Need advice on eyepieces for 18" skywatcher

[raanany]

Hi guys

I need your advice.

I'm new here. It's my first post and I'll be receiving my first scope in a few weeks. Kind of a lifelong dream for me. I'm 43 btw.

What I really wanted was an obsession 18"UC. I was even considering the 15" version but owing to freight and tax (I don't live in the U.S.) the total cost of even the 15" would have eaten my eyepiece budget.

Instead I've ordered a skywatcher 18" 450p from a local dealer for $5K. I hope it works well, there aren't any user reviews that I could find.

My question is actually about eyepieces. The optics on my scope have the same figures as the obsession 18” UC. It's a 458mm objective with 1900mm focal length. This means f/4.15.

My dilated eye-pupil seems to be around 6.5mm

Questions:

For the low magnification end, which of these would you recommend best

Model Fe AFOV TFOV Dep Mag Price
Nagler 26 82 1.055 6.267 73 $615
ES 25 100 1.236 6.026 76 $800
Nagler 31 82 1.266 7.473 61 $650
Panop 41 68 1.387 9.883 46 $510


For the high magnification end, I was thinking about ont of these:
Model Fe AFOV TFOV Dep Mag Price
Ethos 4.7 110 0.270 1.133 404 $610
Ethos 6 100 0.314 1.446 317 $575

Do take into account that I might in the future get a coma correction lens (equivalent to 15% increase in objective focal length)

Thanks,
Raanan

[lamperti]

Raanan,
Welcome to the group!
You make get a variety of opinions but I used to own an 18" before upgrading. For low magnification, a 35mm Panoptic may be a bit less expensive and give you 54x and a FOV a bit over 1 degree which is helpful if you star hop. For the higher magnification, I think the 4.7 Ethos is a bit too high magnification. Perhaps an 8mm to give you 238x, though the 6 mm would be OK too. Given those two for an initial spread, you should be plenty busy. Perhaps a 13mm or 17mm can be on your future wish list.

Al

[Dragan]

Welcome to the group Raanan and congrats on the new scope.

If your primary form of observing will be deepsky, then you may want to look at eyepieces in your budget that will provide you with 1.5-2mm exit pupils. (eyepiece fl ÷ telescope f number = exit pupil) That'll give you optimal magnification for the type of observing you'll find here at DSF. By no means is that the only way to decide on an eyepiece. Its just one suggestion.

And, i'm with Al. The 4.7 may be a bit too much mag depending just on how often you get good seeing. I think you'll get more use out of eyepieces in the 8-12mm range. Have a 6 as your high power EP.

Just suggestions. Good luck with whatever you decide!

[skyraider]

At f4.1 you will want a ParaCorr Type2. It will make a huge difference, especially in any of your wide field / low power eyepieces.

[Don Pensack]

You will need a Paracorr to correct the abundant coma at that f/ratio. That will make your focal length now be 2185mm = f/4.77
So, your nominal range of focal lengths would be from33mm down to 2.4mm
In practice, you should start with a set of at least 3, yielding 75x, 150x, and 225X.
If you add a couple more, add one to yield 300x and 400X.
That means focal lengths (rounded off) of approximately 30, 15, 10, 7, and 5mm
The best one near 30mm for that focal ratio is the 31mm Nagler = 6.5mm exit pupil = 70x A great low power eyepiece.
Around 15mm, the 16mm Nagler or the 13 Ethos (both have almost exactly the same true field). I'd go for the Ethos = 2.7mm exit pupil = 168x This may be your most used DSO eyepiece.
Should you have something in between? Maybe something around 18MM?
Around 10mm, the 10 Ethos or the 10 Delos. = 219x = 2.1mm exit pupil. High visual acuity.
Around 7mm, the 6 Ethos or 6 Delos = 364x = 1.3mm exit pupil. Nice high power uninterrupted by floaters in the eye.
Around 5mm, the 4.7mm Ethos SX or the 4.5mm Delos. = 465x = 0.99mm exit pupil. This will be a practical high power.
Beyond that, use a barlow or get a shorter focal length eyepiece. You won't use it much, so it's only if you can afford to do it:
3.7mm Ethos SX
3.5mm Delos
3.5mm Nagler
2.5mm Nagler


Some additional notes:
The 25x100 ES is a "compromise" eyepiece, with a lot of limitations (I won't list all of them here). You'd be better off with a 21 Ethos or 20x100 ES.
With a 31 Nagler, you don't need anything in this focal length anyways. Your next stop is at 18-20mm.
The Pan 41 would yield too large an exit pupil

[deepskytraveler]

Around 7mm, the 6 Ethos or 6 Delos = 364x = 1.3mm exit pupil. Nice high power uninterrupted by floaters in the eye.



Don, please explain what you mean by "Nice high power uninterrupted by floaters in the eye." I believe I see more floaters at higher powers.

Thanks,

Mark

[Don Pensack]

The high power generated will be of sufficient power to reveal small details, yet not produce exit pupils so small that floaters in the eye intercept the light.
You may have noticed some shadows that appear when watching the moon at high power that are shaped like little dots or amoebas or bacilli. These little shadows always seem to drift in front of the detail you want to see.
They are small protein agglomerations in the vitreous humor of your eye and are casting shadows on the retina.
When the magnification is very high, the size of the light cone entering the eye is very small--small enough for floaters to actually intercept the light nearly completely and cause shadows to appear in the images. This often occurs in older folks when the exit pupils are smaller than 1mm (focal length of eyepiece = f/ratio of scope).
In this case, the exit pupil produced is large enough that floaters likely won't play a role in the image, yet high enough to see details.
For most of us, the usable ends of the magnification range are not really determined by the sizes of our pupils at low power and our lack of retinal resolution on the top end, but by where astigmatism becomes obtrusive at the low end and floaters become obnoxious at the high end.
I should be able to use eyepieces with exit pupils from 5mm to 0.5mm, but to avoid astigmatism and floaters, I pretty much confine it to 3.7mm to 0.8mm and avoid both lower and higher powers. Your own experience will show what range works for you.

[RolandosCY]

Well, I own an 18" f4.5. I have quite an extensive collection of eyepieces, but I tend to use only 3 eyepieces: The 31mm Nagler provides a nice wide field of more than one degree that usually contains enough stars to allow me to proceed to my target even if it is not visible at the low magnification. My second eyepiece is the 13mm Ethos. It provides me with a mag of 159x which often is enough to reveal even tiny faint targets yet again provides enough stars for starhopping and nice starfields. Now, for my high power eyepiece, I moved from the 6mm Ethos to the 7mm Nagler and now to the 7.5mm Takahashi LE. Despite the significantly smaller field at 275x, the images obtained with the LE seem to be more detailed and contrasty. I do enjoy the wide fields of Ethos and Naglers yet the images of the Tak are simply from a different level.

Note that this collection is only for the 18". For my refractors I prefer the 21mm Ethos as my wide field eyepiece. Not a matter of quality or field, but it is something intanglible, the image using the 21mm Ethos is simply more pleasing than that with the 31mm Nagler. On the dob both images are similar so I prefer the 31mm which gives a wider field....