EP Focal length and FOV for DSO viewing?

[Benboy (Ben)]

Hi everyone.

I'm finding asking questions on this forum very helpful sooo...here goes another.

I was wondering what focal length and FOV I should be looking at getting for viewing DSO's on my 6" Dob.

What would be the top focal length recommendation... and the second? and what size FOV is acceptable. I'm on a budget so won't be forking out for any expensive 100 degree eyepieces - but would appreciate any input on what has worked well for others.


Thanks in advance

Ben

[astro744]

If you have a 2" focuser look at http://www.televue.com/engine/TV3_Pa...n=Advice&id=96

If you have 1.25" only then 24mm Panoptic for maximum true field and 13mm Nagler for close-ups on DSO's. Add a 7mm Nagler for close-ups on planetary nebulae and planets too.

Note the above is a premium wide apparent field set that will last you a lifetime. You could stick with 50deg AFOV Plossls and for the cost of two of the above you could get all the Tele Vue Plossls up to 32mm which would be a very nice premium 50deg AFOV set. The wider AFOV is nice if you have an untracked DOB.

If you want one eyepiece to give you that wow factor go for the 13 or 12mm Nagler as your first premium wide field eyepiece. I personally prefer the 12mm but the 13mm is very nice and much smaller.

[mental4astro (Alexander)]

The easiest way to determine the optimal longest focal length eyepiece for a reflector is using the following formula:

longest f.l. eyepiece = f/ratio of scope X 6mm

Therefore with your 6" dob, its focal ratio, say is f/6, then the longest focal length is 36mm. At a stretch a 40mm could be used as this focal length is more common than 36mm.

The reason for not going any longer is that the shadow of the secondary mirror starts to become too obvious and seriously degrades the image.

Quality wise, the first determining factor is the size focuser you have. There is a physical limit to how wide a field of view you can get with a 1.25" EP (eyepiece) in the longer focal lengths. This being determined by the size of the barrel of the eyepiece fitting into the focuser. Hence you will find really wide angle eyepiece (greater than 65 degrees) only in the 2" format in anything over 30mm.

The second determining factor is human eye physiology. Being blessed with human eyes, we have a limiting field of view of 68* until we need to start to move our head to see more. An 80* FOV is enormous, and 100*, well, that is in the relm of birds of prey and antelops.

The third factor is the focal ratio of the scope. 'Fast' scopes, those with a focal ratio typically smaller than f/6, like f/5 and f/4, begin to show an artifact called "coma". This manifests as the stars along the edge of the field of view showing little comet like tails. It is not a defect in either the scope's mirror or the eyepiece being used. It is a result of the eyepiece not being able to deal well with the 'stumpy' cone of light the primary mirror forms, resulting in the eyepiece being incapable of focusing all the light at the edge of its FOV all at the one point. This can be over come to an extent with the use of 'coma correctors', but this adds to the weigth and complexity of using a scope, not to mention balance issues.

The forth factor is $$$. If this is not a problem, then ignore the first three factors.

If $ are a factor, then keep in mind that there are many exceedingly good eyepieces out there that can perform just as well, and fulfill the complexities of the first three factors listed.

To this end I would suggest you look into the TMB Paragon 40mm (my first preference, $300 new in OZ, I've used its 30mm sibling and I want one), and the GSO Superview 42mm (second pref., $60 at Andrews Communications, I have the 30mm version). Both EPs have a 68* FOV, though the later EP is not as good for coma correcton and does suffer from astigmatism.

I must admit that an 82* FOV is amazing. However, it is not for ME. I've used one in my f/4.5 scope, and found it uncomfortable to use, and without a coma corrector, this really wide FOV is not done justice.

Try an 82* and a 68* FOV eyepieces. See what feels most comfortable to use for YOU. A Nagler EP is at least twice the price of your 6" dob.

[dannat (Daniel)]

Ben when looking for Dso's i like to stick with a simple formula..that is size of the mirror in mm divided by 2..in your case 150/2 = 75.
that is the magnification(75x) i would like to use to scan around for dso's with..at this magnification if you used a 68 deg eyepiece you are getting just below 1 deg {some of the 60-68deg ep's around are hyperion, stratus, meade swa, pentax xf, Wo swan..TV panoptic/radian)
i would use this magnification most (or a little less - say between 50-75x) - then for higher mag you might want something about 100x more - this will be to zoom in for planets, faint nebuale or GC's

everyobe has there own preference though - some like to push magnification much more - others just like low magnification that wide view give

[Suzy]

Hi Ben,

With my 6", I use a 22mm (I used to use 25mm- good too) to find an object, then bring it in with a 15mm - like what Astro744 said. However, now that I have a Pentax xw 10mm (70 deg fov), I find myself using that very often as even though it's high mag, I have a great field of view. I think it also depends on the object. When I want to view the Trapezium in Orion, I throw on the 10). But with the Tarantula as it's so small, I first found it with a 30mm and then had to pull it in with a 15mm and when it's viewable again I will actually be putting my Pentax 10mm on it. The Sombrero Galaxy got found in my 22mm and had to pull it in with my 10mm- this was my first viewing of it, and If I had more time would have liked to have tried pulling it more with a 5mm (not sure how that would have worked, but a 10mm was great).

I agree with what Alexander (Mental) said about the 80 + deg fov's. I am not a fan. I have a 85 fov in my 30mm and whilst it's good for sky cruising, it's a lot of work having to move your eyeballs around - my opinion anyway. My Pentax being 70 deg fov and my Vixen Lvw 22mm (65 deg fov) - i find is perfect both for my eyes, and the fact I'm not working the dob as much. My 10mm 70 deg fov (my favourite), and 22mm 65 fov are my hardest working eyepieces on my 6" dob. If you are going to spend "decent" money, my recommendation would be to spend it in these focal lengths. Don't forget you can Barlow them down if you get a Barlow.

Premium widefield eyepieces, like Astro744 said will give you the wow factor, it will feel like you are doing a space walk, not to mention quality and contrast. They will last you a life time. I will never part with my Pentax, and if a thief came up to me to rob me whilst viewing, I would leave my scope and run off with my eyepieces ('cause they cost more than the scope and apparently that's the natural order of things).
Cheap ep's limit the scopes performance - you will see the improvement for yourself when you throw on a "moderate range" ep on your scope, compared to the ones that come with the telescope. Then take a gawk at premium... you'll never, ever, ever, go back!

Enjoy Ben!
Suzy.