Deer Lick galaxies under nigh impossible conditions

[timh]

PDS200 8 inch reflector, Baader MkIII coma corrector, f 1009 mm and ASI294 MM camera with ZWO UV/IR filter. 428 x 10s luminance frames at gain 280 (0.17 e/ ADU) and 0.47 arcsec/ pixel. CEM70 mount with PDS2 guiding via an 80 mm, f 400 mm achromat and ASI120 MM camera.
Image processed with PixInsight preprocessing, Gradient removal, BlurXterminator, Histogram stretch. Arcsin stretch , NoiseExterminator

Took the scope on holiday to a site near Exmoor in Devon, U.K. hoping for dark skies around the new moon. Unfortunately mainly clouds and rain with only two brief usable windows with dark skies but partially covered with fast moving clouds.

As I am sure most are aware the Pegasus NGC7331 galaxy group is not really a group but just a chance alignment. But I always enjoy looking at the distant fuzzies and learning about the identification and distance data. Some good resources for identifying galaxies and abstracting data are the following links.


NGC Objects: NGC 7300 - 7349 (cseligman.com)


Principal Galaxy Catalog (PGC) Objects 1 to 499 (cseligman.com)


http://atlas.obs-hp.fr/hyperleda/

The foreground spiral form galaxy is NGC7331 – also selected by Patrick Moore to feature in the Caldwell catalogue. Based on its recessional velocity relative to the Cosmic Microwave Background radiation of 490 km/sec (and H0 = 70 km/sec/Mpc), NGC 7331 is about 23 million light-years away, in fair agreement with redshift-independent distance estimates of about 23 to 75 million light-years. The HST estimated distance is 45 million light years and NASA described the galaxy as a twin of the milky way. At 25 M light years this would be more nearly so at ~ 90,000 light years across the longest dimension. However if it were at 45 MLY it would have to be quite a lot bigger – up to 180,000 light years across - than our galaxy given its apparent angular size – it also lacks a central bar and the central stars rotate around the centre in the opposite direction to the outer arms – which is unusual.

The three ‘deer fleas’ shown in the closer in image and below NGC7331 are NGC7336, NGC7335 and NGC7337. NGC7336 is estimated to be the most distant galaxy at ~ 390 million light years and about 100,00 light years across in its longest dimension. NGC7335 is estimated to be ~ 275 million light years distant and ~ 110 thousand light years across. Despite the dusty ring between the inner and outer rings, there is no sign of spiral structure, so NGC 7335 must be classified as lenticular. NGC7337 is also estimated to be about 280 million light years distant and also about 110 thousand light years across.

The annotated wider field image includes further nearby galaxies including the remaining ‘flea’ which is NGC73340, a lenticular galaxy at ~ 280 million light years distant about 70,000 light years across including the outer halo. The other NGC designations are spurious and correspond to close double or triple stars. The un-annotated irregular galaxy at the bottom left is PGC2051985 and is estimated to be about 45 M ly distant - at this distance it may be a genuine member of the NGC7331 galaxy group which also includes NGC7320? The remaining galaxy at the top left is PGC69281 which is estimated to be very distant at 450 million light years.


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Notes on the imaging

The dark sky background was down at ~ 21.6 mag/ sq arcsec as compared with typically ~ 18.7 at home. Thus, potentially, the SNR from even a single hour’s imaging at the darker sky site could exceed that from a dozen or more hours at home.

Calculating an exposure/ gain setting that would at least 10X swamp the read noise (as per Robin’s teaching) suggested that an exposure time of at least 100s at unity gain would suffice while maximising dynamic range. (Actually I used a convenient tool based on Robin’s approach that has been provided by @Steven Bellavia who has created a bunch of spreadsheets for different CMOS cameras and filter combinations at : https://drive.google...U9CKj5GilnZW8VJ - note some bugs and SNR figures inaccurate after changing the specified filter)
However because of the fleeting clouds and rapid time-scale of the sky variability exposures as long as 100s were impossible. So to be able to gain any sort of image at all I reverted to high gain , short subs and a strategy of only capturing frames with good brightness and sharpness (FWHM filter) – thus initially rejecting about 50% of frames using the Sharpcap filters – and then further rejecting about 50% of these during preprocessing in PixInsight.

This was a rather horrible strategy for a dark sky location but it was a question of needs must. The SC frame log indicated few contiguous runs of ‘good’ 10s frames that might have justified taking a longer exposure strategy.

In the end - even given the undesirably high contribution from read noise using 10s exposures at gain 280 – with RN then contributing perhaps 45 % rather than < ~ 9% of the total noise at 100s - there was still a large benefit from imaging under dark sky as opposed to home conditions (I,e maybe up to a 10x rather than the full 15 X advantage that could have been realised with longer subs under more ideal conditions). Some imaging was a lot better than none!

[Menno555]

Great write up, Tim. And again proof on how a dark sky is the best filter (together with gasoline ).

When I captured this group, I did some reading about it. Especially on the naming cause I was curious on how this looked like "the lick of a deer".
Turns out it is a place, Deer Lick Gap. The author of hundreds of astronomic books (don't recall the names) had one of his best observing nights there and and called this group after it

Menno