ASI294 overexposed? Vignetting/weird colors even with flats

cjules
Posts: 5
Joined: Sun Apr 07, 2019 8:57 pm

ASI294 overexposed? Vignetting/weird colors even with flats

Post by cjules » Sun Apr 07, 2019 9:03 pm

New to this - currently using an ASI294 with a Stellarvue Access 80 f/7 (.8 reducer) on an AZ-GTI in Alt-Az mode and Sharpcap for everything. Seems like lots of folks have gotten good results with similar setups.

Main issue seems to be where the histogram starts in live stacking mode. The peak seems to appear way too far to the right which according to Sharpcap, indicates overexposure. That's crazy, right? I'm doing 15-60 sec subs, mostly 15-30. This happens in all sky settings, from red to yellow zones, with or without the Optolong L-Pro filter.

I've been using 50 darks averaged by SC with the capture tool, and 50 averaged flats. The darks seem to be doing their thing, but flats don't seem to be fixing all the light issues. I'm wondering if whatever is causing the histogram to be so far to the right may be the bigger issue, and the flats can only do so much to correct the saturated pixels. I know some folks have had issues with flats and Sharpcap with this camera, but no matter what I do adjust making flats, I get lots of vignetting/color artifacts. The problems seem to get worse the longer it stacks. Would love to hear any ideas! I've tried messing with the brightness and gain - images seem pretty unusable with >250 gain so stick with unity or 200.
Image

BlackWikkett
Posts: 216
Joined: Sat Jun 23, 2018 8:48 pm
Contact:

Re: ASI294 overexposed? Vignetting/weird colors even with flats

Post by BlackWikkett » Sun Apr 07, 2019 11:17 pm

The image attached to this post didn't render

ASI294 for flats try these settings. 0 brightness, gain 150-300, exposure 250 - 350 ms. Exposure time and gain settings really depend on what you're using to illuminate your scope to take your flats. Use the SharpCap flats tool and turn on the histogram. In SC flats tool use Bias, and mono. Adjust the exposure and gain to that the white channel peak is in the middle, 50% on the horizontal scale, and your highest color channel doesn't clip the right side of the histogram.

If you haven't already have a look at this thread. viewtopic.php?f=5&t=1282

cjules
Posts: 5
Joined: Sun Apr 07, 2019 8:57 pm

Re: ASI294 overexposed? Vignetting/weird colors even with flats

Post by cjules » Mon Apr 08, 2019 8:03 am

Thanks. That sounds like what I've been doing. I did read that thread last week so thanks for doing all that testing! Does your live stacking historgram peak show up in the middle or right side? I haven't seen this is people's screenshots so not sure what's causing it. Hopefully this link shows up.
https://imgur.com/a/UY3wzIa

BlackWikkett
Posts: 216
Joined: Sat Jun 23, 2018 8:48 pm
Contact:

Re: ASI294 overexposed? Vignetting/weird colors even with flats

Post by BlackWikkett » Mon Apr 08, 2019 1:35 pm

The imgur link showed up this time. My Live Stack histogram peak is usually in the middle. I noticed you have logarithmic selected in the histogram if you uncheck this you'll see the linear curve mostly hugging the left side of the histogram. Your histogram looks fine for the settings you have applied.

From you image i notice that you are adjusting your red and blue color balance. When you create darks you'll need to make sure that the red and blue levels are the same as your light frames. This is most likely causing the results you are seeing.

For darks make sure your brightness, red and blue level are the same as the light frames.

BlackWikkett
Posts: 216
Joined: Sat Jun 23, 2018 8:48 pm
Contact:

Re: ASI294 overexposed? Vignetting/weird colors even with flats

Post by BlackWikkett » Mon Apr 08, 2019 1:37 pm

The imgur link showed up this time. My Live Stack histogram peak is usually in the middle. I noticed you have logarithmic selected in the histogram if you uncheck this you'll see the linear curve mostly hugging the left side of the histogram. Your histogram looks fine for the settings you have applied.

From you image i notice that you are adjusting your red and blue color balance. When you create darks you'll need to make sure that the red and blue levels are the same as your light frames. This is most likely causing the results you are seeing.

For darks make sure your brightness, red and blue level are the same as the light frames.

Creating flats leave the red and blue at 50 / 50 and brightness 0. I've had the best results with these settings.

cjules
Posts: 5
Joined: Sun Apr 07, 2019 8:57 pm

Re: ASI294 overexposed? Vignetting/weird colors even with flats

Post by cjules » Mon Apr 08, 2019 3:39 pm

Ok lots to try - really appreciate the tips. The tips about darks makes sense. Would you mind sharing your process? Since different targets are going to need different R/B adjustments, it sounds like you'd need to frame the target, adjust levels, and then capture darks?

I've been trying to capture darks/flats in advance, and with the EAA nature of the 294, it's great to be able to go from one target to another in one session. With the method above, assuming 60s exposures, it would be adding ~1hr between targets to capture the correct color balanced darks. Am I missing something?

Again thanks for the tips - you've clearly done a lot of the heavy lifting on this camera and calibration settings.

BlackWikkett
Posts: 216
Joined: Sat Jun 23, 2018 8:48 pm
Contact:

Re: ASI294 overexposed? Vignetting/weird colors even with flats

Post by BlackWikkett » Mon Apr 08, 2019 5:57 pm

You're correct about varying white balance levels and needing to adjust from night to night and object to object.

I usually broadcast at https://www.nightskiesnetwork.com/ as do many others that use one shot color (OSC) cameras. Some use the red and blue color sliders to get a rough color balance while I leave mine at the default of 50/50 for R/B. Both process will deliver similar results with the adjusted R/B tends to give a better white balance.

Most of the guys that adjust the R/B will setup their system with LPS filters and get a general setting for R/B. Usually since most are using an LPS filter this tends to severely limit the red channel so their setting for red will usually be 99. This usually doesn't perfectly line up the red histogram curve with the green and blue but gets close. Then you'll adjust the blue channel and there is usually enough headroom to get almost perfect alignment with the green channel. While making these adjustments make sure you have the smart histogram open and watch the red, green and blue mean readout and try to match this as close as possible. You'll need to get the best settings based on your skies and equipment.

Once you have your base line for red and blue color balance you can create your dark library. I would setup multiple exposure and gain settings and shoot 10-20 subs to make the master dark. Make sure you are using the same R/B and brightness settings that you intend to use for Live Stacking.

Once you start Live Stacking with your R/B and brightness adjusted master dark applied the histogram becomes live once you have 2 frames stacked click the auto stretch and auto white balance you should see a fairly color balanced and corrected stacked frame displayed.

My process is just as above except I leave the R/B at 50 and make all the color adjustments in the Live Stack histogram. I use the red, green and blue vertical sliders to align the peak for each curve.

I tend to use the same master dark library for several months and will usually create new masters darks as I notice new hot pixels. If you're using a cooled 294 that's another advantage since you'll be able to control your temp and have consistent thermal noise.

My settings for my rig RC6 f/9 1370 FL IDAS D1 LPS filter and ASI294 MC Pro cooled to -15c (I'm able to maintain this temp year round)
Brightness: 4
Red: 50
Blue: 50

usually use gain of 300 and exposure of 15, 30 and 60 sec

create new flat nightly once I've fine tuned focus for the night.

Hope this helps!

cjules
Posts: 5
Joined: Sun Apr 07, 2019 8:57 pm

Re: ASI294 overexposed? Vignetting/weird colors even with flats

Post by cjules » Mon Apr 08, 2019 8:34 pm

Great resources and tips again. Just gave it another go. This time I did a flat captured with t-shirt method, making sure the white peak was at 50, brightness 0, R/B 50/50. When it got dark I framed my target, and used the sliders in live-stack to align the peaks. Once that was done, I captured 10 30 second darks with those sliders adjusted.

Seems I'm still getting lots of red on the right side of the image. This is super apparent when I use GIMP's Colors->Levels-Auto Import Levels and try to set a black point. Haven't gotten around to proper processing yet but I'm not sure it's worth it until I figure this out. No filter used tonight to keep it simple.

https://imgur.com/a/jKWnB5K

cjules
Posts: 5
Joined: Sun Apr 07, 2019 8:57 pm

Re: ASI294 overexposed? Vignetting/weird colors even with flats

Post by cjules » Mon Apr 08, 2019 8:39 pm

Just re-reading your post. I think I'm confusing the sliders. When I say I adjusted the sliders once I framed my target prior to capturing darks, I mean the R/B sliders on the right hand side under image controls. In live stack mode, do the vertical RGB sliders also align the histogram peaks? I've either not tried or not noticed that.

BlackWikkett
Posts: 216
Joined: Sat Jun 23, 2018 8:48 pm
Contact:

Re: ASI294 overexposed? Vignetting/weird colors even with flats

Post by BlackWikkett » Mon Apr 08, 2019 10:27 pm

The gradient in your image could be caused by several things most likely light pollution in that area of the sky. If you you're using a good flat this should not only remove dust but most of the gradients caused by your optical train. Anything left after that is usually just gradients from sky glow. Your image is heavily stretch so this will tend to over emphasize any flaws in the image.

From reading your post it seems you took the flat before it got dark. Which would be OK if you didn't have to change your focus. The 294 is very sensitive and your flats will not calibrate the image correctly if you change anything except maybe very very slight focus changes due to temperature variation.

The dark seems to be working well, I'm not seeing hot pixels or the 294's characteristic amp glow.

The vertical R/G/B and saturation sliders(in the the large Live Stack histogram) are post processing and will only change the small display histogram and image displayed on screen.

Post Reply

Who is online

Users browsing this forum: No registered users and 1 guest