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Flats and darks with SharpCap questions

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#1 Biggen

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Posted 27 May 2019 - 07:40 AM

So doing some reading, darks need to be captured at the same temp, exposure, gain, etc... setting that you would be using when imaging the target.

 

But what about the flats? Can I not take flats ahead of time and store them? Or do they also need to match all the same settings as the target and darks?

 

Im trying to figure out the work flow with Sharpcap for darks and flats.  Once I’m happy with a target but BEFORE I livestack, am I supposed to capture darks and flats at once and then begin live stacking?

 

I understand that darks/flats aren’t required for livestacking, but I’m trying to learn the various ways to do things if I ever want to jump more into the AP realm. Currently using an ASI224 with a WO61 and AZ GTi.


Edited by Biggen, 27 May 2019 - 07:42 AM.


#2 makeitso

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Posted 27 May 2019 - 08:03 AM

Here is the information I have in how to take flats. This is copy and paste, not my words. I’m pretty sure it’s correct.

 

“The Flat Frames are used to correct the vignetting and uneven field illumination created by dust or smudges in your optical train.

To create good flat frames it is very important to not remove your camera from your telescope before taking them (including not changing the focus).
You can use a lot of different methods (including using a flatbox) but I found that the simplest way is to put a white T shirt in front of your telescope and  smooth out the folds. Then shoot something luminous (a flash, a bright white light, the sky at dawn...) and let the camera decide of the exposure time (Av mode),

The flat frames should be created with the ISO speed of the light frames. The temperature is not important.

Take a few of them (between 10 and 20 is usually enough). DeepSkyStacker will combine them automatically to create and use a clean master flat frame.”

 

Jack



#3 Biggen

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Posted 27 May 2019 - 08:21 AM

Hmm..  Well I'm more confused now. 

 

Reading through the Sharpcap Manual about flat frames, it says:

 

Setup Camera Settings

Ensure that the camera is set to the correct settings at this stage. If you intend to image at 1600x1200, bin 1, RAW12 then set the camera to those settings before capturing flat frames. There is no need to use the same exposure or gain for flat frames (in fact this would not normally work). Since flat frames should have as little noise as possible, it is usually a good idea to set a low gain value.

So perhaps I answered my own question a bit.  According to the Sharpcap manual, it doesn't seem matter what exposure/gain your light (target) frame was captured as so long as the resolution, binning, color depth mode, etc are set the same.

 

Reading a bit futher there is also this: 

 

Capture and Subtract Bias Frames – If selected, SharpCap will set the camera exposure to minimum after capturing the flat frames and then capture an equal number of bias frames. The master flat frame will then be made up of the average of all the bias frames subtracted from the average of all the flat frames. Selecting this option should give better flat correction across a wide range of image brightness, but requires that the Black Level/Offset/Brightness controls of the camera are not modified between capturing the flat frame and capturing the target image frames.

So if I check that box, I cannot adjust the Brightness slider in Sharpcap after capturing and subtracting bias frames?


Edited by Biggen, 27 May 2019 - 08:23 AM.


#4 makeitso

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Posted 27 May 2019 - 09:11 AM

What I posted is general information not specific to sharpcap, more of a rule of thumb.

 

I am definitely not the expert here in this. More will chime in I’m sure, and probably with more specific information for you.

 

Jack



#5 mikenoname

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Posted 27 May 2019 - 12:16 PM

Here is some info I have gleaned on the subject. Haven't actually tried to use flats yet, though.

 

I take flats, at sunset, with the scope pointed about 15 degrees east of the zenith. No t shirt needed.
Exposure time ranges from 10ms-30ms (adjust the time so the peak of the white histogram is at 50%). You turn the histogram

on by clicking at the upper right corner icon.
I capture 10.
Oh... and you can't capture flats if you have darks already applied.

 

More info:
https://forums.sharp...hp?p=4753#p4753


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#6 DSO_Viewer

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Posted 27 May 2019 - 12:17 PM

What I posted is general information not specific to sharpcap, more of a rule of thumb.

 

I am definitely not the expert here in this. More will chime in I’m sure, and probably with more specific information for you.

 

Jack

Flats must be taken at all the same settings as your light except for the exposure time.

 

Steve



#7 Biggen

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Posted 27 May 2019 - 01:04 PM

Flats must be taken at all the same settings as your light except for the exposure time.

 

 

What about gain? That is supposed to be keep at a minimum when taking flats according to the manual.



#8 Rickster

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Posted 27 May 2019 - 01:42 PM

One thing to keep in mind is that using dark, flat and/or bias frames will always (yes, always) add noise to your image.  So it is important to do it in a way that minimizes this added noise.  Otherwise you may unintentionally make your image worse rather than better.  http://deepskystacke...lish/theory.htm

 

If your original image has objectional vignetting or amp glow, etc, it may make sense to use dark, flat and/or bias frames.  But too often, people use darks/flats/bias frames because they think it is the thing to do, without understanding when it makes sense and when it doesn't.  Which is to say that they expend extra time and effort to make their images worse.

 

Personally, I don't use darks, flats or bias.  Instead, I prefer to set up my imaging train to produce images (lights) that do not need darks, flats or bias frames. I figure that the better I can make my data, the less I have to mess with it.

 

For example.  Take a look at the images in this post.  https://www.cloudyni...-dso/?p=8477462  They are straight out of the camera (a circa 2012 DSLR, no less).  If dark, flat and/or bias frames were added to those images, the net effect would be negative.


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#9 Biggen

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Posted 27 May 2019 - 01:55 PM

Rick thanks for that reply!

 

Yeah, I think I'll take a step back and get stacking working properly first.  I went down the rabbit hole reading the other day.  

 

I would think that using flats and blacks for EAA would be a bit cumbersome if you are wanting to view several targets in an evening.  If you are expected to take flats/blacks with the same settings/temperature as your lights, then you would have to take them before each target you wanted to look at.

 

I could see doing this if you wanted to focus HEAVILY on one target for the evening.  But if you wanted to see a dozen targets in one session, this would take too much time to setup every time.


Edited by Biggen, 27 May 2019 - 02:05 PM.

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#10 Biggen

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Posted 27 May 2019 - 01:56 PM

Here is some info I have gleaned on the subject. Haven't actually tried to use flats yet, though.

 

I take flats, at sunset, with the scope pointed about 15 degrees east of the zenith. No t shirt needed.
Exposure time ranges from 10ms-30ms (adjust the time so the peak of the white histogram is at 50%). You turn the histogram

on by clicking at the upper right corner icon.
I capture 10.
Oh... and you can't capture flats if you have darks already applied.

 

More info:
https://forums.sharp...hp?p=4753#p4753

So once you capture your flats, those are for that evening only?  You start the process over again next viewing session or do you reuse?



#11 DSO_Viewer

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Posted 28 May 2019 - 11:11 AM

What about gain? That is supposed to be keep at a minimum when taking flats according to the manual.

No every setting including the gain, again with the exceptions of exposure time.

 

Steve



#12 DSO_Viewer

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Posted 28 May 2019 - 11:15 AM

One thing to keep in mind is that using dark, flat and/or bias frames will always (yes, always) add noise to your image.  So it is important to do it in a way that minimizes this added noise.  Otherwise you may unintentionally make your image worse rather than better.  http://deepskystacke...lish/theory.htm

 

If your original image has objectional vignetting or amp glow, etc, it may make sense to use dark, flat and/or bias frames.  But too often, people use darks/flats/bias frames because they think it is the thing to do, without understanding when it makes sense and when it doesn't.  Which is to say that they expend extra time and effort to make their images worse.

 

Personally, I don't use darks, flats or bias.  Instead, I prefer to set up my imaging train to produce images (lights) that do not need darks, flats or bias frames. I figure that the better I can make my data, the less I have to mess with it.

 

For example.  Take a look at the images in this post.  https://www.cloudyni...-dso/?p=8477462  They are straight out of the camera (a circa 2012 DSLR, no less).  If dark, flat and/or bias frames were added to those images, the net effect would be negative.

I also agree to keep your imaging train is best as possible for example clean with no dust, proper spacing for your reducer to prevent vignetting etc. You cannot control dark current noise caused by thermal heat generated by the camera esp. during the hotter seasons. Having dark's will make a big difference and really shows when you zoom in tight.

 

Steve



#13 saguaro

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Posted 28 May 2019 - 11:25 AM

So once you capture your flats, those are for that evening only?  You start the process over again next viewing session or do you reuse?

You can re-use your master flat between sessions if you don't remove or change anything in your imaging train, i.e., camera position, filters, focus, etc.


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#14 Rickster

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Posted 28 May 2019 - 02:15 PM

Here is some good info on using darks.  http://www.clarkvisi...ion-technology/



#15 davidparks

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Posted 28 May 2019 - 02:47 PM

All calibration images should be taken at the same size/resolution/bin level.

 

Darks - Same exposure, gain, and temperature as the lights.  Use a 50/50 Red/Blue setting with OSC cameras to maintain a baseline color balance.

The purpose of Darks is to remove camera noise, including Amp Glow, Thermal Noise, and Hot/Cold pixels.  This is all about the camera electronics (not about the image/optics/light).  EAA generally uses short exposures, so Darks will have very little impact, however, you can create darks for your common settings and use them over and over, known as a Dark Library.  This is much easier if you are using a cooled camera.  Without a cooled camera, you may want to take Darks just before or after your Lights, in order to match the temperature. 

 

Flats - Same brightness as Lights, monochrome, bias, 50% luminance on histogram.  Use the gain setting that produces the least noise (know your camera, it might be 0, it might be 120, or 350, depends on camera)(exposures can be very small if using a light panel, milliseconds, just shoot for 50% luminance).

The purpose of Flats is to remove light gradients, vignetting, dust motes, etc.  This is all about the image/optics/light.  The orientation of every piece of glass in your imaging train needs to be the same as your Lights, including focus.  Don't fret about the very fine/minute focus you might make because of temperature changes or seeing conditions, but take your flats at the same "infinity focus" as your Lights.

“Brightness” is also called “Black Level”, and sometimes “Offset”.  If you use the Sharpcap Smart Histogram Brain Window, it can measure your sky brightness and tell you the Optimal Black Level.  For me this is 4.  It will even automatically set the Brightness slider for you.  Take your Flats and Lights using the same Brightness.  Flats will make a great impact on your EAA imaging, I highly recommend them.  You will need to figure out when the best time in your workflow is to take Flats, this will depend on what method you use (t-shirt, twilight, flat panel, etc).  Personally, I use a cheap light panel which is simple and fast.  I take Flats "on-the-fly" just before I start imaging Lights.   Generally, you do not reuse Flats from night to night, although they can be used on all targets in a single session as long as you don't rotate any glass in your imaging train.  Theoretically you might reuse them, but in practice something always changes... the scope is jiggle in transport, a dust speck falls on your lens, etc.  Learning to take Flats "on-the-fly" with a light panel makes this calibration frame fast and easy, so much so that you can rotate your camera to frame your target "just so" and quickly take new flats just for that target.

 

There are many threads about calibrations frames, methods, and equipment for such.  I can also be more detailed here if needed.


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#16 makeitso

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Posted 28 May 2019 - 06:59 PM

Flats must be taken at all the same settings as your light except for the exposure time.

 

Steve

You did read the post before you posted this right? You would have noticed it said don’t change anything including the focus if you would have.

 

Thanks, Jack


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#17 Biggen

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Posted 29 May 2019 - 08:35 AM

Thanks for all the help guys.  Its all coming together!  I played around with Sharpcap last night on M106 and was able to easily integrate darks into the livestacking feature. It greatly reduced the hot pixels vs when I tried to livestack M106 with no darks.

 

I ordered an LED lightbox on Amazon today to play around with flats.  I had round "dust motes" after stacking on the final image.

 

Lots of fun anyway!


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#18 Biggen

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Posted 29 May 2019 - 09:33 AM

All calibration images should be taken at the same size/resolution/bin level.

 

Darks - Same exposure, gain, and temperature as the lights.  Use a 50/50 Red/Blue setting with OSC cameras to maintain a baseline color balance.

The purpose of Darks is to remove camera noise, including Amp Glow, Thermal Noise, and Hot/Cold pixels.  This is all about the camera electronics (not about the image/optics/light).  EAA generally uses short exposures, so Darks will have very little impact, however, you can create darks for your common settings and use them over and over, known as a Dark Library.  This is much easier if you are using a cooled camera.  Without a cooled camera, you may want to take Darks just before or after your Lights, in order to match the temperature. 

 

Flats - Same brightness as Lights, monochrome, bias, 50% luminance on histogram.  Use the gain setting that produces the least noise (know your camera, it might be 0, it might be 120, or 350, depends on camera)(exposures can be very small if using a light panel, milliseconds, just shoot for 50% luminance).

The purpose of Flats is to remove light gradients, vignetting, dust motes, etc.  This is all about the image/optics/light.  The orientation of every piece of glass in your imaging train needs to be the same as your Lights, including focus.  Don't fret about the very fine/minute focus you might make because of temperature changes or seeing conditions, but take your flats at the same "infinity focus" as your Lights.

“Brightness” is also called “Black Level”, and sometimes “Offset”.  If you use the Sharpcap Smart Histogram Brain Window, it can measure your sky brightness and tell you the Optimal Black Level.  For me this is 4.  It will even automatically set the Brightness slider for you.  Take your Flats and Lights using the same Brightness.  Flats will make a great impact on your EAA imaging, I highly recommend them.  You will need to figure out when the best time in your workflow is to take Flats, this will depend on what method you use (t-shirt, twilight, flat panel, etc).  Personally, I use a cheap light panel which is simple and fast.  I take Flats "on-the-fly" just before I start imaging Lights.   Generally, you do not reuse Flats from night to night, although they can be used on all targets in a single session as long as you don't rotate any glass in your imaging train.  Theoretically you might reuse them, but in practice something always changes... the scope is jiggle in transport, a dust speck falls on your lens, etc.  Learning to take Flats "on-the-fly" with a light panel makes this calibration frame fast and easy, so much so that you can rotate your camera to frame your target "just so" and quickly take new flats just for that target.

 

There are many threads about calibrations frames, methods, and equipment for such.  I can also be more detailed here if needed.

Thanks David!  I was able to use darks last night when I was livestacking M106.  I have a light panel on order so I'll play with that when it comes in this week.

 

I'm using the ASI224.  It appears its read noise drops sharply at 60 gain according to the manual and continues to decrease until flattening out around 250 or so.

 

I did the sensor analysis the other day in Sharpcap and the read noise graph it plotted virtually lines up with the ZWO graph in the manual for the 224 so at least I know the camera is working properly.

 

I haven't yet messed with the Brain or Smart Histogram yet as it seems a bit daunting.  But it sure would be nice to let Sharpcap figure out the gain/exposure/brightness and then take my flats/darks with those settings instead of me having to guess.


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#19 davidparks

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Posted 29 May 2019 - 01:10 PM

That’s great news B waytogo.gif   The Sharpcap Brain does a really good job at measuring your sky brightness.  Just specify a lower and upper limit, like .1 to 60 seconds for exposure, choose unity or dynamic range for gain (I recommend dynamic range), and a total integration time.  Sharpcap will do it’s thing and recommend a good exposure, gain, and brightness level.  (I don’t think different integration times really change the numbers, it’s all about sky brightness).   But there you go, now you know the Brightness level you should set your Lights and Flats too, regardless of exposure/gain you plan to use. I’d stick with max dynamic range for your gain, and then “round up or down” the exposure to the nearest thing that makes sense.  You can make a Dark Library using all of SC’s ‘quick pick’ exposures:  1, 2, 4, 8, 15, 30, 60 for the one or two gain setting you end up using for lights.


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#20 Biggen

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Posted 02 June 2019 - 07:57 AM

That’s great news B waytogo.gif   The Sharpcap Brain does a really good job at measuring your sky brightness.  Just specify a lower and upper limit, like .1 to 60 seconds for exposure, choose unity or dynamic range for gain (I recommend dynamic range), and a total integration time.  Sharpcap will do it’s thing and recommend a good exposure, gain, and brightness level.  (I don’t think different integration times really change the numbers, it’s all about sky brightness).   But there you go, now you know the Brightness level you should set your Lights and Flats too, regardless of exposure/gain you plan to use. I’d stick with max dynamic range for your gain, and then “round up or down” the exposure to the nearest thing that makes sense.  You can make a Dark Library using all of SC’s ‘quick pick’ exposures:  1, 2, 4, 8, 15, 30, 60 for the one or two gain setting you end up using for lights.

David I was playing around last night and used the Brain.  I set it to a minimum of 2 second subs and a maximum of 10 second subs over the course of 60 minutes max.  I had it set to dynamic range.

 

It came back and told me my skies are 3 e/pixels/s and told me I needed 600 six second exposures over the course of an hour with a gain at 61 and an offset at 12.

 

Well I did exactly what it told me but the images are so dark that DSS can't even detect a single star when trying to stack them.  I have to slide the upper right brightness slider in DSS all the way to the left it order to see anything in the 1st sub of my stack.

 

I'm assuming that with the parameters I told the Brain to image, the image is just too dim.  For dynamic range to work, would I need to either up the subs or lengthen the total stacking time? 


Edited by Biggen, 02 June 2019 - 08:01 AM.


#21 mikenoname

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Posted 02 June 2019 - 11:32 AM

David I was playing around last night and used the Brain.  I set it to a minimum of 2 second subs and a maximum of 10 second subs over the course of 60 minutes max.  I had it set to dynamic range.

 

It came back and told me my skies are 3 e/pixels/s and told me I needed 600 six second exposures over the course of an hour with a gain at 61 and an offset at 12.

 

Well I did exactly what it told me but the images are so dark that DSS can't even detect a single star when trying to stack them.  I have to slide the upper right brightness slider in DSS all the way to the left it order to see anything in the 1st sub of my stack.

 

I'm assuming that with the parameters I told the Brain to image, the image is just too dim.  For dynamic range to work, would I need to either up the subs or lengthen the total stacking time? 

 

I had the same experience when I tried playing around with it. The settings it recommended were WAY too dark to get acceptable images.


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#22 davidparks

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Posted 02 June 2019 - 12:09 PM

Let’s look at the specific graphs for your camera,  I believe you said you were using the ASI224

 

2954ak0.jpg

In the 4th graph, Read Noise, we can see that at 61 gain, the read noise takes a nice drop, while the DR (dynamic range, 3rd graph) jumps back up to it’s highest point. Thus 61 is highest dynamic range.  However, it appears that all the reports that the 224 is great high gain camera are true, as you can see in the 4th graph, as you increase gain, noise is reduced even further (brighter images without increasing noise), however, you also loose a bit of dynamic range.  Dynamic range is the difference between the lightest and darkest areas, you might think of it as “how many shades of grey” between black and white.

 

We can also see, on the 2nd chart, that Unity Gain is 135.  Unity Gain is the setting whereby the sensor produces 1 electron for each photon of light (in a given pixel).  I recommended you try Highest Dynamic Range to start with, but it seems the 224 actually performs better at higher gains for deep sky objects.  You can try Unity Gain at 135, the Sharpcap Brain can use that too, but I would also consider both 200 and 350 as viable gain settings to compare with.  All of these (135, 200, 350) will get you much brighter images at the same exposure.  You will loose a little range, but it may not be noticeable.  Since in general, for EAA, you may observe a target for 5 or 10 minutes (instead of hours), you may opt to use the higher gain of 350, to get as deep as you can in a smaller integration time.  If you don’t mind letting a target stack for a bit longer, you might enjoy a more modest 200 gain.  In any case, it’s obvious that 61 won’t work for you, so choose a small handful, like 135, 200, 350, and do a couple comparisons.  For each gain setting, choose the common exposure quick pick that just begins to show stars in the un-stretched view.  This should allow Sharpcap (or DSS) to stack successfully.  After stretching a 5 minute stack at each of your chosen gain settings, you will know exactly which gain works best for you, and the sky where you are at.  You can then build your workflow, and Dark Library, all around that gain setting(s).

 

Hope this makes sense smile.gif


Edited by davidparks, 02 June 2019 - 12:14 PM.

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#23 Biggen

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Posted 03 June 2019 - 11:32 AM

Let’s look at the specific graphs for your camera,  I believe you said you were using the ASI224

 

 

In the 4th graph, Read Noise, we can see that at 61 gain, the read noise takes a nice drop, while the DR (dynamic range, 3rd graph) jumps back up to it’s highest point. Thus 61 is highest dynamic range.  However, it appears that all the reports that the 224 is great high gain camera are true, as you can see in the 4th graph, as you increase gain, noise is reduced even further (brighter images without increasing noise), however, you also loose a bit of dynamic range.  Dynamic range is the difference between the lightest and darkest areas, you might think of it as “how many shades of grey” between black and white.

 

We can also see, on the 2nd chart, that Unity Gain is 135.  Unity Gain is the setting whereby the sensor produces 1 electron for each photon of light (in a given pixel).  I recommended you try Highest Dynamic Range to start with, but it seems the 224 actually performs better at higher gains for deep sky objects.  You can try Unity Gain at 135, the Sharpcap Brain can use that too, but I would also consider both 200 and 350 as viable gain settings to compare with.  All of these (135, 200, 350) will get you much brighter images at the same exposure.  You will loose a little range, but it may not be noticeable.  Since in general, for EAA, you may observe a target for 5 or 10 minutes (instead of hours), you may opt to use the higher gain of 350, to get as deep as you can in a smaller integration time.  If you don’t mind letting a target stack for a bit longer, you might enjoy a more modest 200 gain.  In any case, it’s obvious that 61 won’t work for you, so choose a small handful, like 135, 200, 350, and do a couple comparisons.  For each gain setting, choose the common exposure quick pick that just begins to show stars in the un-stretched view.  This should allow Sharpcap (or DSS) to stack successfully.  After stretching a 5 minute stack at each of your chosen gain settings, you will know exactly which gain works best for you, and the sky where you are at.  You can then build your workflow, and Dark Library, all around that gain setting(s).

 

Hope this makes sense smile.gif

David you are a wealth of information.  Thank you so much!

 

I'm getting more comfortable now picking exposure/gain via using the histogram.  I'm still unclear how to set the brightness/offset level though.  It seems to be more subjective unless there is a way to measure it out properly using the histogram.

 

At any rate, here was an EAA session last night of M51 with my ASI224 and my WO61.  Using the AZ Gti in EQ mode.  I was doing 30 second subs, 1x1 binning, gain 275, and brightness of 49.  Total live stacking time was about 10 minutes.  No darks or flats.

 

MTx8SNgh.png

 

I'm pretty happy with my first successful EAA session.  


Edited by Biggen, 03 June 2019 - 11:42 AM.

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