Best 3-eyepiece set for 8" dob?

[morls (Stephen)]

Hi,

I've decided I want to think long-term about building my eyepiece collection....After 6 months of getting to know both my telescope and my personal preferences for observing, I've reached the following conclusions:

1. I am very happy with the dobsonian, and I can see some beautiful things with the 8". The only change I can see myself making here is perhaps getting (or maybe even building) something larger, but this is down the track...

2. I enjoy looking at the planets, but I have a greater interest in exploring DSO's.

3. I've done some modifications to my scope, an 8" F6 collapsible Skywatcher, so collimation is now much easier. I've also got the Catseye gear, and the difference is startling....so, I'm now beginning to understand how much enjoyment good gear can add...

So, with this is mind, I've researched and come up with the following list of eyepieces. I don't want a huge collection, and premium eyepieces are expensive, so I've decided to try and restrict myself to 3. I'd be interested to know what people think of my choices, and would really appreciate any feedback/suggestions...

1. 24mm Panoptic
2. 16mm Nagler T5
3. 10mm Pentax

I currently own a Meade 2x Telextender, which I believe performs the same as a T.V. Powermate

I'm also hoping these eyepieces will be suitable should I eventually move on to a larger scope...(up to 16".....dream on...)

Thanks for reading my thread...

Stephen

[Paddy (Patrick)]

Hi Stephen,

I salute your choice to take time to know your scope and to go for quality eyepieces. I do have a couple of thoughts. Firstly, assuming your scope is fl 1200, the nagler 16 will give you a TFOV of 1.1 degree and the panoptic 24 will give you 1.3. These may be a little close. I sold my 24mm pan when I got my 17mm nagler as they were just too close. Also you mkay not want much more than one degree for a wide field ep, so you might be able to do without the Pan, lovely eyepiece though it is. The other thought is that some people (myself included and I don't wear glasses for observing) find the eye relief on the 16 mm type 6 Nagler a little tight. Others don't mind. I also found it a bit harder to get the eye position right on the 16. I bought the 17mm type 4 Nagler instead and really like it. At Vicsouth last year I got to compare the 2 in my f4.9 scope and was very pleased that I got the 17mm and didn't like the 16 nearly as much. However when we compared them in Tim Nott's f3.6 22" scope, the 16 mm type 6 performed better - there was a bit more astigmatism in the 17 mm using the faster scope. So this might be an issue if you upgrade. But then, if you find the 17 mm doesn't cut it, you would have no problem selling it on IIS I suspect. For the loss of $75-$100 you might have some years of more pleasurable observing.

Sooo... I wonder if it might be worth going for the 17mm Nagler, the 10 mm Pentax and then seeing whether or not you need a wider field eyepiece. You might save about $300. Only thing is you have a 2" ep instead of all 1 1/2" but you just leave the adapter on the 10mm while observing and no problems.

Again, you may be quiet happy with the 16 mm type 6 as many people are, but i would suggest looking through one before you buy.

[morls (Stephen)]

Thanks for the thoughtful reply Patrick...

Yes, it's a 1200mm focal length dob..and maybe I should clarify a bit - when I said DSO I meant globs, open clusters, nebulae etc as well as galaxies....

It's a very good point about the TFOV, and now I'm wondering exactly how wide a TFOV I need....I was thinking the 24 Panoptic would cover all the wide field needs.

I chose the 16mm T5 because I read in a post (from 2005!) by Dave47Tuc (#15 http://www.iceinspace.com.au/forum/s...ead.php?t=1209) that 2-2.5mm exit pupil is desirable for best contrast and brightness in DSO. The 16mm T5 would give me an exit pupil of 2.7mm...But your point about eye relief is something that I had considered as well, and maybe I should think a bit more about this one....The 17mm does sound very good....

Another point I was considering was that with my 2x telextender (1.25") the 16 becomes an 8, but this might be a bit unnecessary with the Pentax 10mm.....

hmmm, food for thought....

[Paddy (Patrick)]

I would suspect that 8mm and 10 mm are so close that you wouldn't notice the difference. The 2X with the pentax would give you a nice high power set up. My personal experience is that 1 degree FOV is plenty for a wide field eyepiece.

Anyway, I'll be very interested to hear what other thoughts people have as there are many on this forum that have a much more extensive knowledge and experience with eyepieces than myself and I'm sure you'll get more opinions.

[Steffen]

Originally Posted by morls

The 16mm T5 would give me an exit pupil of 2.7mm...

There isn't much between a 16mm and a 17mm eyepiece in terms of exit pupil. On an f/6 scope they're 2.7mm and 2.8mm respectively, so I wouldn't worry about that too much.

Cheers
Steffen.

[morls (Stephen)]

Thanks Steffen.

So, am I right to think that the 17mm Nagler will give me a view with more contrast compared to the 24mm Panoptic, at the expense of 0.2º TFOV?

I suppose another question is how significant is the difference between 1.1º and 1.3º TFOV? This is something I just don't have the experience to know...

[Steffen]

And yet another question could be, why are Tele Vue still making the 24mm Panoptic instead of advising customers to just get the 17mm Nagler? It surely can't be the $100 price difference (peanuts by Tele Vue standards)…

Cheers
Steffen.

[Satchmo]

Thats an odd question eg . With an F5 scope the only way to get a nice bright 5 mm pupil is with a 24mm eyepiece - a 3.4 mm pupil ( ie with a 17mm ) just doesn't shine for some objects -and apparent field is a matter of personal taste.

[morls (Stephen)]

Now I'm geting a little confused....

I'm not sure how exit pupil affects what is actually seen through the eyepiece. On my f6 dob, the following would apply.....

1. 24mm Panoptic - exit pupil 4mm
2. 17mm Nagler T4 - exit pupil 2.8mm
3. 10mm Pentax - exit pupil 1.7mm

So, if the TFOV between the 24mm and the 17mm are very similar, what is the effect of the different exit pupil sizes? Does a smaller exit pupil mean less detail, like looking at a smaller vs larger computer monitor?

[morls (Stephen)]

OK, having given this a bit more thought, I've come up with an alternate plan...

The Televue site has a section specifically for 6" - 10" dobsonians with f/l 1200mm, and one of the mid-range eyepieces they recommend is the 13mm Nagler Type 6. I think I'll go for one of these - my thinking is this will also give me a 6.5mm option when used with the 2x telextender. It's also an eyepiece that will be more affordable for me in the medium term.

I'll eventually get a premium low power, wide field eyepiece, but there seems to be a few different ways to go here, and I think I'll take my time and try to have a look through a few before I decide, especially considering the cost involved.

Thanks Patrick, Steffen and Mark for your input, it's much appreciated!

[dannat (Daniel)]

if you go the 13mm, think about the 8.5mm pentax XF - not as wide as the xw series but will provide a high power view

[morls (Stephen)]

Hi dannat,

yeah, I was thinking I would have to revise my high power options too...was thinking maybe the 7mm XW would do the job....

I haven't thought about the XF, but I'll check it out, thanks

[Paddy (Patrick)]

I think you'll find the 13 mm Nagler a very useful ep.

I have to say I normally think about eyepieces in terms of magnification and FOV and less about exit pupil. Perhaps because optics has always been a daunting area for my simple mind. So it was very interesting to read Mark's thoughts which I understand to be that using a lower power ep with smaller FOV delivers larger exit pupil (which relates to magnification) and therefor more light. But would this tend to reduce contrast as the background light is also brighter? Any illumination appreciated. Also, does this mean that the eyepiece aficionado has several eyepieces for the same FOV - higher mag ones for detail and contrast and lower mag for brightness. Could get expensive.

[morls (Stephen)]

yeah, the exit pupil part of the equation has got me wondering as well...I'm kinda thinking out loud here paddy, so sorry if I'm paraphrasing you...

going back to the 24mm Pan vs 17mm Tele figures

24mm Panoptic - field stop=27mm; TFOV=1.29º; exit pupil=4mm
17mm Nagler - field stop=24.3mm; TFOV=1.16º; exit pupil 2.8mm

For the sake of trying to figure out how exit pupil affects what the object being observed actually looks like through the eyepiece, I'm going to assume the TFOV of both eyepieces is the same....

So, with this assumption in mind, what we are dealing with is the same amount of information (light) gathered by each eyepiece, are we not? If so, then if that amount of information is spread over a larger area, would it not be easier to see detail? Does this also mean, however, that any background 'noise' would also be more obvious?

So, does the larger exit pupil eyepiece give a more detailed view of a given object, with less contrast, than an eyepiece with a smaller exit pupil, given the same AFOV between the two?


I know this doesn't take into account the way the eye behaves...all I'm trying to do here is understand how exit pupil works....there are probably a multitude of other factors I'm not even aware of that enter the equation...

Any help/insight greatly appreciated....

it also makes me wonder if, rather than thinking of magnification, I'm better off choosing eyepieces based on TFOV/exit pupil....perhaps try to match TFOV while having a selection of various exit pupil sizes....

I got my TFOV figure from a post by gargoyle_steve:
TFOV = (180/Pi)*field stop / Scope focal length.
180/Pi converts the calculation from radians to degrees - approx 57.296

[Waxing_Gibbous (Peter)]

Ok,
I've just been out with all of these EPs.
To my eyes the 24 Pan beats the 17 Nagler for the following reasons:
Noticably flatter field of view. The Nagler, while sharp over 90% of the FoV had noticable curvature toward the edge.
24 Pan exhibits a brighter image with more contrast
For me (I like to squish my eye close to the lens), the 17 suffers from blackout or "kidney-beaning" unless you are at just the right distance. The 24 is more forgiving. I also found this with the Pentax XW EPs, but others don't seem to mind.
If you don't want to go too wide the 19mm Panoptic is a real jewel. My most used eyepiece in my 8" Newt.
If I was picking just 3 I would have:
19 Panoptic
13 Nagler
7.5 Takahashi LE / ED
The latter is pretty dear (about $320), and has a narrow (c.50* FoV), but has fantastic throughput with no flaring and is just the bees knees on not just planets, but bright Nebulae and close clusters.
Just my humble opnions, but although the raw numbers are a good guide, they don't always reflect the "experience" of viewing.
Peter

[morls (Stephen)]

Hi Peter, thanks for the comparisons, a great help....and good to hear how these eyepieces perform in a 200mm newt....is yours an F6?

It seems the choices for the longer f/l eyepiece are a bit more involved than the shorter ones....I hadn't thought of the 19mm Panoptic, but it sounds very good, and I think I have to consider this one too...

At the moment I'm trying to scrape together the $ for a 13mm T6 Nag; this eyepiece seems to rate very highly in a lot of peoples' opinions...(one advantage of these things costing as much as they do is it forces me to take my time when deciding.....)

[koputai (Jason)]

The T6's are the cheapest they've ever been as Bintel have dropped the prices in the last few days.

I'm hoping to head in there today!

Cheers,
Jason.

[morls (Stephen)]

Well, I've just gone and bought my first premium eyepiece - a 13mm T6 Nagler...can't wait to try it out

fingers crossed now for some clear skies.....