Eyepiece advice for a C925 Celestron.

[The_bluester (Paul)]

Actually I think we are at cross purposes here somewhat (Also you have the terms wrong way around there, that is the TFOV formula not AFOV)

what I am is if you have the same mag (EP FL) and equivalent field stops, do different AFOV designs produce a difference in apparent contrast/sky darkness.

Looking a little deeper I think some marketing approximations might be leading to my question anyway. Compare a Nagler 31 and a 30mm ES 82 degree EP. Both supposed to be 82 degree apparent fields. Generally it is accepted that higher mag will result in smaller TFOV.

The ES 30mm has a larger field stop than the Nagler 31mm and thus both greater mag and greater TFOV in the same scope. I reckon either the Televue has a slightly smaller AFOV than stated or the ES has slightly larger.

I think my question has probably grown out of comparing things which don't really compare.

[Amaranthus (Barry)]

Yes, you are right, I mis-typed AFOV instead of TFOV.

But my point still holds. (As far as I'm aware) It is not possible to have two different eyepieces with identical FSD but different TFOV. This is because TFOV is determined solely by the FSD (for a given telescope).

If you can point me to such a comparison, I'll withdraw in shame But as I understand eyepiece design, what you suggest is impossible. Maybe the confusion here is 'equivalent' vs 'identical'.

[The_bluester (Paul)]

I think I have miss explained what I was asking. I am after the other way around though I am not sure even that applies as FSD seems to grow with AFOV too.

What I mean is, if you had the same TFOV between two eyepieces (So same field stop) but different apparent fields (Say an 82 degree EP compared to a 68) do you get a darker background?

I suspect that it is probably a moot point as deeper digging seems to indicate that the smaller AFOV eyepieces have correspondingly smaller field stops and thus TFOV than equivalent FL wide AFOV ones.

The ones I am comparing directly don't quite make sense to me. The ES 30mm 82 degree has a larger field stop than the Nagler 31 and so should have a larger TFOV. How can you take a large TFOV and spread it over the same apparent field while having slightly higher mag? The field stop is directly measurable and hard to fake so I suspect that the AFOV of one or both is not actually 82 degrees, one is larger or one is smaller or both.

[Camelopardalis (Dunk)]

You can have two eyepieces that will show more or less the same field of view, but one with a larger AFOV will be shorter focal length and smaller corresponding exit pupil, and magnification. This would result in a darker background.

But you'd be better choosing the exit pupil depending and what and where you observe from. I find that anything much more than about 2mm just sucks in the City sky glow and spoils the view, not so at a dark site

Regarding the ES30/Nagler 31...the field stop isn't necessarily the same as the exposed glass at the bottom of the barrel...

[The_bluester (Paul)]

I was taking the stop dimensions for both of those from the respective websites, but again, who knows if they are a truly equivalent measurement.

Regards exit pupil and LP, I live in a relatively dark area, not truly dark sky but hugely better than suburban Melbourne. I live about 6km out of Kilmore, so about 40 away from the urban fringe. I still miss the childhood skies though, we used to live about halfway be tween Echuca and Rochester in country Victoria. Black, black skies and just a hint of a light cone from the towns.

[casstony]

Seems like you're getting unnecessarily confused by technical aspects of eyepieces Paul. A 34mm ES 68 degree with a narrow band filter would be good with your scope accompanied by one or two 82 degree shorter focal length ES's.
I tried the 6.3 reducers a few times and thought the contrast suffered in comparison to using a longer focal length eyepiece alone.

The 31 nagler and ES30 performance is very similar in the scopes I've used them in.

[The_bluester (Paul)]

Probably, but I am an ex tech and a nerd at heart so if something does not quite sound right to me that I have been told I tend to chase that rabbit hole to the bottom until I understand or debunk.

[brian nordstrom (As avatar)]

My Celestron Axiom LX 31mm is as good as a friends 31mm Telemangler in my C9.25 ,an awsome eyepiece at nearly 1kg a Monster 82* eyepiece .
I know these are not made any more but they pop up on CN;s from time to time at good prices , I don't need a TV 31mm Nagler , ., but I find my 19, 22 24 and 27mm Panoptics get the most use in my C9.25 , great views that allow the Cat to shine .

Brian

[Camelopardalis (Dunk)]

Agreed Brian, the Axiom LX eyepieces, at least in the longer focal lengths, are a great match for an SCT

I think the take home for the OP is that either of these three is going to knock the ball out of the park in the scope, arguably the Nagler is better...it must be, it's more expensive I use the ES 30 in my scope and it's a great eyepiece.

[The_bluester (Paul)]

Ah well, starting with the 18mm ES to get a taste of how their 82 deg works in my 925 at a relatively low cost.

Hopefully the 2" E-Lux set I have in the trader section sell in reasonable time. Once they go I can consider selling the 68 degree Meade I have as well and look at the ES30 to replace it if I like what I see in the 18mm.

I am keen to consolidate the EP line, I can not justify having hundreds of dollars sitting in the EP case if they don't get put in the diagonal at least on the majority of nights. TFOV has become an issue as my scope often ends up used as an advanced spotter so to speak when a friend comes out for an AP session, we often use mine visually to compare star fields an check he has his pointing right in a test sub. Nothing I have comes all that close to his FOV on the chip.

[Camelopardalis (Dunk)]

Don't pre-judge the 30 based on the 18mm. I've heard that the 18mm has short eye relief. The 30 is a very comfortable EP to use.

[The_bluester (Paul)]

They list 13mm which should be fine for me, About the same as my existing Nagler 11mm which I don't have any problems with, I don't wear glasses but an observing mate does and he still gets good views out of it.

[The_bluester (Paul)]

Well, a rough and ready first light (For me) with the ES18 and I am pretty happy so far. I pulled the scope out and briefly set up in the shadow of the garage to keep the moonlight off me and had a quick look though it.

There is some slight false colour on bright stars in about the last 10% of the FOV (If that, maybe less) but it seems to have the best match for the field curvature of my SCT of any EP I own. Stars are noticeably sharper in the outer portion of the FOV than in my 68 degree Meade EP.

Pretty happy so far, now to save up for it's bigger brother.

[Tony_ (Tony)]

G'day Paul,

I'm glad you like the ES 18mm - it's by far the main eyepiece I currently use. I would like to get the 30mm one day too.
The outer 10% doesn't matter much - when you are looking at the main central part the outer edge sort of just blends in. The ES eyepieces seem to get mostly good reviews and are reasonably priced.

Regards,
Tony.

[The_bluester (Paul)]

I would note that I did not notice th false colour when I swung around and had a quick look at the moon, which pretty much filled the FOV, the moon was under a quarter but you could see the unlit portion by the scattered light easily.

[The_bluester (Paul)]

OK, moving on, some eyepieces sold and now I am looking into the ES30. Has any one had recent experience buying from VTI or ExtraVision?

VTI seem to have the 30 as unavailable and have done for months. Do they only order them in on receipt of an order?

Either way, it is one case where if an Australian supplier is smooth to deal with, I would have trouble getting one in from overseas any cheaper.

[Andy01 (Andy)]

Pardon hacking your thread mate, but I had a good look at M42, Tarantula and 47 Tuc through a 13mm Ethos on a 12" dob at VicSouth on Sat night - OMG!!!!!
It was 3.00am - seeing was great and I have to say it was gobsmakking - even after looking earlier in the evening through 25" and 18" Obsessions.
Vote 1 for Ethos

Andy

[The_bluester (Paul)]

Lol, not going to that level of pain (And I could never hide a lump of glass that big from the wife!)

I have had a look though an Ethos (Sitting in a PST of all things) and it was very nice!

At a guess, the 30mm ES in my F10 will probably give a similar TFOV as a 13mm in an average 12" dob, but it will certainly miss the AFOV of the Ethos. But is it a lot of $$$$ for an extra 12 degrees!

[Camelopardalis (Dunk)]

About 20% wider than the 13E in a 12" f/5...the ES30 is a hard to beat for price and performance in your scope

[The_bluester (Paul)]

The things which happen on forums.

I sold some Celestron EPs to another IISer as I ended up using them rarely. That thread (Or maybe this one) prompted another member to message me to see if I was interested in selling my 11mm Nagler, which I agonized about for a while as I really liked it, but I decided it did not get enough use to keep so we agreed a price and it is in a box as we speak (if you are reading this Rod I am just waiting on a tracking number from my wife, will message it to you soon)

And then another IISer offered me an ES30, which I have just bought and am keenly awaiting a tryout of.

I should sell my Meade 68 degree as the ES is likely to completely replace it as the goto EP.

Time to get new pick and pluck foam for my EP case, nothing fits any more!