# Creating a CMD from a B Image and V Image

**URL:** <https://community.openastronomy.org/t/creating-a-cmd-from-a-b-image-and-v-image/230>\
**Category:** Astropy\
**Created:** [December 6, 2021, 5:29pm UTC](https://community.openastronomy.org/t/creating-a-cmd-from-a-b-image-and-v-image/230 "2021-12-06T17:29:12Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![collinsinclair](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/collinsinclair/32/147_2.png) [@collinsinclair](https://community.openastronomy.org/u/collinsinclair)\
**Post date:** [December 6, 2021, 5:29pm UTC](https://community.openastronomy.org/t/creating-a-cmd-from-a-b-image-and-v-image/230/1 "2021-12-06T17:29:12Z")

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Hello! I’m wondering if astropy offers an API to create a CMD (color-magnitude diagram) given an image (FITS) taken in B and an image taken in V. Namely, I’m looking for something that identifies stars in the region, performs aperture photometry to determine the relative apparent magnitudes of the stars, and plots them (or returns a list of values for me to plot myself). Thanks so much!

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**Author:** ![mwcraig](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/mwcraig/32/37_2.png) [@mwcraig](https://community.openastronomy.org/u/mwcraig)\
**Post date:** [December 6, 2021, 10:26pm UTC](https://community.openastronomy.org/t/creating-a-cmd-from-a-b-image-and-v-image/230/2 "2021-12-06T22:26:14Z")

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A couple questions:

1. Are your images aligned and/or do they have WCS? In other words, are the stars at the same location in both images?
2. Do the magnitudes need to be calibrated in some way or is it ok to just use the instrumental magnitudes for for the CMD? The latter might be fine for a first look at, say, an open cluster, (and is what I’ll likely do for the intro observational course I need this for tomorrow).