NGC 3621 Hydra

[Placidus]

We were much inspired by Paul Haese's exquisite marathon version of NGC 3621, a half-face-on galaxy in Hydra.

Our version has just 14 hours of exposure (9 hrs 30 min of luminance and 1 hr 30 min each of RGB), taken in snatches before moonrise over the last five nights.

Big one here (https://photos.smugmug.com/Category/Astrophotography-at-Placidus/i-z39RwLJ/0/48cf2726/O/NGC%203621%20Hydra%20Lum%209h30%20R GB%201h30%20each.jpg)

Apart from slight differences in colour, we are very pleased to see a quite similar galaxy to what Paul found. It is nice that there is something you can trust not to change every time you look.

A galaxy of contrasts: The core is flocculent, with only suggestions of spiral structure. The very centre does not appear especially bright. The outer spiral arms are very different: well-formed, almost grand design, but extremely faint. There is strong star formation occurring in the outer edges of the spiral arms especially toward 4 or 5 o'clock.

There is another very small grand design spiral about 50% of the way toward 12 o'clock which is worth a look, as well as the usual smattering of distant fellows.

Aspen CG16M at -30C on 20" PlaneWave on MI-750 mount. Processed with GoodLook.

Very best,
Mike and Trish

[Joshua Bunn]

Always admire your star colour Mike and Trish.. . And such a vibrant colour full galaxy.
What's going on with the green stars and green at 8 o'clock on some of the brighter stars?

Lovely image!

Josh

[strongmanmike]

Great work guys! It is an intriguing galaxy for sure and the difference in brightness between the inner regions and the arms is significant. My decon meter needle waaaas quivering in and out of the green and into the yellow zone but that's all, it didn't move anywhere near the red zone... :P

Mike

[Placidus]

Thanks, Josh.

The green smudge toward 8 o'clock on the bright stars is a complete mystery. :confused2: :shrug: We saw it in our second-most-recent galaxy shot also.

Perhaps it's time to check the filters for spider eggs, check the rim of the main mirror or the corrector lens for wasps nests. This is all to be done three metres up a wobbly ladder, and getting at the filters involves rather a lot of tight and awkward bolts, and the risk of introducing dust, so we'd be thrilled if anyone could point us in the right direction.




Thanks Mike. We shall consider ourselves warned that we're pushing the envelope. :D

[Geoff45]

Great work guys. I tried this one a few years back and decided that it was just too difficult. Well done!

[atalas]

:eyepop:what a wonderful deep field guys....little fellas where ever you look :thumbsup::thumbsup::thumbsup:

[Paul Haese]

That's a confirmed colour there MnT. Soooo much larger resolution than my rendering. :thumbsup:

[gregbradley]

That's some spectacular processing you've done there. That image really sparkles. Some real vitality and transparency achieved in this one.

I like this galaxy also and its a bit of a tough one as whilst its a spiral it seems to have some remnants of spiral arms. Perhaps its an old merged galaxy.

Greg.

[Ryderscope]

It certainly does sparkle and the faint outer arms are captured nicely.

[Placidus]

Many thanks Geoff, Louie, Paul, Greg, for your kind comments.

Cheers,
Mike and Trish

[RickS]

Well done, M&T! Great colour and I love those faint spiral arms.

Not sure about the green blobs but I think it's unlikely to be something on the filter since it affects bright stars all across the field.

Cheers,
Rick.

[Placidus]

Thanks Rick.





I think I might have figured out what the green blobs are. They are residual or ghost images in the green sub from the immediately previous red sub.

Examining the raw green sub, I can see that they are perfectly formed stellar images, all offset by precisely the same amount across the field on any given sub. They are not smears, nor diffraction spikes. The amount of offset varies from sub to sub.

We use a poor-man's dithering between subs. We do a focus run between subs, and rely on the scope's pointing accuracy being only of the order of 11 sec arc standard deviation. We don't explicitly dither by a large but controlled amount. We did something like 20 luminance frames, and statistical rejection effortlessly gets rid of after-images, especially after-images from the usually brilliant focus star. But with RGB, there are only three subs in each colour, and that is not enough to do anything meaningful.

Dr Mike proposes the following cure:

(1) Stop looking for wasps nests (there was one, and a big one too, but it wasn't the culprit).
(2) Dither by a larger and more controlled amount.
(3) Do more RGB frames, perhaps 6 shorter ones rather than 3 longer ones. Then our usual Winsorized mean statistical rejection technique will get rid of the blobs.

Some folk use infrared pre-flash to get rid of after images. That is important if you are doing spectra at a chip temp of -100C, but close to useless on our chip. At -30C the pre-flash hugely increases the background noise. We care much more about grit-free faint spiral arms and distant faint fuzzies than about green blobs next to the odd bright star.

Thanks everyone for their thoughts on The Blob.

Mike

[Peter Ward]

Technical quibbles aside....(which would drive me dotty) a delightfully deep field. :thumbsup:

[RickS]

Didn't occur to me that it could be RBI. I get a little on my U16M but mainly just a single ghost star after focusing. I've never seen anything that dramatic before.

We use RBI preflash on the PL16803 at SRO and the extra noise is a PITA.

Cheers,
Rick.

[strongmanmike]

I can't say I have noticed any RBI issues with my FLI16803 :question: I use -30DegC permanently all year, I do not utilise pre flush and just dither 3 pixels and use darks (although these are probably not necessary really) :shrug:

Mike

[Andy01]

Great image guys, lots to see and enjoy there :thumbsup:

[multiweb]

Subzero cool Mike. The field is great and the details in the main subject amazing. So many faint fuzzies around too. :thumbsup::thumbsup::thumbsup:

[Placidus]

Hi, Mike. So same chip as yours, but with ours we get two images for the price of one.



Thanks Andy.



Cheers, Marc. Hunting for the faint fellows can be almost as much fun as taking the main image.

[markas]

Great image:thumbsup:

Mark

[Placidus]

Thanks Mark.

[Atmos]

Absolutely lovely MnT! The detail, the colour, the faint fussies and yes, even those random green blobs hehe

[Placidus]

Thanks Colin. We think we know what to do to stop the green blobs now. Haven't done it, of course. ;)

[topheart]

Lovely galaxy!!
Cheers,
Tim

[cometcatcher]

Every one of your images looks like Hubble deep field in the background. Galaxies everywhere.

I want a 20" now. And clear skies to use it. We must have had 4 semi-clear nights all summer. I'm thinking of ditching visual astronomy and doing radio astronomy.

[Placidus]

Thanks, Tim.



The first part's a wonderfully encouraging comment and much appreciated, Kevin.

How are you doing for actual rain up there? We're in the grip of a severe drought, and the paddocks are turning to dust, so while the astrophotography is going well, the farming is completely on hold, other than the domestic veggie patch and the chooks.

[cometcatcher]

We have received just over half the yearly average to date, which is still quite a lot (656mm /1114mm). But the trend is for night time cloud, even when it's not raining, which sucks. Winter is usually clear up here so hoping for better conditions soon.