SGP and Flats


After I finally had a chance at a couple of nights of clear skies I was able to get out and gather some data. I did my Lights, Darks and Biases using SGP and we got *.fit files. For the Flats I could not figure out how I could do it using SGP since I have been taught that the Flats are shot using aperture priority and while the optics is pointing to some bright light, and SGP only works with camera on Manual mode. So I gathered my Flat data using aperture priority and my iPad screen. And the file format is NEF Raw.

I am having some issues using my Flats in Pixinsight and I am trying to figure out what is going on so that I am better prepared for the next clear night.

What do I need to know about taking flats using SGP when I am using a DSLR and I am not able to hang around until dawn?

Thanks a lot.


Aperture priority is really just a convenience method around settings that are available in manual mode. Aperture priority is just setting the shutter speed based on a fixed aperture. You can do the same by changing the exposure in SGP. Or you can use the Flats Calibration Wizard in SGP to find the flat values for you. Although since DSLRs aren’t completely linear it can take a little bit to get the ADU where you need it.

This is not surprising when applying different formats of data. The FITs from SGP have been scaled to 16bit and the NEF have not. I’m not sure if the NEF has overscan data in it or not so the images might be even differently sized. Additionally how the data is stored in each file (top down vs bottom up) may also be different so the flats could be rotated 180 degrees as compared to the FITs. All in all it’s highly unlikely that calibration frames taken in one format would work for the other. It’s always best to take the same for everything. If you want to be safe you should save images in FITs and RAW:

It’s also worth mentioning that FITs from different applications will likely not be compatible with each other either.

Hope that helps,


I just checked out the wizard and while not in an imaging session. I don’t
see how the wizard will figure anything out, and it looks like it simply
adds the event to the target. I have installed the 2.6. Is this Flat Wizard
topic covered in the manual?


You want the “Flats Calibration Wizard” not the “Flats Wizard”. The SGP manual is pretty deep, you can find it in there.


Thanks a lot.