Riding The SNR Curve

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oopfan
Posts: 1321
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Riding The SNR Curve

#1

Post by oopfan »

I was fortunate to have had two clear nights in a row, a perfect opportunity for experimentation. I have a mono camera and filters. I like the flexibility, but it also presents some challenges. My biggest challenge is my refractor. It does not focus blue very well. I've learned that conventional LRGB imaging results in bloated blue stars, but I discovered a remedy. I simply eliminate blue by using a Wratten #12 filter (minus blue) for luminance, and standard red and green filters. So, why do my images show blue? (See attachment.) It is the way that I mix the channels in Astro Pixel Processor (APP).
M51 compare 60s-60s to 150s-300s (2).jpg
M51 compare 60s-60s to 150s-300s (2).jpg (227.17 KiB) Viewed 6891 times
At first, I had trouble processing these images of M51. I was following the same technique I use for star clusters, but I was not getting the result I expected. The image had a severe red bias. In a topic that I started a few days ago, entitled "Exposure's Effect on Color Quality," I speculated that the exposures I used were not long enough. I was wrong. The problem was with APP. It turns out that I need to run Star Color Calibration on galaxies when I use the Wratten #12, red, and green filters. Now that I have that behind me, I can move on with my analysis.

Looking at the attachment, your first reaction might be "I like the left-hand image best." I don't disagree with you. I like the HII regions in the arms (the red highlights.) The right-hand image has some red, but it is muted. The reason for that is due to the order of filters. East of the meridian, I start with red, then green, and finally Wratten #12. Red light wavelengths pass through the atmosphere more easily than green and blue at a given altitude above the horizon due to Atmospheric Extinction. The unfortunate consequence of starting with red is that the atmosphere is still unsteady after sunset. So, in the right-hand image, the atmosphere was turbulent when I captured reds. However, on the night before, the left-hand image, the atmosphere was steadier, and therefore red light was stronger.

If we overlook the loss of red highlights in the right-hand image, we can see that the galactic core and arms are brighter. How can this be? I spent 151 minutes on the left-hand image, but I got a worse result. The right-hand image took only 110 minutes, but it is so much brighter.

Oh no! There goes Brian again with his 300-second exposures. Let me say, that 300 seconds is completely arbitrary. I just wanted to make it significantly long enough so that the difference was obvious. Furthermore, I have a paltry 71mm aperture. A lot of you guys and gals have 8 inches and greater. You are scooping up far more photons than I. I don't doubt that you are saying to yourself, "Brian, that's a pretty cr**py image for 2 hours of effort." Yes, that's what I get for having a small aperture. Believe me, if I could start all over again, I'd go straight for an 8-incher.

The point that I want to make is that you can get a brighter image in less time if you increase exposure. And there is the proof. Now of course there could be many reasons why you don't want to go longer. Among those are: severe light pollution, poor guiding, dodging clouds, and aircraft. Those are all valid reasons. Thankfully, most nights, I don't have those problems.

If this sounds like something you would like to experiment with, then use The Brain to determine the exposure, but change the noise tolerance to say that you have a low tolerance for noise. The Brain should offer you a longer exposure.

Finally, circling back on my original topic entitled "Exposure's Effect on Color Quality," I still believe that longer exposures have an additional advantage. I don't see it in these two images, but I suspect the reason is that they are still too noisy. I am hoping that with more integration time, it will become clear.

Brian

EDIT: I chose not to run a de-noising filter on the images, nor did I enhance them in any other way except for a small black level adjustment. I wanted to show them in their raw, unadulterated form. They will look much better after running them through Affinity Photo.
timh
Posts: 515
Joined: Mon Aug 26, 2019 5:50 pm

Re: Riding The SNR Curve

#2

Post by timh »

Hi Brian,

I hope that you are doing well.

A bit of an aside ..just a thought on a possible alternative approach to the problem with the achromat not bring blue stars to focus? Not an approach that could help with galaxies or star clusters but might work with some of the HII regions like the Pelican etc. Perhaps take the RGB image - bloated blue stars and all- apply as much masking and star reduction as make sense but then swap in the luminance from a NB HA image. I am finding that as well as adding greatly to nebula detail and definition it is also a very good way of shrinking down star sizes so that they don't overwhelm nebulae. The star colors remain true but the stars are shrunk - which should mitigate the problem you have in the blue - at least for stars in these kinds of objects? Making image luminosity dependent on an HII filter obviously means that the cooler red and orange stars will artificially appear somewhat brighter relative to blue stars. But it doesn't seem to be that noticeable an effect in practice though -

best wishes ..TimH
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oopfan
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Joined: Sat Jul 08, 2017 2:37 pm
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Re: Riding The SNR Curve

#3

Post by oopfan »

Hi Tim,

Weather here continues to be challenging. As soon as a storm passes through, we get one day of sunny weather and then a Canadian system rushes down from the north and spoils the party. So, I could pick up one night of imaging every once in a while, but I prefer stretches of two or more nights. Just not getting those these days.

I am planning on a new kit, something that is a real photon bucket to take advantage of single-night opportunities. A Celestron RASA 8" running at f/2.2. That is a major improvement over my f/5.9 refractor. They both have the same focal length, so the improvement will be dramatic. I'll pair that with a ASI533MC and Celestron CGX mount. The 533 has got a dynamic range 2.5 stops greater than my Atik 314E and the Peak QE goes from 54% to 80%. Pixel size goes from 4.65um to 3.67um, so star quality improves. I am going to purchase the camera first and test it out on my William Optics refractor. I am hoping to capture the IFN and M81/M82 without blowing out the galaxies. Let's see what happens with the poorly focused blue! Also, thinking about switching over to PixInsight. Sometime this coming spring or summer, I'll pick up the RASA and mount.

Thanks for adding to this conversation. I am hoping that PI will improve my photos with the refractor. It would be nice to go back to using a luminance and blue filter.

Brian
timh
Posts: 515
Joined: Mon Aug 26, 2019 5:50 pm

Re: Riding The SNR Curve

#4

Post by timh »

Hi Brian,

Thoroughly recommend PI even though it is quite expensive. I just like the logic that permeates its approach throughout - just the process of using the program helped me to understand image processing more deeply. A useful guide written by Rogelio Bernal Andreo was good to get started. A friend of mine has the 533 and I think it is a good camera - the pixel size is an advantage over my 296 for some set ups--although aesthetically I like the 'window box' shaped framing of the 296 better.

Over the last couple of years I have a had a good opportunity to compare different set ups. The second-hand market is quite good here so have been buying and selling - with a rule that at least one has to go out before another comes in. Aperture and speed make a huge difference to doing astronomy here in England - big bright images as fast as you can get them within short windows of opportunity - and serious attention to dew. While resolution may usually be seeing limited anyway above above D ~ 100 mm bigger aperture scopes do - as you would expect - just seem to add deeper color information so much quicker. Personally I like trying to image some of the smaller objects in greater detail (as opposed to big vista shots) so am now awaiting a 12 inch F4 Newt that is just light enough to work OK on my existing mount (having sold expensive binoculars and a 10 inch Dob to fund it). On the other hand an 8 inch F 2.2 should be just great for the bigger sky shots - I thought that the SharpStar astrographs looked quite interesting in that space too - personally I think that these kind of bigger aperture reflector options make more sense than apo refractors for short focal length work.

Anyway good luck with your plans!

TimH
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