should show up with a flat frame aswell, given that that the flat frame is controlled with something like a light box and not the sky and sun with a shirt ectt . generally a cheshire works pretty acurate on getting the depth adjustment for the secondary in the correct position ,

i rack my focuser out somewhere near to the same focus postion as my camera and then look through the cheshire , you might want to try different focus position just to generaly get the fov a good fit around the secondary mirror edges.

if it isn't comming out in your flats then really the problem is most likely not with the scope but an outside source ,moon ectt, street light .

[SkyHiker]

I had some bad gradients with my DSLR when imaging the horsehead no matter how I rotated it. Eventually I switched to an astro camera and the problem is gone. I attribute that one to Alnitak hitting a shiny part of the camera.

I also had reverse vignetting when I started using an astro camera using just flats and no biases. The DSS forum straightened me out that I really need biases in order for the stacking math to work. This never was a problem with my DSLR. I suppose this is not your issue since it happens with uncalibrated lights as well.

So my best guess is some sort of reflection. I hope you find the cause.

[Larry 1969]

Thanks for all the replies and suggestions. I get so few clear nights to test anything out. I was able to get out for a few hours this past weekend and I spent it on NGC 1333. It was low in the east and I think there was some terrible sky glow in that area so the image is crap, but I took new flats at 32,000 ADU and the vignetting seems solved. I couldn't deal with the noise in this but I didn't notice any vignetting to speak of.

Larry

[yobbo89]

Thanks for all the replies and suggestions. I get so few clear nights to test anything out. I was able to get out for a few hours this past weekend and I spent it on NGC 1333. It was low in the east and I think there was some terrible sky glow in that area so the image is crap, but I took new flats at 32,000 ADU and the vignetting seems solved. I couldn't deal with the noise in this but I didn't notice any vignetting to speak of.

Larry

it's actualy some pretty reasonable detail and sharpness in the center , it looks like longer exposer time will seatle out the noise in the outer regions,but you do face skyglow comming in doing so ..