NGC4889 and coma supercluster

[timh]

SW200 PDS Newtonian, Baader flattener F 5.0. CEM70 mount, ASI120MM guide camera (50 mm, f = 160mm guide scope)
PDH2 guiding, Sharpcap (livestack) capture, PixInsight processing

ASI294 PRO MC camera, gain 124 at -10C

An opportunistic mix of frames taken on clear and moonless moments over March and April and all stacked together in PI.

40 frame master flat and master darks SC

83 x 70s + 41 x 40s lights taken under Bortle 6-7 skies

30 x 70s + 33 x 44s lights taken under Bortle 4 skies

The frames taken under dark skies were dramatically deeper having less background and light artifacts to remove in processing. Oh for more access to dark skies! Thus in building the final stack the dark sky integration was weighted 70% despite comprising fewer frames. Nevertheless the Bortle 6-7 skies - although less transparent - showed much better 'seeing' and were therefore sharper (lower FWHM). Indeed the seeing was so poor under the dark skies that most frames were rejected and it took about 4h to gain about 1h of acceptable lights. Incidently this is one simple practical reason why I capture in SC livestack -- because it is so simple to to set FWHM and brightness thresholds to avoid filling the hard drive with too much dross.

Anyway I was quite happy with the final result although a struggle to get it....dark skies and good seeing now there is a rare thing.

The Coma galaxy supercluster - neighbour of Laniakea - is ~ 320 M ly away and thus far enough away to be significantly red-shifted by Hubble flow (about 16 nm at 656 nm) and far enough back in time that the first reptilian land animals were only just appearing here on Earth. Nearly everything in view is an entire galaxy and, to me, they look like beads moving along invisible strings and revealing of some of the underlying large-scale cluster structure of matter and dark matter threads.

NGC4889 at the centre of the frame is apparently a true behemoth of an elliptical galaxy - bigger and with more globular clusters than M87, a central black hole some 5000 fold or more bigger than at the centre of the milky way and 8-15 trillion (mainly population II) stars. The other big elliptical near the centre is NGC4874 and what must be a huge face on spiral is NGC4921.

Tim

[oopfan]

Hi Tim,

I tried it two years ago but I underestimated how much integration time it really needed. That's what I get for seeking inspiration from NASA APOD images! I read that you picked up a bunch of frames at a Bortle 4 site. Just curious, how does the Bortle 4 stack compare to the Bortle 6-7?

Thanks,
Brian

[timh]

Hi Brian,

Bortle 4 allowed me to detect things that were not detectable at Bortle 6-7 - or to be specific I should say that I could find additional faint detail after 60 min on M51 that I have not previously managed to detect over about 2.5h of images accumulated at Bortle 6. The core of M51 looks good under any skies but under darker skies you can detect more extension to the spiral arms as well as the fainter hood. Similarly 50 minutes or so of Coma frames under Bortle 4 were much better than 2.1 h of accumulated Coma frames at Bortle 6. Even to the extent that when I combined them in Pixmath and weighted roughly according to the SNR it was 70:30 in favour of the darker sky site (as in the picture above).

At Bortle 6 I have never needed to add offset to move the 40s exposure histogram away from the vertical axis whereas at Bortle 4 I needed either to add about 2% of offset - or, alternatively, take the exposure time up to about 120s. Obviously the best solution would be the longer exposures -- however the real problem with the Bortle 4 site which was by the sea was that the seeing was awful on this occasion (actually a score of zero on one astro weather werbsite) - so PHD2 was going mad. My solution was to only go for objects near the zenith and take 40s exposures which worked but even then, using SC to reject high FWHM values, it took about 3-4h to gain an hour of usable frames. And even then in this case I was trading 'blurriness' for detection.

Unfortunately I didn't stop to take sky measurements (e-/ pixel/s) - I should have done but had little time. But I am guessing from the above that the Bortle 4 site was about 4-5 fold darker. For my CMOS camera, even with 40s exposures, the sky shot noise at my normal Bortle 6 site must massively exceeds the total read noise and the sky brightness be the major limitation to being able to see really faint stuff - which is why I am skeptical about the possibility of detecting something like the IFN from my home site.

Comparison below on the coma data ---Bortle 4 v Bortle 6. It is hard to do in a fair way though because the real problem at Bortle 6 is that it is not just the fact of there being more light but also of its directionality and variability ..i.e. in practice the problems vary from night to night - and the real work comes in processing and what you are left with after removing uneven light gradients.

My overall impression though. I would much rather have 1 hour of frames from a Bortle 4 site than 4-5h of frames from Bortle 6 and especially so if my target is a broad light spectrum emission nebula or a galaxy --i.e something that filters will be of only marginal help with.

Below pictures are at gain 124, as above comparing 57 min at Bortle 4 with 125 min at Bortle 6
--------------------------
Postscript . Actually- to be more quantitative here are the numbers and SNR stats calculated using the face on spiral galaxy as signal and an adjacent star free area as background, lined up identically across 4 different integrations taken at the two sites and at two different subframe exposure times. Taking into account that there were fewer frames comprising the dark site integrations the differences are quite stark. For the same total time of exposure SNR is improved more than 5 fold at Bortle 4 compared to Bortle 6


30 x 70s frames at the Bortle 4 site (ADU units and stats measured using PI stats process. SNR in 3 channels = (signal - bckkgrd)/ SD bdckgrd

RED (49.113 - 40.145 )/ 1.518 SNR = 5.901
GREEN (38.989 -32.835 )/ 1.039 SNR = 5.923
BLUE (19.552 - 16.038)/ 1.021 SNR = 3.442

83 x 70s frames at the Bortle 6 site

RED (328.054 - 319.927)/ 2.214 SNR = 3.67
GREEN ( 295.129 -289.592 )/ 1.587 SNR = 3.489
BLUE (185.842 - 182.657)/ 1.597 SNR = 1.994

33 x 40s frames at BORTLE4

RED (17.896 - 12.336)/ 1.251 SNR = 4.444
GREEN (16.255 - 12.495 )/ 0.835 SNR = 4.503
BLUE (14.786 - 12.632 )/ 0.778 SNR = 2.768


41 x 40s frames at BORTLE 6

RED (165.465 - 160.139)/ 2.400 SNR =2.219
GREEN (156.491 - 153.655)/ 1.776 SNR = 1.597
BLUE (102.809 - 101.010)/ 1.808 SNR = 0.995





TimH