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NGC 4236 Barred Spiral Galaxy in Draco

Contains: NGC 4236, NGC 4120

Technical card

Resolution: 3880x3008

Dates: May 10, 2017

Frames:
Astrodon 3nm OIII: 10x2340"
Blue: 45x392" bin 1x1
Green: 39x240" bin 1x1
Astrodon Ha 3nm: 10x1710"
Astrodon L-NIR: 28x1029"
Red: 10x570" bin 1x1
Astrodon Sloan i': 40x332"

Integration: 32.0 hours

Avg. Moon age: 13.95 days

Avg. Moon phase: 99.25%

Astrometry.net job: 1576808

RA center: 183.522 degrees

DEC center: 69.391 degrees

Pixel scale: 1.076 arcsec/pixel

Orientation: 179.946 degrees

Field radius: 0.734 degrees

Locations: The Hills Observatory, Manchester, United Kingdom

Description

The galaxy is a member of the M81 Group, a group of galaxies located at a distance of approximately 11.7 Mly and an angular diameter: 22.9' x 7.2'

I seem to be picking really faint difficult targets to image at the moment, objects not easy even under dark skies. I am trying prove to myself that deep sky imaging under poor city skies is possible. In fact of the 14 separate nights I imaged this galaxy the average SQM-L (Sky brightness) was 18.2 with three of those at 17.75 and the best at 18.75.

This galaxy is depicted rather less blue than most images you will find, this is down to my inclusion of narrow band filter images using 3nm Ha and OIII Astrodon filters. 6.5hr of OIII data added little to the final image as OIII and Ha emissions occur close together and with the limited resolution my equipment gives on this scale, they tend to merge together. But the ~5hr of Ha data makes quite a difference.

As with all my images North is at the top and East to the left. There are various different exposures and bins used so the times in the data above are average figures.

For example Blue exposures were (all bin1x1)
16 x 300s
4 x 360s
24 x 450s
1 x 600s
The reason for the variation is due to adjusting the exposure length depending on the sky conditions to try and get a batch of similarly exposed images, i.e. with approx. the same mean ADU. This makes working out the total exposure (& filling in the Astrobin form) more difficult especially when different bins are used, which I also change depending on the seeing conditions. I am going to try and keep to standard exposure lengths in future, but if I find I don’t get the results I want I will return to mixed exposures again.

I have offset the main galaxy to include NGC 4120 in the frame on the far right. If you look close there are many other small galaxies in the image.

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Author

Mike Oates
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NGC 4236 Barred Spiral Galaxy in Draco, Mike Oates