apparent size?

[neil groves]

Is there a website, app, star atlas, ANYTHING that will allow me to visually compare objects? I read this so and so is so many arc secs and the other so and so is that many arc secs apparent size, but these figures mean diddly squat to me, I need to be able to compare them visually side by side?

Neil.

[neil groves]

or do I need to measure in degrees?....i'm talking specifically about galaxies and nebula?

Neil.

[Schorhr]

Try Stellarium or http://www.12dstring.me.uk/foveyepiece.php

:-) WIth the later you can Open two tabs or windows to compare objects.

I hope that'll Do.

[neil groves]

That website is great .....

thanks

Neil.

[acey]

There are 60 arcseconds (denoted 60") in 1 arcminute (1') and 60 arcminutes in 1 degree. So if an object has a diameter of, say, 5 arcminutes then that's 5/60 = 0.083 degrees or 5x60 = 300 arcseconds.

To compare sizes just convert to a single unit: arcsec, arcmin or degrees. It's just like comparing times, which can be in seconds, minutes or hours. For DSOs the most useful unit is usually arcmin.

Bear in mind that if two galaxies are of the same shape and one is twice the diameter of the other than it will have 4 times the area. Also bear in mind that with galaxies (which are generally elliptical in profile) the quoted size may be the "major axis" (the diameter along the longest axis) or the "semi-major axis", which is half of that (i.e. from the centre to the edge). Last thing to bear in mind is that quoted sizes for DSOs are usually their size as measured on images - what you see at the eyepiece may be smaller (sometimes a lot smaller).

[ronin]

Not astually that easy, just about everything gives a picture of an object and then gives a size. Catch being all the pictures are the same size so you get no real impression of the difference.

The other problem is they are not always measured to the same "boundry".

M42 is sort of a good (bad) example of this, I have read about 4 different sizes for this and each had an image to show it. Size can I suspect therefore depend on the scope and exposure length used. A 2 hour exposure will show bits that a 30 minute exposure will not.

Where does a nebula end ?

It is gas/dust and and sort of exponentially decays, no defined edge to say this is it.

Wouldn't it be easier to find out what the scope will capture then go looking at the pictures and read the sizes and decide what will fit in and what will be nice and big?

Damn it Neil , you are asking about observing and I expected imaging.

Try ignoring the stuff, probably not relevant to 98% of what you want.

[webboid]

I would recommend Night Sky Observer’s Guide by George Robert Kepple and Glen Sanner. These books give apparent sizes of objects as viewed in different size telescopes. They are far more realistic than info given on websites & other resources. Particularly useful as well is to know what the true field of view of your eyepieces are then you can guestimate how much of it will be seen in that eyepiece. Example you have an eyepiece that gives you 30 arc min (0.5 deg) tfov & you are looking at an object that is 3 arc min across it would be in the middle 10% of the view. The moon in the same eyepiece would fill it completely.

HTH.

[neil groves]

Thanks guys, I was just trying to get an idea of what to look for when hunting galaxy's, I found M102 quite by accident the other night while hunting M101, it wasn't till I processed the image that I was aware I had missed my target and I was so excited at the eyepiece, so if I see a galaxy and see how big it appears, then I will compare it in a star atlas or something and see what to expect when I find what I am looking for.

Neil.

[AlexB67]

Hi Neil,

This book is quite good.

http://www.amazon.co.uk/Illustrated-Guide-Astronomical-Wonders-Observer/dp/0596526857

or check some sketches In prep of your observing sessions is very useful too.  I mention the book because it has images in it that were prepared in a way to more represent what targets look like visually, instead of the long exposure images.  I got it mainly for double stars though as it has a nice list of them.  Often books geared towards visual observing  will actually  provide tables of apparent sizes and  surface brightness, and they will often quote smaller dimensions you will see quoted in the wiki or other places more relevant for imaging.

For example,  http://www.clarkvision.com/visastro/  this book has an extensive appendix with surface brightness figures and sizes suitable for visual work as well.

[acey]

Here for what it's worth is how the NGC deals with size. It was published by Dreyer in 1888 and all the object descriptions (based on visual estimates by the discoverers) use the abbreviation system devised by William Herschel. John Herschel suggested equivalent values for the size estimates, which Dreyer reproduced. All size estimates (assumed to be maximum diameter) are merely approximate, so there are gaps and overlaps in the proposed numerical values, but they gave some idea of the sizes you might expect to see, based on its NGC description.

eS (excessively small): 3-4"

vS (very small): 10-12"

S/cS (small/ considerably small): 20-30"

pS/pL (pretty small/ pretty large): 50-60"

cL/L (considerably large/ large): 3-4'

vL (very large): 8-10'

eL (excessively large): 20' or more

For clusters, Dreyer noted that a wider extent is usually assumed; 1' would be vS, one of 15' or 20' would be L. Herschel's scale can be a bit confusing because it's based on 18th-century English, and having "pretty small" and "pretty large" as adjacent steps in the scale seems odd nowadays. But if you just think of it as a series of steps then it makes sense - and it's still widely used by amateur astronomers.