[gregbradley]

Hi Richard,

Thanks for writing your paper on flats. Very infomative.

I remember various posts that were made after you published this paper. Your recommendation
for a typical Proline 16803 camera if I recall correctly was to go for about 40K ADU levels. Your
paper indicates around 5-10 subs should do (for an ML4022).

---I don't recall the ADU levels but I described how to determine the camera gain by making a photon transfer plot to character the main performance parameters of the camera. I always work in terms of electron units and to be able to convert from ADU to electrons you need to know the gain (units of e-/ADU)

One thing I didn't understand was the recommendation for 250Ke to 1000Ke. I use CCDSoft and the reported
values are in ADU. How do you convert Ke to ADU?

----1Ke- = 1000e-
ADU * gain (e-/ADU) = e-. When you multiply these two items together the ADU in the numerator cancels the ADU in the denominator leaving behind only e-. That, by the way, is called "dimensional analysis" and is taught in the first semester of the freshman year in engineering school which is where I learned it way back at Texas A&M in 1972 (dating myself).

---Last time I measured a Proline 16803 I found a gain of about 1.5e-/ADU. So for 40,000 ADU that means you have about 60,000e- or 60Ke-



I use a CDK17 often and it has bad vignetting and tends to have a hot spot in the illumination in the centre.
I have minor light pollution when imaging rising objects in the east. This results in hard to process images
and flats do not seem to get rid of all the vignetting but a lot of it.

What would you consider best practice for flats in this situation?

---Flats are easy in principle but difficult in practice. If you are imaging a rising object in the East and have Light Pollution, why not wait until it is higher and then begin shooting? If that doesn't work for you then you may need to first calibrate those low shots, remove the gradient with a gradient removal tool and then set them aside for stacking later when you have all the data you want to stack. My open tube home built Classical Cass is hard to shoot flats for because I cannot do the Mid-Day method I can use on my refractors because the OTA is not light-tight. I found a workaround by using a ML4022 camera which has a KAI series interline sensor. Using the electronic shutter I can take flats that are 100milliseconds (0.1 second) long and not have any mechanical shutter transition artifacts. That is a good method for an interline chip. The problem with removing the gradient from individual frames is that the SNR is usually low and when you start doing mathematical manipulations on low SNR images you can make a noisy mess out of the result.



Do I use flat dark subtract when making the master flat or simply use bias subtract when applying the flats?

--you can do either. I usually use darks (no RBI mitigation!). My flats will be exposed from 5 to 12-13 seconds typically so I may shoot 20 darks of 6 seconds and use that to calibrate each raw flat prior to stacking (using normalization turned ON).

I seem to get better results using bias subtract when applying the min/max clipped master flat. But perhaps I am not
doing something 100% there. I use dusk flats usually pointing to the opposite side of the sky to the sunset area
and usually only 3 flats per filter (I see you recommend more than that). It is a bit of a rush to capture dusk flats
but I find also I get the same results by using a white cotton cover over the scope and take flats during the day
in my internally black painted observatory as a small amount of light filters in at the roof/wall junction.

---That is precisely why I recommend having a fast downloading camera. You are screwed on flats otherwise. The best flats use the sky as the light source, your task is to come up with a scheme that works. 3 is entirely too few. Look over my treatise on flats again and see how the FFPTC shows you how effective your flats are at removing the FPN (can be sensor based FPN or can be optical non-uniformity including the center hot spot (really cosine to the fourth power rollof) and dust motes.

Do small well cameras require more subs in their flats than deep well cameras?

---No: again looking at the FFPTC information (flat field photon transfer curve) you can immediately see that for low signal level in the images you don't need much in the way of flats. For high signal level you need really good flats. So the moon is a worst case but a faint galaxy may be the best case from the perspective of making do with a small number of flats. Signal level is what is important.
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Best regards,

Greg.


Greg Bradley















Reply, Reply All or Forward | More






MeHi Richard, Brilliant. Thank you for taking the time to answer me there. Much appreciated. Best regards, Greg. Greg Bradley To Richard Crisp


Today at 12:58 PM

Hi Richard,

Brilliant.

Thank you for taking the time to answer me there. Much appreciated.

Best regards,

Greg.


Greg Bradley




Show message history


On Saturday, 9 November 2013 9:50 AM, Richard Crisp <rdcrisp@sbcglobal.net> wrote:

answers inline below.
I am including our new buddy Mark Striebeck of Google on this reply because I suspect he will gain some benefit from this discussion.
rdc




From: Greg Bradley <bradgregley10@yahoo.com>
To: "rdcrisp@sbcglobal.net" <rdcrisp@sbcglobal.net>
Sent: Friday, November 8, 2013 11:46 AM
Subject: re; Flats


Hi Richard,

Thanks for writing your paper on flats. Very infomative.

I remember various posts that were made after you published this paper. Your recommendation
for a typical Proline 16803 camera if I recall correctly was to go for about 40K ADU levels. Your
paper indicates around 5-10 subs should do (for an ML4022).

---I don't recall the ADU levels but I described how to determine the camera gain by making a photon transfer plot to character the main performance parameters of the camera. I always work in terms of electron units and to be able to convert from ADU to electrons you need to know the gain (units of e-/ADU)

One thing I didn't understand was the recommendation for 250Ke to 1000Ke. I use CCDSoft and the reported
values are in ADU. How do you convert Ke to ADU?

----1Ke- = 1000e-
ADU * gain (e-/ADU) = e-. When you multiply these two items together the ADU in the numerator cancels the ADU in the denominator leaving behind only e-. That, by the way, is called "dimensional analysis" and is taught in the first semester of the freshman year in engineering school which is where I learned it way back at Texas A&M in 1972 (dating myself).

---Last time I measured a Proline 16803 I found a gain of about 1.5e-/ADU. So for 40,000 ADU that means you have about 60,000e- or 60Ke-



I use a CDK17 often and it has bad vignetting and tends to have a hot spot in the illumination in the centre.
I have minor light pollution when imaging rising objects in the east. This results in hard to process images
and flats do not seem to get rid of all the vignetting but a lot of it.

What would you consider best practice for flats in this situation?

---Flats are easy in principle but difficult in practice. If you are imaging a rising object in the East and have Light Pollution, why not wait until it is higher and then begin shooting? If that doesn't work for you then you may need to first calibrate those low shots, remove the gradient with a gradient removal tool and then set them aside for stacking later when you have all the data you want to stack. My open tube home built Classical Cass is hard to shoot flats for because I cannot do the Mid-Day method I can use on my refractors because the OTA is not light-tight. I found a workaround by using a ML4022 camera which has a KAI series interline sensor. Using the electronic shutter I can take flats that are 100milliseconds (0.1 second) long and not have any mechanical shutter transition artifacts. That is a good method for an interline chip. The problem with removing the gradient from individual frames is that the SNR is usually low and when you start doing mathematical manipulations on low SNR images you can make a noisy mess out of the result.



Do I use flat dark subtract when making the master flat or simply use bias subtract when applying the flats?

--you can do either. I usually use darks (no RBI mitigation!). My flats will be exposed from 5 to 12-13 seconds typically so I may shoot 20 darks of 6 seconds and use that to calibrate each raw flat prior to stacking (using normalization turned ON).

I seem to get better results using bias subtract when applying the min/max clipped master flat. But perhaps I am not
doing something 100% there. I use dusk flats usually pointing to the opposite side of the sky to the sunset area
and usually only 3 flats per filter (I see you recommend more than that). It is a bit of a rush to capture dusk flats
but I find also I get the same results by using a white cotton cover over the scope and take flats during the day
in my internally black painted observatory as a small amount of light filters in at the roof/wall junction.

---That is precisely why I recommend having a fast downloading camera. You are screwed on flats otherwise. The best flats use the sky as the light source, your task is to come up with a scheme that works. 3 is entirely too few. Look over my treatise on flats again and see how the FFPTC shows you how effective your flats are at removing the FPN (can be sensor based FPN or can be optical non-uniformity including the center hot spot (really cosine to the fourth power rollof) and dust motes.

Do small well cameras require more subs in their flats than deep well cameras?

---No: again looking at the FFPTC information (flat field photon transfer curve) you can immediately see that for low signal level in the images you don't need much in the way of flats. For high signal level you need really good flats. So the moon is a worst case but a faint galaxy may be the best case from the perspective of making do with a small number of flats. Signal level is what is important.

Best regards,


Greg.