Any truth(or weight) in the statement that the best marriage for ep's is, if you have a convex objective, you should go for concave EP' tops and visa-versa?
Thanks
Steve
No. Many EPs have a flat eye lens, and then what? Plus with many other EPs the reflections seen through the eye lens may not allow you to determine the shape of the eye lens top surface. Many retailers would not be able to answer that question for you either as they will be most reluctant to open a factory sealed EP.
Unfortunately it is a case of reading LOTS to work out what is what. While most people who write "reports" about eyepieces don't understand optical matching, what they write can be used as a tool to help you work things out. Most people will use ONE eyepiece in a given scope, say for this purpose a Newt. Forget their opinion that the EP is crap... What is important is to try to work out what aberrations they recognise.
The one single most important aberration to recognize for Newts is astigmatism! This will appear as little seagulls all concentric to the optical axis. Astigmatism does not mean a poor eyepiece for your scope though. If it appears slight and right at the very edge of an 80deg AFOV, then for my money this is a good eyepiece if it is at a good price! NO ONE does any observing right at the very edge of the FOV. NO ONE. You may look at the edge to see what it says, but when you are actually observing YOU DO NOT LOOK ALONG THE EDGE OF THE FOV - you will move the scope.
But for some people this is just not good enough, and good luck to them
Field curvature is unavoidable, especially with faster focal ratios, and not something that should concern you unless it is really grotesque. Then this could be more of a symptom of a mismatch than anything else.
Reading what people write between the lines is your best friend, as will be your tolerance to certain aberrations. And DO NOT MAKE A PURCHASING DECISION BASED ON ONLY ONE EYEPIECE FROM A WHOLE LINE THAT WAS USED IN ONE SCOPE TYPE, ESPEICALLY IF IT IS NOT THE FOCAL LENGTH EYEPIECE YOU ARE KEEN ON!!!!!!! Contemporary EP design is just too complex and involved and to pass judgement on a whole eyepiece line based on one individual focal length in just one scope only shows the ignorance of the author.
There are a number of comparisons between the Baader Genuine Orthos (BGO) and the Baader Classic Orthos (BCO).
The BGO's often rate a hair better but on the 18mm the BCO seems to rate better. BCO's have a 52 degree FOV. Depends on your preferences for the size of the FOV. I definitely like a wider view but I am not super experienced here so I may change my mind on that. I just got the 18mm Fujiyama so I can post my impressions about that here next clear night.
What focal length eyepiece and scope do you use?
Greg.
Greg
I have a pair of 12mm BGOs' that are mainly used my 5" f7.5 apo (950mm ) and 5" f9.4 (1200mm ) achro for lunar and planetary. Using the binoviewer in the 10" newts is problematic because of the weight and balance issues and also focuser position when the newts are EQ mounted. I have 1.6X and 2 X OC to use in the binos, allowing a for range of magnifications, 121X & 152X in the apo and 153X & 192X in the achro and they give nice crisp images in both scopes, the image in the apo is better colour corrected of course but the achro is quite acceptable, when teamed with a Baader Semi Apo filter.
Have never looked through the BCOs' so can't add anything there but the wider FOV does make them more desirable for some. I also have an 5mm TMB which is a good performer in the fracs and the newts and also several Circle T volcano top orthos', 25, 18, 9 & 7mm. The early volcano tops are fine eyepieces but suffer a little from internal reflections and narrow fov of 40/42 deg.
Thanks Saturnine: so many designs! I haven’t tried any orthoscopics so far, never thought I had a use case that differentiated them significantly from my plossls, but I’m still a relative novice with a very small collection, and will surely try at some point. I’m a fan of Baader!
Thanks Alex: these TMBs certainly have an interesting back story. I don’t think that the term clones necessarily implies any skull duggery. There are lots of re badged, housed, rebranded Astro stuff. All legitimate, and all clones. The sky watcher Az eq6, Orion Atlas pro, Saxon AZEQ6 I would describe as the same mount or clone. [/QUOTE]
Good quality Plossls should not be left out of the mix, with their simple design and less glass for the light to pass through, they provide great views, with 50 / 52 deg FOV. Like the orthos', unfortunately as the focal length gets shorter say from 7mm down, so does the eye relief which makes them difficult if not unusable for old eyes.
Its the 28mm RKE with its seemingly half meter of eye relief that creates an effect that the image is floating above the eyepiece itself. Its a lot of fun but no body is making any serious observations with it.
Yes but that's hardly unique to the 28 RKE. Serious observations are now done by camera. Unless something serious happens to be going on and no camera is handy. Then an RKE might provide a serious advantage over a non-observation. Seriously
At the very low end of ep's - a little RL of what Alex has said
Hello,
When I first went looking for ep's I read many a forum trying to find out about eps' and light cone angles, lenses, groupings, designs and such. So many pretty bling things to read about and ponder. While doing so I saw a pair of knight owls(?) at 30mm and 15mm on ebay going for a song - no bidders, zero interest.
Off to research said ep's and man did they ever get bagged out, both here and at cloudy nights. My favourite quote which has stuck in my head was along the lines of "there not seagulls at the edge, or even eagles but screaming pterodactyls". But not to be put off I kept searching for an article to reinforce my idea of getting them - as you do, read the stuff that you already agree with/reinforces your opinions.
Eventually I found a high school grade author who pointed out at F6 and above there not the worst ep you could get, but a little scratchy at the edges. As a novice I am more into the view at the center of the ep anyway and was keen to get a cheap wide field to try (still on my x2 ep's as supplied with the scope at that stage).
$50 later I had both ep's with my 2" barlow I had 30/20/15/10 and 7.5mm in an 80 degree field. I won't say there the best ep I own now by a long way, but at F6 there are useable to a newb and his dob and I would say a 6" F8 dob they would be very useful for a novice, or those with a limited budget wanting extra wide FOV. (But get the 20mm it has a removable x1.5 barlow and gets you a 30mm too boot)
These ep's are still sold off very cheaply and if you have a slower scope and a tight budget, I'd say get them if they're as cheap as mine were(up to $40 a pop maybe for the 20/30mm anyway). I'm still looking mostly at the center of FOV at the object of interest and even with adverted gaze the edges really aren't a big worry for me, or that bad imho.
Too many people want to defend spending what amounts to a decent second car for most family's on a set of premium ep's. If you get a slower scope (F6+) and are new to this hobby, cheap eps are fine if you do a little research, and very good if your not chasing extra wide FOV's ep's. After you have spent a few hours out in winter at sub zero temperatures, you'll know if your a keen observer and can look at migrating up after that.
Blessed be the astronomy club that lets you try all these ep's first!
Steve
Ps. I have wanted to try and capture the images from my 30mm knight owl's (read apm cheapest wide field "clone/replica") and the images from my Tele Vue 32mm wide field I was donated.
The 30mm knight owl shows far more nebulosity on M42 than the older Tele Vue 32mm does, but I am assuming that due to scatter control or a lack of it. The Tele vue image is 60/70% of the nebulosity but has a lot nicer blackness within the cloud for contrast, something that is almost absent in the knight owl.
But till I got a better ep, I was none the wiser, and for M42, I tend to use the knight owl over the older style Tele Vue anyway as I want to see nebulosity in my clouds even if I only get 3 stars in the Trapezium Cluster.
Pps. I still troll pictures of M42 trying to find out what it should look like, but with AP you get so many options, not just based on scope, aperture and camera settings but post processing and such its hard to know what a "true image" looks like, even at grey scale. The Knight owl best duplicates sites such as "astronomy.tools" FOV calculator image imo.
Steve, those Knight Owl eyepieces that you mention and your reading about them are exactly what I mean about "reading between the lines" when it comes to figuring out optical matching of eyepieces.
That people were describing "pterodactyls" to me says they were using those eyepieces in fast Newts/dobs. WRONG scope for those eyepieces. And you then use them in a slower Newt/dob, and this astigmatism aberration is milder, still there but far less aggressive.
Put those eyepieces into a refractor, especially a slow refractor, and they will perform much better. Won't be "perfect" but much improved.
The GSO SuperViews are similar in that way. Inexpensive but for value for money very good. They do show astigmatism in Newts, and the faster the Newt the more significant the seagulls will be. In an f/5 Newt, this aberration may well be just fine for an individual person In refractors and SCTs they still do show tangential astigmatism rather than sagittal (seagulls), and the slower the frac the less significant is as well.
One thing I really like with these SuperViews is they are excellent for novices/newcomers to astro as they are so easy to use. I bought the 30mm and 15mm more than 12 years ago, with the 30mm being my first 2" eyepiece. In the subsequent years I have bought and sold on soooo many eyepieces but I have NEVER even considered selling my two SuperViews. I don't use them for my own observing, but they are my EPs of choice for outreach.
I came across a picture that demonstrates what is mean by "seagulls" as a symptom of astigmatism due to a mismatch between eyepiece and scope. In "Astronomical Optics", Bruce MacEvoy has a picture that shows the different appearance between Sagittal astigmatism (seagulls) and Tangential astigmatism. The page that deals with aberrations seen at the eyepiece is highly recommended. You can follow the maths or you can just keep it simple with reading and following the diagrams. Understand the appearance of these different aberrations will go a long way to help you finding the better eyepieces for YOU. I have my own set of preferences in what I look for in eyepieces, and it would be wrong of me to insist that MY preferences are the "right" ones. I am not you, nor am I in charge of your purse strings. All I can do is show you what things are through the eyepiece and you can then make your own choices.
Mirko, I agree that the effect looks more radial, but it relates to how this particular type of astigmatism is produced. See the actual astigmatism section of the article:
That's a terrific resource Alex, thanks for sharing. 99% of it is well above my pay grade to comprehend, but it's great to have a definitive list with diagrams of all the types of aberrations out there.
One thing about aberrations - their presence/appearance does not necessarily indicate that there is something wrong with a given eyepiece. Instead their presence is an indicator of many different conditions predicated by the scope-eyepiece-eyeball combination at play.
Some aberrations are unavoidable, such as coma in Newts, and eyepieces will be able to deal better or worse with it. How severe the coma is is also a function of the focal ratio of the primary mirror, and coma correctors are optimized for specific focal ratios, say centred around f/4 or f/5, with a half f/stop either side. So an f/5 corrector in an f/4 Newt won't clean up all the coma present. Like wise an f/4 corrector will over correct in an f/5 Newt.
Other aberrations will be noticeable as a result of an excessively fast focal ratio that is beyond the design parameters of the EP.
Aberrations do not indicate a poor eyepiece either. What it can do is offer really good value for money if what aberrations are visible are totally acceptable to YOU. A wee amount of astigmatism along the very edge of the FOV of an 80° EP is no real problem as you do not ever do any observing along the very edge of such an EP. You don't. You move the scope. So such a $250 EP may be an absolute bargain for you And if you rather drop $1000 on an EP, great too
Alex.
Last edited by mental4astro; 03-02-2021 at 08:55 AM.
NO ONE does any observing right at the very edge of the FOV. NO ONE. You may look at the edge to see what it says, but when you are actually observing YOU DO NOT LOOK ALONG THE EDGE OF THE FOV - you will move the scope.
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