help please....

[thersites]

Hi all

need to know the lowest useful magnification for a 12" dob for wide field views of globulars and diffuse nebula; hence want to know what fl ep is best for wide field views, and is a 2" better than a 1.25"? fl of telescope is 1500mm. is 50 or 42 mm ep too much?
ta
adam

[rmcpb]

The lowest useful magnification is usually limited by the exit pupil of the eyepiece. This is the diameter of the image produced by the eyepiece/scope combination. Now the best dilation of eyes is about 7mm in young people, this lowers to about 5mm by 50 years so if you don't want to lose any light then the lowest effective magnification would give an eye relief between these values.

How to calculate is discussed in this thread on CN (http://www.cloudynights.com/ubbthreads/showflat.php?Cat=&Board=beginners&Number=1416713&Forum=f76&Words=exit%20pupil&Searchpage=0&Limit=25&Main=1415142&Search=true&where=bodysub&Name=&daterange=1&newerval=1&newertype=m&olderval=&oldertype=&bodyprev=#Post1416713).

So using this formula Exit pupil = Focal length eyepiece/ Focal length scope a 30mm eyepiece would give you a 6mm exit pupil with your f5 scope.

Hope this helps.

[thersites]

so a 42 or 50 would be needlessly excessive?

[janoskiss]

You can use a 42mm superwide (which is the widest you can go in 2" format) but don't spend much on it. You just need to be aware that it will not let you use the full aperture of the scope. It will be like having a 9 to 10" scope with a large central obstruction instead of a 12", but the EP should still be useful for finding things and taking in the widest fov possible. One benefit of the exit pupil cutoff at your eye is that the effective f-ratio of the scope will be increased, which means that you can get away with a less well corrected EP. A 42mm Superview should be okay.

The widest TFOV "serious" EP, ie one that uses the full aperture of your scope, should have an FL not much greater than 30mm (maybe less, depending on your eyes, age, skies ...). How much $ do you have to spend?

[thersites]

don't have heaps to spend. i know i need a high quality ep to get the best out of an f/5. i guess what i was asking is what is the best wide field of view ep for this scope, which maximises the aperture but doesn't excessively magnify.

[janoskiss]

At the wide tfov end high quality EPs get very pricey. I want to try a 28mm UWAN which sells for a bit over $400. If it's as good as everyone says (except Mike :P), it would seem to be the best bang for buck for an f/5-ish, and it's an ultrawide!

30mm Stratus / Scopos is the only cheaper option I know of that should still work fairly well. Saxon are also meant to be importing their versions of these EPs.

[thersites]

so if the lowest mag ep i have is a 25mm 1.25", would this suffice for globs and diff nebula until i save up some more dough?!

[janoskiss]

Actually for most globs and nebs you'll want something about 10-13mm... sounds like you need a bit of hands on experience before you start shopping for more expensive eps. :)

For a finder EP a 25mm Plossl is okay but a bit limiting with a 1500mm FL scope. You might want to grab a 32mm which gives the widest tfov possible in the 1.25" format. I recommend you start out with 32mm, 20mm and 15mm plossls and a 2x barlow. That will be plenty to keep you happy for quite some time. :)

[33South]

MAS do a pretty good budget ($129.00) 2" 30mm widefield, its a rebadged Moonfish.

I only use mine at f10 and f15 its great, but the report by Steve gives it a good rap for a 10" f5.

http://www.myastroshop.com.au/products/details.asp?id=MAS-174

[iceman]

Well I didn't like it.. the other people who looked thru it that night seemed to have the same opinion as me :P

If you've got a 25mm, that will do until you get some experience. Get a cheap 32mm GSO plossl to cruise around with if you want something lower power in the meantime.

[paninaro]

Or you could get a GSO 30mm 2" Superview 5 element from andrews($59) or bintel ($79), I have used one in an 8" and my 10" dobs and they're excellent for the money.

[thersites]

the gso 12" comes with 25, 15, 9 and 6mm plossls. do i really need to get another (32)? or will these four do me for a while? will the images race across the ep in a 25 mm?

[rmcpb]

Just stick to the set you have for now and get some eyepiece time BEFORE starting out on the accessory trail. Before more eyepieces I would say you need a red torch, planisphere (the large Chandler one is great), observing chair (otherwise you will get dobber's back :) ), cheshire eyepiece (for collimating), a Telrad, a good basic atlas like the Bright Star Atlas 2000.0, a variable polarising moon filter and a basic moon book like Discover The Moon by Jean Lacroux, Christian Legrand, Christopher Sutcliffe. With this little lot you will be well and truely off and running.

Another thing to do is to get along to your local astro society and see what is used and why in the accessory market.

[janoskiss]

Yes, yes, yes to what Rob just said. All of it. :)


I had similar exp with some other EPs that receive raving reviews and comments on another even larger astro forum. :whistle: I wonder out of every 100 recommendations you read for an EP how many people actually looked through and properly assessed the EPs concerned and how many are just regurgitating what others told them or what they want to believe. :confuse3:

[chunkylad]

Ditto What Rob, Steve and Mike have said.

In my experience, you gets what you pays for in widefield eyepieces. At f5, low-cost eps just don't cut it.

My most used eyepiece for DSOs is my 19mm Panoptic, soon to be followed by its big brother, a 27mm Panoptic. I have a 30mm GSO Superview, and it's great if you like seagulls (in other words, at f5, only the centre of the field of view is usable, thus reducing its value as a widefield eyepiece). I have not heard anything else on this forum, or seen anything else myself to suggest otherwise.

Cheers

[glenc]

I mainly use a Baader 13mm Hyperion Eyepiece (115x, 40' field) on my 12" dob. If I was buying two I would get a Baader 21mm Hyperion Eyepiece (71x, 65') and a Baader 8mm Hyperion Eyepiece (188x, 25'). I have a 30mm GSO Superview but rarely use it. There is more info at:
http://www.myastroshop.com.au/products/baader-ep.asp