Calibration question... calling all gurus

[bianry]

Update.
If I use the first 8 lightframes with a masters that I make from selected frames with sigma max +-1 then I can make a decent integrated luminance.
With the rest of the frames I can not get a good calibration, any combination I have tried.

Still. The integrated calibrated frames I rcvd from iTelescope gives a picture with better SNR and a better look. But the background is more uneven. Something definitely going on with the flats.

regards

Mats

[pfile]

Update.
It does not seem to be so simple a solution as we thought. No matter how I stack the flat frames I can not get a decent calibration. And I do not think they are flipped. The scratch marks that can be seen in the upper right corner are there. And they move. But not simply by rotation. They move across horizontally and sometimes you can see other scratches on the lower half of the frame.
I inspected 30 flat frames from their FTP-server and they vary a lot in brightness. All have the same streaks but the vignetting is all over the place and not at all uniform.

I rcvd a confirmation from iTelescope that they are checking the data but still no response after that.

regards

Mats

you have misunderstood me. the lights are flipped over. the flats are not. the flats provided belong only to those first 8 light frames. this is why you can get a good calibration of the first 8 frames, but not the rest.

if you load your master flat, use the geometry menu to rotate it 180 degrees, and save it out as a new file, then calibrate the last 9 images or whatever with that rotated flat, they will come out correctly.

i did blink the flats to make sure they were all the same, but the key thing is blinking the lights. you can see the CCD artifacts/dust moving at the meridian flip boundary (or rather, one frame late because the first E file is actually still from the W side of the pier)

rob

[bianry]

Duh!!

Will try that. Report later.

regards

Mats

[Carlos Milovic]

If the lights are rotated, then you should check also if the darks/bias also match. I would just rotate the lights.

[pfile]

fair enough but darks/bias orientation should not matter since no light is entering thru the OTA, right?

i mean you can take darks/bias on your desk with the camera hanging upside-down and it won't change what the darks and bias look like...

rob

[bianry]

OK.
If I rotate all the WEST-frames 180 degrees I can get the same result as the calibrated frames from iTelescope. Even slightly better noise estimate.

Still no masterpiece but at least now I understand what is happening.

Thanks to all who helped and especially Rob

regards

Mats

[Carlos Milovic]

Hi rob,

Yes, it doesn't matter, BUT, in this case, the lights frames seems to have being rotated either automatically, or by the guys at itelescope, so, most probably bias and darks don't match either.

[bianry]

If I rotated the lights before calibrating should that not put everything "right way up" then?

regards

Mats

[pfile]

everything is correct now, i think all that carlos is saying is that since they delivered you an incomplete set of calibration frames there's reason to suspect that they have delivered you the wrong bias and darks as well.

rob

[Carlos Milovic]

No no, I'm saying that if he rotates the lights 180º, they would be at the same orientation at they were captured, so then all calibration frames should work fine.
I have the impression that they did not use the rotator to change the orientation, but just rotated the files to get the same orientation of the object.

[bianry]

Quote. Reply from iTelescope

Your last message nails what I believed had been the issue.   Indeed, as a courtesy, we do a 180° rotation on images from post-meridian, so that all images are oriented the same direction.

If you are doing your own calibration, then you must rotate either the uncalibrated west side images, or the calibration files prior to applying them to west side images. In fact, in this instance, the last EAST image (-009) also needed rotating.

The "pattern" you see is indeed an optical aberration in the system that can be "flatted out".  The reason you were seeing it remaining in your image, and in symmetrical mirrored locations, is that your East side images calibrated correctly, while the west side ones did not.  The west side calibrated images showed the original pattern (the flat didn't eliminate it, since it was rotated 180°), and at the symmetrical location, the un-rotated flat backed out an aberration that "wasn't really there", thus creating a NEW aberration.

In addition, your dark files were upside down, likely leaving your west side images not only streaked, but lousy with hot and cold pixels.

I used CCD Stack to calibrate your uncalibrated images, with master calibration files I created from the raw calibration files, and got a very good result.  The data is excellent, and I think you'll find that if you work as above (easiest way is to probably just rotate the uncalibrated light frames back, and let Pixi handle the stacking its own way), you'll get great results as well.

Please let me know if you have any additional questions.

Best Regards,

Jeff Woods
iTelescope.Net Imaging Specialist

[pfile]

okay, so carlos and i were talking about two different things. if they indeed did not use a rotator then yes, you have to rotate the lights back to the original position angle.

yeah, here is an additional question: "why the hell would you do that?"

if there's no rotator involved it makes no sense to rotate the images "as a courtesy". it just creates confusion like what happened here. if they want to rotate their calibrated images, fine, but the raw lights? that's just dumb.

additionally how would the dark files be "upside-down"? indeed the fits reader direction can cause files to be read in upside down but that will happen with all the frames, not just one type of frame. the lights are upside-down, because they went out of their way to make them that way.

conclusion: itelescope has no idea what they are doing or saying.

rob