What's the magnification of an astro image?

General astronomy-related discussion (publicly viewable)
User avatar
Pete
Astro Day Coordinator
Posts: 3999
Joined: Fri Jun 13, 2003 9:03 pm

What's the magnification of an astro image?

Unread post by Pete »

How can we correlate visual magnification to our photographs? We all know that the magnification of a telescope equals the focal length of the telescope divided by the focal length of the eyepiece. BUT. What about astro images? How to best figure the magnification of a picture of a nebula? And does this question even make sense?

We might know that a star cluster is 20,000 light years across and measures 2 inches across on a print. So what’s the magnification? Quoting from an old S&T article the most accurate approach is to
define magnification as the amount an object is enlarged in comparison to how large it would look to the unaided eye at a given distance.
T
The best I can figure, if we know the size of the image FOV (measured in arc-minutes) and we view this image on a monitor or on a sheet of paper, we can approximate the magnification. How? At a comfortable 15" reading distance I’ve estimated that the image spans an angle of 60°. If my camera setup has a diagonal dimension of 21 arc minutes, the magnification = 60°/21’ = 3600’/21’ = 171X There you are. The magnification of my uncropped images is 171X. Approximately, as I'm rocking in my chair and the monitor isn't.
Pete P.
TimW
Posts: 775
Joined: Wed Oct 18, 2017 1:15 pm

Re: What's the magnification of an astro image?

Unread post by TimW »

When I get asked questions from the curious onlookers about an image I am viewing whether it be through an eyepiece or on my laptop, I always compare it to the approximate size of the full moon. Alot of people believe that these objects are super tiny in the sky and must require a great deal of magnification. When comparing an objects apparent size in the sky to that of the full moon they get a sense of just how large these objects are in the sky and that most are just too dim to see with the unaided eye.
Tim W.

Laurie J. Wood
March 22, 1967 - August 23, 2019
Forever in my memory. Forever in my heart.

Scope: 8" Celestron SCT on an AVX equatorial mount
Solar Scope: Lunt Solar System 60 mm Double-Stacked H-Alpha
Solar Imaging Camera: ZWO ASI174mm monochrome
Imaging Camera: ZWO ASI1600MC-Cool
Autoguide: PHD2
Image Capturing: SharpCap
DebayerIng: PIPP
Planetary Stacking: Autostakkert
Planetary Post Processing: Registax
Deep Sky Stacking: Deep Sky Stacker
Deep Sky Post Processing: Photoshop
Bruce D
Life Member
Posts: 5624
Joined: Sat Jun 14, 2003 6:10 am

Re: What's the magnification of an astro image?

Unread post by Bruce D »

Makes sense to me Pete. At 171x how big would M31 be on the monitor?
Bruce D
User avatar
Pete
Astro Day Coordinator
Posts: 3999
Joined: Fri Jun 13, 2003 9:03 pm

Re: What's the magnification of an astro image?

Unread post by Pete »

Bruce D wrote: Mon Jul 20, 2020 9:08 pm Makes sense to me Pete. At 171x how big would M31 be on the monitor?
M31 actually spans 3° To fit diagonally on my 21" monitor that's situated 15" from my eyes (which results in a 60° viewing angle) there's only a magnification of 60°/3°, or 20X. So given the fixed 60° working FOV for reading or viewing, and using my approach, M31 just doesn't fit into a 60° FOV at 171X.

Normal magnification is what you see through an eyepiece (which is normally a 60° apparent FOV. Image size, on a screen or on a print, is a different animal entirely. And that's what I'm trying to examine here.
Pete P.
Post Reply