Eyepieces for my Skywatcher 70 x 900

[Darth Wader (Wade)]

Hi all,

I've got a Skywatcher 70 x 900 refractor and I've been using a 10mm eyepiece for planetary viewing as well as a 25mm with a barlow for star clusters and nebulae (I generally don't use the barlow with my 10mm because the images are usually unclear). Being especially interested in the planets I'm looking to get some new eyepieces and I came across some 4mm & 6mm 1.25" Guan Sheng ones through the Andrews communications website. I don't want to spend a fortune for two reasons - I've really only just started out with the scope and I don't want to buy the eyepieces only to discover that they're not up to scratch. So two quick questions for you:

- Are the Guan Sheng eyepieces any good?

- Will the magnification factor be too high for my little 70 x 900 and give me nothing but disappointment? I calculated the magnification at 225x for the 4mm and 150x for the 6mm but I have no idea if this is pushing it under suburban backyard conditions.

I really appreciate your help!

Cheers
Wade

[dannat (Daniel)]

the eye relief is too short on the gso ep's imo. If you don't want to spend a lot of money i would look at either the long perng planetary or william optics spl on the andrews website. the long perng are 99, WO 119 but both come from the long perng factory. The eye relief is good - and while the fov is not that of a nagler, the 55-60 deg is more than enough for planets. kkep in mind at 70mm the most you could squeeze is 150x magnification, which would be the 6mm,I'd be inclined to get a 9mm, i think you'll use it more for more objects and it won't matter a smuch what the seeing is like

[wavelandscott (Scott)]

I agree with the comment about plossls and eye relief especially for me once you get below 10 mm.

I think a pretty good "budget" eyepiece are the TMB Planetary eye Pieces (or whatever they are called now). Daniel at Frontier Optics did handle them. While they are not quite in the same class as the Pentax, Televue range, I consider them above many others (plossls etc.) as the have good eye relief and a larger field of view.

The Pentax XF eyepieces also deliver value with outstanding performance and a lower price point.

I've always encouraged people to avoid buying lots of cheap eyepieces if they think they are serious about the hobby. On any given viewing night I will tend to use just a couple of eyepieces...

Get a couple of good ones instead of several average ones...the money you spend will be about the same. Also if you decide to exit the hobby, the less than $50 eyepieces will be hard to give away...the higher end stuff will retain a much larger portion of its re-sale value.

Clear Dark Skies!

[Darth Wader (Wade)]

Hi guys

Thanks for the replies! I'm just starting out so I don't know what you mean by "eye relief" and "the fov is not that of a nagler, the 55-60 deg is more than enough for planets..." Help!

Cheers
Wade

[wavelandscott (Scott)]

Originally Posted by Darth Wader

Hi guys

Thanks for the replies! I'm just starting out so I don't know what you mean by "eye relief" and "the fov is not that of a nagler, the 55-60 deg is more than enough for planets..." Help!

Cheers
Wade

If that is the case, then before you spend any money at all, get to a viewing night with other people and try some different gear out. A little observing time will give you a much better understanding of your likes/dislikes and help ou get the right gear for you to enjoy the hobby.

[dannat (Daniel)]

eye relief is how close/comfortable to look thru the eyepiece, bigger number the better - ie 6m eye relief is very tight and your eye will be pressed up against the piece, 20mm is comfortable and your eye doesn't have to be glued to it
fov is field of view - or how much the eye piece lets you see out - bigger is better, think about looking thru a peephole in a plane compared with a porthole - the porthole lets you take a lot more in

[Coen]

I too have a 70x900 skywatcher refractor. Apart from the two basic eyepieces and barlow that came with the scope I have:
A better quality 25mm plossl it higher contrast than the 25mm that comes with the scope but narrower field-of-view so the clusters do not fit entirely in the eyepiece compared to the basic eyepiece. I also think it does better showing the colour of some of the redder stars than the basic eyepiece. Having said that I do not use either 25mm often.
A 15mm GSO SuperView, this has almost the same field of view as the 25mm plossl but at 60x. It suffers some degradation of the star shape (i.e. no longer pin-points) towards the edge of the view but for the refractor it is a good match and is the default eyepiece living in the scope.
A 6mm GSO plossl for that once in a while good seeing moment. As noted this eyepiece does need the eye close and not all of the field of view is visible at once i.e. need pan the eye around a little. I have it for splitting that double star that is tough and for the occasional look at the planets especially when attempting Io's shadow transit on Jupiter.

Regarding other things, if you have not already:
- I'd get yourself some good star maps, there are a number around that are free as well as ones you can purchase.
- A pair of binoculars, star maps and the 'scope. Start with something you know in the sky and on the map, pan around with the binoculars, see something interesting zoom in with the 'scope and find on the map. Great way to learn your way around and prepares you for the moment you want to find that object you've read/heard about.

I note in another thread you discuss viewing Jupiter, once you are comfortable with the planet, viewing a transit of a shadow can be fun. There is software out there that predicts the times and so use that to plan an observation. Ganymede is the easiest for the 'scope. Io is tough in the 70mm. Some colour filters also help bring features out on the cloud deck. Light blue and light yellow work for me, light green is supposed to be good too. You can attempt to see the "dent" in the cloud deck where the red-spot is. You will not see the red-spot itself due to its low contrast.

As you spend more time at the eyepiece you will begin to see more things too.

BTW over in the Observation Reports section you'll see some write-ups of what I have been up to with a 70mm, that might be helpful (although I am looking at some obscure things at times).

An object selection that I am sure you will enjoy:
M42 - the Orion nebula (middle "star" in the sword), taking careful note of the structure of the gas cloud and the trapezium star in the middle.
Omega Cent - a big globular cluster that looks like a grainy ball of fluff that you can look at for a long time seeing the salt & pepper of stars.
NGC 5128 - the hamburger galaxy, bit tougher but looking at it for a while and you will see parts of the dust lane bisecting the cotton ball looking thing.
Eta Carina - cluster plus gas cloud with a really large star (eta Carina itself) glowing orange.
Around the Carina region there are wonderful star clusters to view including NGC3252.
Kappa Crucis - Jewel box cluster (near the arm of the southern cross)
Acrux - the foot of the cross, the star is a close double (for the 70mm) of two nearly equal stars.

Good to see you are enjoying yourself!

[Darth Wader (Wade)]

Thanks for all the informative replies guys.

Coen, my lovely wife got me Astronomy 2009 which I now couldn't live without! I also have a planisphere and My Dad's old binoculars which I use to scan the skies for points of interest unless I know what I'm going to look at straight away.

The only problem I have at the moment with Jupiter is that it's fairly tiny (not that I expect much more from a small scope of course) so I doubt I'll be able to see a transit until I get a decent eyepiece. I might pick up a 6mm as mentioned above, if it's working for you in the same scope then it should be OK for me. Hopefully the 6mm will allow me to see saturn with a little detail, at the moment it's not much more than a dot!

I've checked out M42 but again, not much to see compared to the first time I viewed it through an 8' dob at a star party! The 25mm eye piece doesn't really do it any justice so down the line I may look at something else for nebulae/star clusters. Will definitely note down the objects you've named, I'm always looking for new things to view.

Cheers
Wade

[Coen]

Well, as mentioned I have the two basic 25mm and 10mm that came with the telescope, and with "training" (i.e. practice and experience teaching your eye and brain how to look) you can see the shadow transits. I have seen, at different times, the shadow transits of all 4 principle moons of Jupiter. The one that was toughest and not consistently seen was Io. Ganymede is a distinct black spec that is clearly visible with the 10mm eyepiece. The 15mm GSO SV shows them clearly also. The picture in the 6mm was not crisp but suspect, as Jupiter was low in the sky, that it was a function of atmosphere. Best viewing is of objects as close to overhead as possible - especially true when trying to spot small shadows.

The size of Jupiter in the eyepiece - well it is a long way away . For comparison, in the 70mm the 10mm eyepiece gives a FOV of about 36 arc minutes. Jupiter is about 35 arc seconds across at the moment. In other words your eyepiece FOV is 36*60/35 Jupiter's wide (rounding gives 60 Jupiter's across) at about 90 magnification. The same eyepiece in a F5 8" reflector gives magnification of 100 with a true field of 33 arc minutes i.e. Jupiter will look a little bigger but not a lot with respect to the total field size. However there will be more light gathered so it will look brighter, also the resolution will be much better in the 8". In the 70mm the best angular resolution if everything is 100% efficient & designed is 1.65 arc seconds or, to go on a tangent, lunar features to a bit over 6 km in size and at Jupiter's distance you get 21 "chunks" across (35/1.65) i.e. best you can get is a resolution of a feature 1/20 of Jupiter's width or about 1/2 an Earth (I think there about 11 Earth diameters to Jupiter's?). The red-spot is about two Earths across so 4 resolution chunks (give or take). When Jupiter is closer it grows to about 45 arc seconds or 27 "chunks" across i.e. features to about a bit bigger than 1/3 an Earth in size. For an 8" (200mm) it becomes a resolving power of 0.58 arc seconds or lunar features a little over 2 km in size or in Jupiter's case (at 35 arc seconds) 60 "chunks" or 1/5 Earth in size. More aperture gives you better detail and more photons i.e. a brighter object in the eyepiece.

M42 is an object that has detail no matter what aperture including the 70mm but dark skies help a lot. No dark skies = little to no detail no matter the aperture which means you go more towards clusters and double stars to look at.

P.S. My maths might be rough but I trust you get the idea of what I mean, plus remember Jupiter is about 860 million kilometres away at the moment.