Calibration question... calling all gurus

[pfile]

yes i agree when using a single dark/bias/flat on this data the resulting calibrated image is pretty clean. the flats look terrible but i think they represent what's going on with the sensor - dust all over it (or maybe on the filter) and there are some 'grinding' marks which admittedly are usually more likely to be found on back-illuminated CCDs but i suppose they can happen on front illuminated CCDs as well. all of those artifacts cleaned up nicely in the calibrated image.

rob

[Phil Leigh]

But only if you use a single calibration file of each type?
That seems odd to me. In fact it makes my head hurt.

I have to be honest, if my uncalibrated light frames or flats looked like that I'd get another hobby!

[bianry]

Ok. Chris and Rob, you are telling me that 1 frame, 1 dark, 1 bias and 1 flat makes a good result and that the iTelescope master is not so good. Check!
But if you look at my first post in this thread. The frame on the lower right. That one is calibrated with BPP-script and I fed it not masters but individual darks/bias/flats. And there is a lot of streaking as you can see.

Should I try calibrating with just one flat? Not making a master of them?

Any advice. Other than giving up and buying my own scope and camera and mount 

r

mats

PS. Could someone upload a "normal" master flat so I could have a look at it

[Phil Leigh]

Yes i did what Bianry did and the result was dreadful. How come you cant make masters from the cal files... Ouch ouch ouch....

[pfile]

well using one calibration frame is sure to inject way more noise into your calibrated light than necessary. let me try downloading some of the calibration subs and see what happens.

rob

[pfile]

i need to go read itelescope's descriptions, but i think what's going on here is this: i21 has a rotator. there is a meridian flip involved here (i assume that's what the W and E means in the light filenames) - since the orientation of the DSO is the same between the E and W-named images, the rotator turned 180 degrees after the meridian flip.

that's fine, but it means you need 2 sets of flats; one for the E side and one for the W side, since the camera orientation changes when the rotator rotates. all the flats provided in the dropbox are for the E side; the W side is missing. calibrating the W lights with the E flat is probably making them look absolutely awful given how crazy those flats are.

also it looks like one of the lights is misnamed - E-180-009 looks like it should have been named W-180-009. also if the "180" is supposed to represent the rotator position angle, then something went wrong in the file naming at iTelescope because half of the images should have PA = 0.

anyway, i assume that's the source of your troubles. i guess for now you can just calibrate and stack the E images.

rob

[Harry page]

Hi
you are true jedi  8)

I was just looking why some worked and others did not and I did not think of this 

Harry

i need to go read itelescope's descriptions, but i think what's going on here is this: i21 has a rotator. there is a meridian flip involved here (i assume that's what the W and E means in the light filenames) - since the orientation of the DSO is the same between the E and W-named images, the rotator turned 180 degrees after the meridian flip.

that's fine, but it means you need 2 sets of flats; one for the E side and one for the W side, since the camera orientation changes when the rotator rotates. all the flats provided in the dropbox are for the E side; the W side is missing. calibrating the W lights with the E flat is probably making them look absolutely awful given how crazy those flats are.

also it looks like one of the lights is misnamed - E-180-009 looks like it should have been named W-180-009. also if the "180" is supposed to represent the rotator position angle, then something went wrong because half of the images should have PA = 0.

anyway, i assume that's the source of your troubles. i guess for now you can just calibrate and stack the E images.

rob

[pfile]

haha thanks harry. the clue was that when using the boosted STF stretch you can see the polishing marks on the CCD in the top right corner. when blinking thru the lights you can see them disappear from the top right and reappear in the bottom left corner.

i guess if the PA is really perfectly 180 degrees (StarAlignment says one of the W images is 0.33 degrees off) then perhaps bianry can simply make his other flat by rotating the master flat 180 degrees, and it will  probably work.

it's pretty deceiving because the rotation visually looks pretty spot-on as well, so looking at the DSO it just seems like it was all imaged on the same side of the pier.

rob

[bianry]

Wow.
I lift my non existent hat for you Sir.
Yes, there is a meridian flip involved and that is the W or E in the filename. The 180 on the other hand is the exposure in seconds so that is correct. They have in fact a very good naming convention for the file names. Once you decode it you have all the info you need right in the filename.
t21-bianry-ngc5033-20140222-024836-luminance-bin1-e-180-004.fit.zip
Telescope-user-object-date-time-filter-binning-e/w-exposure-running number.
And PI:s system of adding _c or _r as a prefix fits right in.

I will use this info in my communication with iTelescope if you don't mind.

regards

Mats

[pfile]

oh - duh… 180s. okay. well so maybe something they could add to the filename is the rotator position angle. that would make everything a little more obvious. for that matter they should say "180s" instead of 180…

as i mentioned just above you might be able to (or they may be expecting you to) rotate the master flat by 180 for the post meridian-flip exposures. the rotator looks to be pretty precise, so that might be their expected flow.

also if you can turn on dithering on t21 you should try that. it makes it easier to reject any residual hot pixels or column defects.

rob

[bianry]

I will try to run a new batch tomorrow as you suggested.

Will post the results here.

Dithering was supposed to be turned on. At least I ordered it in the script. Can you tell from the lights that it was not?

A big thank to all of you who helped me with this and especially you Rob who cracked the code.

regards

mats

[pfile]

when you blink unregistered lights that have been dithered, you should see the stars going around in box-shaped patterns. generally dithering is set up to move each image about 10-15 pixels or so, but that's of course up to whoever has set up the auto guiding and what controls the capture program has.

your unregistered lights look like they've already been registered and the only motion i can see is a small drift, which is probably due to differential flexure between the imaging camera and the guide camera.

thx for the rose

rob

[pfile]

this is what a dithered set of lights looks like… you might have to click it to start the animation.

rob

[bianry]

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

[Phil Leigh]

Rob - that is genius. Never would have thought to blink the flats...