Eyepiece selections?

[Nab (Darren)]

Is there a general rule of thumb when considering eyepiece size selection? Going beyond low-medium-high.. It is wise (for example ) to not quite double your magnification between each eyepiece? 20mm/10mm being the Skywatcher supplied kit seeming to suggest this - although it does seem (observationally) a large jump from 60x to 120x..

Using my 8" dob scope, and playing around in Stellarium with top shelf eyepieces…
Would (again for example) a 24 (50x), 13 (92x), 7 (171x), 5 (240x) seem like a good combination? - which the Stellarium simulation seems to suggest it is.

I'm also assuming that practically 240x is going to be about the maximum usable accounting for tracking if nothing else.

Looking forward to your views..

Thanks.
Darren.

[malclocke (Malc)]

I'm no expert in these things, but one thing I've read before which does make sense and I've tried to follow is to *not* buy in doubles, as if you acquire a 2x barlow that will give you an equivalent of double magnification for each of your eyepieces.

My scope came with a 30mm and a 9mm, I bought a 2x barlow and subsequently a 26mm, so I now have 30mm, 26mm, 15mm, 13mm, 9mm and 4.5mm.

Malc

[barx1963 (Malcolm)]

Not all EPs are created equal. The cheap plossls supplied with scopes are acceptable at low power and usually rubbish at higher powers.

The factors that effect eyepiece decisions are:

- The size of the scope, both aperture and focal length
- Your budget
- Targets you are interested in
- Seeing conditions where you are
- Do you wear glasses?

The way these affect you are

1) Size of your scope. There is an oft quoted rule of 50x per inch of aperture as a "theoretical" maximum power. In reality it is more like 20x. So for an 8" scope 160x is about it except on those really exceptional nights.
2) Your budget. Spending more money on EPs gives you more scope. In my scope the supplied plossls were completely useless at focal lengths below 15mm, which only gave me 100x. The 12mm, 9mm and 6mm plossls were never used. I now have a 10mm Ethos and a 6mm Delos, both of which are used regularly. So if you can afford good wide field EPs, you will find more flexibility with choices. That said, a 6mm Delos still requires pretty good seeing to be any real use, just that a plossl is almost never usable.
3) If you are primarily after DSOs, most of the targets that are reachable with an 8" are not ones that need much power. The exception is the smaller planetaries. On the other hand if you are a solar system observer, power is your friend, but again you must be very patient for good conditions.
4) Following on from points above, if good seeing is rare where you are, you are better off going for DSOs and concentrating on low power.
5) If wearing glasses when observing, cheap, high power EPs are a waste as the eyerelief is usually poor.

My recommendation would be get a couple of high quality EPS, one low power and one moderate power. I am a Televue user can only give opinions on them. The 24mm Pan is a great economical EP with gives 50x in your scope and will give years of pleasure. For moderate power a 13mm Nagler(92x) or 14mm Delos(86x) are also good choices.

Others have opinions on EPs and their favoured brands, but only knowing
much about TV, I cannot comment on other brands. Good EPs are a lifetime investment.

Malcolm

Originally Posted by Nab

Is there a general rule of thumb when considering eyepiece size selection? Going beyond low-medium-high.. It is wise (for example ) to not quite double your magnification between each eyepiece? 20mm/10mm being the Skywatcher supplied kit seeming to suggest this - although it does seem (observationally) a large jump from 60x to 120x..

Using my 8" dob scope, and playing around in Stellarium with top shelf eyepieces…
Would (again for example) a 24 (50x), 13 (92x), 7 (171x), 5 (240x) seem like a good combination? - which the Stellarium simulation seems to suggest it is.

I'm also assuming that practically 240x is going to be about the maximum usable accounting for tracking if nothing else.

Looking forward to your views..

Thanks.
Darren.

[Rodstar (Rod)]

Hi Darren, you are well on the way with your original post.

Many experienced observers take the view that increments of about 1.6x is ideal.

I am assuming your telescope's focal length is 1200mm. Your sweet spot is likely to be around 100x, which would make a 12mm or 13mm EP about the right mid-point. Assuming you go with the 13mm, then 1.6 longer will give you a 20mm for wider field views, and 1.6 shorter gives you an 8mm EP for closer work.

In my scope, my sweet spot is around the 20mm (125x), and so my war horse is my 20mm Nagler. My wider field EP is the 31mm Nagler, which is 1.55x the focal length. Below the 20mm I use a 13mm Ethos (1.53x shorter), and below that a 9mm Nagler (1.44x shorter again).

One reason that people will often choose not to get EPs in steps of 2x is that you can achieve the same effect with a 2x barlow. If you get EPs in 1.6x steps, then that, in combination with a decent 2x barlow will give you a lot of options.

In terms of what is your upper useful limit, in an 8 inch scope more often that not you will not get to 240x unless the conditions are particularly still. Probably 100-150x is where you will spend most of your time.

Good luck!

[MattT]

Hi Darren,
You are on the right track. My set goes 5,7,9,11 16, 20, 28. Add a 2X Barlow and there is not a lot left to fill in the magnification stakes.
For more education on eyepieces I suggest you join Cloudy Nights in the US and cruise the forums there. No offence here to any one, as someone once said .."you just can't shut them up on Cloudy Nights" I know a lot of IISers are on CN. Have a look at this thread. http://www.cloudynights.com/ubbthrea.../o/all/fpart/1
Before you spend on eyepieces you need to know what is out there IMO.
It's a learning curve for sure.
As the thread says all sorts of things come into play...not just the eyepiece.... For what it's worth I eventually settled on the Explore Scientific eyepieces along with some GSO plossls.... after owning some very expensive ones too. I know this is a lot to digest for a beginner, assuming you are?
Hope this is of some help and not confusing.
Matt

[cmknight (Chris)]

I've got a 6" Mak with a 1900mm fl. I like low power work, so my eyepiece choices are the Meade Series 5000 SWA's. I've got the:

40mm (47.5x)
34mm (56x)
28mm (68x)
24mm (79x)
20mm (95x)
16mm (119x)

I do have shorter fl's than the 16mm, but I hardly ever use them any more.

[Wavytone]

Darren,

One way is to establish the highest and lowest useful magnifications of your scope, and choose eyepieces that span this range. Some can get away with just 3 eyepieces (low, medium, high) but I'll guess that for these people the range between highest/lowest is no more than 5:1.

For more than 3 eyepieces use steps of 1.5X - 2X between focal lengths.

The highest useful power for most reflectors is between 1.2-1.5X per mm of aperture, 240X to 300X for your scope, and at a guess a 4-5mm eyepiece will achieve that.

The lowest useful power is that which gives an exit pupil around 6mm, the eyepiece focal length corresponding to this = 6mm x the focal ratio of the scope. For an f/6 scope, this means a 36mm eyepiece, and your lowest power is 33X.

So... to span this range using 5 eyepieces I'd suggest: 5, 8-9, 12-15, 18-21 and 30-35mm. You could omit the 5mm, the number of times you'll use it in a year will be countable on 1 hand.

For a set of 3, I'd choose 7, 15 and 30mm. Adding a good-quality 1.4X barlow gives you effectively 5, 10 and 21mm which makes a fair bit of sense.

The last problem is how much you're willing to spend vs what sort of eyepieces that will buy, and new vs secondhand. There are some bargains to be had too if you are patient and shop around, and there's nothing wrong with good secondhand ones either.

[Nab (Darren)]

Thanks everyone there is some good advice here.
I see one of the big challenges in acquiring new equipment is to weigh up all the variables: technical, personal traits, price/performance, preferred targets etc.. with some food for thought it helps wading through all the other information that is out there..


So.. my current thinking would be:
scope: 8” 1200mm f6 (price/transportability/storeabilty balance)
me: glasses, prefer deep sky, moon & planets more peripheral, visual only.. not interested in photography although I will be dabbling (reviving a past skill) in sketching.


so a selection might be (from the TeleVue range – why not
24 Pan 50x
13 Nagler 93x
8 Delos 150x
with the supplied plossls at 20mm (60x) and 10mm )120x) as in-fill – actually I have been surprised at how good (relative?) the 20mm plossl is. I didn't expect to be able to resolve so much detail in the LMC with it. I suspect a UHC filter is going to enhance this even more..


and possibly (Depending on how things look at 150x) a 6 Delos to go to 200x and/or an Orion edge on planetary 5 for 240x – although the small fov looks like it might be frustrating in simulation.. and how often would I really use it..


I have picked up how much the seeing varies from night to night and thus how important it is to not rush into making assumptions about what will work and what will not..


looks like a fully clouded out weekend out this way


Cheers..

Darren

[wavelandscott (Scott)]

Do you wear your glasses to observe?

If so, make sure you look for eyepieces that are compatible with glasses and have sufficient eye relief.

[Rodstar (Rod)]

Sounds good Darren.

You won't be unhappy with any of those eyepieces, and the spacing seems fine. And you'll get the benefit of the Televue 10% off sale on all of them!

Let us all know how you get on with them.

[Wavytone]

Darren,

The high-power eyieces (basically anything under 9mm) are mainly used for observing planets, double stars and occasionally the moon. In these cases A wide field isn't important - what matters more is sharpness, extremely good contrast free of scattered light and internal reflections, and these are achieved by having as few air-glass surfaces as possible. This is why older designs like plossl or orthos (and modern variants such as the TMB planetaries) and even the humble Edmund RKE are still popular in short focal lengths.

[Nab (Darren)]

I was thinking that a relatively small fov at high magnification on a non-driven scope would lead to a lot frustration as the target would seem to be zipping out of the eyepiece all the time.
At the 200x + end of the scale the matrix seems to be price/optical perfromance/fov.

Cheers.
d.

[Rodstar (Rod)]

Darren - I agree with your comment re FOV. My 7mm Ortho, which does not get a lot of use, is very limited by its narrow FOV. Objects, as you say, do "zip" through the FOV, unless of course you have tracking (which I don't).

From reports I have read about the Delos, it is pretty sharp across the entire FOV, and at 72 degrees, should be more satisfying to observe with in a practical sense than a narrower FOV EP.