Understanding Bias Files

[mike1485]

I am pretty new to this hobby and trying to gradually improve my understanding of what is going on "under the bonnet" (or "under the hood" for our cousins across the pond!)

I have taken single bias frames with my ASI533MC camera, setting the gain at 100 and the offset at 70 (this value is set behind the scenes in the ASIAIR Pro).  One bias frame with exposure time 0.001s was taken using the ASIAIR Pro and one using Nebulosity.  So far as I can tell all settings are the same the only variation being the software used for capture.  Analysing image statistics in Pixinsight shows:

                      ASIAIR.           Nebulosity

Median:           2800                 2856

Std dev:          6.5.                    1073.6

Subject to some random variation I would have expected these parameters to be very similar irrespective of the method of capture.  That seems to be the case for the median value but the standard deviation is very different. Can anyone shed any light on this to help me understand what is happening?

Many thanks

[vlaiv]

Try adding SharpCap to the mix of apps to test with.

ASI provides two types of drivers - native drivers and ASCOM drivers. SharpCap can utilize both type of drivers so you can take bias with each and compare with above results.

Something tells me that this has to do with driver used. I know that darks are not compatible between drivers - could be the same with bias subs.

[mike1485]

Thanks. That sounds like a useful avenue to check. I am not a SharpCap user but I can probably figure it out!  Do I need SC Pro or does the free version give me the necessary functionality?

[vlaiv]

48 minutes ago, mike1485 said:

Thanks. That sounds like a useful avenue to check. I am not a SharpCap user but I can probably figure it out!  Do I need SC Pro or does the free version give me the necessary functionality?

Free version is sufficient.

Here are results from ASCOM driver and native driver for ASI185 - bias file (0.125ms exposure):

Quite different results for both mean / median and stddev

[mike1485]

Interesting results. I didn’t realise the driver could give such different results. I have not managed to figure out how to switch drivers to test my camera. I think it is currently using the native driver in sharpcap. I’m not sure what the ASIAIR would use as that is Linux based I think. Presumably something different again.

[powerlord]

asiair uses zwo drivers. it only uses indi drivers for dslr. I believe folk have tried to inject other indi drivers for example to support non ZWO astro cameras without success.

[mike1485]

1 hour ago, vlaiv said:

Free version is sufficient.

Here are results from ASCOM driver and native driver for ASI185 - bias file (0.125ms exposure):

Quite different results for both mean / median and stddev

Just to check - you are running the same offset setting in both trials?

[mike1485]

Results from SharpCap consistent with Nebulosity. Median 2827, StdDev 1081. Still haven’t worked out how to use Ascom Driver in SharpCap. 

[vlaiv]

7 hours ago, mike1485 said:

Results from SharpCap consistent with Nebulosity. Median 2827, StdDev 1081. Still haven’t worked out how to use Ascom Driver in SharpCap. 

Make sure you have it installed and then select this option:

If you have more than one camera - it will give you "selector" when you first click - so you choose the right camera and after you can set it up via ASCOM options dialog.

[mike1485]

3 hours ago, vlaiv said:

Make sure you have it installed and then select this option:

If you have more than one camera - it will give you "selector" when you first click - so you choose the right camera and after you can set it up via ASCOM options dialog.

Thank you. I will give that a go later and see what difference it makes. 

[mike1485]

16 hours ago, vlaiv said:

Make sure you have it installed and then select this option:

If you have more than one camera - it will give you "selector" when you first click - so you choose the right camera and after you can set it up via ASCOM options dialog.

So, using SharpCap I have the following results.

                      ASIAIR.           Nebulosity      SharpCap(Ascom)      SharpCap(Native)

Median:           2800                 2856                   2800                           2827

Std dev:          6.5                     1073.6                 6.4                              1081

So it appears Vlaiv has hit on a very plausible explanation for the different StdDev figures.  Thanks Vlaiv.  

One thing I note with the Ascom driver is that there is no way to change the offset value.  Apparently this is a known issue with the current version of the driver as distributed by ZWO and results from a problem with compatibility with the ASCOM platform on some older computers.  I believe 70 is the Ascom default used for this camera (this is the value in the fits header for ASIAIR fits files). So this is the value I used for the "brightness" parameter in the Sharpcap native test and in the Nebulosity "offset" parameter. 

I am not sure what all this means in practice or why one driver should result in such different values from another.  Any views/thoughts welcome!

[vlaiv]

Interesting that ASCOM driver does not let you set offset value.

I have 3 ASI cameras and use older version of driver at the moment (haven't updated in a while) - but every version of driver so far let me set offset on every camera.

In any case, when that thing with driver gets resolved - I've found that ASCOM driver is better / more stable for long exposure imaging, while native driver is preferable option for planetary / high frame rate imaging.

[mike1485]

34 minutes ago, vlaiv said:

Interesting that ASCOM driver does not let you set offset value.

I have 3 ASI cameras and use older version of driver at the moment (haven't updated in a while) - but every version of driver so far let me set offset on every camera.

In any case, when that thing with driver gets resolved - I've found that ASCOM driver is better / more stable for long exposure imaging, while native driver is preferable option for planetary / high frame rate imaging.

Older drivers did support offset.  Discussion on ZWO forum is here: https://bbs.astronomy-imaging-camera.com/d/12956-latest-zwo-ascom-driver-isnt-supporting-offset-correctly