DSO - Field of View

[ee001]

I'd like your opinions on what sort of 'true field of view' I should be looking at when observing DSO, particularly galaxies. For example, with my dob an 11mm 50degree eyepiece would give a true field of view of 0.4degrees and a mag of 139X. Is that too narrow/too wide?

Is there a way of finding out how much of the field of view different objects occupy so that I can decide which eyepieces would serve me best?

Thanks

[BlueAstra]

If you get a good reference book (I use Philip's Deep Sky Observer's Guide) it will generally tell you the typical angular extent of a DSO (eg M8 is 45' x 30'). This should give you a guide to the magnification required to frame the object if you know the FOV of the EP.

[John]

It all depends on the DSO - some are relatively small (eg: M57, The Ring Nebula) and some pretty large (eg: M31, The Andromeda Galaxy). When I'm using my dob I tend to use a red dot finder to get the scope pointed in the general vicinity then a wide field eyepiece (eg: somthing that gives 1 -1.5 degrees FoV) to find the object. I then use more power as needed but seldom more than 100x - personally I like to view DSO's with some surrounding sky.

John

[mdstuart]

Well I have observed about 180 galaxies so far and I would always start with a 20mm eyepeice to get a wide field view and then perhaps around 7mm for a closer look. In most cases this works for the galaxies I have seen so far.

[Aza]

Does this help:

VSA - Visual Sky Assist 1.1 - Check deepsky objects and print your own list

[ee001]

Does this help:

VSA - Visual Sky Assist 1.1 - Check deepsky objects and print your own list

That was very helpful, particularly the fact that it gives the size of each object.

Thanks to all of you.

[brianb]

Usually with DSOs start off with a very low power (about 5x per inch), this is the only way to see faint nebulosity. Higher powers up to approx. 20x per inch may or may not be useful depending on the target. The field of view is a red herring, with very few exceptions galaxies will fit in a 10' field, most star clusters will happily sit in 30'. It's the magnification which is important - low magnification gives more contrast, higher magnification digs deeper into structures like globular clusters.

[MeerkatinBKK]

There's a good piece of software, Astro Planner, that also gives dimensions for DSOs. The basic version (Win/Mac) is here and is free.

If you enter your scope, eyepiece and any reducers/barlows you have into it, it will also recommend which to use for each one, as well as producing a starmap with the FOV outlined. So as an example with my details entered, and clicking on M69, it suggests: 29x (Best: Plossl 32mm + f6.3 focal reducer at 40x. Best E/P: Plossl 32mm at 63x).

It does a bunch of other useful things too, and has become an essential part of my astronomy software arsenal.

Nick

[ee001]

There's a good piece of software, Astro Planner, that also gives dimensions for DSOs. The basic version (Win/Mac) is here and is free.

If you enter your scope, eyepiece and any reducers/barlows you have into it, it will also recommend which to use for each one, as well as producing a starmap with the FOV outlined. So as an example with my details entered, and clicking on M69, it suggests: 29x (Best: Plossl 32mm + f6.3 focal reducer at 40x. Best E/P: Plossl 32mm at 63x).

It does a bunch of other useful things too, and has become an essential part of my astronomy software arsenal.

Nick

I just had a chance to try this out and I must say what a great find! It answers my initial post but as you say has many more uses.

Thanks a lot.