Why so blue?

Posted by: @astrogee

So it seems the background is coming up a rather strong blue. What would be the astrophysics reason for this? Or is my camera (ASI294 Pro) doing something wrong? Or is APP doing something wrong?

Hi!

Difficult to say why with no more additional indication, but just calibrate the background after integrate (or neutralize background in the preview module for a live view) and all is done.

Thanks for the reply @Steph but I want to know why the raw image has so much blue. I don't think its colour balance because usually blue and red need to be boosted for colour balance - note that the camera doesn't provide colour balance information.

Posted by: @astrogee

Thanks for the reply @Steph but I want to know why the raw image has so much blue. I don't think its colour balance because usually blue and red need to be boosted for colour balance - note that the camera doesn't provide colour balance information.

It must be a fits header issue, like "ROWORDER" "BAYERPAT", "COLORTYP" , etc tag, who must be writen or read badly.

It depends of your acquisition software, driver version, or simply of yours settings

@steph Thanks! I'll check that out with my software vendor (Cloudmakers). Can you tell me what you see for the same target - assuming you have probably imaged M16 😉 I mean where do you see the red, green and blue content on the histogram, relatively speaking. (I think have a DSLR based image that I can check also.)

EDIT: I found an old DSLR image of M16 and here is the Rawdigger histogram output. It shows that for my Pentax K-5II, the red is the lowest - even though it is modified for Ha - then blue, then green (G1 & G2). I expect most OSC cameras like my ASI294 to have a very similar profile.

@astrogee I've got a lot a frame of M16...but all in narrowband data 😉 . I also well know the IMX294 sensor, but the Altair version, with less fits header compatibility issue.

But consider that your camera works well, just a little fits header issue. What software did you use, have you got the last SDK ZWO, did you use the ASCOM driver ? Check this points first.

I open your image with AsifitsView, and as you can see is well balanced...no problem

I've  open the image with Pixinsight and SiriL, the same problem of blue color appears.

@steph Interesting! I use a mac and found some very good software called AstroImager from Cloudmakers. But it connects remotely to a linux based RPi running "Indigo" (rather than ASCOM) with ZWO drivers. But like you say, there is probably an issue there. I'll contact them and show them this thread. Thanks for your help!

@astrogee You can use KStars on Mac. It comes with a module called Ekos which is very similar to the Cloudmakers software you're using. It uses INDI drivers (instead of IndiGo) which are also very similar. At the heart of it all lie binary drivers provided by ZWO. KStars for Mac includes the binary drivers for Mac. There also is a Linux version which runs on Raspberry Pi. The difference between KStars and the Cloud akers software is that KStars is free 😉

In any case, all ZWO OSC cameras use RGGB as Bayer matrix. Please check the FITS headers of your images and make sure that that has been written in there. If that's the case then don't make any changes to tab 0 since APP will be able to figure out the Bayer matrix on its own. If it is not the case then set the pattern in tab 0 to RGGB and leave "force Bayer/X-Trans CFA" unchecked. That should solve your issue. If it doesn't then we probably will want to have a look at your raw FITS images to see what's wrong with them.

@wvreeven Thanks, I used Kstars/Ecos a few years ago for my Pentax but found Cloudmakers and Indigo more reliable as its built for the mac, since then I switched to ASI294 Pro for the camera - makes things a lot easier.

I tried selecting RGGB but no difference.

I included the fits file link with the original post but can't see where the discrepancy might be. I think Cloudmakers guys need to look at it. I'll also try the camera directly with ASI Studio. 

@astrogee You can install KStars with all INDI drivers and Ekos (not Ecos) now via homebrew. It doesn't get easier than that.

I may be able to have a look at the raw FITS files in a few days. 

Cloudmakers responded to this issue and explained that ZWO boost the blue signal in the ASI cameras. Here's that thread: https://bb.cloudmakers.eu/viewtopic.php?f=7&t=826&p=6695#p6695

@astrogee Yes, ZWO set the proportional gain of G to 0.5, of R to 0.52 and of B to 0.95 or thereabouts. Sorry, I didn't realize that your first image was a screenshot of a raw light, not of the integration result. With flats the proportional gain should be corrected again and the resulting image should not have such a blue cast.

In Ekos/INDI it is possible to manually set the proportional gain of each channel. Does the Cloudmakers software have a similar possibility? In the end it shouldn't really matter of course, unless your B channel saturates while the others don't. In that case you may want to reduce the proportional gain of the B channel.

Posted by: @wvreeven

@astrogee Yes, ZWO set the proportional gain of G to 0.5, of R to 0.52 and of B to 0.95 or thereabouts. Sorry, I didn't realize that your first image was a screenshot of a raw light, not of the integration result. With flats the proportional gain should be corrected again and the resulting image should not have such a blue cast.

In Ekos/INDI it is possible to manually set the proportional gain of each channel. Does the Cloudmakers software have a similar possibility? In the end it shouldn't really matter of course, unless your B channel saturates while the others don't. In that case you may want to reduce the proportional gain of the B channel.

Thanks for the tip. I'll be careful with the flats. I typically don't use them, keeping the sensor clean and the images are pretty flat, but it's good to know. (I found out that yes indeed the Cloudmakers software can adjust the proportional gains too, which I think I'll try.) 🙂