Help with flat frames

Discussions of Electronically Assisted Astronomy using the Live Stacking feature.
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sammedina1
Posts: 2
Joined: Tue Feb 23, 2021 1:22 pm

Help with flat frames

Post by sammedina1 »

HI all,



I'm relatively new to EAA and would appreciate assistance with SharpCap flat frames. I had some early positive results with flats, however, I recently attempted to image IC2024 with a new filter (Baader UHC-S/L Booster) and got poor results: dust halos, vignetting and color abnormalities. I'm stumped because outside of the new filter, I used same equipment and followed same approach for producing the flats. Below is my equipment:



ES102 APO FCD100
Stellarvue flattener/reducer (SFFR102-25)
ASI294 MC Pro (cooled between -15 to -20)
AVX mount[/list]


My approach and settings for flats

White t-shirt method
LED tracing light box (set to lowest illumination)
Per SharpCap instructions, I take care to ensure there are no changes to the imaging train
Exposure 2.5s
Gain 120 - 125
My histogram shows white channel peak @ ~50%...red, blue and green channels are between 40% and 80%


It never occurred to me to inspect my master flats when I produced decent images (prior to attempting flats with the new filter). Unfortunately, I did not save those flat files and am left with questions re what may be happening with my recent attempts. I've included a link below to a master flat I produced during a test last evening. You'll note many dust halos. This makes sense as I do know my filter needs to be cleaned; possible my camera sensor also needs to be cleaned. I'm uncertain how to interpret the balance of the master flat file. Appreciate any thoughts and insights. Happy to provide additional info as needed.



Link to master flat file: https://drive.google.com/file/d/1fi9Mvt ... sp=sharing



Thanks

Sam
Last edited by Menno555 on Tue Feb 23, 2021 4:50 pm, edited 1 time in total.
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admin
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Re: Help with flat frames

Post by admin »

Hi,

the sample flat frame worries me for a few reasons:

1) the vignetting in the corners is extreme – I think you will struggle to get good correction in these very dark corner areas regardless of how good your flat frame is

2) there is a strong colour cast difference from the top to the bottom of the frame – are you using an ADC? If you aren't then I'm struggling to understand why the colour cast is present as it would imply that the illumination is uneven.

3) there's an awful lot of dust - it might be worth a session with a rocket blower to try to reduce the amount

Anyway, I'd suggest changing the settings when capturing the master flat to enable the option to use monochrome flat frames and also to make sure that the option to subtracting bias frame is enabled. You could almost certainly increase the brightness of your illumination and reduce the exposure length too – as far as I'm aware there's no good reason to take long exposure flats - 10ms will do if you can get the illumination level right.

Finally, if you are using an ADC then I'd suggest doing some research about whether or not it should be used for flat frames – not something I have experience with.

Cheers, Robin
sammedina1
Posts: 2
Joined: Tue Feb 23, 2021 1:22 pm

Re: Help with flat frames

Post by sammedina1 »

Thanks for the reply. I made a few changes based on your response and additional insights I've gained as I've continued to troubleshoot my flats. In short, I made the following adjustments:

1. Turned up the brightness of my LED panel and reduced exposure time to 1500ms
2. I changed the output format from FITs to TIFF
3. Completed a light cleaning of my filter

The combination of the above produced a much improved flat. Regarding number 2, I experimented with this change because I noted that the master flat generated in FITs format has a stretch applied to it. I've attached a photo of the master flat with the histogram shown. Note that I did not adjust the histogram...I'm assuming this is an auto stretch. I'm still developing my knowledge of histograms and cannot determine if in fact this stretch is appropriate. Appreciate any thoughts you have.
Test Flat Histogram - FITs.jpg
Test Flat Histogram - FITs.jpg (130.19 KiB) Viewed 334 times
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admin
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Re: Help with flat frames

Post by admin »

Hi,

Glad to hear that you are making progress – I see what you mean about the auto stretch – it looks like your viewing software is automatically stretching the image from the darkest to the brightest values. This is both enhancing the apparent strength of the colour cast and also making the corners look far more vignetted than they really are.

Cheers, Robin
Ronald
Posts: 27
Joined: Sat Feb 29, 2020 9:51 pm

Re: Help with flat frames

Post by Ronald »

Hi Sam
I have very similar equipment the asi294osc as well the AVX mount. My scopes are the skywatcher evostar 80ed and the celestron c6. After trying long exposures 2 -4s my flats looked like yours. Then I tried short exposures between
380ms-450ms aiming at the sky just before sunset adjusting the histogram to around 55% peak, then i take a set of darks adjusting the luminance to clear the left side of histogram with the brightness/ offset/ then take around 50 darks I use those for dark subtraction with the flats I select 400 to average in the mono mode for the flats.the histogram will move down slightly as the sunlight diminishes but when complete I have created a nice even flat with the histogram close to 50% peak which works perfectly for livestack it takes about ten minutes from start to completion. With the Celestron c6 I use a white t shirt one layer over the end of the scope to keep the exposure at 200-500 ms levels. I hope this helps you
Ronald and Kim
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