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INDI Library v1.8.6 Released (21 Aug 2020)

August 2020 release of INDI Library v1.8.6 introduces new drivers while providing fixes and improvements to existing devices and core framework.

Bias frames - No data/empty?

8 months 3 weeks ago
andefeldt
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Bias frames - No data/empty? #47322
Hi,
I've been capturing everything with KStars/Ekos by selecting either Bias, Flats, Darks or Lights as my image type. Up until recently I've stacking in DSS but I've just got PixInsight and I want to do everything in here (I'm running Mac). When trying to stack my Bias frames, I get an error telling me that the frames doesn't contain any data and the process fails (PI: Zero or insignificant signal detected (empty image?). I've tried opening a Bias frame in other programs and they indeed look very empty even when fully stretched. So I guess I've produced all my images so far without Bias frames.

What am I doing wrong in Ekos when capturing Bias frames? The header of a Bias frames states the following:
SIMPLE  =                    T / file does conform to FITS standard
BITPIX  =                   16 / number of bits per data pixel
NAXIS   =                    2 / number of data axes
NAXIS1  =                 4144 / length of data axis 1
NAXIS2  =                 2822 / length of data axis 2
EXTEND  =                    T / FITS dataset may contain extensions
COMMENT   FITS (Flexible Image Transport System) format is defined in 'Astronomy
COMMENT   and Astrophysics', volume 376, page 359; bibcode: 2001A&A...376..359H
BZERO   =                32768 / offset data range to that of unsigned short
BSCALE  =                    1 / default scaling factor
INSTRUME= 'ZWO CCD ASI294MC Pro' / CCD Name
TELESCOP= 'EQMod Mount'        / Telescope name
OBSERVER= 'Unknown '           / Observer name
OBJECT  = 'Unknown '           / Object name
EXPTIME =         3.200000E-05 / Total Exposure Time (s)
CCD-TEMP=            -2.52E+01 / CCD Temperature (Celsius)
PIXSIZE1=         4.630000E+00 / Pixel Size 1 (microns)
PIXSIZE2=         4.630000E+00 / Pixel Size 2 (microns)
XBINNING=                    1 / Binning factor in width
YBINNING=                    1 / Binning factor in height
XPIXSZ  =         4.630000E+00 / X binned pixel size in microns
YPIXSZ  =         4.630000E+00 / Y binned pixel size in microns
FRAME   = 'Bias    '           / Frame Type
XBAYROFF=                    0 / X offset of Bayer array
YBAYROFF=                    0 / Y offset of Bayer array
BAYERPAT= 'RGGB    '           / Bayer color pattern
SCALE   =        -9.551690E+02 / arcsecs per pixel
DATE-OBS= '2019-12-02T22:48:43.287' / UTC start date of observation
COMMENT Generated by INDI
GAIN    =                 120. / Gain

SkyWatcher ED80 Pro
Skywatcher HEQ5 Pro with Rowan Belt Mod with EQDIR cable
ZWO ASI294MC Pro imaging camera
QHY5L-II-C guide camera on 50mm guidescope
DeepSkyDad AF3 autofocuser
Raspberry Pi 4 4GB running Kstars/Ekos

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8 months 3 weeks ago
Herrhausen
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Bias frames - No data/empty? #47325
Don't worry, as long as you take and subtract dark frames, everything should be fine (bias is included in dark frames). I ran a quick test with the same camera and found it works as expected. Did you load the bias frame in FITS viewer and what does it look like and, more importantly, how do statistics look like (MAX for R,G,B in particular)?

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8 months 3 weeks ago
andefeldt
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Bias frames - No data/empty? #47327
I've tried opening the files in various programs and they all give me an empty file. I just tried in KStars fits viewer which just gives me the same result.

All values are 0!

Light frames, dark frames and flat frames work fine, but bias frames doesn't.

What am I doing wrong?


SkyWatcher ED80 Pro
Skywatcher HEQ5 Pro with Rowan Belt Mod with EQDIR cable
ZWO ASI294MC Pro imaging camera
QHY5L-II-C guide camera on 50mm guidescope
DeepSkyDad AF3 autofocuser
Raspberry Pi 4 4GB running Kstars/Ekos
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8 months 3 weeks ago
El Corazon
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Bias frames - No data/empty? #47329
I have run into the same problem before.

You are trying to capture a frame at EXPTIME = 3.200000E-05 / Total Exposure Time (s), which may be less than the camera can handle. When Alfred captured his bias frames, it looks like he used a 1 s exposure time.
Try increasing the exposure time for your bias frames to something greater than 0.000032s. Then it should work.

Jo

Atlas Pro AZ-EQ, ASI1600MM-Pro, ASI120MM-S, ES102ED, WO-Z61, Nikon D3300, ASI-EFW, ZWO LRGB,Ha,O3,S2 filter set

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8 months 3 weeks ago
andefeldt
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Bias frames - No data/empty? #47330
Actually, Ekos selects that exposure time for me. I (by accident) sat the time to 3.725 seconds last time, but Ekos just exposed the bias frames as the lowest possible (the 0.00003 s that’s written in the header). The file was named with the 3.725 label though.

SkyWatcher ED80 Pro
Skywatcher HEQ5 Pro with Rowan Belt Mod with EQDIR cable
ZWO ASI294MC Pro imaging camera
QHY5L-II-C guide camera on 50mm guidescope
DeepSkyDad AF3 autofocuser
Raspberry Pi 4 4GB running Kstars/Ekos

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8 months 3 weeks ago
El Corazon
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Bias frames - No data/empty? #47331
Have you tried shooting them as light frames and progressively setting a shorter exposure time? Then you find out when your camera hardware becomes limiting.

Atlas Pro AZ-EQ, ASI1600MM-Pro, ASI120MM-S, ES102ED, WO-Z61, Nikon D3300, ASI-EFW, ZWO LRGB,Ha,O3,S2 filter set

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8 months 3 weeks ago
andefeldt
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Bias frames - No data/empty? #47334
No, but I don’t think that’ll help since Ekos automatically sets the bias exposure time to 3.2E-5 s, no matter what I enter as exposure time.

SkyWatcher ED80 Pro
Skywatcher HEQ5 Pro with Rowan Belt Mod with EQDIR cable
ZWO ASI294MC Pro imaging camera
QHY5L-II-C guide camera on 50mm guidescope
DeepSkyDad AF3 autofocuser
Raspberry Pi 4 4GB running Kstars/Ekos

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8 months 3 weeks ago
El Corazon
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Bias frames - No data/empty? #47335
I understand.

That's when you set the tab for 'Bias'.

What I meant was that you should set the tab for 'Light' frames, then take images at the shortest exposure your camera will allow (Probably 1/1000s). Then use those as bias frames.

Atlas Pro AZ-EQ, ASI1600MM-Pro, ASI120MM-S, ES102ED, WO-Z61, Nikon D3300, ASI-EFW, ZWO LRGB,Ha,O3,S2 filter set

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8 months 3 weeks ago
ChrisRowland
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Bias frames - No data/empty? #47336
Let's try Science.
There's a hypothesis that the camera collects no data if the exposure is too short.
Try taking dark frames at shorter times until you see that the data goes to all zeroes.
Or try lights with the camera covered, it's the same thing.

Maybe there's a point where if the exposure is too short the camera does nothing.

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8 months 3 weeks ago
Copello
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Bias frames - No data/empty? #47337
There is an excellent book/tutorial CCD Data Reduction Guide
that explains in very great detail with Python (astropy) code how actually an astronomical CCD image is formed.
In chapter Understanding an astronomical CCD image
is written:

An offset voltage called bias is applied to the CCD chip to ensure there are no negative counts during readout. There are small variations in the value of the bias across the chip, and there can be small variations in the bias level over time.


In sub chapter Construction of an artificial (but realistic) image
is explained in detail how a offset (bias) image is formed.

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8 months 3 weeks ago 8 months 3 weeks ago by Herrhausen.
Herrhausen
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Bias frames - No data/empty? #47351
My bias frame was (automatically) taken with 3.200000E-05 exposure time (pulldown menu set to BIAS in Ekos) too and it is NOT empty. So something different is not right with andefeldt's setup.

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8 months 3 weeks ago
wvreeven
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Bias frames - No data/empty? #47352
It is possible that nothing is wrong with the Bias subs but rather with the way they are processed with PixInsight. Perhaps this should be asked in the PixInsight forum first? Either this leads to a correct way of processing them or to the conclusion that indeed the bias subs are wrong. In my experience it is best to start debugging at the end of the chain rather than at the start.


Wouter

ASI6200 and filter wheel with a SkyWatcher Esprit 80 ED on a SkyWatcher HEQ5-Pro
ASI1600MM-Pro Cooled and filter wheel with an 8" TS Ritchey-Chrétien on a SkyWatcher EQ6-R

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