Hi,

I'm pretty new to astronomy, and I have been reading a lot of books about it in general. I'm curious about the apparent diameter of an object. The way it is explained is that the apparent diameter of an object in the sky is how wide (or how big) it appears from earth. For example, Jupiter is listed as having an apparent diameter of 30 arcseconds.

But I don't really understand how I would *know* that an object is X-arcseconds wide. When I look at Jupiter at low-power, of course it looks smaller. When I look at it at high power, of course it looks bigger. So isn't the apparent diameter all relative to what magnification you're viewing at?

If anyone is up to the task of explaining it, that would rock. Otherwise, if you know of a good book or online source I could look at, that would be really great.

Thanks in advance.

I understand the confusion!

The apparent visual diameter is measured in arc-seconds, where there are 60 seconds in a minute, 60 minutes in a degree, and 360 degrees in a "great circle" in the sky. So, for example, if you measure along the ecliptic you get 360 degrees and if you measure along the meridian you get 360 degrees at right angles to the ecliptic. The idea is that the apparent diameter is "as perceived against the backdrop of stars" -- but that works with magnification too, since the "backdrop" likewise is magnified to the same extent.

In other words, through an eyepiece both the object and the "great circle" are magnified the same amount. However, since the eypiece has a smaller field of view than you get with the naked eye -- or some different eyepiece -- the size of the target relative to the field of view is different.

Another way to think of this is that if the object were plotted on a star chart marked in degrees, minutes, and seconds, then the object's diameter could be measured directly from the star chart. If you then used a copier to enlarge the portion of the star chart that includes the object, it would still have the same apparent visual diameter even though it would appear to be larger (on the copy).

In mathematical terms then (simple geometry), an object's apparent visual diameter depends solely on two things:

• the object's true physical diameter
• the object's distance from the viewer

The latter parameter explains why an object like a planet has a different apparent visual diameter on different dates -- because both it and the Earth are moving, so the distance between them changes.

Does that help?

Ahhh, ok. I think I understand that.

So is there any way to determine how many arcminutes/seconds in general you can see through your scope at any given time? Or do you just have to focus in on something you know the apparent diameter of, like Jupiter, and then apply that scale to everything else viewed at the same magnification?

Thanks.

Sure. There are formulas, but I like the "hands on" approach.

One way to find out is to time how long it takes a star near the celestial equator to drift across the center of your eyepiece field of view. This will be different for each eyepiece  because each eyepiece has different magnification and therefore a different field of view. Furthermore, two eyepieces of the same magnification may have different designs and therefore different fields of view.

So, set up your scope and aim at a star you can see well and that is near the celestial equator.

Position the star near the edge of the field of view and then watch it drift through the eyepiece's field of view. If your scope has a motor drive, you can use it to aim the scope, but then turn off the RA drive while you're timing the star moving across the field of view. Do this until you get the star drifting right across the center of the field of view. Once you have it going right across the center, time it with a stopwatch several times, then average the times.

The background star field drifts at 360 degrees per 24 hours, or 15 degrees per hour. If it took one minute for the star to drift across the field in a given eyepiece, then it would have a field of view of 1/4 degree (15 degrees per hour divided by 60 minutes per hour = 1/4 degree per minute). Similarly, if you used an eyepiece of the same design but twice the magnification the field of view would be half that, or 1/8 degree. You can convert from degrees to arc-seconds by multiplying by 3600 (60 arc minutes per degree times 60 arc seconds per arc minute).

There's a little math any way you go!

The other formula requires you to know or to measure the field stop of the eyepiece and that measurement could be less accurate than timing the star crossing the field of view.

Once you know the size of your field of view, you can estimate the size of objects you view with it, or you can determine which eyepiece to use for a given size object.

These are the formulas Jeff is referring to:

TFOV: True Field Of View
True Field Of View is the Field Of View Seen through the EP when used with a specific telescope. measured in degeees.

There are a few formulas that can be used to determine the True Field Of View

First we have determined the magnification an EP will yield when used in a telescope of a specific focal length..

Telescope Focal Length divided by the EP Focal Length

example: 1200 / 25 = 48x magnification
Provided you already know the manufactures specifications for the Apparent Field Of View we can use the magnification to calculate the True FOV.

EP apparent FOV / Magnification of View = True FOV

example if the EP has an apparent FOV of 50 degrees and the EP yields a magnification of 48x

50 / 48 = 1.04 degrees True FOV

Or You can aim the telescope at a star near the celestial equator. Time the star as it moves from on side of the FOV to the other.
Mulltiply that time in minutes by 4
If it only take 15 second for the star to cross the FOV (.25 minutes)

4 x .25 = 1 degree True FOV

One of the more accurate is to messure the Field Stop located inside the Bottom end of the EP barral that secures the lens elements with in the EP. The bottom Lens element we have Already discussed is the Field lens. The Field Stop is the ring you see holding those in place. Measure across the diameter in millimeters.

Example: a 25 mm EP will have a field stop at or near 22mm

The formula to calculate the True FOV with this measurement is as follows.

Field Stop Diameter divided by Telescope focal length multiplied by 57.3

22 / 1200 x 57.3 = 1.05 degrees True FOV.



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