Eyepiece purchase advise

[jdkstars]

Although advanced in years I am quite new to observing the stars with a telescope.

I live in an area with significant light pollution and for the last several months have been using a 130mm tabletop (Zhumell) Dobsonian, which I have been upgrading with eyepieces.  

MY present eyepieces are:

1. GSO super (68*) plossl 20 mm

2. Paradigm 12mm

3. Meade 5000 UWA 8.8

I am now interested in an eyepiece to maximize my ability to view whatever DSO could to found (given the light pollution, my old eyes and my current scope). In the future I may consider filters (or moving/driving to dark sites) but not yet

Am looking in the general price range of $200 (us) and so far (as to currently available products)  initially thinking about the Explore Scientific 82* 14mm or 68* 16 mm.

I would appreciate any advice as between those EP or any other that would  best counter the light pollution.

Thanks

John

 

 

 

 

[KP82]

You could use the following link to check the fov of the eyepieces you plan to get:

https://astronomy.tools/calculators/field_of_view/

I believe the ES82 14mm would yield about the same tfov as your existing GSO 68-degree 20mm. The ES68 16mm will be too close to both the 20mm and 12mm. Since the max fov on your 130mm dob is limited by the size of the focuser and 20mm 68-degree is already approaching that (24mm 68-degree is the max), my suggestion would be get a UHC filter to improve your views of DSOs.

[AlexK]

Welcome to the forum, John!

Kudos to your z130 decision. It is a truly solid choice! Pun intended as the Solid OTA is the way to go in any size until you can't lift it.

The best DSO EP for it given your age and light pollution would be a 25mm focal range one.

Because 130mm / 5mm (average dark adapted pupil size in elderly  ) = 26x, 650mm/26x = 25mm
(or even simpler, as it's 1:5 mirror you can do 5mm(pupil) * 5 (aperture) = 25mm)

Just look for the widest AFOV you could afford, which is not large for a 1.25" barrel, thus should not be uber-expensive at 25mm.

THough the key for DSO observing in the light polluted location is actually not expensive eyepieces and not even any "magic" filters, but your skill of eyes darkness adaptation and its maintenance. I would rather dig into that direction first, as the collection you have is quite adequate already (I doubt you can get to a 5mm pupil size yet, at 4mm your 20mm plossl will work ideally).

On a side note to figure your actual pupil size do the following:

1. get a ruler (preferably with millimeters) and a camera with a flash (smartphone is OK and even better as it has the voice shutter release) to your telescope.
2. Make sure the "Red eyes" feature of the camera is off, setup it on a tripod if you fancy.
3. When done observing (or after at least 20 minutes of observing) something uninterrupted at night, your eyes will be dark adapted as much as they can in the given realistic environment so pupils will be widened to the max (or you can just sit outside for 20 minutes, just don't look at bright lights directly).
4. Now turn On the camera (don't look at its screen!).
5. Take off glasses (if you use tem).
6. Get the ruler in one hand and camera in another.
7. Press the ruler (numbers out and down) to your eyebrows, and open eyes wide (look into the camera against dark background).
8. Smile and make a shot of your face with flash. You should get something like this: 😃
9. Then it should be easy to estimate your dilated pupil size on the image (if not, you will have to wait another 20 minutes in the dark, so take it seriously).

In theory, you could achieve 6mm pupil size if perfectly dark adapted and even better (all eyes are different but older eyes are less flexible). But in high light pollution it will shrink back just from looking through the EP, so being a bit conservative on that pupil size is beneficial in two ways: you keep the light narrower prolonging un-vignetted view and also bumping the zoom which is in turn darkening your view, preserving your darkness adaptation for longer.

The math rule I have described above is trying to ensure that ALL the lights your mirror (or objective lens)  is focusing is transferred by the eyepiece through the pupil opening to your retina in most efficient way, so the EP is not limiting the light energy your brain perceiving by shrinking the light beam. But the darkness adaptation is an independent and much more important factor.

Edited January 28, 2021 by AlexK

[jdkstars]

Your advice is greatly appreciated

One of my problems is having spent more time reading about observing than actually observing.

Based on reading (not observing) I thought that a higher power ep with a wide afov would perhaps allow for a greater darkening of the light pollution enhancing the ability to locate any accessible DSO. So even through a 14mm 82* ep has slightly less fov than a 20mm 68* ep it’s higher power (46 vs 32x) would produce a slightly darker sky.

I do (sort of) understand the exit pupil factor and that it puts a limit at the lower end of magnification.

Notwithstanding what I “think” I know  I do understand that I know very little about this subject, particularly as to how things work in the “field” ( or in my case back deck peering mostly through the trees)

thanks
 

john

 

[AlexK]

6 minutes ago, jdkstars said:

Based on reading (not observing) I thought that a higher power ep with a wide afov would perhaps allow for a greater darkening of the light pollution enhancing the ability to locate any accessible DSO. So even through a 14mm 82* ep has slightly less fov than a 20mm 68* ep it’s higher power (46 vs 32x) would produce a slightly darker sky.

That's true. The darkening is indeed a noticeable factor, that's why I'm suggesting to reduce the exit pupil against the widest viable size but only a little bit which means more magnification. Though 15-16mm corresponds to a ~3mm exit pupil, which is for me kinda too much a sacrifice already for the fast 1:5 scope. My z12 is also 1:5, and I indeed love my 17mm most, but rather for its 100 deg AFOV, as best bright galaxies views I had with it were with the stock 26mm EP so far after a meticulous darkness adaptation (in the city only for a couple of seconds at a time). 

Then, as you have mentioned the desire to go with NB filters soon, I would ditch the zoom factor dimming, as filters are darkening the sky glow overwhelmingly better than that (however limiting you to hot gas DSO's). Filter views would rather benefit form the darkness adaptation skills and the exit pupil size than from a powerful eyepiece.

In addition, with the higher zoom, you will have your targets "smeared" wider too (together with the sky glow) i.o.w. get "darkened" as well, thus that zoom darkening is truly beneficial only for faint stars and very compact DSO observing (e.g. PN). A wide field EP is not needed for these. But still, I would recommend wider AFOV EPs as they have other benefits, e.g. for pointing (star hopping, ID) and tracking (observing in the field transit), and simply giving a more pleasant impression from views.

Would like to play with the darkening? Your EPs line is already quite good for that, so perhaps, get a 30mm (almost the top end of your usable range) and a 2x Barlow for more "steps" in between.

[Alan64]

8 hours ago, jdkstars said:

Although advanced in years I am quite new to observing the stars with a telescope.

I live in an area with significant light pollution and for the last several months have been using a 130mm tabletop (Zhumell) Dobsonian, which I have been upgrading with eyepieces.  

MY present eyepieces are:

1. GSO super (68*) plossl 20 mm

2. Paradigm 12mm

3. Meade 5000 UWA 8.8

I am now interested in an eyepiece to maximize my ability to view whatever DSO could to found (given the light pollution, my old eyes and my current scope). In the future I may consider filters (or moving/driving to dark sites) but not yet

Am looking in the general price range of $200 (us) and so far (as to currently available products)  initially thinking about the Explore Scientific 82* 14mm or 68* 16 mm.

I would appreciate any advice as between those EP or any other that would  best counter the light pollution.

Thanks

John

I would suggest a 1.25" 32mm 50° Plossl, and for the widest view possible.  Your GSO "SuperView" 20mm that you have already offers a view almost as wide, but not quite.  The 32mm(yellow) and the 20mm(red) compared...

I have this GSO 32mm, and it's very good...

https://agenaastro.com/gso-32mm-plossl-eyepiece.html

It's a hot-seller, routinely going out of and into stock.  Currently, there are only 5 left.

Also, for the sharpest images, especially at the higher and highest powers of which the 5" aperture is capable, ensure that the Newtonian's collimation is as accurate as possible.