Beginners Eyepiece experience

[StutzJr]

So I'm still getting to know my brand new skywatcher 6" dobsonian, first relatively cloudless night in a while here. I have two Skywatcher "SUPER" http://www.skywatcher.com/swtinc/product.php?id=264 achromat/kellner eyepieces (25mm/50° "wide angle long, eye relief" and 10 mm/52°) that came with the scope. My workmate has kindly lent me two more eyepieces to try for a short while so that I can get an idea of what to expect. The first one is an Agena ED 9.5mm http://agenaastro.com/agena-9-5mm-ed-eyepiece.html
which has a 55° AFOV, and the second is a TMB 6mm Planetary II https://www.astronomics.com/tmb-6mm-...ce_p16540.aspx which has a 58° AFOV.

I was able to view Jupiter and Saturn, the moon and what I think was the orion nebula (I saw a small question mark shape of 4 stars close together over a cloudy smear that did not move). I found that the Agena 9.5mm seemed to have a noticeably larger field of view than the sw super 10mm, and as expected both had similar quality views of the planets in terms of size/sharpness/contrast, perhaps the agena was marginally better. The TMB 6mm indeed gave more magnification, perhaps 1.5 times larger views of planets, but much harder to get in focus. I struggled for a while with the focuser before discovering I could slowly screw the EP itself out from the barrel for a finer adjustment, although this made the eyepiece quite loose. Even at best, while giving a slightly larger view, I saw no improvement in sharpness or contrast, perhaps slightly worse than the 10mm sw/9.5 mm agena. Of course this may depend on seeing conditions and the aperture of my 6" scope. Either way, even given ideal conditions, I wouldn't want a smaller focal length eyepiece with a dobsonian mount, I'd want an equatorial or fine control AZ mount to be able to more easily track my target as it quickly moves out of the field of view.

I found that for both the loaned eyepieces, the more complex lens configuration not only made the EP longer and wider, they also were less forgiving of the eye not being exactly exactly aligned to the focal point. Initially I almost got a headache trying to keep my head in the right position to look down the EP without getting a tunnel effect cutting out part of the field of view. I assume that I would see a similar effect If I used my eyepieces with a barlow lens?

[rogerco (Roger)]

I would be a bit wary of "slowly screw eyepiece out from barrel" sounds like you are unscrewing the eyepiece optics from the nose-cone and risking having the eyepiece optics fall off. High power eyepieces like 9 and 6 are very difficult to use as the "seeing" that is how stable the atmosphere is rarely is good enough. You may find using a 12mm or bigger with 2.5x Barlow easier to use.

Give some consideration to Televue Plossl eye pieces, they are moderately priced and heaps better than what came with my dob.

All the above with a grain of salt as I am mainly interested in photography.

[raymo]

Hi StutzJr,
Rogerco is spot on. Dobs were primarily designed for mainly
low power wide field work. This means that eyepieces of less than about
8 or 9mm are of very limited use. As Rogerco has said ,you can get
higher powers without the hassles of shorter focal length eyepieces
by using low power ones with a 2x barlow. I have been using Dobs for many years, and I have found the following to be the best arrangement
for people not wanting to spend a fortune on optics.
Seben[ or similar] 8-24mm zoom eyepiece- about $60
2x barlow lens "$40-50
When you want to spend a bit more and get views like looking through
a window,
2" 30mm eyepiece with screw on 1.5x barlow about $80-90
2" 15mm " useable with the above 1.5x barlow. About$80
These two eyepieces, especially the 30mm will blow you away.
I think Andrews sell both of the 2" eyepieces, and the Seben zoom
can be got on ebay frequently. You will find that your viewing will
be much easier and relaxed using the above gear.
Hopes this helps
raymo

[StutzJr]

Thanks for the comments Roger, I did have a suspicion that the barrel wasn't supposed to be unscrewed like that in use. I'm still not exactly clear on why a 12mm EP with a 2 x barlow for example would be "easier" to use than a 6mm EP - will I have an easier time focusing and viewing with this configuration? Won't I have halved my field of view?

There certainly isn't a shortage of options with eyepieces to choose from.. I saw an interesting video today on YT http://www.youtube.com/watch?v=S5oIBZ1KD2g that appeals to me - as his second example is the exact pair of eyepieces I have. His suggestion is to consider either a 9-13mm or 15-17mm plossl eyepiece as a first supplement to the basic pair of modified achromat EPs that come with the scope. From what I've read here and elsewhere, that sounds like good sensible advice. I just have to decide which one (probably around 13mm) and settle on a brand that suits my budget. I'm also keen to get a moon filter pretty soon, I guess a variable polarizing filter would be a good choice for long term usability regardless of whatever I might get in future..

Just saw raymo's post as I was about to click submit - I was thinking about 2 inch eyepieces too - I'll have to look at those as well..

[Wavytone]

Hi StutzJr,

Be very wary of unscrewing eyepieces especially high power ones for several reasons:

If you unscrew it, part of it may come off in your hand. That's the good news. The bad news is if you aren't very careful, something else will be left behind in the focuser, and could easily hit the ground or worse, the primary mirror of the dob, marring the surface.

If it turns out you unscrewed part of the optics, high power eyepieces tend to collect bits of dust and fluff very easily that will appear as big black blobs in the field of view, attempts to "clean" will be less than successful. Lastly, if you have dismantled it, after reassembly they never work as well as they did originally.

Bottom line is DON'T, if you value them.

[StutzJr]

Let me clarify right now that I won't be doing that again, it was just a comment about my first experience of reaching the performance limit in terms of resolution whether due to equipment or conditions. I just wanted to know if it was possible at all to achieve focus given that condition or whether I was just struggling with my rack & pinion focuser.
Now I know that's a bad thing to do, fair enough.

[rogerco (Roger)]

If your dob does have a rack and pinion focuser you might consider getting a low profile Crayford 10:1 focuser as it will greatly increase your focusing ability. The low profile one will also assist if you want to attach a camera later.

Re using a 12mm with 2x barlow as against a 6mm. I don't know the optics but I have found the combination much easier to use, something about eye relief and exit pupil width ? I was looking at Saturn tonight with the C8 which is around 2000mm with a 2.5x barlow and a 23mm eyepiece very nice, seeing wasn't good enough for a 12mm with the barlow, and the 12mm by inself wasn't as easy to use as the 23mm and barlow.

[StutzJr]

Cheers Roger,

I think upgrading the focuser is a bit overkill for my first 6" dob. May need to upgrade the user first..
Thanks for the answer on the Barlow. My first impression was that it was a cheap workaround, but it's sounding more and more like a necessary tool in the toolbox so to speak.

[raymo]

Hi again StutzJr,
Regarding your query about field of view[F.O.V.]. I'l try and keep it short, but here goes. There are several benefits to be had
by using a barlow. The light passing through a barlow diverges and typically doubles or triples the focal length of the scope being used.
The longer the focal length, the less obvious any optical aberrations
in the optical train become.[ can only be good].
The longer the focal length of a scope, the easier it is to achieve
a really good focus.[short F.L. Newtonians can be fiddly to focus well].
A barlow significantly improves the eye relief of any eyepiece it is
used with, meaning that, especially with short F.L. eyepieces, you don't
need to have your eye jammed up against the eyepiece, trying to look through a tiny 5 or 6mm diameter lens.
If you look through a 12mm E.P. you will have a certain F.O.V. If you
add a barlow, you will double the magnification and halve the F.O.V.
which is exactly what a 6mm E.P. does. So in short, why would you
use a 6mm when you can have a better experience with a larger lens
to look through and better eye relief with 12mm +barlow,[provided
that the barlow is of good quality].
Incidentally, I really love my Seben 8-24mm zoom, the quality
is amazing for the price.
Hope it all helped.
raymo

[rogerco (Roger)]

Thanks Raymo, we have both learnt something, great explanation.

[StutzJr]

Thanks raymo for the really informative post! That's exactly the answer I was looking for, much appreciated!

[raymo]

No probs.
raymo