ASI071MC processing

Hi Janos,

I'll have a look tomorrow 😉

Did you enable "force CFA" for these frames in 0) RAW/FITS ? Do you get different results when applying RGGB or GBRG as CFA pattern for instance?

If you set another CFA pattern and reload the current image in the image viewer, by using the image viewer dropdown box (above the image viewer panel) then the newly chosen CFA pattern is automatically applied.

(The asi071MC has the same sensor as the Nikon D5100 I believe)

Mabula

Hi Mabula,

Thanks for the tip, I have now reprocessed the image set with Force CFA on and RGGB, now it gives good calibration and integration results. I will post the final image when I finished it in PS CC. I am still curius about your findings with this camera based on the frames I provided,

Cheers,

Janos

Hi Janos,

I will definitely took a look later today 😉

Looking forward to your result!

Mabula

Hi Janos,

I am looking at your data.

I have one advice:

You provided darks with the same exposure as the lights. That's fine

But the flats need either bias frames or dark frames with the same exposure as the flat frames to correctly remove the bias pedestal form the flats.

But the calibration does seem to work fine still. If you were to add bias frames or dark frames (so called dark flats) for the flat frames then I think the data calibration should improve.

The combined master dark looks rather odd though, (but no problem when applying it). But what's really odd is that the R,G,B channels seemed to be scaled for some reason.

First image, the master dark, second image the master dark with BG-N applied (neutralised background) in the preview filter. Third screenshot showing strange patches on the masterdark:

I created a bad pixel map, masterdark and masterflat and a calibrated frame looks fine:

Integration of the 10 light frames that you provided with a quick post processing using

• remove light pollution
• calibrate background
• calibrate star colors
• selective color

looks pretty good to me 😉

Hi all. This is my first post at this forum.

@ Mabula: You wrote above: "But the flats need either bias frames or dark frames with the same exposure as the flat frames to correctly remove the bias pedestal form the flats."

This statement really confused me. I was always under the impression that flats were included to remove imperfections in the optical train and that exposure time and ISO/gain was irrelevant as long as the flat frames were normally exposed.

Bias frames I have always taken with the camera's shortest exposure time (1/4000 sec on my DSLR) and the same ISO (not sure if that matters) as my lights.

I know that this tread is not about DSLR, but generally it would be the same,  would it not?

Please explain.

Helge.

Hi Helge,

Welcome to the forum 😉

Yes, I can understand why it can be confusing in the way this was written..., to be absolutely clear, it is:

"But the flats need either

• bias frames of the same iso/gain as the flat frames or
• dark frames with the same exposure and the same iso/gain as the flat frames(these are called flatdarks or darkflats..)

to correctly remove the bias pedestal form the flats."

Indeed the exposure time of the flats is irrelevant, although longer exposed flats tend to be of better quality and usually give less problems. Just make sure the histograms don't clip on the right side of the histogram.

And yes, the iso/gain of the flats is irrelevant, but lower iso/gain tend to give better results and less problems because the exposure time can be made longer without clipping the histogram on the right side.

So for correct flat-field calibration, the flats need to be corrected with either

• bias frames of the same iso/gain as the flat frames or
• dark frames with the same exposure and the same iso/gain as the flat frames

And yes, there is only one way to make bias, that is with the shortest exposure time your camera has. But the ISO needs to be the same as the ISO of the frames that you calibrate.

So if you use bias frames for your lights, the bias frames need to have the same ISO as the lights, but if you use bias frames for your flats (that might have different ISO/gain) you need to make sure the bias have the same ISO/gain as the flats. So if the flats have different ISO/gain than the lights, you need 2 different masterbias frames. One for the lights and one for the flats.

Yes, data calibration rules are the same for different types of cameras, but depending on the sensor characteristics, different calibration paths can give better results, really depens on the sensor and also the ability of your gear to apply dithering between frames.

All rules can be found in this thread:

https://www.astropixelprocessor.com/community/tutorials-workflows/astronomical-data-calibration-priciples-must-read/

Let me know if this clears the confusion for you 😉

Kind regards,

Mabula

Hi Mabula.

Thank you for the extensive explanation. You have made it all very clear and it is pretty much as I thought it was except that I need bias frames for both lights and flats. That was new to me. Then again, if I shoot using ISO 200 (or whatever) for all three types of frames I should get away with one set of master bias. Correct?

I will sit down and read the info your link and watch the videos. Looking forward to learning and using this piece of software.

If only we could have a clear night or two. 🙂

Helge

Hi Helge,

You're most welcome 😉

that I need bias frames for both lights and flats. That was new to me. Then again, if I shoot using ISO 200 (or whatever) for all three types of frames I should get away with one set of master bias. Correct?

Yes that would be correct.

Yesterday, I added a new tutorial on calibration for a DSLR, that is probably interesting to watch 😉

https://www.astropixelprocessor.com/a-dslr-data-calibration-workflow/

Mabula

Hi Mabula.

Just watched the video, very informative.

Thanks for your help so far.

Agreeing with Terminator; "I'll be back". 🙂

Helge

You're most welcome Helge 😉

Hi Mabula,

Thanks for the information.!

If the flats have different ISO/gain than the lights, one needs 2 different masterbias frames. One for the lights and one for the flats. I assume that calibration because of two types of bias frames should be done separately?. .Or can I put them altogether in the Bias map and will the software automatically assign the right bias frame to the flatframes and lightframes with the different Gain.?

Dick Van Tatenhove

Good question, I'm not 100% sure and would have to try that out myself, it might. Safest would be to calibrate them separately.

One can create two masterbias frames.  But then.....? Perhaps Mabula knows the right workflow.?

Well, you can load in the flats and calibrate them with the right bias frames. That's about it, then start a new session where you assign the flats to the correct light frames.

Ok...in that second session I load  the calibrated Masterflat plus the Bias frames for the lights and Masterdark. So the calibratedmasterflat will not be calibrated again by the bias frames with the gain value of the lights.? I will give it a go... 

Btw my deepsky and calibration frames with the same gain value are perfectly calibrated by APP except for the "pain in the head" galaxy  😉 Ngc 4725 with different Gain values. To be continued.

The ISO/gain used for flats is irrelevant. The flat is not used for that kind of calibration. It is only used to even out anomalies in the optical train.
Iso/gain also does not matter for bias frames because they are so short that dark current does not have time to develop.

Helge.