Observation by Nick James: Hubble's first Cepheid in M31

Uploaded by

Nick James

Observer

Nick James

Observed

2021 Aug 10 - 23:52

Uploaded

2021 Aug 15 - 11:16

Objects

The Andromeda Galaxy (M31)

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Constellation

Andromeda

Field centre

RA: 00^h42^m
Dec: +41°13'
Position angle: +1°26'

Field size

0°43' × 0°30'

Equipment

ASI6200MM + Celestron HD11

Exposure

10x60s

Location

Chelmsford, UK

Target name

M31 V01

Title

Hubble's first Cepheid in M31

About this image

I was taking to Nick Hewitt the other day about his talk on the Great Debate and Hubble's discovery of the first Cepheid in M31 which led to the first accurate distance estimate of that galaxy. There is a nice finder chart taken with the INT here and here it is on one of my patrol images taken with a much smaller telescope. It looks to be faint at the moment at below mag 19. I'll keep an eye on it.

Files associated with this observation

m31_v01_20210810_2352_ndj.jpg (1.2 MB)

Like this image

Jeremy Shears, Ron Arbour, Graham Winstanley, David Basey, Dominic Ford, Nick Hewitt, Andy Wilson, Ron Johnson, Nik Szymanek, Grant Privett, Dr Paul Leyland, Callum Potter, Peter Carson, Eric Watkins, Steve Knight

Comments

Nick James, 2021 Aug 15 - 17:13 UTC

I managed to find an online catalogue of Cepheids in M31 here and have overlaid positions for objects brighter than 20 at max on a mosaic of my patrol frames of M31 taken on 2021 August 10 shown here. Each of the panels in the mosaic is around 40'x30' so this covers most of the disc of the galaxy. The image has had the diffuse background light from the galaxy removed. There are objects in quite a few of the circles.

Dr Paul Leyland

Dr Paul Leyland, 2021 Aug 21 - 16:17 UTC

Could you explain how you removed the diffuse background from your image please?

It is something I often want to do but have only used ad hoc home-brew solutions up to now.

Thanks,

Paul

Nick James, 2021 Aug 22 - 09:32 UTC

For every pixel compute the median of the pixels within a specified radius of this point and then subtract this from the pixel value. It is quite compute intensive since you need to calculate as many medians as there are pixels in the image but it is effective. I tend to use r=6 with my setup (corresponding to a radius of around 8 arcsec) and it takes about 20s to do this for a 2400x1600 image on a fast machine. I have started playing around with the Nvidia CUDA API and that will be a *lot* faster.

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