Preprocessing Help on Narrowband

[pwatson]

Hello.  I have just started using LRGB and narrowband filters. I imaged NGC 6960 using LRGB first.  I used PI for all processing and it went fine.  I then proceeded to image using Ha and OIII filters. Imaging went well and the raw images looked good in PI. I then tried preprocessing using the BPP script, the same way I used it for my LRGB. For some reason the integration of the light frames keeps failing and I get the message shown in the .png below.

It looks like the calibrated images are lacking data. Just for fun I integrated the lights by themselves and the result had all the data in the image though it was less than a quality image as one would expect.
Does anyone have any ideas on what is going on here?

I later tried using master flats, master dark and a super bias and ended up with the same results. Here is a link to my data.
https://www.dropbox.com/sh/yxgsrcffmr94ovs/AAA5wfUWZpO3C6y1uZpnggPba?dl=0


Thanks,
Pat
 

[Greg Schwimer]

Hi Pat,

I didn't do a deep analysis but I did notice that both your master bias and master dark have a higher median signal than the light frame I looked at:

Using 16-bit counters:

  Bias median: 772.230
  Dark median: 821.895
  Light median: 368.0

My guess is this explains your problem. Did you use a different setting on the camera for these lights relative to what you captured the dark and bias with?

You can add a pedestal to increase the median of the light frames so you don't end up with pixels with negative values. I tried 0.02 and ran pixelmath to remove the dark and bias signals. Looks pretty good.

How I did this with pixelmath (EDITED FOR ORDER):

Add pedestal to light: $T + 0.02
Remove bias from dark: $T - bias
Remove bias from light: $T - bias
Remove dark from light: $T - dark

I'd find out why the values are less than the bias and dark. They should be *at least* as bright, and ideally a bit brighter so you overcome the noise in the frames.

 -Greg

[pwatson]

Thanks for the response Greg.  I started astrophotography a year ago and used strictly a DSLR up until now.  I imaged the same object using LRGB and everything was fine. This is the first narrowband and only the second time imaging with the ASI1600MM Pro. So this is all new to me and I have tried to learn as much as I could through forums.

All the frames were taken at 139 gain, 50 offset and -10 temp.  I used -10 because it has been hot outside and I wanted to make sure the camera could handle it. Is it your opinion the medians are too high in the dark and bias?

Pixel math is definitely a stretch right now but I'll see if I can figure out what you are telling me to do. Can I add pedestal and then use normally in preprocessing?

Thanks again.

[Greg Schwimer]

I'm happy to help. I used pixelmath as a quick way to try out calibrating with the dark & bias. Normally you'd just use the usual suspects to get the job done. I checked the median values with Statistics.

I don't think there is a way to add a pedestal in BatchPreProcessing but maybe its hidden somewhere in the settings. You can do it using the ImageCalibration tool by adjusting the output pedestal. I've not used it myself but it may do the trick for you.

As to the dark and bias frame median being higher than the light frame, normally I'd expect to see a light frame median that is at least a little bit higher than an uncalibrated dark, as the light frame would contain the bias signal, the dark signal, plus the background of the image. In narrowband it can be less of a difference than LRGB due to the lower amount of background light getting through.

The difference I see in your frames makes me think maybe something changed with the camera settings between capturing the bias & dark and the actual light frame exposure, or the lights were already calibrated possibly. At least it begs the question why a dark or bias frame w/ no light might be brighter than a frame that was exposed to light.

[pwatson]

When I took my darks and bias frames I did not use the Ha filter because I didn't think it would matter since it was a "dark". However, I just hooked my computer up to my rig and this time set the Ha filter and took some bias and dark frames.  The median value came down to 200 from the 750 range which puts it lower than the light frames.

Could this have been the problem all along? I'll take a set of darks and bias frames and try again. Now I understand what those charts were I read in a Cloudy Nights post.

Thanks again.

[pwatson]

I think I figured it out Greg.  My offset may have been set differently by mistake. It may have been at 15 for my lights and then 50 for my original darks and bias. It should have been at 50 for my lights. Does the offset have to be the same for darks, bias and lights?


Thanks.

[Greg Schwimer]

When I took my darks and bias frames I did not use the Ha filter because I didn't think it would matter since it was a "dark".

This is true.

However, I just hooked my computer up to my rig and this time set the Ha filter and took some bias and dark frames.  The median value came down to 200 from the 750 range which puts it lower than the light frames.

Verify all of the settings are the same as before the filter change. Also, if your camera does not have a physical shutter I'm guessing this could impact it. Not sure though.

My offset may have been set differently by mistake. It may have been at 15 for my lights and then 50 for my original darks and bias. It should have been at 50 for my lights. Does the offset have to be the same for darks, bias and lights?

I'm not sure. I don't know the details of how your camera works but I think you may be on to solving the problem. For calibration frames to do the best job they must accurately match the light frames. That is, all camera settings (binning, exposure, temperature, etc, etc) should be identical. This is the same as with a DSLR - if you change ISO, you need new calibration frames. An allowable difference may be with darks where they can be scaled from different exposures, but that doesn't always work as well as the "real thing".

[pwatson]

Greg, you saved me.  I reran some darks and bias frames with the lower offset setting (15) that I believe my lights were taken at by mistake.  This brought the median value down below the median value of the lights. I ran the BPP script this morning and everything worked fine. This is a lesson well learned.


Thanks again so much for looking at my data and discovering the issue.

Take care.

Pat

[Greg Schwimer]

Excellent! I'm glad we figured that out.