Creating Flats with Optolong L-Extreme Filter?

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poolemarkw
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Creating Flats with Optolong L-Extreme Filter?

#1

Post by poolemarkw »

What's the best process and settings for capturing flats using the dual-narrowband Optolong L-Extreme filter on a ZWO ASI2600MC Pro color camera? There is so little light allowed through the filter that I have to either brighten my light source significantly and/or use a much longer exposure time.

WITHOUT the filter I have had great success using Gain = 0, Brightness = 0, Red balance = 50, Blue balance = 50, selecting "Capture and Subtract Bias Frames" and selecting "Create Monochrome Flat Frame". I set my exposure to center the white histogram above 50 and that is typically between 800ms and 1400ms depending upon which telescope I'm using.

My first attempts to create flats for the L-Extreme I simply raised the brightness of my light source and still had to use 8 second exposures (rather than around 1 second) and my resulting stacks are vignetted (brighter to the outside edges) when stretched.

Thanks!

Mark
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Re: Creating Flats with Optolong L-Extreme Filter?

#2

Post by admin »

Hi Mark,

if you are seeing brightening at the edges in the final images then that tends to mean that the flat frame is overcorrecting for the vignetting - that would usually either be caused by no or incomplete dark subtraction when capturing the light frames or excessive dark/bias correction when capturing the flats.

In practical terms, there is nothing stopping you from turning up the gain when capturing your flat frames to keep the exposures shorter. It does mean that you will need to capture more frames to bring the noise level under control however (so, for instance, 80 x 1s frames with higher gain rather than 8 x 10s frames at minimum gain). Another thing to try is to use the option to subtract full dark frames when capturing the flat (this is relatively new in SC 4.0)

cheers,

Robin
poolemarkw
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Re: Creating Flats with Optolong L-Extreme Filter?

#3

Post by poolemarkw »

Thanks Robin!

The issue isn't caused by the lack of Darks during the capture of the Lights. I had the proper Dark included during that time. What would cause the second issue "excessive dark/bias correction when capturing the flats". Regarding the noise issue, I captured 20 flats and the gain was set fairly low. On my next test I'll just keep the light source low, zero gain, and do much longer flats.

Mark
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oopfan
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Re: Creating Flats with Optolong L-Extreme Filter?

#4

Post by oopfan »

Hi Mark,

Up until recently I never bothered capturing Dark Flats. Now I do it for the Ha filter. My Flat's exposure time runs 25 seconds using a bright EL panel. After doing this I noticed an improvement in my final image, albeit small. From what I read, Dark Flats really aren't necessary unless your Flats exposure time is more than a few seconds. With your CMOS camera you may experience greater noise reduction than with my noisy CCD.

Hope this helps.

Brian
poolemarkw
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Re: Creating Flats with Optolong L-Extreme Filter?

#5

Post by poolemarkw »

Thanks Brian. I don't think SharpCap has an option for me to create Dark Flats. How would I do it?

Mark
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Re: Creating Flats with Optolong L-Extreme Filter?

#6

Post by oopfan »

Hi Mark,

It depends on if you Live Stack or stack offline with PI, APP, DSS, or a host of other programs. I've been using APP the past couple years, so it works fine. I'm not a Live Stack practitioner so that would be a question for Robin. Sorry I can't help more.

EDIT: As for the mechanics of taking a Dark Flat, you first need to know the exposure that you used for the Flat. In my case an Ha Flat takes 25 seconds. Next, to take Dark Flats, I cover the lens like I would for regular Darks, set the exposure to 25 seconds, and capture 30 to 50 frames.

Brian
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Re: Creating Flats with Optolong L-Extreme Filter?

#7

Post by admin »

Hi Mark,

the very latest SharpCap 4.0 versions do have a dark flat option
Capture.JPG
Capture.JPG (50.56 KiB) Viewed 1045 times
To be honest, I can't work out how you could easily overcorrect for dark/bias when capturing the flats either, but it remains a faint possibility.

cheers,

Robin
poolemarkw
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Re: Creating Flats with Optolong L-Extreme Filter?

#8

Post by poolemarkw »

Thanks Robin! I found this feature yesterday afternoon (I was still using version 3) and tried it last night on the Rosette nebula. It appears to have done a much better job.

Mark
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Re: Creating Flats with Optolong L-Extreme Filter?

#9

Post by Menno555 »

Hi

Just want to add (and confirm) on what Brian said.
Here too I have flats of between 6 to 12 seconds with the L-eXtreme filter depending of the gain/offset.
With this filter taking longer exposure flats really does help. I did compare one time flats of 1 second and flats of 10 seconds, and the 10 sec ones gave way better results, including no vignetting whatsoever.

Menno
poolemarkw
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Re: Creating Flats with Optolong L-Extreme Filter?

#10

Post by poolemarkw »

Thanks Menno! It's good to hear a consistent result from several sources.
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