Eyepieces for 8" dob

[Tigaroo]

Just treated myself to a set of 5 1.25 Plossl's , so will finally have a full range of ep's after managing for 12 years with the supplied 25mm and 10mm and 2x Barlow that came with both my scopes! Looking forward to clear skies to start trying them out. I have a new 4" cassegrain on eq mount and an 8" Dobsonian , I've been wondering what advantage there would be in getting any 2" EP's for the dob and if so what powers ? Or save up for a Hyperion zoom? 

[estwing]

if you've put up with 2 ep's for 12yrs that set should do you for life!

[Ricochet]

The only reason to get any 2" eyepieces is to get a wider true field of view than a 1.25" barrel allows. Until you've got some experience using your new eyepieces you won't know whether you need a wider field for the targets you enjoy looking at. A 32mm plossl has the widest true field of view for a 1.25" eyepiece if one of those is in your set. 

[Tigaroo]

thanks ! Yes , I have the 32mm and a set of coloured filters to try too....so rain rain go away!  

[Alan64]

Hello Tigaroo,

The 2" 38mm 70° within the following listing would provide you with the lowest power(32x) and the widest field-of-view; the largest area of the sky that can be viewed with an 8" Newtonian in other words...

https://www.firstlightoptics.com/skywatcher-eyepieces/skywatcher-panaview-2-eyepieces.html

Such would aid your finder in finding objects to observe.  It would also allow you to observe the galaxy in Andromeda and the Pleiades in the fall and winter; and scan the congested star-fields of the Milky Way in summer in the hunting for objects of interest, then to pop in a shorter focal-length eyepiece for a closer look once one is spotted.

An 8-24mm zoom ocular would render your new Plossls rather redundant with the 8" telescope.  Zoom oculars are chosen, however, for their convenience, and in not having to lug out a set of individual eyepieces at every outing.  They also serve as a teaching tool, and in helping to select individual eyepieces, according to the powers used most often with the zoom.

For your 4" Maksutov, the 40mm or 30mm within this listing might be of interest, if those focal-lengths are not already among your new Plossls ...

https://www.firstlightoptics.com/vixen-eyepieces/vixen-npl-eyepieces.html

I have the 30mm and 6mm from that line, and I'm rather fond of both.  The 30mm has an extendable/retractable eye-cup, for an improved viewing position, and an enormous field-lens through which to observe...

Clear skies to you, and enjoy!

[Tigaroo]

Thanks Alan, found that info really helpful. Will consider a 2" 30mm at some point , or even a zoom ...would still use the Plossl's in the other scopes ! Looking forward to some dso hunting ! 

[John]

Apart from the instant ability to change the focal length, you might not find that the zoom delivers anything, performance-wise, if your plossls are decent ones. You would get a slightly larger field of view at the short focal length end with the zoom (most offer around 60 degrees over the plossls 50) but you will loose field width at the longer end (24mm) where zooms tend to have a more resticted field of view than the equivalent plossl. Slightly frustrating because the longer FL is just where you want a nice wide field of view !

Have you considered getting a UHC filter ?. With the spring and summer months coming, the views of the Veil Nebula in Cygnus alone with your 32mm plossl and a UHC filter would be worth the filter cost I reckon

Many other nebulae would benefit from the UHC filter too.

[Alan64]

9 minutes ago, Tigaroo said:

Thanks Alan, found that info really helpful. Will consider a 2" 30mm at some point , or even a zoom ...would still use the Plossl's in the other scopes ! Looking forward to some dso hunting ! 

With many DSOs being rather dim, you want the brightest view, and the 2" 38mm will serve for that. 

Cheers.

[John]

4 minutes ago, Alan64 said:

With many DSOs being rather dim, you want the brightest view, and the 2" 38mm will serve for that. 

Cheers.

Surely a 30mm will show a darker background sky though and help the faint DSO stand out a little more ?

Personally I find a 5mm exit pupil more effective for DSO spotting from my back yard than a 6mm plus one.

 

[Tigaroo]

Think I'll have to save up for the uhc a bit longer John, thanks for the advice . My new new lenses are accompanied by a selection of Kodak Wratten coloured filters  , I guess some of them help with dso's , or are they best utilised for lunar and planetary viewing? 

I struggle to understand the whole ' exit pupil'  , ' eye relief ' thing! I just juggle the ep's until I find the best view for me! 

[John]

7 minutes ago, Tigaroo said:

Think I'll have to save up for the uhc a bit longer John, thanks for the advice . My new new lenses are accompanied by a selection of Kodak Wratten coloured filters  , I guess some of them help with dso's , or are they best utilised for lunar and planetary viewing? 

The coloured filters are used by some to help tease out planetary detail. I don't find them that useful to be honest but see what you think.

They won't help on deep sky objects though. No filter really improves the views of galaxies or clusters and it's nebulae that are the main targets when using UHC or O-III filters because the filters only let through the bandwidths of light that the nebulae principally emit. The effect is not to make the nebula brighter but to remove the other light bandwidths which helps the nebulosity to stand out more, in some cases quite dramatically so. Good tools to have, when you feel inclined

 

[Tigaroo]

Sounds like the next bit of kit to save up for then ??

[Charic]

Welcome to the SGL.

I'll second the 32mm 2" fitting, Sky-watcher Panaview. Great for wide field viewing on the Dob.
If you take the Dobsonians focal ratio (I'm assuming f/6) and multiply by your dilated pupil, you will find the longest focal length eyepiece that is suitable for your eye.
I'm about 29mm but the 32mm is just fine. Bought it mainly to see Andromeda M31, as my other eyepieces at that time were not wide enough?

[Louis D]

36 minutes ago, Charic said:

Welcome to the SGL.

I'll second the 32mm 2" fitting, Sky-watcher Panaview. Great for wide field viewing on the Dob.
If you take the Dobsonians focal ratio (I'm assuming f/6) and multiply by your dilated pupil, you will find the longest focal length eyepiece that is suitable for your eye.
I'm about 29mm but the 32mm is just fine. Bought it mainly to see Andromeda M31, as my other eyepieces at that time were not wide enough?

I really don't know everyone is so uptight about making sure you don't use an eyepiece that produces an exit pupil larger than your entrance pupil.  Sure, there will be some "wasted" aperture, but you'll be able to fit the entire Pleiades or Collinder 70 into one field of view with a widest FOV eyepiece like a 40mm 70 degree eyepiece.  Sure, you could go with a 3-inch 30mm ES-100 to keep the exit pupil down, but that's not really doable in a Newt.  A 31mm Nagler or 21mm Ethos will both produce a smaller true FOV, clipping some of the view.  I regularly use eyepieces with "too large" of an exit pupil for my entrance pupil because I haven't fully dark adapted yet.  The world doesn't end and neither does my eye nor my telescope explode because I "wasted" some aperture.  I always recommend a 2" eyepiece with a field stop that is as close to 46mm as possible to max out what can be seen with a 2" focuser.

[Charic]

My comment was based on theory and suitability, not sure why my answer has rattled the cage. PM if there's an issue.
 

[John]

1 hour ago, Louis D said:

I really don't know everyone is so uptight about making sure you don't use an eyepiece that produces an exit pupil larger than your entrance pupil.  Sure, there will be some "wasted" aperture, but you'll be able to fit the entire Pleiades or Collinder 70 into one field of view with a widest FOV eyepiece like a 40mm 70 degree eyepiece.  Sure, you could go with a 3-inch 30mm ES-100 to keep the exit pupil down, but that's not really doable in a Newt.  A 31mm Nagler or 21mm Ethos will both produce a smaller true FOV, clipping some of the view.  I regularly use eyepieces with "too large" of an exit pupil for my entrance pupil because I haven't fully dark adapted yet.  The world doesn't end and neither does my eye nor my telescope explode because I "wasted" some aperture.  I always recommend a 2" eyepiece with a field stop that is as close to 46mm as possible to max out what can be seen with a 2" focuser.

No problem about using very larger exit pupils. There are no rules about it but it's also OK to say when you have tried something and it's not worked so well. All we can do is share our experience on here. You share yours, I share mine. They probably won't all match up but thats how it is. I guess people reading the posts have to work out which situation their own circumstances and preferences are likely to match the best, go with it and see how they get on.

My back yard has some moderate light pollution issues like many suburban homes will have in the UK. I find the exit pupil that the Ethos 21 creates with my F/5.3 dob (3.96mm) is more effective most of the time for finding and viewing the fainter DSO's than my 31mm Nagler's 5.85mm. So thats what I can report on the forum.

"Your mileage may vary" as the saying goes

 

[iPeace]

3 hours ago, Tigaroo said:

I struggle to understand the whole ' exit pupil'  , ' eye relief ' thing! I just juggle the ep's until I find the best view for me! 

Works for me....

[Charic]

.....the joy of owning several eyepieces, one of them will/should give the best view on the night, given suitable conditions for observing.

[Tigaroo]

You've all been really helpful, thank you for your input ? and I've just had the message to say all my new kit arrives Tuesday ! Whoop! Very excited to get playing and viewing . As I've had great pleasure for many years with just two basic ep's and a 4" scope I reckon I'm going to be more than happy playing about with all the new options and getting even better views from my Dob which I've only had two years but absolutely love using ... Such an absorbing hobby and inexhaustive in where you can go with it,  but even just looking up at a starlit sky with my own two eyes never ceases to blow me away , everything else is just a massive icing and cherry on the top! ??

[Charic]

46 minutes ago, Tigaroo said:

looking up at a starlit sky with my own two eyes never ceases to blow me away

What about some low powered binoculars, you'll be blown even further. Stargazing this way is mind blowing, especially at a darker site.  At my darker site, its difficult to see the main constellations on a Moonless night, due to the fact I can see thousands (give or take one or two) more stars! My first to grab are the 8x40s 

[Alan64]

9 hours ago, Tigaroo said:

Think I'll have to save up for the uhc a bit longer John, thanks for the advice . My new new lenses are accompanied by a selection of Kodak Wratten coloured filters  , I guess some of them help with dso's , or are they best utilised for lunar and planetary viewing? 

I struggle to understand the whole ' exit pupil'  , ' eye relief ' thing! I just juggle the ep's until I find the best view for me! 

Eye-relief is simply the distance that you would need to hold your eye away from the field-lens of an eyepiece, and in order to see the full field-of-view...

For instance, with my 6mm Plossl, I have to hold my eye to where my eye almost touches the lens of the eyepiece.  Once I get close enough, I may then see the full field...

But if I pull my eye back away from the eyepiece, the view becomes narrower...

Incidentally, that feature of the Moon is known as "The Moon Maiden Looking Out To Sea"...

Eye-relief is really only a concern for those who must wear prescription-eyeglasses whilst observing.  The thickness of the lens and frame of the eyeglasses necessarily pushes the eye outward and away from the eyepiece.  Therefore, eyepieces with 15mm to 20mm of eye-relief are preferred in that instance.

Plossls shorter than 10mm -- 8mm, 6mm, 4mm -- have very short eye-relief, like the 5mm-distance shown in the first image, whilst other types among those shorter focal-lengths have longer eye-relief.  For example, this 4mm has a most generous 16mm of eye-relief...

http://agenaastro.com/bst-1-25-uwa-planetary-eyepiece-4mm.html

Such eyepieces contain built-in barlows.  Plossls, Kellners and orthoscopics do not, and therefore have tight eye-relief.  With a 2x barlow, one may combine it with a 10mm or 12mm Plossl , and for a simulated 5mm or 6mm, respectively, but whilst retaining the more generous eye-relief of the 10mm or 12mm.  In some cases, the barlow may even increase the eye-relief of an eyepiece further when used.

To wit, the importance of choosing eyepieces can be very much akin to that of prescription-eyeglasses.  I must wear eyeglasses when driving, but I don't need to whilst observing with a telescope, therefore I may make use of practically every eyepiece on the market.

 

Exit-pupil...

The diameter of the pupil of the human eye, which receives light, varies.  During the day, it can be quite small, especially when the sun is shining brightly...

...but it then opens up considerably at night, and up to 7mm or more.  When the eye is fully dark-adapted, and after being out in the darkness for thirty minutes or more, the pupil is then dilated out to its maximum diameter.  At 52, my pupils still dilate from 8mm to 9mm maximum.  Then, as we age, the maximum diameter decreases, generally, and to about 6mm or so.  Such, however, varies from person to person.

In theory, you simply don't want the exit-pupil of an eyepiece to be larger than that of the pupil of your eye.  It's not a hard-and-fast rule, however.  When the exit-pupil diameter of an eyepiece exceeds that of your eye's pupil, then some of the light gathered by the telescope is lost and falls upon the iris instead which does not receive light...

It can be as though that you're observing with a 6" aperture instead of an 8" with such an eyepiece, and due to the light lost.  But as others have stated, it's really no big deal.  One would not expire from the event.

You may find the exit-pupil of any eyepiece simply by dividing the eyepiece's focal-length by the focal-ratio of the telescope.  For example, your 8" Newtonian is an f/6...

38mm ÷ f/6 = a 7mm exit-pupil

Therefore, if your own pupil dilates to 7mm or more at night, once dark-adapted, then your eye will receive all of the light, every last bit, gathered by the primary mirror of your Newtonian, and when using a 38mm eyepiece.

Then, take this 2" 60mm for example...

https://www.harrisontelescopes.co.uk/acatalog/masuyama-60mm-2-inch-eyepiece.html

60mm ÷ f/6 = a 10mm exit-pupil

I don't think anyone's eye on Earth dilates up to 10mm at night.  The pupils elsewhere in nature would, and even more, like that of an eagle, and perhaps those of an owl or two.

On the other hand, that 60mm would give you a low, low power of 20x with your 8", and binocular-like.  An eyepiece like that, however, is best suited for an 8" f/10 Schmidt-Cassegrain, or SCT as it's commonly known.  With an 8" SCT, therefore, that 60mm would produce a 6mm exit-pupil, and well within the maximum of one's own pupil once dark-adapted.

I would think nothing of giving that 2" 38mm a whirl with the 8" in future, as the light-loss would be negligible at worst, if any at all.

Given another poster's concern that 32x would not produce a dark enough sky background to detect the dim and even dimmer fuzzies, you could also go with a 2" 32mm, and for a power of 38x.  I happen to have that very eyepiece myself, and for my 4" f/8 refractor...

Cheers.

[Tigaroo]

Wow! Never heard it explained that way. Very helpful, now I understand why it's uncomfortable to see properly at times . although I've now started wearing vari-focal's I still look though scope without , I guess this might change soon and therefore have more of an impact on eye relief. The joys of aging! 

[Alan64]

I hope that you can continue to observe without having to wear glasses in future.  If you do find that you need more eye-relief, and for future reference, the oculars needed would not break the bank...

http://www.365astronomy.com/Planetary-Eyepieces/

Most of those have eye-relief from 16mm to 20mm, and cover the moderate-to-high range of magnifications.  Generally, eyepieces that provide lower powers, like the 2" 32mm and 38mm eyepieces previously mentioned, always come with generous eye-relief, and even greater than 20mm.  The 2" 38mm, for example, has 28mm of eye-relief.

Always check the specs within an eyepiece's listing for the eye-relief before considering.

I'm going to take a wild guess and say that you got this kit...

http://www.brookstone.com/pd/Celestron-Telescope-125-Eyepiece-and-Filter-Kit/864308p.html

The most useful colour-filter within those kits is the #80A, and is used for Jupiter and the Moon, and in teasing out detail.  Colour-filters are generally for planetary only.  

Here's a guide as to which colours are for which planets, and towards the bottom of the page...

http://www.lumicon.com/pdf/3filterspec_prnt.pdf

Incidentally, many years ago I went hog-wild and got every colour that I thought might be useful...

I then found that I hardly used any of them at all, but they certainly are pretty.  I did use one of the reds on Mars during its last opposition, and it did help a bit in discerning the maria that much more.

The two yellows are helpful for reducing the effects of false-colour with faster achromatic refractors.

Your kit may come with a fixed-13% Moon filter.  The filter blocks 87% of the Moon's light, no more, no less.

But there may come a time when you would prefer to vary the amount of incoming light...

http://www.rothervalleyoptics.co.uk/antares-variable-polarising-filter-125.html

Let's say that the Moon is at half-phase, but still too bright for comfort.  You then pop in the fixed-13% filter, but then the Moon is too dark.  That's where a variable-polariser comes into play.  You can adjust the amount of incoming light from 2% up to 40%...

Have you ever seen Jupiter and Venus appear like this through your 8"...

You can use a variable-polariser to dim those down, too, and to where you can see colours and details on Jupiter's surface, and the Moon-like phases of Venus.

I have a 6" f/5 Newtonian, which is the next step down in aperture from an 8".  The images are dimmer than those of an 8", of course, but even when observing Jupiter at a moderate-to-high power one night, the planet appeared all-white and devoid of features.  Also, the spider-vanes of the secondary assembly caused flares to jut out from the planet, with Venus here as the example...

I then popped in the variable-polariser, and dimmed Jupiter down to where colours and surface-details appeared, the festoons and whorls within its equatorial bands.  The flares, also, disappeared.

The sight blew my mind.

If you ever consider getting a safe solar filter for the front of the telescope, like this...

...then you can use the variable-polariser to dim down the Sun, too, but only with a safe solar filter fitted over the aperture at the front.  Safety is most important when observing our nearest star.

[Tigaroo]

Yes , educated guess there Alan ?? that's the kit. Should be artriving any moment ! I had only recently purchased a set of three filters plus a moon filter ,after accidentally smashing my trusted moon filter by popping it into my pocket and then drooping a bunch of keys on top ??...can't be without a moon filter as I love lunar viewing. However  with the new kit arriving these are now surplus to requirement and will be passed on to a friend or resold , it's been cloudy ever since I got them so they haven't even been used yet. This  ep kit seemed to be a reasonably affordable companion set for my scopes and the inclusion of a good set of filters was secondary, but pleasing . I shall read the article , and I think the kit also comes with a guide to which filters to use in which situation. I'm impatient for some clear skies to get playing ! 

[Tigaroo]

All your information has been really enlightening , I shall be referring to it lots! Thank you