SG capturing at a different resolution than native CR2?


#1

I tried my first imaging run using SG Pro last night. I used it to capture the lights, darks, and bias frames. I took the flats by hand with the Canon 60DA after disconnecting the computer from the mount.

Today while trying to remove the bias from the flats using Nebulosity 3, I got an error message that the images were not the same size. Sure enough, the bias frames shot by hand were at the normal CR2 resolution of 5184x3456 pixels. However, the lights, darks, and flats taken with SG were at a resolution of 5202x3465. That is 18 pixels wider, and 9 pixels higher than usual. I have no idea how this is possible, but it stopped me from using the flats for the image. I have just finished stacking 12 5 minute subs of the NA Nebula, calibrated with 9 darks. I have a really bad crosshatch pattern throughout the image. I’ve never seen this before, and don’t know if it is related to SG, or a mistake I made in processing. Google here I come…
If anyone has any idea why SG would not be shooting at the normal CR2 resolution, please let me know. Thanks!
Dean


#2

Dean,

The bottom line here is that different applications can read RAW data in different ways (not actually modifying the data per-say), but each making their own decisions on how many pixels to crop (physical pixels vs usable pixels) etc. The bottom line is that you should never use different applications to take light and calibration data. One application could vary slightly in dimensions (your case), or even worse, flip the X or Y axis entirely on read out (this would result in catastrophic calibration results)… there are no standards here. If you choose to switch the application you use for primary light capture, you will need to redo all of your calibration data as well.


#3

If you would like to avoid this your best bet is to either:
1 - Take all frames in the same application
2 - Capture and process CR2 frames which will not be altered or converted.

Thanks,
Jared


#4

Thank you Ken for the lightning fast reply! That makes sense, and I will use SG for all calibration images from now on.

Dean


#5

Thanks Jared! I will take all frames using SG from now on.

Dean


#6

One thing I do with my Canon, though, is just always save and use the RAW files for calibration/integration. What Ken/Jared say is certainly true/good advice, but the one difference with Canon/DSLR is that the RAW files are generated by the camera, not the app. All programs, as far as I know, read that RAW file (CR2) to generate a new image (say, a FITS) and that’s where the problem lies. Some libraries can do it differently. In the case of SGPRO, it is including the overscan area, and other apps might not, as you’ve run into.

In PixInsight (and possibly Nebulosity), it’s important to not use this overscan area and indeed PI has overscan parameters for just this (ignoring these regions). So even if you capture all data with SGPro, you should know how the image “looks” in each program. It really helped me to take daylight pictures as it made it obvious is there were any flipped axis, overscan, or debayering issues.

I’ve successfully combined SGPro FITS from my Canon with darks/bias CR2 from BYE in PI by using the overscan settings. Having learned all this though, I just stick with CR2 files as that’s the closest to the original as you can get.

FITS files have some advantages, too (primarily the metadata), and when I suggested a RAW+FITS option to Jared I think he had it implemented in about 5 minutes. So if you’re really unsure, save both RAW and FITS. It’s easy enough to delete later.


#7

Thanks Bhwolf,

Lots of good info there that I didn’t know. I will switch to capturing in CR2 again. It does sound like less possible trouble, without any major downside.

Dean


#8

The downside to CR2 is that it does not contain any “astronomy” metadata. With FITS we populate things like the sensor temp, RA/Dec, image angle, etc. If you don’t need these then CR2 is probably your best bet. The fields we populate are generally beneficial when plate solving and using previous frames as reference images.

Thanks,
Jared


#9

Thanks Jared,

I am just getting familiar with the plate solver in SG, am very impressed, and am not willing to give it up. :slight_smile: I will keep capturing in FITS if its info is needed by the plate solver.

Dean


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