Error: No correlation between master dark and target frames

[Farzad_k]

Hello,

I am still trying to figure out why my Flats don't work. So I am looking at warning messages during flat  calibration process. I am getting the subject warning. Does anyone know what the warning means? he darks were captured in .fit format and the Fats were captured in NEW RAW. Is this what the warning message is about? Attached is the stretched original and stretched calibrated files. They look very different.

Thanks a lot.

Farzad

[Farzad_k]

...and here is how they look like when I convert the NEF flats into .fit before calibration:

[pfile]

were these frames captured with SGP? raw files and fits files produced by SGP are not compatible.

rob

[Farzad_k]

Rob,

I capture Flat data outside of SGP because SGP apparently can only do it if the scope is aiming at the dawn sky which I am not able to do. So I gather the data using the same settings and by pointing my scope to a bright uniform display of my iPad ornSurface Pro.

I did get different results after I converted the NEF raw files into .fit before calibrating, but the Master Flat still doesn't work.

[vicent_peris]

Hi,

How long is the exposure time of your flats?

Best regards,
Vicent.

[Farzad_k]

Hello, Vince.

The Flats which are NEF RAW files used a shutter speed of 1/320th of a second.

[vicent_peris]

Hi,

Don't worry at all about this correlation problem between the dark and flat frames. The exposure time of your flats is very short, so the dark signal is minimal. This warning means that the dark signal is so insignificant compared to the signal in your flats that there's no difference whether we subtract or not the dark from it. Everything is right in your calibration workflow.

Best regards,
Vicent.

[pfile]

my point is that a .fits file captured by SGP has been "re-linearized" by DCRAW. the 14-bit data coming from the camera is expanded to fill the space of 16-bit integers. so you can't mix frames captured by SGP as fits and NEF files because PI will interpret the NEF files as 14 bits. it's not the file format, it's the fact that SGP has changed the raw camera data.

you have it backwards on SGP - SGP does not know how to do sky flats. it only knows how to do panel flats. so you should be able to do your panel flats with SGP and get them in "16-bit" fits format from SGP.

rob

[Farzad_k]

Hi,

Don't worry at all about this correlation problem between the dark and flat frames. The exposure time of your flats is very short, so the dark signal is minimal. This warning means that the dark signal is so insignificant compared to the signal in your flats that there's no difference whether we subtract or not the dark from it. Everything is right in your calibration workflow.

Best regards,
Vicent.

Thanks, Vincent. It is good to know that the warning is something that can be left alone. I don't think there is a way to make a manual for this software like there are for other programs.

Farzad

[Farzad_k]

my point is that a .fits file captured by SGP has been "re-linearized" by DCRAW. the 14-bit data coming from the camera is expanded to fill the space of 16-bit integers. so you can't mix frames captured by SGP as fits and NEF files because PI will interpret the NEF files as 14 bits. it's not the file format, it's the fact that SGP has changed the raw camera data.

you have it backwards on SGP - SGP does not know how to do sky flats. it only knows how to do panel flats. so you should be able to do your panel flats with SGP and get them in "16-bit" fits format from SGP.

rob

Thanks, Rob. I don't know what panel flats are. I have been taught that the flats are to be taken with the DSLR set to aperture priority and SGP seems like it works only with the camera in manual mode.

I have posted a support question on the SGP on this issue and would like to get their opinion on what I need to do next time to gather compatible data.


Farzad

[pfile]

SGP are aware of the issue with the re-scaling; they have chosen not to change it for now. basically you need to collect all your frames from SGP in fits mode, or else collect them all in NEF mode in which case the NEF files will be compatible with NEF files captured with other camera control programs.

the way you figure out the flats is to use the flat wizard to compute the duration of the flats based on the brightness of your flat panel (or laptop screen or pad or whatever you are using - panel flats mean any artificial light source was used for the flats, vs. sky flatss or t-shirt flats which all use the sun as a source of illumination)

rob

[Farzad_k]

So the best thing to do for now is to have SGP save data in NEF RAW format. I will definitely remember that one. Thanks a lot.

[pfile]

well, yes, but you are giving up something - when saving as FITS you can get all the fits keywords that give the target, exposure time, center coordinates, etc. if you save as NEF you might be able to get a nice filename but there won't be any FITS keywords.

another strategy is to just use SGP for everything and understand that it's going to modify the data coming from the camera if you use fits. key thing is just to not mix calibration frames in NEF format and SGP fits.

rob

[Farzad_k]

It makes sense. However, I still have to figure out how to get SGP to do the Flats for me. Ill have to play with it a little bit.

Farzad

[Farzad_k]

SGP has notified me that there is a wizard that helps with the Flats and they also say the best is for all the files to be compatible. So I guess I am trashing all the data I collected and look forward to another clear sky opportunity.