SharpCap Forums

Robin,
I am helping a friend with his new camera, and we are coming across a problem with the offset. When I set it to 40, the image is gray and the histogram is at 40%. I attached the logs for you. Connecting the camera through ASCOM does not show this problem. Do you know what is happening?

Eric

Hi,

I feel a bit like I'm asking about the Emperor's new clothes here… Why would you set the offset to 40 if that gives you an image that is too bright? There is no real magic value for the offset control on any camera – you just pick the smallest value that nicely separates the histogram peak of a dark frame from the left-hand side for your selected exposure and gain settings. If I remember rightly, for a lot of qhy cameras the offset seems to be applied before the gain, which means that you need to pick different offsets at different gain values.

Cheers, Robin

Eric

See https://www.qhyccd.com/uploadfile/2018/ ... 334403.pdf OFFSET section.

Dave

I use offset of 40 because it is what I use for my QHY224C and QHY168C cameras. I selected 40 using this table: https://www.qhyccd.com/index.php?m=cont ... =31&id=199 Now I am seeing their starting Offset is 200 with the QHY247C

I did not use that table last night, however, I did see that when connecting using ASCOM, the default offset was 79. It was in the same ballpark as the offset I was using for my cameras so I guessed that that was some recommended starting value. The offset sensitivity with the QHY247C seemed very sensitive. Especially compared to the ASCOM controls that pop up in a new QHY window in Sharpcap.

Eric

Eric

In 2019 there have been 12 updates to the qhyccd sdk. This could make the older information in the tables obsolete.

What I have been doing to establish a reasonable offset value for DSO imaging is for a given gain: try an offset value and take a bias frame looking for a centralised histogram (or at least the min value being > 0). If histogram not central, then adjust offset and repeat. If an SDK changes, then I recheck my offsets for the gain settings I use.

[Note: for lunar/planetary I set offset=0 (or lowest value available).]


Interestingly the bias frames from my Atik CCD and Canon DSLR (both of which offer no offset adjustment as this is taken care of in hardware) look just like the image above (centralised histogram).

Dave