Eye pieces

[Simmo74]

I have recently purchased a 6" scope & have a 20mm & 10mm eye piece.
What eye pices should i be investing in to be able to see the rings of Saturn, Jupiter etc ?

[iborg]

Hi

With the scope and eyepieces, I think you shold be able to disks (small ones) of both Jupiter and Saturn. With the 10mm, I think you should also see Saturns rings.

Try using an eyepiece calculator (link below, or use google)) to get an idea of the magnifications with your setup.

You will need to know
1: Telescope apeture (6", 150mm)

2: Telescope focal length (?, and number written like F6)

3: Eyepiece focal length (10 or 20mm in your case)

Have fun

Philip

[Wavytone]

Simmo, if you know the focal ratio (fr) then you will want one eyepiece with a focal length in mm of fr for sure. The shortest useful eyepiece will be about 0.75 x fr or perhaps 0.6 x fr, in mm. If you try ones even less than this, yes the image will be bigger but dimmer, and less sharp, so you won't see more detail.

For example, with say a 6" f/5 reflector, you'll want one about 5mm and another perhaps at 3.5mm. Of if you have a 6" f/15 maksutov, 15 mm and 10mm.

[Nab]

With a new scope to learn you might want to focus on working out your mount - assuming you got the EQ, and the two eyepieces you have. Both of these will show you Jupiter and the rings of Saturn (which you can make out in decent binoculars). Learn to use what you have well before investing more.
Cheers.
Darren.

[AEAJR]

We would have to know the focal length of the scope to know if what you have now is adequate. Anything over 60X will see the rings. Getting it up to about 120X would be better. typically a 6", 150 mm scope has the potential to go to 300X though the atmosphere will typically limit you to something around 200X most of the time.


Let me offer some ideas for a plan for your eyepieces since you just purchased your telescope. This will give you an approach to plan your eyepiece purchases. You can sub in the FL and aperture of your telescope and do the calculations for yourself.


For the purposes of this discussion it doesn’t matter what kind of telescope you have; refractor, Newtonian, Mak or SCT. Manual vs. GoTo also does not matter. You can substitute in your aperture and focal length to get the eyepiece range for your telescope.


Assumptions:


1.25” focuser – There is a minor change at low power if you have a 2” focuser.
You want to view everything so you want to exploit the full range of your telescope, lowest power to highest power. We will address practical limits later.


Your lowest power widest view will be a 32 mm plossl which his based on the 1.25 mm focuser. Use 2X the aperture of your telescope in mm for your highest power target. You won’t always be able to go this high but it is a good target for planning.


Let’s run an example.


Aperture 130 mm
Focal length, FL, of 700 mm.



So top power for this example would be 260X (2X aperture in mm)



This will be an important forumula: FL telescope / FL eyepiece = Magnification You can adjust the magnification targets I set based on what eyepieces came with your telescope and if you plan to continue to use them or plan to replace them.



Approaches:


single FL eyepieces
eyepiece + barlow
zoom + barlow.


I suggest you have at least 4 magnifications, but that is up to you. I use 6 in the example.


Single FL Eyepieces

Example: FL = 700 - I am going to use a range of magnification targets and approximate FL for the eyepieces. Nothing magic about these magnifications, just an example.


22X = 32 mm (useful for star hopping, large DSOs like the Pleiades and the Andromeda Galaxy)
50X = 14 mm
100X = 7 mm
150X = 4.6 mm
200X = 3.5 mm
260 X = 2.7 mm


So, you could buy those 6 eyepieces and have a good range. Or you could buy 3 eyepieces and use a barlow which gives you 2 magnifications for each eyepiece.



Using a 2X barlow for this 700 mm FL telescope.


32 mm - Barlow for 16 mm
7 mm – barlow for 3.5 mm
5 mm – Barlow for 2.5 mm


Based on 3X barlow:


32 mm – Barlow for 10.7 mm
14 mm – Barlow for 4.6 mm
8 mm – Barlow for 2.6 mm


Zoom + Barlow - Based on an 8-24 zoom and a 3X barlow for this example

8-24 zoom = 29X to 87.5X and everything in between
w/ 3X barlow - 87X to 262X and everything in between.


You see that you have 3 approaches that will give you a range of magnifications for your telescope. Obviously you can mix and match as you like.



Atmosphere - Note that atmospheric conditions will often be the limiting factor rather than your telescope or your eyepieces. Many days I can’t use anyting over 220X even in my 8”/203 mm telescope. The turbulance in the atmosphere causes so much distrubance of the image that going higher does not provide more detail. Using the 2X aperture formula I would target about 400X for my telescope but I don’t have an eyepiece for 400X. Anything over 220X I use a barlow. I have used 400X on the moon but even on the best night the image has been very unsteady.



REFERENCE LINKS


Selecting an eyepiece - Orion telescope
This is a very general discussion of eyepieces and why there are a variety of designs
https://www.youtube.com/watch?v=m7u9Q5hV7yc

Eyepiece Designs - This is the one I turn to when I am trying to understand or explain the
differences between the various designs. There are many different designs, Many are named
for their original designer, such as Huyghens, Ramsden, Kellner, Plossl, Konig, Erfle, Branden and Nagler.
http://www.chuckhawks.com/common_eyepiece_designs.htm


zoom eyepiece review – Includes the Celestron zoom
http://www.chuckhawks.com/comparison_zoom_eyepieces.htm


Baader Hyperian Mark III Clickstop zoom review
http://www.weasner.com/co/Reviews/2014/Baader_Zoom_Eyepiece/index.html

[AEAJR]

Did you get your new eyepieces?