Overscan

[John_Gill]

Hi,

Just got my new QHY268m camera and there is an "Overscan" area. During ImageCalibration ---> Overscan ---> Image region, I define the top, right, bottom and left pixel (like a preview). Do I need to need to define the Source and Target regions? I do not take Bias frames, so the Dark frames will be of a different size... so should I crop the Darks to the same size as the overscan area?

Thanks
John

 

[bulrichl]

Hi John,

During ImageCalibration ---> Overscan ---> Image region, I define the top, right, bottom and left pixel (like a preview).

No, please take a look at the mouse-over help texts: You define left pixel coordinate, top pixel coordinate, width an height (like a preview).

The ImageCalibration Reference Documentation, chapter 2.5, contains somewhat more detail:
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2.5 Overscan
Each overscan allows you to define arbitrary source and target overscan regions in CCD pixel coordinates. Up to 4 overscan regions can be defined. Only the pixels parts of the final image region will be kept in the target image. The same overscan parameters are applied to all master images with the option calibrate and to the target frames (but each image is processed individually).

Image region
The image region defines a rectangular area of the CCD frame that will be cropped after overscan correction. It usually corresponds to the true science image without any overscan regions.

Source region
A source overscan region is used to compute an overscan correction value (as the median of all source region pixels) that will be subtracted from all pixels in the corresponding target region.

Target region
Define the target region corresponding to the source region above.
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Do I need to need to define the Source and Target regions?

Source and Target regions have to be defined.

I do not take Bias frames, so the Dark frames will be of a different size... so should I crop the Darks to the same size as the overscan area?

I don't understand this statement. As stated in the Reference Documentation, if you decide to apply overscan correction, it must be applied (with the same parameters) to ALL calibration files as well. That means that your dark frames have to be overscan corrected. Thereafter, they have the same dimensions as the overscan corrected light frames. The same holds for flat frames and flat-darks.

Are you sure that you will need overscan correction for this camera at all?
What does the manufacturer of the camera recommend?
Overscan correction is necessary when the bias offset of the camera is unstable over time, otherwise you will not need it.

Bernd

 

[John_Gill]

Thanks Bernd, this seems like a lot of fiddling - perhaps I should just do my Darks, Flats and Lights and then use a process container and crop all the images, then process as normal.

John

 

[bulrichl]

If you don't want to perform overscan correction and the goal is only to get rid of the overscan region, there is an easier way. Cited from the camera manual ( https://www.qhyccd.com/index.php?m=content&c=index&a=show&catid=30&id=279 ):

"If you do not use this area, you can select the option "ignore overscan area" in the ASCOM driver or select a ROI of effective area range in SharpCAP."

If you had this feature disabled in the camera driver only for some frames which shall be used further, you can crop these frames in PixInsight. According to the manual, the total image area is 6280 x 4210 pixels, including overscan (at the bottom) and optically black area (at the left).
According to the Baader website, the effective usable region is 6252 x 4176 pixels (no guarantee).

Bernd

 

[John_Gill]

Hi Bernd, thanks again for your input. I use Sharpcap and when changing to the ASCOM driver to disable the overscan area, I then found the Filterwheel and Focuser stopped working. I guess I could mess about and change everything to ASCOM drivers.... There is a warning in Shapcap that certain functionality will be lost using the ASCOM driver. Anyway I will look at Sharpcap ROI but I think a DynamicCrop of all images in PixInsight is a lot easier and I will have better control.

Regards
John

 

[FlapAstro (aka rakla)]

John,
there was a thread some months ago regarding the use of the overscan area, and the correct setting for a QHY268C. I realise you have the mono version but the settings are probably the same.
If you decide to use overscan, please note that the color version of the camera shows a 4 pixel wide black border on the right side of the chip when using the QHY Ascom driver. There is no mention in the reference documentation from QHY, however their support confirmed its existence.
/Ralf