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Author Topic: Assitance with Calibration Frames  (Read 830 times)

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Offline Uplooker

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Assitance with Calibration Frames
« on: 13:35:06, 07 September, 2016 »
Hello All,
Further to posting my image the Eastern Veil image, I am aware that I need to bone up on taking calibration frames. For that particular image I only used Flats - no bias, darks, which were taken using a Huion light panel. I have subsequently taken bias & darks. When I took the Flats I aimed for an ADU value of 19-20,000, based on what I had gleaned from here and other sources.
When I look at the specs of my QHY9 the Full Well is given as 25.5ke-. This got me thinking that I need to reduce the ADU value that I take flats to 50% of the Full Well i.e. 11-12,000 ADU.

Am I correct?

On a slightly different point, I read a post on here, I think within the last 6-9 months,  that was made by Chris Bailey where he talked about and gave rational/formulae for calculating the max exposure based on the CCD Full Well capacity.

I am unable to find it and would appreciate it if someone could point me in the right direction to where the post is located.

Regard,

Ian
Ian

Scopes: SW 150P Newt, ST80, Celestron C8 XLT, TAL 100RS / PST Stage 2 Mod & BF10, SW 300PDS (soon to be on a Dob Mount)
Cameras: SPC900, QHY5, Canon 450D, DMK31AU03.AS
Mounts: NEQ6 powered by EQMod, EQ3-2

Online chris.bailey

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Re: Assitance with Calibration Frames
« Reply #1 on: 14:48:40, 07 September, 2016 »
Ian

I don't recall the post you are referring to.

It is all a little confusing. Full well is quoted in electron count. ADU is just value range which for a 16bit image is 0-65,536. It can take more than one electron to make a count of 1 ADU and this is related to the gain of the device. In very simple terms for a gain of 0.5, it takes two electrons in a well to give a count of 1 ADU. In an ideal world then a count of around 30,000 ADU would give you around half full well but that does depend a little on the sensor architecture and the controlling electronics. The suggestion of half full well is a little historical in that CCD's were not linear over their entire range and this linearity tended to break at around half full well. Modern sensors are much better and my tests on the 8300 chip suggests they are linear up to around 90% full well. The 50% advice has stuck as there are other considerations.

So why not just take very short duration, low ADU flats? They would have low signal to noise ratio and add noise to the images being calibrated so the aim is to have decent S/N flats to avoid having to take too many of them.

For my Atik 383L I find exposing flats to 30,000 ADU peak is a little hot and it can lead to some degree of over correction of dust shadows. Quite why this is is apparently down to the way the images are read out but I dont have a decent answer, it just is. I then aim for a mean ADU of 20,000 ADU which gives me a peak of around 22,500. Some people suggest a mean of 15,000 for the same camera so it is a matter of a little trial and error. If you have very string vignetting it might make sense to go a little hotter with the flats to avoid noise in the shadow regions.

Chris

ps I don't know if you have SGPro but it has the best Flats taking arrangement I have seen in any software package.

pps The graphic below is a pretty good explanation
LX200|ZS70|FSQ85|FLT110|Altair DF250RC|EQ6 Pro(Rowan Belt Mod)|ParamountMX
ATIK383L+/EFW2/OAG|Lodestar|Baader 36mm LRGBHaSIIOIII
Starlight Express SXVRH16/ONAG/FW|Lodestar X2|Baader 2" Filters
Starlight Express SXVRH814/ONAG/FW|LodestarX2|Baader 2" Filters
Lunt LS60PTBF1200|DMK41|Quark Chromo
Samyang 135mm f1.8

Offline tomhow

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Re: Assitance with Calibration Frames
« Reply #2 on: 16:54:44, 07 September, 2016 »
I think Chris covers everything there.

One thing i would encourage is testing your calibration workflow carefully. 

It can be very difficult to decide if the calibration is perfect when looking at a finished stack from many sub exposures taken at various different elevations and phases of the moon and weather. e.g. a smeared LP gradient can look like poor calibration. Not to mention background nebulosity.

Every so often I will "check" my calibration with a few subs of a  dull region, more or less straight up to avoid as many gradients as possible, with no moon and no nebula. If I can't flatten a featureless bit of space then I stand no chance on a dusty bit of milkway :)
Tak Sky 90, Atik 490, Homemade Mount, OAG, Lodestar

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Offline Uplooker

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Re: Assitance with Calibration Frames
« Reply #3 on: 22:44:54, 07 September, 2016 »
Thank you for the replies. I use ATP. I find it very good for my needs ATM. It does have a Flats aid, whereby I set the ADU I want to achieve, which filters I want to use and it will run through all the filters taking flats that fall within the range I specify and ultimately creates a Flats Plan that I just select and off it goes.
I must say the big advancement I have made is buying the Huion light panel. Prior to that I was trying to take sky / twilight flats. Gawd, that is hit and miss :canadian:

I normally run my QHY9 at -10degC and am pleased to see the darks I took had far fewer hot pixels that I thought I would get. I may try Chris's use of cooling to -20degC.

@ Chris,
I will try to give a better description and / or search for the thread that I only have a vague recollection of. It may of been around the max exposure for light frames. That said, you take considerably longer subs that I do, so I am unlikely to reach the max for the QHY9 considering the similarities between it and the Atik 383.

Thank you all again

Ian

Scopes: SW 150P Newt, ST80, Celestron C8 XLT, TAL 100RS / PST Stage 2 Mod & BF10, SW 300PDS (soon to be on a Dob Mount)
Cameras: SPC900, QHY5, Canon 450D, DMK31AU03.AS
Mounts: NEQ6 powered by EQMod, EQ3-2

Online chris.bailey

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LX200|ZS70|FSQ85|FLT110|Altair DF250RC|EQ6 Pro(Rowan Belt Mod)|ParamountMX
ATIK383L+/EFW2/OAG|Lodestar|Baader 36mm LRGBHaSIIOIII
Starlight Express SXVRH16/ONAG/FW|Lodestar X2|Baader 2" Filters
Starlight Express SXVRH814/ONAG/FW|LodestarX2|Baader 2" Filters
Lunt LS60PTBF1200|DMK41|Quark Chromo
Samyang 135mm f1.8

Offline Uplooker

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Re: Assitance with Calibration Frames
« Reply #5 on: 13:28:34, 08 September, 2016 »
Woooo, that's pretty darn deep and complex stuff. Will need to read a few more times to start to understand some of what's contained in there. Unfortunately that is not the one I was thinking of.
Don't worry yourself Chris, I will do a bit more searching and hopefully find what I am after. Thank you all the same.

Ian



Sent from my Fone
Ian

Scopes: SW 150P Newt, ST80, Celestron C8 XLT, TAL 100RS / PST Stage 2 Mod & BF10, SW 300PDS (soon to be on a Dob Mount)
Cameras: SPC900, QHY5, Canon 450D, DMK31AU03.AS
Mounts: NEQ6 powered by EQMod, EQ3-2

 

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