Help with terminology

[matt]

Of late... I've been getting a little more involved with the whole science/physics/voodoo of planetary CCD imaging.

I was wondering if anyone has a link to something explaining arcseconds, particularly how these apply to seeing and pixel resolution?

Actually, I'm not even clear on what an arcsecond is?

I'm not really clued in on these (and other) units of sky measurement.

Sorry for the dumb question

Thanks

[ballaratdragons (Ken)]

Dumb Question? I think it's a good one Matt. I don't know what an Arcsecond is either. Or an Arcminute!

[ballaratdragons (Ken)]

or a Parsec!

[matt]

Glad I'm not the only one, Ken

Arcseconds... arcminutes.... archeadache

[leon]

I'm not a 100% on this one but i think an arc minute is fraction of one degree, and an arcsecond is a fraction of an arc minute, but i could be wrong here.

Cheers leon

[matt]

Yeah. I think you're right, leon.

I have a vague idea of what they are but I really want to nail that down in terms of exactly what it represents in the sky in real terms, in terms of what each unit covers in area at the eyepiece or through the camera.

Cheers, mate.

[Dennis]

Hi Guys

As a starter, in angular measurement, there are 360 degrees in a whole circle.
1 degree = 60 minutes
1 minute = 60 seconds
Therefore 1 degree = 60 x 60 seconds = 3600 seconds.

If we look at the full Moon, its apparent size is 1/2 degree, that is, it subtends an angle of 1/2 degree.
1/2 degree = 1/2 of 60 minutes = 30 minutes.

To differentiate between minutes and seconds of time, and minutes and seconds of angular measurement, the terms arc minutes (arcmins) and arc seconds (arcsecs) are often used.

Cheers

Dennis

[matt]

So the full moon would be 1800 arcseconds?

[ballaratdragons (Ken)]

Ahhhhh, thanks Dennis

I probably could have googled about them but never really needed to know, so I didn't.

[Dennis]

Another example:

If you step outside and gaze at the Southern Cross (Crux), have a look at the “bottom” star, Acrux and the top star, “Gacrux” which define the long axis of Crux.

The angular distance between these 2 stars is approx 6 degrees. So, you could fit 12 full Moons in between these 2 stars. (12 x ½ degree = 6 degrees).

Generally, this is how amateur astronomers measure distances between objects, using degrees, arc minutes and arc seconds.

Here are some examples:
Full Moon is approx 30 arc mins in diameter.
Sun is approx 30 arc mins in diameter.
Jupiter ranges from 30 arc secs to around 45 arc secs in diameter.

Cheers

Dennis

[Dennis]

Originally Posted by matt

So the full moon would be 1800 arcseconds?

Let’s see. Full Moon is approx 1/2 degree, which is 30 arc mins.

So, 30 x 60 = 1800 arc secs - yep spot on Matt! I needed my calculator.

Cheers

Dennis

[Dennis]

According to Starry Night Pro, the apparent angular size of Saturn's disc right now is 20 arc secs, so Bird did a fantastic job of getting such a great image the other morning!

Cheers

Dennis

[avandonk]

One degree equals 60 minutes and one minute equals 60 seconds. So there are 3600 seconds in one degree. They are call arc minutes and arc seconds to not get them confused with seconds of time.

Fuller explanation here
http://en.wikipedia.org/wiki/Arc_minute

Bert

[Dennis]

Just like we write terrestrial Longitude and Latitude in degrees, minutes and seconds, we can also write something like:

At 3:00am on Saturday morning, Saturn will be at an Altitude of 47 degrees 20 minutes 10 seconds, or in shorthand:

Saturn will be 47° 20’ 10” above the horizon.

Tomorrow, we can look at arcsecs and seeing.

Cheers

Dennis

[Dennis]

Back to the original question:

"I was wondering if anyone has a link to something explaining arcseconds, particularly how these apply to seeing and pixel resolution?"

Hi Guys

Well, as the thread hasn’t been added to, I’ll continue.

Just like a telescope/eyepiece combination has a field of view (FOV) that can be expressed in deg, mins & secs, a telescope/sensor combination can be described in the same way. Here are some FOV examples:

• Celestron C9.25 at F10 + Baader 21mm Hyperion = 36.5 arcmins.
• Celestron C9.25 at F10 + Vixen LV 5mm = 5.74 arcmins.
• Celestron C9.25 at F10 + Philips ToUcam = 5.27x3.95 arcmins, or 5.27’x3.95’.
• Celestron C9.25 at F10 + Pentax *istDS = 34.38’x22.97’.
• Vixen 102mm at F9 + Pentax *istDS = 80.77’x53.97’.

So, let’s take the C9.25 at F10 with a ToUcam and a FOV of 5.27’x3.95’.
We know the following about our ToUcam:

• Chip size = 3.6mm x 2.7mm.
• Resolution is 640x480 pixels.
• Pixels are 5.6um square.

A FOV of 5.27’x3.95’ is the same as 316”x237” which would fit in approx 9 Jupiter’s with the current angular size of Jupiter being around 32”.

So, we can image a field of 316”x237” with our 640x480 pixel chip.
316/640=0.49
237/480=0.49

We are just dividing the FOV by the number of pixels on our chip to find out what each pixel can "see" or record. This means that in theory, the smallest detail on Jupiter’s disc that each pixel can resolve would be 0.49”, or ½ arcsec. So, you would record a feature on the disc if it appeared to be say, 2” in size and this feature would occupy 4 pixels in your image. You would not be able to record any detail that is say, 0.25” in size.

I’ll leave it there for the moment as I’m concerned I’m hogging the post and of course, as always, I could be wrong or over complicating things, so I always welcome corrections, better explanations and constructive feedback.

Cheers

Dennis

[davidpretorius]

dennis, you are excellent!

thanks heaps, make heaps of sense.

[sheeny (Al)]

Good on you Dennis.

Just found this thread, but I see you've got it covered!

Al.

[matt]

Great stuff Dennis.

Hog away!!!

It don't get much better

[[1ponders] (Paul)]

Onya Dennis

And for those of us who don't have a head for these things (or are just too lazy to sit down and work them out... yep that's me) you can try Ron Wodaski's CCD Calculator It's great as you can add your own equipment if it isn't already included.

Frankly I'd be lost without it

[Dennis]

Hi Paul

That’s the software I used for the calculations above, plus one running on my PDA for the eyepiece calculations, called “Boenkyo” which is a gorgeous piece of free software. Gee, it’s good to stand on the shoulders of giants, who have already done all the hard work for us.

We have not yet covered the effects of seeing on imaging, nor the Nyquist sampling theorem, which we’ll hopefully get to later as the post builds on itself.

Cheers

Dennis