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Old 07-07-2018, 08:24 AM
NTgazer (Chris)
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Planetary eyepiece for f6 newt

Hi I am looking for some advice or recommendations please on eyepiece selection. I have an 8 inch f6 Newtonian and am looking for a high magnification eyepiece for planetary viewing? I sometimes see comments about some eyepieces along the lines of 'good in refractors but not newts' etc, is this to do with the construction or FOV/angle? My budget is only around $150 unfortunately. Thanks
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Old 07-07-2018, 03:13 PM
Wavytone
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For your scope 6mm down to 4mm. There's a set of 5, 6 and 7 TMB Planetaries for sale here on IIS, maybe buy the set and sell the 7mm if you don't like it.
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Old 07-07-2018, 05:03 PM
astro744
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The 5,6 & 7mm focal lengths are ideal for 8" f6 and planetary viewing giving 240x, 200x & 171x with 1200mm focal length and exit pupils of 0.83, 1 & 1.17mm at f6.

You will find the 7mm very often used on nights when seeing is above average but not exceptional. 171x gives a good image size on Jupiter and Saturn. In fact 8mm would be useful too giving 150x which is still ample for the larger planets and provides for a sharper view when seeing is average which happens quite a bit. (120x too is great on Jupiter).

If you want more power then get a quality 2x Barlow and you will have 8, 7, 6, 5, 4, 3.5, 3 & 2.5mm but the shorter focal lengths are more suited for a short refractor and will give a smaller exit pupil at f6. Start without the Barlow though as you may find 5,6 & 7 is all you need.
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Old 07-07-2018, 05:55 PM
Tropo-Bob (Bob)
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Hi Chris,

What EPs do U currently own and how are they performing at higher magnifications?
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Old 07-07-2018, 06:01 PM
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mental4astro (Alexander)
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The reason some eyepieces are better suited to one scope type than another has to do with the way different telescope designs come to focus.

We first think that a scope comes to focus at a single point. Well, it does if the point is a single star. But there are stars across the whole field of view! not all in the one spot. So, each star really is an individual point of focus which really is a focal plane, not single focal point. AND this focal plane is NOT FLAT, but curved, somewhat like a bowl.

The shape of this curve is either concave or convex, and which it is is determined by the design of the scope - Newtonians give a concave focal plane, refractors, SCTs and Maks produce a convex focal plane.

Then, how "deep" the focal plane/bowl shape is is given by the focal ratio - the faster the focal ratio, the deeper the bowl shape, the slower the f/ratio the flatter the shape.

A clue to this is given by the use of "field flatteners" in astrophotography. The chips in cameras are flat. But as the scope produces a curved focal plane, an image produced by a neat scope will be sharp in the centre of the chip but become increasingly out of focus further out from centre. A field flattener alters the light path coming from the scope so that it is in focus across the whole chip, effectively flattening the focal plane.

Eyepieces in turn are first designed with a particular focal plane shape in mind. As it is, it is easier and cheaper to design and make an eyepiece for a convex focal plane than a concave one. And so, an eyepiece will work better in one scope/focal plane than another. This is called "optical matching".

If you use an eyepiece in a scope that is not an optical match, you will see a number of aberrations such as astigmatism, pincushion, spherical aberration and also field curvature.

Not understanding optical matching and you will find many people dismiss a whole eyepiece line as "crap" when really they used that eyepiece in the wrong scope type. There are so many eyepieces lines and brands dismissed as rubbish because of ignorance. Use these same eyepieces in the correct scope design, and they perform brilliantly. Funny that, eh!

Now, with the complex nature of contemporary eyepiece designs, you will also find that within a specific eyepiece line thats firstly designed for a particular focal plane, you may find one or more individual focal length pieces that will actually do a very good job also in the very focal plane shape they are not designed for. They may not be "perfect", but the aberrations will still be very well controlled so to be quite acceptable (depending on what you are prepared to accept).

Take the Baader Hyperion line. These are specifically designed for a convex focal plane - refractors, SCTs and Maks. In these scopes these eyepieces are brilliant. However, in Newtonians all of these are poor to very poor performers, and the visible aberrations become more significant the faster the focal ratio gets. HOWEVER, the 5mm Hyperion actually is also brilliant in Newtonians, including very fast ones such as f/4.

Ok, this explanation is quite long, but it is enough to start seeing how there is more to eyepieces than "this is a scope, this is an eyepiece, so it will work..."

Nothing wrong with this at the start of your astro experience. I too thought exactly like this too. But as my experience grew, so did my awareness of performance differences and ultimately being made aware of optical matching and the oddity of contemporary eyepiece design that can give brilliant eyepieces for bugger all cost But it takes time to learn.

Alex.
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Old 07-07-2018, 08:48 PM
NTgazer (Chris)
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Hi Bob I have 10mm and 20mm plossl and a 2x Barlow. The plossls are unbranded and came with the scope when I purchased it second hand. On Jupiter I can see banding and am wondering what a better quality, higher mag eyepiece will show me


Quote:
Originally Posted by Tropo-Bob View Post
Hi Chris,

What EPs do U currently own and how are they performing at higher magnifications?
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Old 07-07-2018, 09:15 PM
NTgazer (Chris)
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Hi Alex that is an excellent explanation, thanks for taking the time I appreciate it. I will know what to look for in prospective new eyepieces and research accordingly
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Old 08-07-2018, 08:36 AM
Tropo-Bob (Bob)
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Quote:
Originally Posted by NTgazer View Post
Hi Bob I have 10mm and 20mm plossl and a 2x Barlow. The plossls are unbranded and came with the scope when I purchased it second hand. On Jupiter I can see banding and am wondering what a better quality, higher mag eyepiece will show me
Try looking for the Red Spot on Jupiter tonight. It should be easily visible from about 7.30 to 11.30 NT time. There are online sites that U can check for these times and events of Jupiter's moons. For example: https://www.shallowsky.com/jupiter/

Also try your 10mm with the barlow. This will give you an idea on how a 5mm EP may perform at 240x. That magnification would be too high for my local atmosphere, but U can only try it & see. When I owned a 8' Dob, I generally used a 14mm EP with a 2xbarlow for my highest magnification.

If possible, find a local Astronomy Group and U may obtain a much better insight to the equipment that U need (like looking though somebody else's EP). Kinda like trying before U buy.

Good luck and stay in touch.
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Old 08-07-2018, 09:50 AM
astro744
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Have a look at http://www.televue.com/engine/TV3b_page.asp?id=154

and specifically http://www.televue.com/engine/TV3b_p...n=Advice&id=80

Is your 8" f6 Newtonian on an EQ mount or is it a Dob; i.e. doies it have tracking? If you have tracking then you are likely to observe at the centre of any eyepiece so edge of field aberrations are not a concern. If you are observing without tracking then you may consider a wider field eyepiece with good correction and this is where you either have to try a lot before you get something you like or pay up front for a quality brand designed for any telescope, typically down to f4.

Your question though was specifically for planetary eyepieces and usually on axis sharpness is most important for these as typically planets are observed with tracking as far more detail can be noticed as you are not distracted by having to nudge the 'scope as the object moves to the edge of field.

You also have to ask yourself what eye relief do you prefer. If you require eyeglasses when observing then you need 20mm eye relief (and these are very comfortable too for non-eyeglass wearers). If you are fine with putting your eye close to the eyepiece then a Plossl or Ortho is fine and will perfom very well in a Newtonian with excellent on-axis sharpnes and reasonable off axis performance. Buy the best Plossl or Ortho you can afford and for your equipment a Tele Vue 8mm Plossl at $129 from Bintel would be ideal with change to spare.

Another option and perhaps preferable would be for you to buy a Tele Vue 15mm Plossl and then a Tele Vue 2x Barlow (unless your current Barlow is of good quality long design. (Shorty Plus is supposed to be good but a standard short Barlow has too many abberations and vignetting is introduced with most eyepiece combinations but as I have not used either type I'll recommend what I have and a 2x 1.25" Tele Vue is a very fine Barlow). The 15mm/2x combo will give you a 7.5mm eyepiece and 160x and 10mm eye relief which is comfortable for a 50 deg. eyepiece and non-eyeglass wearers. Later you could complement it with an 11mm PLossl for 109x (and 218x with Barlow) and then later an 8mm which will give 150x (and 300x with Barlow).

Note I mentioned exit pupil in my previous post and this refers to the diameter of the cone of light exiting the eyepiece and is directly related to the brightness of the image that you see. It can be calculated by diving f ratio of telescope into focal length of eyepiece, e.g. 8mm/ f6 = 1.3mm exit pupil. It can also be calculated by dividing the magnification into the telescope aperture, e.g. 200mm/150x = 1.3mm exit pupil.

Typically your magnification range will be from 0.5mm exit pupil to 7mm exit pupil (maximum eye iris opening, typically at 7mm). This equates to a 3mm eyepiece to 42mm eyepiece at f6 which further equates to 400x to 28x with your 1200mm focal length telescope. At the smaller exit pupils eye floaters become visible and annoying but it is observer specific as to how much of a problem it is. I can observe down to 0.4mm exit pupil without too much problem below that the image simply is too dim for my liking especially for low contrast objects like planets although I can go lower for high contrast objects such as double stars (i.e. more power).

I mentioned the 5,6,7mm in my previous post and was indirectly referring to the TMB currently for sale as indicated by another member. However as I have not used these eyepieces I cannot comment on their quality and suitability for your telescope but they do represent excellent value and would more than likely be fine.

I don't think you should worry too much about what eyepieces suit what telescope simply because most manufacturers wont tell you or they only tell you half the facts and in most cases it really only matters the most for low power work. Just buy the best that you can afford even if you only buy one at a time. If you buy a reputable brand you wont be disappointed and its a keeper for life and you never know when you may end up with another telescope you can also use them on.

See also http://www.televue.com/engine/TV3b_page.asp?id=214 for some specifications I mentioned.

Whatever you choose, enjoy!
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Old 08-07-2018, 01:04 PM
NTgazer (Chris)
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Thanks astro744, my scope is on an eq mount (g11) and don't wear glasses. I think I am leaning towards the 8mm televue plossl, I can try it with my existing Barlow and see how it goes at 4mm and if it turns out my Barlow isn't so flash I can add a televue Barlow to the shopping list. As you say it will probably be a long term keeper anyway.
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Old 09-07-2018, 11:16 AM
Wavytone
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Well the TMB set are spoken for already. Missed a good deal IMHO.
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Old 09-07-2018, 07:32 PM
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mental4astro (Alexander)
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Chris,

Have a look at these TMB eyepieces. I've bought and tested the entire line on refractors and fast Newts. In refractors they are all really good, except for the 6mm which has always been the weakest of the line.

For Newts, the pick of the crop are the 2.5mm, 4mm, 5mm, 7mm and the 8mm. The other focal lengths still do very well, though they show more field curvature than the ones listed above, and this is just splitting hairs - I tested these in an f/4 Newt which is really punishing on eyepieces. However, in an f/6 Newt there will be even less noticeable field curvature, if any.

I also compared these eyepieces with some high end eyepieces in order to get a more considered determination on their performance. The ones noted for use in a Newt are over all the best of the lot. These threw up an image every bit as good and sharp as the Pentax eyepieces I compared them to. These modest TMB's also had superior transmission to some other high end eyepieces.

And yes, Ebay is the best place to get them from:

https://www.ebay.com.au/sch/i.html?_...fLoc=2&_sop=15

They also have a wider AFOV than plossl eyepieces.

Those TV plossls are certainly good, but not your only option. The biggest problem that plossls eyepieces have is as the focal length gets smaller, the eye relief, or the distance you need to hold your eye from the eye-lens to look into it, becomes shorter and shorter, plus the eye-lens becomes smaller and smaller. You practically need to park your cornea onto the eye lens to look into a 4mm plossl. These TMB's all have the same generous eye relief across the entire line, and the same nice big eye lens. Below is a comparison picture of a 9mm plossl and the 9mm TMB.

Click image for larger version

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A 9mm plossl is not an easy eyepiece to use, and it gets harder as the focal length gets shorter.

Alex.
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Old 11-07-2018, 08:16 AM
NTgazer (Chris)
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Thanks Alex, I think they fit the bill nicely actually. Hopefully I can get one landed in time to see Mars so close! It seems like some Honk Kong/China based e-bay sellers basically point a canon roughly in the direction of Darwin, fire the product out and hope for the best that it arrives. Thanks again for your help and indeed everyone else's too
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Old 11-07-2018, 09:22 AM
brent130
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Hi Chris,

I have recently bought some items from this seller which have arrived to Melbourne in 8 days from China.

http://www.ebaystores.com.au/Eshow-A..._sid=871048572

They don't use China Post which takes more than a month to arrive.

You can make a lower offer on most items to make them more competetive

Also use PENNY5 for 5% of items over $30
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