FOV Why so many increments in mm i DONT get it

[icebergahed]

So i found a telescope FOV simulator online and i thought id put in 3 of the eyepieces i have. 21mm 68deg and a 8.8mm 82 deg and 32mm ?deg . looking at M13. 

Now this is the bit i dont understand. With 24mm in between them what is the need of getting anything between.

I would think you would need 1 high power for planets and 1 ultra wide most of everything else

Doesn't seem much difference between 21 and 32

Maybe i have all i need and invest in other gear?

?????Help me understand????

[Knighty2112]

Your right, you don’t need to have anything in between if you don’t want. The difference is negligible for the FOV. To be honest you could probably live with just three EP’s; high power, mid range and a wide field would suffice if that is all you need.

[NGC 1502]

Eyepiece choice is a very big subject, and there’s a huge range of opinion out there.

If I let you know my own choices, it’s not likely to help all that much. Many of us own several telescopes, each with varying capability, so that confuses the issue even more.......

Having said that, many would say that with a given telescope, you don’t need more than 3 or 4 eyepieces. I tried to make do with just two, but found that didn’t work very well and over time I need at least 3, a low power for finding objects and to view large ones, medium and high power. So for my 10” reflector, low power is 44x, medium 92x, high 171x. That covers 90% of my observing time. As I said, that suits me, but other folk may  well find it doesn’t suit them.

One point I have found to be valid, I’d far sooner have a small number of top eyepieces than a large number of inexpensive ones.

See what others say, Ed.

[Alan White]

You might find that you do ned a step between the 21mm and 8.8mm on nights of poor seeing.
The 8.8 at 170x magnification may be overdoing it at times and the 21 is a big step back.

The exit pupil at under 1mm on the 8.8 may also show floaters within your observing eye, depends upon your eye health and age.

I must add a comment as ED posted a moment before I did as I way typing.
Many on here are eyepiece collectors as much as users, so only 3-4? really, why you could have loads of decent ones and an empty pocket 

Al Nagler is reported to have said you only need 3, 5mm, 3mm and 1mm Exit pupils.
How true that is I do not know as I have only read it as he says type posts......

[Timebandit]

32 minutes ago, icebergahed said:

   

I would think you would need 1 high power for planets and 1 ultra wide most of everything else

 

Maybe i have all i need and invest in other gear?

?????Help me understand????

 

 

 

A lot IMO depends on what scope, or scopes you have and focal length,ratio ect can effect what eyepiece or eyepieces you want ,need, or prefer.

If you take those with fast nd big scopes like the Dob Mob boys then they mostly prefer 100d eyepieces as they specialists in DSO. And they mostly have three 100d eyepieces like Ethos and a powermate. They feel this is all they need to cover there DSO observing. That is there best choice in DSO for there fast big scopes to get as much as the object in views as possible.

If you take a planetary observer,then fov to most is not the main factor, as a planet is not spread over a large fov area like numerous DSO. A planetary observer is more likely to have something like a Ortho ,plossl or something in the 70d fov. But they will probably have numerous eyepiece, such as 3mm ,4 ,5 ,6 ,7 ,8,mm to deal with viewing and seeing conditions,to get the sharpest image possible for that particular conditions or planetary target.

Personally the most used eyepieces i have is 3. 5mm ,4 ,5 ,6, 7 ,8 ,9, and 10 mm at 40d and 70d  fov for mostly planetary and lunar, some DSO . And for the larger DSO I have 20mm and 28mm in 82d fov

 

Horse's for courses I suppose on eyepiece choice? 

 

 

 

 

 

[mikeDnight]

Although there may not appear to be much difference in true field to warrant buying numerous long focal length eyepieces, there is a notable change in contrast between a 21mm and a 32mm eyepiece, which is brought about by the darker sky background in the 21mm. It is true though that you won't need many eyepieces in the longer focal length range, but you'll probably need several short focal length eyepieces  so as to fine tune the higher powers to the seeing conditions.

[alanjgreen]

As said above. The problem is higher powers can perform badly under bad atmospheric conditions.

This means we end up having to cover several bases at the high power end as we try to get the magnification as high as needed to frame the object nicely in the FOV with no loss of clarity due to over magnification.

Or if we are really lucky, the conditions may be so good (rare) that there is an opportunity to use more power than normal. So we may have an EP for that too.

There is no happy set of EPs for all scopes. It's only by using your scope with your EPs on your favourite targets that you get the feeling that you want to get closer or you are too close! Then you may also be forced back as the view is suddenly worse than it was before (changing conditions).

Looking at your FOV chart, there is a big gap between x70 and x170, I would want to put an EP in there.

As also said above, the more you increase magnification, the blacker the background will become. This can enhance some objects but eventually they will become too dim.

FYI, Your plossl will be 50 degree FOV by the way.

Alan

[icebergahed]

5 minutes ago, alanjgreen said:

As said above. The problem is higher powers can perform badly under bad atmospheric conditions.

This means we end up having to cover several bases at the high power end.

Or if we are really lucky, the conditions may be so good (rare) that there is an opportunity to use more power than normal. So we may have an EP for that too.

There is no happy set of EPS for all scopes. It's only by using your scope with your EPs on your favourite targets that you get the feeling you want to get closer or you are too close. Then you may be forced back as the view is worse than it was,

Looking at your FOV chart, there is a big gap between x70 and x170, I would want to put an EP in there.

As also said above, the more you increase magnification, the blacker the background will become. This can enhance some objects but eventually they will become too dim.

FYI, Your plossl will be 50 degree FOV by the way.

Alan

Right..... now i can see some logic in regards to high powers and how good or bad seeing is☺ i did also notice the 'blackening' effect in the 8.8 compared to the 21. I did prefer the lighter actually. Although there was detail in the 8.8 that couldnt be picked out with 21 (globular cluster).

I agree the gap between 70 and 170 is big. I think i will get the in between as i was planning to. But thats probably it.

Thanks

 

[alanjgreen]

 

Quote

Doesn't seem much difference between 21 and 32

AG> that's because the fov of the two EPs is not the same. If they were both 68 degree then the 32 would be much larger!

Give it a try...

A larger fov allows you to use more magnification and keep the same amount of sky in the eyepiece.

A Televue example. The 21mm Ethos 100 degree shows almost same sky as 31mm 82 degree Nagler which shows almost same as a 41mm 68 degree Panoptic

Its wonderful to keep the fov but with increasing magnification levels. The target get bigger but the overall immersive spacewalk view is achieved.

If you redo your example but change all EPs to same fov then you will better see the magnification difference to the fov. 

Manufacturers allow us the luxury to pay more to take back the fov that we lost by using more magnification, that's because some objects look better framed if you can also see their setting.

[John]

If you have 3-4 eyepieces covering low / medium low / medlum high and high powers then you are off to a good start. As you observe different objects under different conditions you may well find yourself thinking "well I could have done with something else in there" or  "I'd like to see a wider field of view here" and if that starts happening often you know that you need to think about expanding your eyepiece set a bit. It's only by practical experience that you develop a feel for what works for you, and what does not

 

 

[25585]

Though I do have a Smaug-ish pride in my hoard of eye pieces, particuarly Japanese glass, I feel I'm more akin to fly fishermen who have all the fly baits they might need, even if some are never used.

That aside, my ep buys are usually multiples of 10 and/or the focal length of a given scope. Easy to think of what magnification I am using then.

[Stu]

If you only have one scope, or your scopes are all similar focal ratios then it is easier to keep to a smaller set as they give similar results in each scope. For instance if I have a 60mm Tak and a Skyliner 200p they might seem like they would need very different eyepieces, but because their focal ratios are nearly the same (f5.9/6), eyepieces will give the same exit pupil in both scopes, so a similar range is useful in each scope. For instance, a 30mm 82 degree eyepiece will give a 5mm exit pupil which is a good low power for most people under reasonable conditions. The field of view and mag will be different in each case but that is less important. At the other end of the scale, a 3mm eyepiece will give a 0.5mm exit pupil giving around top end mag in both scopes, useable under very good conditions perhaps although probably too extreme for the 200p except in the moon. That's why the dob mob can get away with only two or three eyepieces.

If you have two scopes of very different focal ratios then more eyepieces are needed to cover the same bases. I used to have an f20 Mak, and an f5 refractor. To get high power in the refractor you needed something like a 2.5mm eyepiece to give you x200 with a 0.5mm exit pupil. Put this eyepiece in the Mak and you get x1600 mag (!) and a rather dim view!  

I choose eyepieces for both rational and irrational reasons, but I will stick to the rational here. For me it is a combination of the three factors they affect when in the scope i.e. Magnification, field of view and exit pupil.

I used to dismiss Exit Pupil as just being the same as magnification, but it is not. It has a real meaning in terms of what you see at the eyepiece. Too small and you will get a dim image with floaters showing up in the view (normally trying too high power in a smallish scope), too large and the view is likely to be washed out, certainly if you have any LP around. Get it just right (at around 2mm) and it can really help with the contrast you see on deep sky objects, galaxies etc

In your collection, something around the x100/120 mark would be useful on globs for example as they may be too dim at x171 in your scope. Anything more than this would depend upon how bothered about planets you are. If you are, then stacking in higher power eyepieces between x150 and x220 ish might be useful to make the most of the seeing conditions. Or get a zoom eyepiece to cover these ranges which makes it very easy to dial in the mag you need.

Apologies for rambling....

[icebergahed]

3 hours ago, Stu said:

Apologies for rambling

Thats fine the more i can get into this sponge people call a brain the better?

[Lowjiber]

15 hours ago, mikeDnight said:

Although there may not appear to be much difference in true field to warrant buying numerous long focal length eyepieces, there is a notable change in contrast between a 21mm and a 32mm eyepiece, which is brought about by the darker sky background in the 21mm. It is true though that you won't need many eyepieces in the longer focal length range, but you'll probably need several short focal length eyepieces  so as to fine tune the higher powers to the seeing conditions.

That pretty much sums it up, especially when galaxy hunting.  Because their surface brightness can have a wide range of variance, a small change in eyepiece length can bring out details.  However, too much of a change can make the details worse (dimmer).  As magnification increases, surface brightness decreases.

Clear, Dark Skies

[jabeoo1]

16 hours ago, mikeDnight said:

It is true though that you won't need many eyepieces in the longer focal length range, but you'll probably need several short focal length eyepieces  so as to fine tune the higher powers to the seeing conditions.

 Hence the popular 3-6 mm Zoom & discontinued 2-4mm version. 

[niallk]

My main workhorse options 31-17-10-6mm (100° afov, except for the 31mm which is 82°).  These give mags of approx 65x/115x/200x/330x and exit pupils of 6mm/3.3mm/1.9mm/1.1mm.  I do see annoying floaters in my eye if I go towards 0.5mm

I like to observe all sorts of targets, and have other combinations available with a 2x PM. I also decided to slip in an 8E that I got for a good price between the 10mm and 6mm for when seeing is not quite as good. 

[25585]

With a short fl, a long fl ep can make a finder unecessary. Thus my 500mm F5 TV frac uses a 55mm to give 9x magnification and around 5 deg TFOV. Its the only scope I can do that with, and makes short fl /rich field scopes very handy for terrestrial use as well.

Lottery win etc = TV NP127!

 

 

[John]

With my 12" dobsonian, which is F/5.3 so 1590mm focal length, most of my observing is done with a set of four 100 degree eyepieces of 21mm, 13mm, 8mm and 6mm. These deliver 76x, 122x, 199x and 265x and show true fields from 1.32 degrees down to .38 degrees. I do have a 31mm / 82 degree eyepiece which shows a touch more sky at 51x and some higher powers for certain tasks but the 4 x 100's are by far the mainstays.

 

[25585]

When I first started astronomy, 30 years ago, my ep focal lengths were; 32, 20 (both Erfle), 13mm Nagler T1 and a 2x Barlow for use with the 20, so effectively having a 10mm also. Later I bought a 10mm Vixen LV.

Those focal lengths give an adequate range of magnifications. The Nagler whetted my appetite for wider field of view at higher magnification, but had too poor effective eye relief for me eventually. I would love an 80+ degrees  AFOV low teen ep with 20mm or more effective eye relief. 

 

 

[Alien 13]

I like the idea of one decent eyepiece and perhaps five or six scopes 

Alan

[Stub Mandrel]

Be honest.

It's a man thing.

It's like asking why you have more than one of anything.

The collecting of anything only becomes meaningful when you can discriminate between them.

Seeing the difference between those EPs becomes part of the hobby; I suspect some folk get as much pleasure from comparing the views of an object as they do from the seeing the object itself.

No different to me photographing the same object at multiple image scales.

 

 

[Louis D]

It's like anyone who has a hobby and starts collecting various examples for their subtle differences.  Think about musicians with multiple instruments, motorcyclists with multiple bikes, target shooters with multiple pistols/rifles, equestrians with multiple horses, woodworkers with multiples sets of hand tools, etc.  I enjoy comparing multiple eyepieces in multiple scopes to see how they present the same objects differently.  The objects in the sky aren't changing much over the years, so I change up how I look at them.

[laowhoo]

We found two approaches when beginning, one according to exit pupil (Mel Bartels favors this) and one according to optimum detection thresholds for DSOs based on Blackwell's studies for the military (Clark). These actually coincide pretty closely and Bartels and Clark worked together on the question of how to see the most. So we went w/ a 1.5x Barlow (recommended by Clark and wh/ is in keeping w/ EP mfrs increments in their various series) and for our 12.5" it went in about 50x steps. Interestingly enough, Clark's own book on the matter includes some 65 DSOs with his drawings and notes, and 90% of these were at 188x. Most of these included another EP. Bartels says look to O'Meara's example of the Herschel 400 (3 EPs). We splurged with some low and high that we don't use often, but when conditions are right...And one novelty EP (Edmund RKE 28) and one specialty EP (Ortho). Otherwise, three EPs with a 1.5x Barlow give us our 4mm exit pupil starting point at 79x (for most conditions) and carry us through the 250x range in 50x increments. But strictly limiting it to only 3 you'll be wanting fine tuning in the range, or to just pump up the mags to really test your eyes and tease out every bit of detail the changing conditions will allow. Framing comes into it to, unless maybe you can afford 100* EPs, so that otherwise, and even where higher mags don't yield more detail, the different mags can also give better framing w/ changing FOV. If you travel w/ the scope to different sites, LP becomes more of a factor too. Anyhoo, a decent simple 2-element Barlow goes a long way and can even improve eye relief. More abstractly, I want the scope to have all the gears it can use.

[25585]

Eye relief is my #1 consideration, so ordinary Barlows are an asset. But then wide field as using a Barlow will cut that down. And optical performance of both optics as any issues will be magnified.

So disregarding price, physical size and weight, what of all makes and designs of EPs and ordinary Barlows, is the supreme combination giving effective longer eye relief (at least 22mm) to an effective short focal length (13mm max), but still having a large AFOV (over 70 deg), an optical combination that would surpass any manufactured prime EP?

 

[Mr Spock]

With my C9.25, 2350mm focal length, I have for widefield 42mm and 22mm for a bit closer. For planetary I have 10, 9, 8, 7, 6 - that means I can get the most out of the conditions and/or have different magnifications for different targets. For example, I'll use the 10mm on Jupiter, the 8mm on the moon and 7/6mm on double stars.