M94 an in-progress fool's errand (and an experiment)

[dx_ron]

It is very difficult to do justice to M94 from my SQM 18.8 backyard. I really should have waited for dark-site trips, but I don't do very many of those despite easy access to better skies ~1 hour drive away.

The experiment part came about due to playing around with exposure settings. A couple of months ago I was shooting for "a few hundred" saturated pixels, which with the f/7 scope + 571c camera on northern galaxy-season targets has me standardizing on 120s subs, HCG (gain 101 for ZWO folks). For these targets in star-poor regions 4-500 saturated pixels meant a few to several stars get quite saturated. When I process an image with the pre 565beta those stars gave the "peak+plateau" artifact we discussed in the Image Troubleshooting forum - so I switched mid-project to 60s LCG subs (gain 0 ZWO), which results in only 40-60 saturated pixels.

I now have roughly equal total integration with both 4.5 - 5.5 hours, so I processed then separately with *roughly* the same approach/settings.

Here is the 120s HCG version:

M94_HCG_day-flat_171x120s_50bin-cropped.jpg (146.34 KiB) Viewed 5433 times

and the 60s LCG version:

M94_LCG_281x60s_50bin_tight-crop.jpg (138.08 KiB) Viewed 5433 times

Both are cropped in quite tight. Both also were subjected to SS Dim Small at 80% strength - hopefully I will eventually add enough time to not be tempted by SS. Both also easily swamp the read noise in each sub, despite the higher read noise and more subs of the LCG set.

The HCG version has a bit more integration time. Overall, though, I'm not seeing a huge difference in snr, though there are slight differences between the versions that probably arose in Optidev. Processed now with 565 the bright stars are fine, of course. The LCG version has more pronounced pepsi stars, but I did not run either stack through rgb realignment.

The biggest difference during processing was that the 60s LCG stack has very few green-centered starfishies for PSF sampling. I ended up selecting every green star (about 8 or 9, as I recall) plus another 15 or 20 non-green samples. SVD seemed to work fine that way.

Of course I wish I could combined the subs into one big stack, but I realize why that's a bad idea in ST. I'll just pick one and hope to keep adding time in hopes of eventually doing justice to the diffuse halo that makes M94 so cool in the first place.

[edited to prove that I do know my M94 from my M97. One of my other unfinished projects is M97]

[fmeireso]

Looks not bad, not bad at all, Ron

As for LCG and HCG modes, for the moment , no idea to use which mode for what...
Can't see to remember what the differences are, really...but i realize the Toupteks seem to have quite some possibilities on how you use them...

For the moment the weather is the biggest issue here, haven't got a clear night in ages and we are now in 'summertime' period means it gets only dark after 22:00 hr ...

That kind of targets should perhaps do well in my RC 8", but i stil have to figure out a couple of things before that one will be 'productive'

[dx_ron]

Thanks Freddy. I'm happy with the details and colors in the central part - but it's the faint halo of stars that makes M94 special. That's going to be hard from B6/7-land.

I imagine it takes a while to wrap your head around gain and 'conversion mode', coming from only using a dslr for so long.

The simple version is that HCG has much lower read noise, but the higher gain saturates stars much more quickly. Everyone uses HCG for narrowband.
With LCG you can expose longer before you saturate the same number of pixels. If you have dark skies, then maybe the higher read noise becomes a factor, but what I glean from various calculators is that my light-pollution skyglow will swamp the read noise by 10x in ~20 seconds even with LCG. So the advantage of LCG with broadband is fewer subs to store and stack.

There is no advantage to using any gain other than 100 at either LCG or HCG. The loss of fullwell value is greater than the gain from lower read noise.

[Stefan B]

Great image, Ron. I know it's not an easy target (https://www.astrobin.com/m47syf/) and I love the amount of detail in the core and the dust lanes you got. I tried to shoot with 5 min subs but overexposed the core. So I ended up with shooting 1 min subs for the core and layering both images (as is commonly done with M 42). Still didn't look as nice as your result.

There is no advantage to using any gain other than 100 at either LCG or HCG. The loss of fullwell value is greater than the gain from lower read noise.

I second that. That's also my conclusion from reading up on the Touptek 571c.

Regards
Stefan

[dx_ron]

Thanks, Stefan. I like the rich colors you got in your version.

I've made some progress. I decided to go forward with the high-conversion gain, 120s subs. The knee-jerk CN reaction to that would be that that is the wrong choice, especially from B6. But I'm only saturating fewer than 100 pixels, and it results in many more choices for stars to sample in SVD compared to the 60s low-conversion gain stack.

This stack is a bit over 10 hours. For the first time, I tried using background noise as the major criterion for rejecting subs, so I was mostly keeping subs +/- about 90 minutes from the meridian. Hopefully more time to be added - but there's rain in most of the 10-day forecast and by next new moon cycle it will already be an hour past the meridian by darkness.

M94_311x120s_HCG_2024_v2-40bin-crop.jpg (464 KiB) Viewed 4877 times