33 replies to this topic

[pjmulka]

I am looking to buy a new eyepiece for my 10" f/7.2 truss dob build. 95% of the time this will be used for planetary viewing. I thought I was beginning to understand how the FL to FOV relationship effected the view at the eyepiece however, after using a eyepiece simulator now  I'm not so sure. The way I thought I understood it any given eyepiece of the same FL would produce the same size image and the FOV determined how much dark space was around the image. Also, again the way I was thinking about it, a larger FOV would have a larger sweet spot. 

 

Using the eyepiece simulator what I found was as the FOV increased the image got smaller. 

For Example:

The image in a 8mm eyepiece at 228.5 magnification with a 100 degree FOV looked very small

The image in a 8mm eyepiece at 228.5 magnification with a 72 degree FOV looked bigger than the 72 FOV

The image in a 8mm eyepiece at 228.5 magnification with a 50 degree FOV looked looked the biggest

All 3 eyepieces had the same magnification yet the size of the images were all different.

 

I'm wondering if the eyepiece simulator is reducing the image when a larger FOV eyepiece is used in order to accurately show the amount of black space around the image or if the image is actually smaller and I just don't understand how eyepieces work. 

 

Questions:

Does the FOV effect the physical size of the object projected in the eyepiece i.e. does the physical size of an image decrease as the FOV increases?

 

[Tangerman]

At equal magnifications, the objects will look the same size in a telescope. The simulators tell you absolutely nothing about magnification. They give you the field of view. You can zoom in on a screen all you want to make an object bigger on the screen, and that's what you're doing with the small field of view, especially if you're using the circular boundaries as your screen border: the smaller that field of view, the more you zoom your screen, which has nothing to do with magnification at a real eyepiece. 

[vtornado]

The way I thought I understood it any given eyepiece of the same FL would produce the same size image and the FOV determined how much dark space was around the image.  --- This is correct.  Under actual viewing some folks think planets look smaller in a wide field eyepiece because of the amount of black around them. 

 

The sweet spot is a function of the f ratio and optic quality.

 

In a dob you may want a wider field eyepiece to allow for more drift time, before bumping.  However the farther off axis the object drifts, the more coma, eyepiece abberations, and other mirror abberations.  Coma can be fixed with a coma corrector.

Widefield can also help with watching the galilean satellites.

[Astrojensen]

 

does the physical size of an image decrease as the FOV increases?

No. 

 

 

I'm wondering if the eyepiece simulator is reducing the image when a larger FOV eyepiece is used in order to accurately show the amount of black space around the image

This is exactly what is happening. The image in an 8 mm eyepiece is the same size, whether the eyepiece has 50° or 100° AFOV. The simulator just does a poor job of showing it.  

 

Think of it this way: The landscape outside your house doesn't change size, if you put in a bigger window in your wall. You can just see a lot more of it. This is the same thing that happens when you increase AFOV.  

 

But there can be an interesting effect: Some people, myself included, subjectively see objects in a large field of view appearing larger, than if they are seen in a small field of view. Why this is, I don't know, but the effect is real. Interestingly, there are also people who see objects in a large field of view as appearing smaller, than if they are seen in a narrow field of view!! 

 

 

Clear skies!

Thomas, Denmark

Edited by Astrojensen, 13 August 2024 - 03:02 PM.

[pjmulka]

Thank you so much! I was so confused there for a minute using that software. Is there a different simulator that will show the actual size of an object through a given eyepiece?

[havasman]

Image simulators have driven many of their users here with confused impressions. I wonder if the sizes of the outer frames your simulator used to present their impressions of the images remains constant. That would account for the distortion that has confused you.

 

The foolproof arithmetic defining true field of view ignores AFOV!

 

TFOV= (eyepiece field stop diameter / scope focal length) x 57.3

Eyepiece field stop diameters can be found here  -  https://www.cloudyni...e-buyers-guide/

[havasman]

Magnification is even more simple:  magnification = scope focal length / eyepiece focal length

 

Again, AFOV is irrelevant.

[Tangerman]

Thank you so much! I was so confused there for a minute using that software. Is there a different simulator that will show the actual size of an object through a given eyepiece?

No, because you can always move closer to a screen and the object will look bigger

[Starman1]

Thank you so much! I was so confused there for a minute using that software. Is there a different simulator that will show the actual size of an object through a given eyepiece?

https://www.stelvisi...cope-simulator/

[pjmulka]

Thanks to Everyone who replied your time and advice is greatly appreciated!
I remember reading about a “safe” rule with regard to magnification, that 250X is kinda the high end of what will be useful in average seeing conditions. I want to get a really nice eyepiece for a planetary viewing I want to go with the highest magnification that will be useable most of the time.

[vtornado]

I would buy a cheap barlow or zoom to figure out how much power you prefer.

 

The best magnification is depenedent upon your personal eyes/brain physiology.

 

As the exit pupil goes under 1mm the image starts getting farily dim.  Some folks prefer bigger/dimmer images some prefer smaller/brighter.

 

barlow/zoom can also be used to assess how much magnification your scope can handle on a "typical" night.  You may also want to consider what eyepiece/maginfication you want to use on those once a year perfect nights of seeing.

Edited by vtornado, 13 August 2024 - 07:24 PM.

[pjmulka]

I have a 13mm Tele vue DeLite and a explore scientific 2x Barlow although technically I don’t think it’s a Barlow I think they call it a doubler I’m sure what the difference is but it’s beefier than a Barlow. I’m looking to upgrade to either a Delos or Ethos but I’m not sure if I want a 10mm or 8mm

[Scott E]

I am looking to buy a new eyepiece for my 10" f/7.2 truss dob build. 95% of the time this will be used for planetary viewing. I thought I was beginning to understand how the FL to FOV relationship effected the view at the eyepiece however, after using a eyepiece simulator now  I'm not so sure. The way I thought I understood it any given eyepiece of the same FL would produce the same size image and the FOV determined how much dark space was around the image. Also, again the way I was thinking about it, a larger FOV would have a larger sweet spot. 

 

Using the eyepiece simulator what I found was as the FOV increased the image got smaller. 

For Example:

The image in a 8mm eyepiece at 228.5 magnification with a 100 degree FOV looked very small

The image in a 8mm eyepiece at 228.5 magnification with a 72 degree FOV looked bigger than the 72 FOV

The image in a 8mm eyepiece at 228.5 magnification with a 50 degree FOV looked looked the biggest

All 3 eyepieces had the same magnification yet the size of the images were all different.

 

I'm wondering if the eyepiece simulator is reducing the image when a larger FOV eyepiece is used in order to accurately show the amount of black space around the image or if the image is actually smaller and I just don't understand how eyepieces work. 

 

Questions:

Does the FOV effect the physical size of the object projected in the eyepiece i.e. does the physical size of an image decrease as the FOV increases?

Starting at the top of your message:

At f/7.2 your lowest useful magnification is restricted by the exit pupil size, so no more than 50mm eyepiece focal length. I recommend a 41mm Panoptic or similar. for maximum field size. You may use it at higher power most of the time, but I guarantee you'll want more field at times.

 

The size of the sweet spot (assuming you're talking about image quality) has nothing to do with FOV per se. It's all about eyepiece designs and how they interact with the telescope type. The more expensive eyepieces tend to have sharpness across the whole visible field (you get what you pay for), however large that may be. But f/7.2 is a little more forgiving, meaning some of the cheaper eyepieces may still give sharp images across a larger field than if the scope were faster.

 

As others have pointed out but put another way, an 8mm eyepiece will magnify whatever object you look at 228 times, period. An object that appears 0.1° across in the sky will look like it's 22.8° in the eyepiece. The AFOV is just a measure of the size of the circle defining the edge of the sky you see in the eyepiece. A larger AFOV reveals more sky at a given magnification, a smaller AFOV hides more sky. And as a practical consideration, especially if your Dob isn't motor driven, again as others have pointed out, a wider AFOV is usually preferable, if you can afford their generally higher price.

 

And although you didn't ask about maximum power, depending on where you live and observe, seeing usually limits your useful magnification, and that limit will vary. But having said that, sometimes even when the seeing isn't great, I see more at very high power, sometimes on planetaries. And on those rare occasions when the seeing IS great, there's almost no practical limit to the useful power. I have a 2-4 zoom. I almost never use it. But when that time comes and there's detail to be seen on Ganymede, well, it's better to have it and not need it than to need it and not have it.

[Starman1]

I have a 13mm Tele Vue Delite and a Explore Scientific 2x Barlow although technically I don’t think it’s a Barlow I think they call it a doubler I’m sure what the difference is but it’s beefier than a Barlow. I’m looking to upgrade to either a Delos or Ethos but I’m not sure if I want a 10mm or 8mm

The Explore Scientific 2X Focal Extender is a telecentric Barlow with 4 lenses.  It's a good one, but not really advised for the 10mm or 8mm Ethos, as their 2" skirts will mean the eyepieces will only be inserted a small amount into the Barlow and the thumbscrew on the Barlow will be hard to access.  If the Barlow is a 2", you don't have that problem, but then you have to be very careful those eyepieces' 1.25" barrels don't hit the Barlow's lens--measure very carefully before inserting the eyepiece into the 2" hole.

There would be no issues with the 10mm or 8mm Delos in either the 1.25" or 2" Barlow.  Your Barlow is more than strong enough to hold the eyepieces.

 

Of course, if not used in the Barlow, there would be no problems with the 10/8 Ethos or 10/8 Delos in the scope.

 

As for whether the 10mm or 8mm would be the better choice, that depends on the focal length of the scope.

In a 1200-1600mm focal length, the jump from 13mm to 10mm is a reasonable change.

In a 500-600mm focal length, a 13mm to 8mm jump is reasonable.

It depends what magnification you're trying to achieve as well.

[Astrojensen]

Thanks to Everyone who replied your time and advice is greatly appreciated!
I remember reading about a “safe” rule with regard to magnification, that 250X is kinda the high end of what will be useful in average seeing conditions. I want to get a really nice eyepiece for a planetary viewing I want to go with the highest magnification that will be useable most of the time.

It would be best to test that statement for your specific location first, before spending a lot of money. My own site rarely allows magnifications that high on the planets. Most of the time, I'm limited to 130x-150x, 200x can be done once in a while, but 250x is quite rare. It would make more sense to first get a high quality eyepiece in the magnification you use the most, then the second most used, and so on. This requires some experimentation and experience. When I had saved up for my first serious eyepiece set, I bought a whole range of orthoscopic eyepieces, from 25mm to 4mm, to be able to experiment with all sorts of different magnifications on all kinds of objects, to get that experience. 

 

Oh, and just because you're limited to, say, 200x on the planets, that doesn't mean it applies to everything else. On nights where I can use 200x on the planets, I can frequently use 400x on double stars, and even more on deep-sky objects. The reason is that these three object classes are observed in completely different ways and react very differently to seeing. 

 

- planets are very bright, but low contrast, and must be observed with the highest resolution part of the eye. They must not be overmagnified, because this lowers contrast and brightness too much. Diffraction effects from the telescope also hurt the image badly.  

 

- double stars are intrinsically very high contrast, single point images. Here we are actually interested in observing the diffraction effect of the telescope, so we can magnify the image as much as we want, until the diffraction image is clearly resolved. 

 

- deep sky objects aren't observed with the high resolution part of the retina, but the part that is most sensitive to light. This part has very poor resolution, though, and loves a large image more than a bright one, so now we want to magnify objects until we can actually resolve them. This naturally also spread out their already dim light, making them even fainter, but the larger images can often still be easier to see. It pays off to play with magnifications and try and find the balance between size and brightness. Often, very surprising results can be had. 

 

 

Clear skies!

Thomas, Denmark

[pjmulka]

I pulled the trigger on a 12mm TV Delos. I’ll update this post after I get a look through it. I’m still looking at getting a 10mm Ethos as well.
Thanks to everyone for your time and advice!

Clear skies,
Pete.

[pjmulka]

Delos came today, can’t wait to try it out. I also may have pulled the trigger on an Ethos as well, and my maybe I mean it’s supposed be here Tuesday! 

[pjmulka]

Thoughts on first light with the 12mm Delos expectations vs. reality. Typically my go to eyepiece is my TV 13mm DeLite which until now has been my only high quality eyepiece and thus all I have to compare the Delos to.
The first thing that struck me was how crisp and sharp the Delos is, not that the DeLite isn’t sharp and crisp but the Delos certainly more so.
Secondly, and this really showcases how little I understand about how eyepieces work, has to do with what I call the sweet spot. By “sweet spot” I’m referring to how easy, or not, it is to find the image in the eyepiece. In my mind I’ve always equated FOV with that sweet spot thinking that as the FOV increased the comfort of viewing would increase also, ie less image blinking easier to fine the image. Apparently that’s not the case as the Delos is actually more difficult to use, for me at least.
The 10mm Ethos arrived this afternoon just as I was leaving for work I’m hoping for a break in the clouds tonight to check it out.
Clear skies,
Pete.

[davidgmd]

Pete, you're not alone in finding the Delos a bit more demanding when it comes to eye placement to maintain the exit pupil. It was a fairly short learning curve before I became comfortable with it. A few find it more difficult or not worth the effort.

[Starman1]

Thoughts on first light with the 12mm Delos expectations vs. reality. Typically my go to eyepiece is my TV 13mm DeLite which until now has been my only high quality eyepiece and thus all I have to compare the Delos to.
The first thing that struck me was how crisp and sharp the Delos is, not that the DeLite isn’t sharp and crisp but the Delos certainly more so.
Secondly, and this really showcases how little I understand about how eyepieces work, has to do with what I call the sweet spot. By “sweet spot” I’m referring to how easy, or not, it is to find the image in the eyepiece. In my mind I’ve always equated FOV with that sweet spot thinking that as the FOV increased the comfort of viewing would increase also, ie less image blinking easier to fine the image. Apparently that’s not the case as the Delos is actually more difficult to use, for me at least.
The 10mm Ethos arrived this afternoon just as I was leaving for work I’m hoping for a break in the clouds tonight to check it out.
Clear skies,
Pete.

It takes a bit of trial and error on the eyecup height on the Delos until you discover the position that's "just right".

[pjmulka]

So what is the determining factor in the ease or lack there of, which makes an eyepiece more comfortable / easy to use?

[Starman1]

So what is the determining factor in the ease or lack there of, which makes an eyepiece more comfortable / easy to use?

Comfortable eye relief, especially effective eye relief.
Absence of spherical aberration of the exit pupil.
A mechanical shape that comfortably fits the eye socket.

[PKDfan]

Comfortable eye relief, especially effective eye relief.
Absence of spherical aberration of the exit pupil.
A mechanical shape that comfortably fits the eye socket.

+ 1 !! And i engage instantly within the observed field after heeding Dons points and will add that the better the overall image is is the quicker i enter and stay engaged within the eyepiece frame of view but eyerelief seems to be the decider between two same focal lengths- the longer the relief the better it is.

Unless it gets Too long.

High power needs also dictate choices for the size of that frame you wish to enclose it in and will mean something different to each person.


Goldilocks law applies here.





CS
Lance

[SeattleScott]

The Delite and Delos both have 20mm ER but yeah with long ER, if you don't wear glasses, you will typically want to raise the eyecup. 

 

I feel like in general, even with 20mm ER eyepieces, narrower AFOV is perhaps a little easier to use. Some people have had a learning curve for Delos or XW. I find the LHD a little tricky in this regard. But no one seems to complain about 65 AFOV LVWs or 62 AFOV Delites.

[pjmulka]

I have a 13mm DeLite that took a bit of getting used to. Now I don’t even think about it my eye just goes there. I’m hoping for a similar experience with the Delos and look forward to trying out my new Ethos tonight, weather permitting of course.