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Re:NGC 891 - looking to advance

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9+ hrs integration of OSC November captures aligned and stacked with Siril, processed with
StarTools.
File Attachment:


I am new to capturing flats, and struggling, so not included with this integration.
I would greatly appreciate C&C. Thank-you!
The following user(s) said Thank You: Magnus, Eric, Wouter van Reeven
3 years 3 months ago #63902

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This is great. You definitely need flats and darks yes. With 9 hours, I think you should be able to see the small galaxy between the two main stars on the top left of your photo. This requires you to remove noise very carefully, something I really struggle with myself :)

-Eric
The following user(s) said Thank You: Dean
3 years 3 months ago #63975

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Replied by Magnus on topic NGC 891 - looking to advance

Hi Dean.

Can I ask where your struggles in capturing flats are?

Lighting
Settings (gain, exposures)
Calibrating / stacking
Using
Proud owner of Observatory 17b - A remote Linux observatory.
Website: Observatory 17b
Build thread @ SGL: Starting summers observatory project
3 years 3 months ago #64111

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Replied by Dean on topic NGC 891 - looking to advance

Hi Magnus,

I tried capturing flats in two ways but am not satisfied with the results:
1) point the scope vertically with several squares of white paper over the extended dewshield of my William Optics GT71
at an overhead LED rectangular panel in my laundry room. Run Ekos to automate the exposure time in the capture module.
When I do this, there is some kind of periodic interference pattern that shows up as horizontal banding and a very short exposure time.
2) similar to 1) but point the uncovered end of the dewshield flat against a laptop display of a white image and similar
banded results and short exposure time
3) I went back to scheme 1) and manually set the exposure time to 1 sec after trial and error. Attached is a sample as
seen in ASIFitsView

I use one of the Siril scripts to automate processing my OSC lights against bias, flats and darks. When I use these
flats, I see minor noise reduction improvement in my final processed lights. As a result, Im not sure if Im capturing the
flats correctly so that I get the most benefit from them.

I have a USB dimmable LED tracing pad from amazon coming today ( tinyurl.com/y36c6gev ) to try out. I plan
on using a small diameter embroidery hoop with white t-shirt fabric to diffuse the LED light.
Last edit: 3 years 3 months ago by Dean.
3 years 3 months ago #64114
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Replied by Magnus on topic NGC 891 - looking to advance

I see. It's mostly the acquiring..

I have the same (ish) usb a4 lightpad, and I think it works well. No noticable 'noise' from it.
I did read up on using flats with my asi 1600mm-c and found some things that is true for these dedicated CMOS cameras.
Most of my calibrating was based on how you would do with a dedicated CCD, so somethings were needed to change.

First problem I had was using Dark, Biases and Flats. For the noise of the CMOS chips, it is more valuable to do Dark, DarkFlats, and Flats.
This is partly because the biases has too short exposure lenght, and causes some random / varying noise.

Because of this it is recomended to do Flats with a exposure length of 0.2 seconds or longer, to avoid the varying bias noise, and instead of Biases use DarkFlats to calibrate the flats.

So my process now is Lights, Darks, Flats, Dark-Flats. All at same gain, offset, focus and temperature
Darks: same exposure length as Lights (but covered)
Flats: 0.2 second exposure length or longer. (I use 0.2s with the dimmable a4 led pad at lowest setting). You use the same focus as when the scope images the sky, so any light source at the devshield should be seriously out of focus, and a even field.
Dark Flats: same as flats, only covered.

In Siri (from googeling, I use PixInsight), you can stack the Flats as Lights and the Dark Flats as Darks. This gives you a calibrated MasterFlat
Then stack the Lights with all the Darks and the Master Flat. (This would be the same as calibrating all the Lights with a [Dark Flat calibrated] master Flat and a master Dark in PixInsight), before registering and stacking the Lights.

Looks like it's going to happen, fingers crossed!!! Starship sn8 launch!
Proud owner of Observatory 17b - A remote Linux observatory.
Website: Observatory 17b
Build thread @ SGL: Starting summers observatory project
Last edit: 3 years 3 months ago by Magnus.
3 years 3 months ago #64116

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Replied by Dean on topic NGC 891 - looking to advance

thank-you Magnus. I will give the dark flats a try.
3 years 3 months ago #64118

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If you want to leave the flat panel a bit out of the equation, you might also want to rotate it during capture, in order to "dither" your exposures. If you are doing sky flats at zenith with a piece of paper, then also capture on both sides of the meridian to reduce reflection.

I agree bias are really not easy with those cmos sensors. A shutter cancellation integrated to the device can help, probably. I'd also note that getting horizontal periodic patterns is expected, check the levels of them, they are very low contrast and it is easy to get fooled by stretching.

-Eric
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3 years 3 months ago #64161

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good news on flats! I received my new ASI533MC Pro, took some shots of the Flaming Star Nebula,
took flats, bias and darks. Flats were done with the LED panel on low dim sitting on top of 3 layers of t-shirt cloth
fitted into a small embroidery hoop with scope pointed vertically. I was able to match flats with same gain,
offset and temperature as the other series: flat exposure was 0.14 sec. No banding and so easy to do!
The following user(s) said Thank You: Eric
3 years 3 months ago #64299

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