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Strategy for getting my 40mm EP, exit pupil and scope to do fields etc.


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Hello, I joined today, said hello in the hello place and now here with my first bucket of questions for experts here!

 

Not sure how my interests will develop, but currently starting to work through an astronomy 'practicals' book. A lot of section 1 is taken up with the celestial sphere and circumpolar charting etc. And I've gotten interested in this, so I am moving towards looking at starfields and identifying stuff I can see with binos and hopefully a SW Equinox 80 (f6.25).

 

So, I have this 40mm EP which amazed me when I put it in the Equinox, but the exit pupil is 6.4 (btw I'm learning all this stuff at the same time and have only a vague idea what I'm talking about) - this may explain why I can see something amazing as I hover over it, but can't get inside the EP, or too close to the image.  I put the same EP in a 5" f9.4 achromat and it was still pretty amazing with exit pupil of 5.3 which I can cope with (if I understand it, I can get into the EP and the image gets on my retina), but I'm not so good at yoga that I can do zenith and circumpolar stuff with a refractor as big as me, and FOV seems to reduce.

 

So, what are my options?  1. I have a ES 68 deg 24mm which is very nice, but I want a larger fov.  2. Is there any point in trying to extend the focal length of the Equinox to reduce the exit pupil? That also will reduce fov anyway??  3.  Just go for a 30mm with exit of 4.8 ?  or, 4. Go for one of those 82 deg ESs??

 

Any other good combinations EPs and whatnot with f6.25 80ed doublet for detailed wide starfields (and some DSOs hopefully)?  I'm prepared to go to 2" eyepieces if I can get anywhere near the experience this 40mm looked to give me.  I have a 2" diagonal in a box somewhere.

 

Thank you for reading!

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Thanks for the response @NIghtspore - yes the ES 68/22mm is very nice and I use it a lot - I wouldn't say it is sharp right to the edges, maybe I could do something about that.  if the 40mm is a plossl,  hmm I don't know! But the pinpoint bright stars, and "3D" quality image maybe an illusion, maybe i'm mixing up fov with immersion?? Do you think wider deg EP say 82 with a smaller exit pupil EP, would be more 'immersive' than the 68? (if i've understood immersion correctly...lol)

 

 

...and yes the 40mm works better in a f9.4 with smaller exits, but its not a manageable scope for what I want and the view seems more restricted, emphasising 'seems'

Edited by Tufty
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14 hours ago, Nightspore said:

Oh, they're not cheap lol.

Haha no I looked, they're not!  The TV site did have a FOV simulator on that page though which showed only small differences in fov between 24 and 27mm, but 27 is a 2" - ...decisions, decisions!

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Sn0RYe3l.jpg

 

Basically, the ES are copies of these. There's nothing wrong with the ES (made by JOC) but there's a reason the TV's cost what they do. 

 

XREZzQJl.jpg

 

Although were a lot cheaper when I bought them (well before TV raised their retail prices enormously). The Pan's are probably my favourite eyepieces.

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Hi! I'm not sure I understood the "get into the EP" stuff, but it sounds to me as if what you really want is a nice 32-40mmEP in a 2" version, assuming your current 40mm has a 1.25" barrel.

I expect you will get a vignetted image when using you EP if it's 1.25", as the light cone is >1.25" in diameter.

 

With a 2" EP you will also need a good 2" star diagonal unless you're an accomplished acrobat!

 

Don't worry too much about the exit pupil, in my experience it only becomes a problem when it's too narrow (which it won't be with a >20mm EP) or too wide for your eye pupil, which means you lose out a bit on brightness.

 

My largest EP is a 2" 32mm SWA 70deg and it gives stunning views! The largest 1.25" I have used is a cheap 20mm Plossl and it doesn't even compare to the experience of the 2". My scope is a 120mm f/8.3 achro with a 2" focuser.

 

Below are my two longest focal length EPs and my scope with the 2" EP and diagonal mounted.

20221112_135513.jpg

20220919_200207.jpg

Edited by Moonlit
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@Nightsporeyour pictures have got my wallet to start flexing for sure. I think because you have got me itching I will see if I can get a look through these Panoptics and compare with ES, I use an ES 68 24mm and a 10mm the most at the moment.

 

@MoonlitYes! And thanks for your response too. My 40mm is 1.25 and I have to hover about 5cm over the EP which nevertheless presents a very nice view. As I move into it yes! vignette would be a good description of what happens.  Are you saying that  2" 32-40mm eps wouldn't do this??  If so I am sold! I have a 2" diagonal but no 2" eps as yet. I'm very intrigued by what you say about exit pupil, I didn't think it was affected by barrel size. In your telescope I'd think the 32 would come out around 4 but in mine 5.3 . I'm working on the basis my eye is a 5 lol so a 40mm will vignette, possibly a 32 a little bit?  But I am intrigued by your use of the 32 in your scope, what you like to look at with that. My f9.4 achromat is more at home with the 40mm but I'm looking into using a short telescope but with good wide views of the circumpolar/zenith for which my big achro is very awkward but also not so bright views as the short fl 500mm scope

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Hi again @Nightspore firstly that Altair looks very interesting and I don't have a 15mm so I'll be in the market for 15s at some point and I'll put it on the list! 

 

I take your point about telescope first ep second. Well, my f6.25 refractor I was planning to use for the textbook practicals is reputedly a doublet Apo with Schott glass, at least that's what it says on the tube! Do you think I need a faster scope? I got it mainly as a portable and to point overhead and higher altitudes as my big achromat, wonderful though it is, is unwieldy for some tasks. If I'm doing this charting thing I just want to sit at the ep and make small adjustments every now and then. My thinking was a long focal ep 25 to 40 in a fast short scope would give me enough starfield to have one bright star in view while I'm plotting fainter ones and then checking on the charts. But I'm also hoping for very nice "views" through an expansive (possibly expensive!) Ep. I'm very attracted to idea of going to 2 x 2" eps eg like the 27 and 32 you showed me. I attach photo of scope, it has a 2" barrel. P.s I think these are used by astrophotographers but I'm too inexperienced for that

skky-watcher-equinox-80ed-tube-seul.jpg

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Should be fine. I don't bother with AP so an ED doublet is fine. The Schott is probably the 'crown' glass element.

 

8AbYvM5l.jpg

 

My 80ED doublet has a Schott crown, with an FPL-53 'flint' giving it a good Strehl ratio. The focuser is aftermarket (Long Perng).

Edited by Nightspore
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Hmmm I don't know about Strehl ratio. Wait. Just looked it up in my Sidgwick (amazing book) which arrived a couple of days ago.  It has Schott and flint in there and talks about 'transmission fall off' so i guess it's something to do with that?  Strehl is not in there- My Equinox said Schott glass on the side and I just thought well if they wanted to advertise the glass they used there must be a reason! They could have put glass by Anglia Windows on the side and I wouldn't have baulked.  I will pay attention more often now I'm on a forum!

 

Just saw a review of the Equinox it says about it and the ds pro "Otherwise, the lenses are both ED doublets that are Chinese made, but use glasses from premium Japanese/European brands. Sky-Watcher have strongly marketed their use of Ohara’s FPL-53 for the positive crown element and a high-grade flint from Schott for the negative. FPL-53 is one of the best high-fluoride ED glasses around and is used in many of the best apochromatic lenses."

 

BTW talking of brands I notice Pentax are in the ep market, but I don't see many discussions online talking about them. 

 

Looks like your finder mount is on the focuser!

 

The other thing with the Equinox is that I can mount it on a manfroto camera mount and can get right underneath the assembly. But I am thinking of getting it its own equatorial, or altaz to make rotation adjustments easier.  Happy nights ahead hopefully!

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I hadnt noticed that myself! maybe the reviewer made a mistake.  i do see some colour fringing on bright objects like Jupiter, but i think it depends on ep and moisture. I havent really got into filters yet, but need to look into that. 

 

i suppose i ought to find out how to do a star test as well, never done one!

 

the 60/360 looks nice!  An AP instrument I presume.

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17 hours ago, Tufty said:

vignette would be a good description of what happens.  Are you saying that  2" 32-40mm eps wouldn't do this??  If so I am sold! I have a 2" diagonal but no 2" eps as yet. 

I'm by no means an expert, but I'm good with numbers and physics and I THINK I have this figured out 🙂

So I've made a chart to see if we all agree on my logic here.

First, definitions:
1.25" / 2" barrel size: Your Equinox 80 has a 2" barrel because it is capable of giving a very large apparent field of view. The light making up that image actually spans a cone with a rather large diameter inside the telescope, diagonal and EP.
If you truncate this cone by using 1.25" equipment instead of 2" equipment at any point in the light path while using an eyepiece that is capable of showing a wider field than is possible with 1.25", you're going to experience vignetting where the edges are darker and maybe disappears. I don't have much experience with this myself because I don't own any EPs over 20mm in 1.25". I'd guess that a 24mm EP would just about "fit" with a 1.25" barrel, but not a >30mm EP with an apparent FoV over ~50 degrees. 
Exit Pupil: This is the diameter of the light cone emerging from the EP to your eye, at its narrowest. You want this to be slightly smaller than your eyes actual dilation (which can be up to 7mm with full night vision, reducing with age). If it's bigger than your eye you won't really notice anything, but the image won't be as bright as it could be. 
If it's smaller than ~1mm you will have to position your eye just right (sideways and up/down).
Eye Relief: The maximum distance away from the eyepiece in millimeters where you will be able to see the whole Field of View that the eyepiece shows. If you're farther away, the image will shrink, hiding the outermost stars. 
Apparent FoV/Field of View: THIS is the holy grail for me. Assuming your eye can fit the exit pupil and be close enough, a big AFoV will let you see stars across much more of your eyes' field of view, in some cases letting you feel like you are IN space instead of just looking at an image. I treasure this, which is why I got myself a 2" eyepiece early on.

I would guess that what you really want is to experience a wider field of the sky (measured in degrees) so you see more stars in a single view. Googling tells me that a 1.25" 40mm EP like yours has a very small apparent FoV or 45-50 degrees. I think the reason for this is that the 1.25" barrel simply blocks the outer parts of the light.

As far as I can tell, this why there are very few 2" EPs with an EP focal length below 20mm, and very few 1.25" EPs with EP focal length over 30mm.

Exit Pupil is the diameter of the light emerging from the EP, and the barrel size will possibly restrict the light path in multiplace places:
image.png.10cd5972190802d37022d10590cfff41.png

I use a chart to compare eyepieces for myself and I have added a section at the bottom for your Equinox 80 scope with various eyepieces. Note column G, "True FoV". This shows you how many actual degrees of sky you will be able to see through the eyepiece with your scope. 
I can see 2.24 degrees with my 32mm 70deg in my 120mm scope, and you can see 3.6 degrees with your 40mm EP in your Equinox 80, assuming I guessed your EPs AFoV right.

image.thumb.png.a7fdfe9590f928f6d731be8c96c04d78.png

Here is a link to the chart if you want to copy the cells into a chart of your own to play with it:
https://docs.google.com/spreadsheets/d/1_BlGrGfVFR-bS2NdNXEApb5bt1UTy37obwA9VdKilqk/edit?usp=sharing

EDIT: TeleVue has a nice site that explains a bit more about the restrictions imposed by 1.25" diagonals/EPs.
"for 1.25" eyepieces, the maximum field stop diameter is 27mm; for 2" eyepieces, it's 46mm"
https://www.televue.com/engine/TV3b_page.asp?id=79

EP calculator. Note that all EPs above 27mm 60deg are 2" eyepieces:
https://www.televue.com/engine/TV3b_page.asp?id=212&plain=TRUE

 

Edited by Moonlit
Added TeleVue links
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Decided to make another post for this.. 

I simulated how the Hercules Global Cluster and surrounding stars would look in three scenarios:

RED: My own f/8.3 achro with my own 32mm - this is the widest field I can get.
YELLOW: Tuftys existing Equinox 80 with his 40mm 1.25" EP assuming it's 45 deg AFoV and assuming he does not get vignetting from the 1.25" barrel - probably his AFoV will be narrower than this.

GREEN: Tuftys scope if fitted with a 2" 72 deg Williams Optics SWAN 40mmEP

https://astronomy.tools/calculators/field_of_view/
image.thumb.png.7e3640045e00d6610554e003f48dc3cf.png

Edited by Moonlit
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@MoonlitWOw! gosh! Thank you so much -you've done most of my homework for me lol! And Yes, I'd like a wider field as you suggest - these calculators look amazingly useful.  I shall experiment.  There's so much to take in, I shall be reading this for a few days.  So on a first take of what your research indicates, I could easily become happier with, say,  2" barrel eps in range 27 to 35 .... ah! yes, I see - so I put in a 2" ES 30mm ep with 82 deg fov and it gives exit pupil at 4.8 .  

The actual fov is 4.92 degs which would be very useful.  Mind you the ES one is £344... but if it does what it says on the tim. 

-this is marvellous! Thank you for all this, most appreciated.  Thsi forum has delivered!

 

You have a 10" reflector on the way, I'm sure that will be fun!

Edited by Tufty
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11 hours ago, Tufty said:

@Moonlit-this is marvellous! Thank you for all this, most appreciated.  Thsi forum has delivered!

 

You have a 10" reflector on the way, I'm sure that will be fun!


Not at all, I love researching! Thank you 🙂
I'm also already thinking about a replacement for my 4.5" achromat and your Equinox 80 and @Nightspores Evostar 80ED are two strong contenders as I want something a little more portable than my 1 meter long scope. I need either an APO or ED scope to combat the green/yellow discoloring I see on the planets and moon when using high magnification. Most scopes in that segment seem to be fast scopes with a low f-number, I'd prefer something in the f/7-f/9 range. I'm taking my time waiting to find the perfect scope, though.

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@Moonlit Researching is good! I would think thrice about replacing your 4.5" !! You've said you really love it and enjoy it.  I always regret not keeping the scope I had as a kid, a 60mm refractor.  I have a 127mm achromat which is awkward, clunky, difficult F=1200 f9.4 but I would never part with it.  It gives me some purple fringing on the moon with the stock ep but using the ES eps this is not imposing at all.  I use a 10 and 24 and it's great - I have a 2.5 barlow which I've used on occasion, again very awkward but the kid in me who always wanted a 5" refractor still can't believe he got it! (Even after a few years now).  I got the 80ed for the same reasons as you I guess.  But so many times I want to view through it alongside the big achro - would you feel the same if you didn't have yours one night when looking through the 10" reflector? Have you tried some upmarket eps in the achro?  Made a huge difference to mine.  You'll want good eps with the 80ed and I'm sure you'll want to put them in the achro! My 80ed is very bright on planets like Mars and Jupiter so I had to look at ways of dimming them to see any detail, star views though are lovely even if the sky backround also seems a bit bright.  So Moonlit I'm afraid you will have to keep your achro and just splash out on an 80ed!! lol  A friend of mine is into guitars and has 26.  When I asked how did he get into this state he said you always need n+1 guitars where n= the number you already have! Possibly the same for telescopes!  When my interest in astronomy resurfaced a few years ago I was still travelling for work (pre-covid) and on impulse in the Science Museum in London on the way out I bought a National Geographic table top refractor 60mm ? plastic I think.  I couldn't believe I could see the bands of Jupiter on it, and very easy to use, though a bit wobbly maybe.  I still have it and have taken it with me on occasions when travelling - moral is you never know!

 

@Nightsporecheers! That looks amazing useful! I'm going to print it out and stick it over the bed!

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p.s. @Moonlit when you say you get 'green' fringing might this be a kind of 'electric yellow or lime' ?  I think this is the complementary of purple which switches to its complementary during movement etc. I have seen that in my achro but my good eps seem to minimise it. 

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RBuWnBz.jpg

 

Cyan fringing and chromatic aberration essentially depend on the quality of the doublet, triplet (or other) ED glass. CA is also very subjective. My 72 ED DS Pro can show 'purple hazing' on very bright objects, especially Venus. Yet it's absent on my other ED doublets which have FPL-53 and Schott or lanthanum glass. Slower refractors will exhibit less CA.

 

cHvJbCs.jpg

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