Uneven illumination

[johnnyt123]

Hi everyone.

I took this image of M83 tonight with a new GSO RC8 i recently purchased.

I was a little disappointed when it came to processing.

I havent gathered much data:
L 60min
R,G,B 6 min each.

Can anyone explain to me why i have such severe uneven illumination and why the majority of the illuminated region isnt in the center?

I have a William optics 0.8x filed flattener also installed.
could this be the cause??

equipment:
Heq5 pro
GSO RC8
WO 0.8x filed flattener
QSI wsg683-8

Thanks for your help.

John

[johnnyt123]

No one has any idea ey??

[alocky]

Have you calibrated these data? Darks, bias and flats?
The field flattener certainly won't improve things if it's also reducing the efl.
Cheers,
Andrew.

[ericwbenson]

Hi John,

This is most likely due to vignetting, As Andrew pointed out the reducer makes any vignetting worse since it enlarges the FOV, and some reducers are under sized and additionally cut into the light cone.
The non-centered distribution is most likely caused by miscollimation, primarily in this case a non-centered secondary mirror.

However you can recover from this without messing with the OTA, you just need to take flats with the camera attached in the same orientation as you took your lights. The easiest kind, since you have a closed tube, would be Tshirt flats before sunset: Drap one or more layers of white sheet over the front of the tube to attenuate the sky, point it up and away from the setting sun, and shoot ~1-20sec exposures aiming for about 30000 ADU in the center.

A little harder because the opportune window is much shorter is sky flats. Aim the scope at ~75deg altitude and towards the east (for dusk), turn tracking off and shoot 1-20 sec exposures, again aiming for ~30k ADU. Since he brightness is changing rapidly you need to adjust the exposure more often, but the dead spot twilight sky is nearly as flat as it gets (as long as the Moon is not in the area!). Once it is too dark and you need more than 20sec to get 30K ADU background, stars will start to show in your image, you're done until the next dawn or dusk.

The camera needs only to be roughly focused for flats to work (at least for vignetting).

Use Maxim or the like to combine the flats into a master flat, then calibrate the raw object images with the master flat. The flats should have the bias removed, for this you need to gather bias frames, Maxim calibration would take care of all this automatically, other software I don't know.

You can get more general info from the online Maxim manual: see here
http://cyanogen.com/help/maximdl/Calibration.htm

Best,
EB

[Star Catcher]

Hi John
I agree with Eric. You need to use good flats, especially if trying to image from light polluted areas with a focal reducer. I would also recommend separate flats for each colour filter. The focal reducer worsens the vignetting situation since it steepens the light cone.

The off centred nature of the light cone suggests an optical alignment issue. Most likely either in the focuser or the secondary. Looking at the image, the star shapes look reasonable with minor distortion but it is not clear from their shapes how far off the collimation is. I am unsure if this is a cropped image or if the chip is sufficiently small enough to not show up the problems further out. The focal reducer should not be causing the off centred effect.

Ted