True size DSOs

[Granada]

Wondering if there is a website or an image showing the night sky with all visible DSOs displayed as if we could see them with our naked eye, and in their true size (relative to other objects in the sky)?

[mental4astro]

Yes, there's both apps and software that does this.

There are many astronomy apps, such as Sky Safari. Objects are projected to their actual scale/size. Stellarium is a free download program that does the same thing on your computer.

I hope this helps you.

Alex.

[TareqPhoto]

I use Stellarium, when i zoom in on some constellations i can see what DSO targets are there, in fact Stellarium has like an icon or emoticon to click in for DSO to turn on and view it on the screen or turn it off if not needed, in fact i use Stellarium to control my mount and i click on those DSO targets or even solar system targets such as the moon and planets and my mount goes there if it is polar aligned and star aligned.

[Granada]

Thanks Alex. This might sound like a stupid question, but as I zoom in, objects (e.g., Andromeda galaxy) obviously get larger, so what FOV should I set to mimic what you'd see in the night sky? To rephrase, how big would DSOs appear in the night sky relative to other objects (e.g., the Moon, your thumb at arm's length, etc...)?

[mental4astro]

Ok, that's a little different :)

There's a few "measuring sticks" that can be used for comparison. The first is the Full Moon, it's angular size is 1/2°, or 30min of arc (30'). The Andromeda Galaxy is roughly 3° long by 1° wide. So with the Moon as a yard stick, it is 6 full moon's long by 2 full moon's wide.

I copied the following from the website "one minute astronomer". The page also contains some diagrams for the following.

https://oneminuteastronomer.com/860/measuring-sky/

"* Stretch your thumb and little finger as far from each other as you can. The span from tip to tip is about 25 degrees.
**Do the same with your index finger and little finger. The span is 15 degrees.
* Clench your fist at arms length, and hold it with the back of your hand facing you. The width is 10 degrees.
* Hold your three middle fingers together; they span about 5 degrees.
* The width of your little finger at arm’s length is 1 degree."

These are better than using coins as a measure.

I hope this helps some more :)

Alex.

[mental4astro]

As for the FOV that you set, well what I do is know the True Field Of View (TFOV) of my scope with different eyepieces. So depending on what EP I may be using I set the FOV ring to that size.

I have a number of scopes, and I use them for different purposes. Take my 8" f/4 dobbie. I pretty much use it solely for low magnification rich field viewing. My 30mm 82° EP gives me a TFOV of 3°. So when I'm looking through an app while planning a session and that scope is what I'll be using, I will set the FOV to 3°.

Note also that the TRUE size of nearly ALL DSO's is not what you will see through a scope. The outer edge of most DSO's become too faint to see, and this area may actually form the majority of the DSO. So while an object may be described as being 0.5° in size (like the full Moon), you may only ever get to SEE less than half of that again.

The other "trick" to be aware of is the relationship between quoted size of an object and its quoted magnitude: the quoted magnitude is determined by the entire light emitted by the object over its entire size, and put into a single point source, just like a star. So take the Andromeda Galaxy again. It has a quoted magnitude of 3.4. However it is difficult to see naked eye, and we certainly don't see its entire enormous size. That's because its 3.4 magnitude is spread out over that huge size and it is also not distributed evenly - its core being brighter than the rest of the structure. This is a common problem most people make when looking for objects and can't see them, despite the objects being quoted as being "so bright"... :rolleyes: :rolleyes: I did this too and it took me a long time to figure out what was going on :rolleyes:

Oh, and don't forget that the FOV ring that you make for yourself can also be used to find out what objects also fall within the same FOV. For instance my 8" f/4 dobbie with that 30mm eyepiece, can fit M8, M20, M21 and a couple of small globular clusters all in the one FOV! :D Makes for a busy and spectacular sight! :)

And of course, different eyepieces in different scopes will all give different True Fields of View. If you don't know ow to work this out, let me know and I'll post the info :)

Alex.

[astro744]

Here is another list of DSOs with sizes as well as images.

http://www.deepskywatch.com/deepsky-guide.html

Most DSOs are small. NGC 5139 (Omega Centauri) is just over full moon in apparent size and a good example to compare others. It is also visible unaided. Most galaxies are tiny in apparent diameter. Note also when looking at a galaxy you mainly see the core as the outer portion is faint.

[Granada]

Makes sense now, thanks a bunch!