Which high power eyepiece

[starman345]

I'm getting bogged down in all the lingo here--- I"m a newbie, only had my 10" dob for a few months and have done very limited observing.

I have a 18mm meade eyepiece with a 3x barlow that came in the deal. Since then I've bought a 13mm Ethos and now I'm looking for something with some magnification so I can get a little closer to the planets, galaxies, etc.

I've read and re-read Warthogs "eyepieces-- the least you need" and its been a great help but I can't get my head around exit pupil and eye relief, and I"m not sure how important that really is when choosing an eyepiece.

The telescope is a 10" skywatcher dob, I"m 58 yrs old- I think that has an impact on eye relief as you get older? Cost of eyepieces is not really important, I"m not made of money but I like to buy good quality.

According to the Warthog post, if I take my f ratio and multiply by 3/4 that should give my high power needs, which translates to 3.5mm, the online eye relief calculator from one site tells me the eye releif for that eyepiece in my dob is 1.1.....isn't that too low?

Any recommendations for a good high power eyepiece that will be comfortable to use in my dob?

Can anyone deliver me from this black abyss of ignorance?

Thanks

[Moonshane]

hi there

I am also pretty new to all this and others might be able to offer different advice but here's mine. This is based on having a similar scope to yours and also a 13mm Ethos (see signature).

Eye relief is easy - this is the distance your eye has to be from the eyepiece eye lens to gain focus and see the whole field. Therefore a 20mm eye relief, most would find comfy even with glasses on and the shorter it gets, the closer in you are to see the whole field. The Ethos has 15mm which I consider perfect without specs. An ER longer than about 25mm makes it tricky to keep your head in the right position and using a barlow can affect ER as it doubles (and maybe even triples?) when you barlow with a 2x (3x). If you have ER of 20mm already then this can make positioning difficult.

Exit pupil is the size of the light cone which gets through the eyepiece and into your eye. It's a relationship between focal ratio (which I think is f5 in your case) and the focal length of the eyepiece. So, your 18mm meade has an exit pupil of 18 / 5 = 3.6mm. Most people are OK to about 6mm, some say 7mm but after that you can possibly see the secondary mirror as a dark patch in the view and or you are losing light and 'wasting aperture'. Some say don't worry about this though if it works OK for you. The smaller the exit pupil the dimmer the view and hence that with higher magnification you get a dimmer image.your 3.25mm eyepiece would have an exit pupil of 0.65mm which is quite small of course but still OK in some cases (eg the moon).

for my the main issue is the seeing in the UK. I can usually get to about 200x at a push and occasionally 250x but not often with my dobs. on the moon it's different so can get to 500x occasionally and still see good detail and sharpness.

with your scope at a focal length of 1250mm approx, you have the following (ignoring the barlow):

18mm = 69x

13mm = 96x

If you can afford another Ethos (or even two) then I would suggest :

6mm = 208x

8mm = 156x

plus

35mm Panoptic = 35x

This would give you a nice higher power set up and also an excellent finder for galaxies etc. and a nice wide field for open clusters. You can often get Panoptics for £200 and Ethos used for maybe £300-350.

I recently got my best ever views of Jupiter through my 6" f11 dob with my 13mm Ethos which gave 125x approx. so you don't always need masses of magnification. I wish I had a 10mm to give about 160x and this is my next purchase. My method will be buy a 10mm Radian to accompany my 8mm Radian (both about £100 each) and then decide based upon the most used one of the two, which will be replaced with an Ethos of the same focal length.

Although not technically a planetary eyepiece, I feel the contrast, sharpness and widefield are all suited to both manual dobsonain and also planetary observation. I should point out that I have never tried a good quality Orthoscopic or Plossl in my scopes so others with views on these may change your mind.

Hope this helps a bit.

[Moonshane]

in fact there's a 35mm Panoptic on AB&S now U.K. Astronomy Buy & Sell

sorry, just noticed you are in Canada.

[wulfrunian]

If you're after a good high power one that's comfortable to use in the dob, I'd stick to wide fields if you can afford them. Another vote here for the 6mm Ethos if you can run to it. If not, how about the 4mm or 7mm Nirvana, they're very nice eyepieces with an 82 degree field of view.

[starman345]

Moonshane,

Thanks for taking the time to answer all my questions, it is very helpful. I better understand exit pupil and eye relief now, not nearly as complicated as I was making it to be in my mind.

I had in mind the Ethos 6mm, but didn't know if I could go even shorter, a 5 or 4mm or maybe the Ethos 3.7mm, but I don't think the seeing here is all that good and I may not be able to use it that much. I live in a maritime environment and also in a town with some light pollution.

How would things change if I bought a 2x powermate for my 13mm ethos? Would that effectively give me a 6.5mm magnification? I'm curious if it would lessen the fov since the lens would be longer?

I see you are using the Nagler 3-6 zoom, how do you like that in your dob?

Thanks again for taking the time

[starman345]

If you're after a good high power one that's comfortable to use in the dob, I'd stick to wide fields if you can afford them. Another vote here for the 6mm Ethos if you can run to it. If not, how about the 4mm or 7mm Nirvana, they're very nice eyepieces with an 82 degree field of view.

Hi Michael, thanks for the input. I am leaning toward the Ethos 6mm. I see you have a 10" dob and the Ethos 6mm. I agree about the wide field views, I really like my Ethos 13mm with its wide field, it sort of puts you right out there among the stars.

[great_bear]

Although on a very good night, under ideal conditions, one can push magnification to over 300x, this is a very fairly rare treat. Most of the time in the UK, the limit is 200-240x and that's with a big scope like yours - with smaller scopes the maximum magnification is less.

The upshot of this is that a 5.5mm should be the shortest eyepiece to consider for "everyday" use with a 1250mm focal-length telescope (which I assume yours is?).

Although the 6mm would be an excellent choice, many people (and importantly, I'm including those with money to spare) actually wouldn't think twice about using one of the 2x TeleVue Barlows with the 13mm you've already got, rather than buying the 6mm as well.

As to which Barlow, you'd have to ask someone who does that on a regular basis.

Note that you'd improve your Ethos views with a Coma Corrector if you don't already have one.

[Later:] oh - in response to your other enquiry, the apparent field-of-view remains unchanged when you use a Barlow. e.g. a Barlowed 13mm eyepiece has the same view as an unbarlowed 6.5mm eyepiece, assuming eyepiece AFOV specs are the same.

[Moonshane]

No problem at all!

Geat Bear makes a good point about Barlows but the Powermate is not a Barlow, albeit the effect is similar. That said, the FOV would not change with a Powermate either. Importantly, the eye relief does not change with a Powermate unlike a Barlow; in principle you do indeed end up with a 6.5mm Ethos and in theory this is excellent news as the Powermate/Barlow (based on my own use previously of a 2.5x Powermate I'd recommend a Powermate over a Barlow every time) and no other observing characteristics change with one important exception. This one exception applies particularly to dob owners and it's weight /balance. The 2" Powermate is quite heavy and when added to an Ethos it's going to be even heavier and balancing will be a pain in the rear in my view. Additionally, the length of the Ethos is already long and add a Powermate to that and you have virtually a foot long eyepiece sticking out on top of your focuser length - again something that would annoy me personally.

Optically though there's no reason not to get a Powermate.

On the Nagler zoom see my review - I think if you read carefully, you might notice that I like it.

http://stargazerslounge.com/equipment-reviews/109089-review-televue-nagler-3-6mm-zoom.html the main 'review' is post #9

in your scope you would get even more use as the magnification range would be 208x - 416x. I see this as my lunar eyepiece really as often the magnification is a bit high for anything else unless it's an exceptional night. You would use it a bit more widely although it's no Ethos, it's a superb addition to anyone's case.

[Chris H]

Something to remember here is that higher magnification is not always going to give you better views, this is particularly relevant for DSO's. The higher magnification you use the dimmer the image, so for faint objects you want a lower power ep. You already have the 13mm Ethos probably the perfect ep for DSO work. For a higher mag ep I would recommend you go for an 8mm Ethos I own one and find this to often be the limit before the atmosphere starts to cause the image to really look like 'soup' you could go for a 6mm but it may not get too much use. I also have a Pentax 5mm ep and it doesn't often get used can't imagine I would ever use the 3.7mm Ethos in my 10" dob.

A collection of the 21, 13 and 8mm Ethos' ep's combined with a 2x powermate would be excellent.

[starman345]

Although on a very good night, under ideal conditions, one can push magnification to over 300x, this is a very fairly rare treat. Most of the time in the UK, the limit is 200-240x and that's with a big scope like yours - with smaller scopes the maximum magnification is less.

The upshot of this is that a 5.5mm should be the shortest eyepiece to consider for "everyday" use with a 1250mm focal-length telescope (which I assume yours is?).

Although the 6mm would be an excellent choice, many people (and importantly, I'm including those with money to spare) actually wouldn't think twice about using one of the 2x TeleVue Barlows with the 13mm you've already got, rather than buying the 6mm as well.

As to which Barlow, you'd have to ask someone who does that on a regular basis.

Note that you'd improve your Ethos views with a Coma Corrector if you don't already have one.

[Later:] oh - in response to your other enquiry, the apparent field-of-view remains unchanged when you use a Barlow. e.g. a Barlowed 13mm eyepiece has the same view as an unbarlowed 6.5mm eyepiece, assuming eyepiece AFOV specs are the same.

Good point about the magnification, that is exactly why I posted this thread, trying to get a feel for what my limits for magnification might be in the real world.

1200mm is the focal length stamped on the side of the Skywather so a 6mm would give me 200x which is probably close to my limit, except for the moon of course.

Now I've got to search out "coma corrector" to see what that is about:D

Thanks for all your help

[starman345]

Moonshane,

I hadn't thought about the size of the powermate with an eyepiece, that would annoy me as well. I think I would rather get another eyepiece.

The Nagler 3-6mm zoom sounds interesting, I read your review and I may consider that later for moon gazing and exceptional seeing nights as you say. There are so many options, I can see why people buy and sell eyepieces often.

[starman345]

Something to remember here is that higher magnification is not always going to give you better views, this is particularly relevant for DSO's. The higher magnification you use the dimmer the image, so for faint objects you want a lower power ep. You already have the 13mm Ethos probably the perfect ep for DSO work. For a higher mag ep I would recommend you go for an 8mm Ethos I own one and find this to often be the limit before the atmosphere starts to cause the image to really look like 'soup' you could go for a 6mm but it may not get too much use. I also have a Pentax 5mm ep and it doesn't often get used can't imagine I would ever use the 3.7mm Ethos in my 10" dob.

A collection of the 21, 13 and 8mm Ethos' ep's combined with a 2x powermate would be excellent.

Thanks Chris, I"ve been looking at the 8mm Ethos as well, and your point about higher magnifications and dimmer images is well taken. The 6mm gives me 200x, the 8mm, 150x. I'm beginning to see why so many people here have a large collection of eyepieces.

[great_bear]

Now I've got to search out "coma corrector" to see what that is about:D

TeleVue's one is called the "ParaCorr", and Baader's one is called the "MPCC" (Multi Purpose Coma Corrector).

The ParaCorr - in true TeleVue style - has a somewhat heart-stopping price-tag

[Ben Ritchie]

My understanding is that the Paracorr is somewhat different to the MPCC as it 'lives' in the focuser and has a normal 2" barrel for eyepieces, whereas the MPCC is a screw-in design and therefore swapping between eyepieces is a fiddle in the dark (especially as the MPCC needs correct spacers too, which vary from eyepiece to eyepiece). If you need a coma corrector for several different eyepieces then the Paracorr is probably worth the money for general convenience.

[Chris H]

Thanks Chris, I"ve been looking at the 8mm Ethos as well, and your point about higher magnifications and dimmer images is well taken. The 6mm gives me 200x, the 8mm, 150x. I'm beginning to see why so many people here have a large collection of eyepieces.

My scope is the same size as yours 10" diameter and 1200mm focul length. I live in North Wales, I like to look at planets when possible but DSO's are really my thing. I use a 17mm Ethos, a Nagler 12T4, an 8mm Ethos and as mentioned above a Pentax 5mm plus I have a 2x barlow. For an average nights observing DSO's I use the 17 Ethos (x70) the most followed by the 12T6 (x100) for a closer look. When looking at compact star clusters and globs the Ethos 8 (x150) goes in the focuser. I can see coma around the edge of my field of view but I can live with it rather than spend £££'s on a Paracorr. I have considered the 6mm Ethos but don't think it would get enough use to justify the cost. If I get anything it will be a new low power ep such as the 31T5.

For my conditions and the targets I seek these magnifications are more than enough too keep me happy.