M42 - the Good, the Bad and the Ugly

[dx_ron]

The good? 1st light for my recently acquired AT130EDT; the combination of low-read-noise CMOS (IMX517C) plus StarTools' ability to gracefully handle very high dynamic range without resorting to artificial HDR compositing means I am pretty confident that just simply throwing enough 15s subs at the problem can eventually produce a nice image. Four Trapezium stars are clearly resolved and not drowned out by the nebulosity.

The bad? Only 75 minutes and the moon was full; it's probably the only 75 minutes I will devote to it this season, because it goes behind a tree by 2200 local time and I have to set up in a non-standard place in the yard to get even that; I do not yet have the bracket to attach the Rigel NStep focus motor, so this was hand focused.

The ugly? Clearly I stink at hand focusing. Actually, the starting FWHM values were decent, but climbed quite a lot in 75 minutes. I knew time was tight, so I talked myself out of spending 10 minutes refocusing - clearly a mistake; the purple horror! I sure do hope the CA can be tamed by better focus and by the Astronomik L3 uv/ir filter I ordered after seeing this. The L3 is a decent bit narrower, compared to the Optolong uv/ir filter I have now. If not, I am in for a lot of hand-wringing and tooth-gnashing during processing.

M42_03-04-23_300x15s.jpg (736.03 KiB) Viewed 16008 times

[fmeireso]

Picture looks pretty descent to me.

Focussing is also something i must pay more attention to. I never refocus since normally the temp drops not much in the evening allthough there has been occasions my FWHM scored way too high.I decided for the future to abslotuyely check after 2 hours and refocus. I manualy focus with a Bahtinov mask. On my last session again i did not refocus and took 3 hours of M81/M82 but then my FWHM stayed more or less on track.But on my Jellyfish is went from about 3 to 7. That is not acceptable...
Stange you get those blue halo's. Sofar i did not see them in my 130 mm TS Photoline showing up

CS
Freddy

[Mike in Rancho]

Nice. 15 seconds x 300 though, wow.

Does that 517C come with just an AR window? I'm not sure I see typical focus issues, unless that CA is a sign? No little stars have a donut hole.

But it seems logical, and a 5+ inch triplet must take some time to cool down too.

Of course a possible fix, other than perhaps fringe killer on the purple, would maybe be a post-tracking invert-green-cap? Hopefully the narrower UV cut does the job though.

Are you using a reducer/flattener? One interesting thing about all such stars is the aspect of the stellar center to the purple halos. Quite the radial pattern out to the edges.

Great job cleanly separating 4 in the Trap!

[dx_ron]

Quite the pixel-peeper, aren't we? Yes, using the AstroTech 0.8x reducer. It is possible that I could use a tiny fraction more spacer, though I think it would be in the fraction of a mm range.

Yes to the AR window version of the camera. Current uv/ir-cut is an Optolong, which has a passband similar to the L2 (blue cut at about 390nm), vs the L3 that cuts at 420nm.

As to focus, maybe I'm just not yet used to looking at in-focus stars with this setup, but they looked bloated to me from the get-go. FWHM values (via Siril) were a bit under 3" at the start and up over 5" by 90 minutes later. The sub-by-sub values that Ekos was giving in real time did not show nearly as dramatic an increase, but I've never squared those two metrics.

When next year comes around I might well pause at the thought of 10 hours worth of 15s subs and come down off my "No Layers HDR compositing for me!" high horse.

[Mike in Rancho]

Well, sure! Aren't we supposed to look at them pixels for interesting outer space things? Being careful that too much magnification could go past the captured image scaling of course.

Unless it's just art and we have to stand back, to appreciate the ambiance or something.

But here we also had a mystery of purple stars and maybe focus that needed forensic examination too.

Comparison of the L3 to the Optolong regular UV-IR will be interesting, though I'm sure fairly specific to your optics. I too watch the HFD's come in on NINA as a clue that I might need to pause and go slap the B-mask on. But a lot of times it's seeing, which can be pretty dynamic, or at least it has been this bizarre winter. Sort of a vicious cycle too, as seeing by itself will fatten the stars, and also trash the guiding, which fattens the stars...

[Topographic]

Autofocus is one thing I can't do without, mind you my eyesight is poor, not great with specs either. I use the L2 which has been good for me in Bortle 3/4 skies.

I think the Nebula is good, I would like to see more of the dust that virtually surrounds it, the sky seems a bit too black to me.

Part of the fun is having different versions in your library!

[dx_ron]

Yep! I hope to get a 'real' M42 with the AT130/IMX571 combo - my last image of it was with my beginner rig (Z61 / dslr) and beginner processing, of course. This is only 75 minutes, so I didn't spend much time on the processing.

Autofocus will be taken care of soon. I'm a big fan of the Rigel NStep, but having a new focuser does require getting a new bracket / gear set (~$70). I wasn't using my eyeballs to focus that image, though - I was looping exposures in the Ekos Focus tab and manually playing the part of the autofocus motor. I really should have paused the image sequence partway through to tweak focus, but I got distracted and did not.

[dx_ron]

Still waiting on autofocus, but the L3 uv/ir-cut arrived. It seems to take care of the #^!* purple halos. This is only 45 minutes (so I didn't spend any real effort in processing), but the stars look so much less purple. Whew

Some obvious atmospheric dispersion going on, as its not very high in the sky.

[decay]

Hi Ron, this version looks much sharper than the first one. Wow. Probably due to better focus position and seeing conditions? And exposure seems to be lower? So hard to say, whether the 'missing' purple halos are due to your new filter?

I do use the Optolong UV/IR and I've never seen such halos. But I use a modded DLSR with reflector scope, maybe that's a different story ...

Anyway, your M42 looks great now

Best regards, Dietmar.

[dx_ron]

Thank you, Dietmar.

Chromatic aberration plagues lenses, not mirrors. I guess a poorly designed coma corrector could introduce CA into a newt, but I don't think that is common. The purple fringing comes about because the far reaches of blue are out of focus when the central part of the spectrum is in focus. So the focus numbers were better with the more stringent uv/ir filter and the stars look tighter overall - both of which are expected.

Exposure time was the same in both versions (15s), but the 2nd one was at somewhat higher gain (the 1st night I forgot to reset the gain from 100 to 177, which is what I normally use with this camera in the lowest-gain readout mode).