# Thread: Measuring the size of lunar features using afocal photography

1. ## Measuring the size of lunar features using afocal photography

Hi there.

I am going to make observations of 3 lunar features including Copernicus Crater, Apennine Mountains and Eratosthenes Crater using my Meade Polaris 130EQ. I have a mount that I am using to attach my Iphone 6 onto the eyepiece lens and I will take some still images of the Moon.

However, I want to use my images in order to determine an estimate for the size of the lunar features but I am not sure how.

I read online that I should determine the resolution of the image, pixel size , field of view and the pixel scale in arcseconds/pixel but I am not really sure what to do.

Some data of my Telescope and Iphone 6

Telescope:
Objective Lens diameter- 130mm
Focal Length- 650mm
Aperture- 130mm
3 Eyepieces - 6.3mm, 9mm, 26mm
Barlow Lens also included with 2x

Iphone 6:
8-megapixel (3264 x 2248 pixels) iSight camera with 1.5µ pixels
ƒ/2.2 aperture

Does anybody have any solutions?

2. ## Re: Measuring the size of lunar features using afocal photography

an174,

Interesting project you've set yourself.

One question I have for you, the "size" you are referring to, is it in km/miles or angular size or on the chip of the camera or what? I'm not clear on that.

Field of view, that's tricky as it is afocal. But start with the True Field of View of the eyepiece, and take it from there.

TFOV = Apparent Field of View (AFOV) of EP / magnification given by that EP-scope combination

The trick from here is the size of the image on the camera. If the entire TFOV of the eyepiece is visible on the camera, then you need to start working with percentages to achieve the "size" you want to determine. It'll be a tougher set of calculations if only a partial amount of the TFOV is seen by the camera.

Other folk may have other ideas and suggestions

Alex.

3. ## Re: Measuring the size of lunar features using afocal photography

Firstly a warm welcome to the forum. No suggestions though - I'll leave it to the experts like Alex.

Cheers

4. ## Re: Measuring the size of lunar features using afocal photography

I make no claims to particular skills or expertise, but I’ll take a shot at this since the thread is at risk of fading away.

I saw a pdf that spells out an approach, attached. It looks like the document was a student project—you may find much better advice, but this will get you off the end of the dock and into the water! I think you could refine it a bit with a precise determination of the pixel scale of your images, rather than using the approximation in #6.

One way to get the pixel scale with your setup is to plate solve an image of a star field. Take photos of some different star fields and upload one of the jpegs to Astrometry.net. Once an image is submitted, it can take a long time to solve. If it fails or times out, try again later, or try a different jpeg. If you’re lucky, at least one of them will get solved. Part of the output is the resolution of the image in arcseconds/pixel.

Then with that value in hand, pick up the process at #7.

Here you go:--

Attachment 172328

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