32 replies to this topic

[Atlanta AstroView 90mm]

I know there are many articles on cloudy nights about this. I have read a bunch but calculating and figuring this out has been very confusing.

 

First off, my refractor: 90mm aperture, 910 focal length, f/10. Currently I have a 1.25” diagonal but my new focuser allows me to have a 2” diagonal to use 2” eyepieces. Now, here is my question:

 

what is the widest view I can get with an eyepiece for 1.25” and 2” eyepieces based on my refractor?

 

I see a lot of 68°, 82°, and 100° style eyepieces, but I know that with my refractor, some of those eyepieces wouldn’t be utilizing the whole degree of view. So what’s the maximum degree an eyepiece can be for 1.25” on my refractor? What’s the Max it can be with 2” eyepieces? I was thinking of getting an 82° eyepiece but want to make sure I’m not losing out on some degrees because I’m limited by the refractor/aperature/focal length size. Can anybody help clarify this for me?

 

Thanks

[jimhoward999]

If the apparent FOV, in radians, times the eyepiece focal length is less than the barrel diameter (1.25" or 2") with a little margin, which it will be, then your telescope wont limit the AFOV of the eyepiece.

 

Said another way, if you divide the barrel diameter by the eyepiece focal length, that, in radians is slightly more than the largest possible AFOV.

[Atlanta AstroView 90mm]

If the apparent FOV, in radians, times the eyepiece focal length is less than the barrel diameter (1.25" or 2") with a little margin, which it will be, then your telescope wont limit the AFOV of the eyepiece.

 

Said another way, if you divide the barrel diameter by the eyepiece focal length, that, in radians is slightly more than the largest possible AFOV.

Im sorry this just confused me more. Can you give me an example based on a 1.25” barrel and a 32mm focal length eyepiece? I think I’m confused by radians haha thanks

Edited by Atlanta AstroView 90mm, 19 January 2021 - 09:54 AM.

[Glen10]

For your scope~Very roughly & approximately 1.75 deg max for 1.25" ep & 3 deg max for 2" ep.

 

......Probably a wee bit less in practice.

[SloMoe]

the widest you''ll get at f/10 in 1.25" is a 68° 24mm, at 38X and 1.79° fov

 

and in 2" 82° 31mm at 29X and 2.79° fov

 

Here's the big difference, the 24mm 1.25" is the size of a plumb and weighs about 8oz.

 

The 2" 31mm is the size of a Naval Orange will weigh 35oz.

 

I use this online calculator to figure most of my eyepiece choices with.

 

Just fill in three fields, the scopes focal length, the eyepiece focal length and the fov in degrees of the eyepiece.

http://www.csgnetwork.com/telefov.html

 

then I go to one of the vendors sites to check eye piece specs.& weights, some of the 2" wide fields are larger than your scope,,,,,,

[Atlanta AstroView 90mm]

the widest you''ll get at f/10 in 1.25" is a 68° 24mm, at 38X and 1.79° fov

 

and in 2" 82° 31mm at 29X and 2.79° fov

 

Here's the big difference, the 24mm 1.25" is the size of a plumb and weighs about 8oz.

 

The 2" 31mm is the size of a Naval Orange will weigh 35oz.

 

I use this online calculator to figure most of my eyepiece choices with.

 

Just fill in three fields, the scopes focal length, the eyepiece focal length and the fov in degrees of the eyepiece.

http://www.csgnetwork.com/telefov.html

 

then I go to one of the vendors sites to check eye piece specs.& weights, some of the 2" wide fields are larger than your scope,,,,,,

This is perfect. Thank you so much for this info!!! You have helped me so many times now, Moe. I should just come directly to you next time I have a question haha. Thank you

[Atlanta AstroView 90mm]

So as long as I don’t exceed 1.79° for TFOV With this refractor in a 1.25” EP, I will be good? With those calculations, and correct me if I am wrong, but theoretically a 19mm 82° eyepiece will still work with this scope, eh? Or a 16mm 100° would work too? At which point will the degree field be affected by the limitations of my scope’s aperature/focal length?

[SloMoe]

Your talking about fair sized eyepieces, if any I would recommend something in the 2" 18mm 82°, little bit smaller fov but better magnification.

 

BTW, I'm not the only guy here that is trying to help, there are many here and it would be wise to wait for others to chime in, then reason out what you think will work and do some research on reviews online of the different eyepieces you're interested in.

Edited by SloMoe, 19 January 2021 - 11:09 AM.

[Atlanta AstroView 90mm]

Your talking about fair sized eyepieces, if any I would recommend something in the 2" 18mm 82°, little bit smaller fov but better magnification.

 

BTW, I'm not the only guy here that is trying to help, there are many here and it would be wise to wait for others to chime in, then reason out what you think will work and do some research on reviews online of the different eyepieces you're interested in.

I agree. I will definitely wait for others to chime in. I’m very thankful for this site. So many knowledgeable people who like to help n00bs like me.

[SloMoe]

And one of the ways I learned enough about glass was to ask a question, search for the eyepieces, be it on vendors sites or in the classifieds for the focal length and/or fov of the eyepiece, google reviews about it, hit the online calculator to find any benefit of getting it.

 

Soon enough you'll know what you want, then mortgage your house and buy it.

[Atlanta AstroView 90mm]

And one of the ways I learned enough about glass was to ask a question, search for the eyepieces, be it on vendors sites or in the classifieds for the focal length and/or fov of the eyepiece, google reviews about it, hit the online calculator to find any benefit of getting it.

 

Soon enough you'll know what you want, then mortgage your house and buy it.

The calculator is what’s been missing for me. This is a huge help. I’ve done so much research on eyepieces over the last month that my mind is swimming! Haha I read a lot of reviews about different ones on this forum. Such great information here. Looking at the specs from manufacturers has been very helpful. And I’m sure it will be even more that I have this calculator.

[f74265a]

If you put your telescope specs into the online tele vue eyepiece calculator, it suggests that your scope maxes tfov at 1.7 degrees for 1.25 eyepieces and 2.9 degrees for 2 inch eyepieces

[SteveG]

The calculator is what’s been missing for me. This is a huge help. I’ve done so much research on eyepieces over the last month that my mind is swimming! Haha I read a lot of reviews about different ones on this forum. Such great information here. Looking at the specs from manufacturers has been very helpful. And I’m sure it will be even more that I have this calculator.

What is you budget? Have you purchased a 2" diagonal?

[Atlanta AstroView 90mm]

What is you budget? Have you purchased a 2" diagonal?

I have not purchased a 2” diagonal yet. My focuser allows for it. I’m not necessarily in the market for the eye pieces I’m talking about was just trying to get a grasp on which focal lengths at which AFOV are the Max for this scope so that when I am ready to purchase I am educated. Eventually I will upgrade to a few 2” eyepieces. For now I’ll probably stick to 1.25” styles

[Dave Mitsky]

A 41mm Panoptic or a 55mm Plössl both have 46mm field stops and will produce a true field of view of about 2.9 degrees in a 910mm focal length refractor.  There are far cheaper 2" 70-degree five-lens-element designs in the 40mm range that will do the same and will work well enough at f/10.

[aeajr]

Typically, in a 1.25" diagonal/focuser, a 32 mm/50 degree AFOV Plossl will give you about the widest field of view. Some 40 mm Plossls go slightly wider but I usually recommend the 32.  I have this for all of my 1.25" scopes. 

 

If you are going 2", then your budget will be a BIG factor.   A low power wide view for that scope could be $75 or $500.   

 

This article may be helpful - understanding eyepieces

https://telescopicwa...cope-eyepieces/

 

A simplified formula for field of view is:

 

FL scope / FL eyepiece = power or magnification

 

AFOV eyepiece / Mag of eyepiece = field of view -

 

This provides a close approximation using simplified terms.   Close enough for my purposes. 

Edited by aeajr, 19 January 2021 - 03:33 PM.

[Dave Mitsky]

The 42mm GSO has a 65-degree AFOV and comes close to producing a maximum TFOV.

https://agenaastro.c...w-eyepiece.html

The 38mm Agena SWA has a 70-degree AFOV and yields a bit smaller TFOV.

https://agenaastro.c...a-eyepiece.html

Not too long ago there were a number of 40mm 70-degree eyepieces on the market. The 40mm Pentax XW is one that may still be available but like many other items of astronomy gear it's currently backordered. The 40mm Pentax XW has far better edge correction than the less costly 2" models, which is important for faster telescopes, but is also far more expensive. It does cost less than a 41mm Panoptic, however.

https://www.highpoin...-eyepiece-70538

[jimhoward999]

Im sorry this just confused me more. Can you give me an example based on a 1.25” barrel and a 32mm focal length eyepiece? I think I’m confused by radians haha thanks

Sure.  if you have a 1.25" (31.75mm) barrel and a 32 mm eyepiece then the biggest AFOV you could get would be a bit less than 31.75/32=0.99 radians = 56.8°....call it 50°.   That is why you wont see a super wide field 32mm 1.25" eyepiece. 

 

If you are asking what is the biggest true FOV you could potentially get (FOV in object space) then that is a bit less than the (barrel diameter)/scope EFL.   So with a 910mm EFL  your max true FOVs is 31.75/910 = 0.035 radians = 2° with a 1.25" eyepiece and 50.8/910=3.2° with 2" eyepieces.  You cant reach these maximums because the eyepiece field stop must be smaller than its barrel diameter; so take a little off these figures. 

 

Sorry I might be confused too, wasn't sure if you were asking about AFOV or TFOV.

[Atlanta AstroView 90mm]

Sure.  if you have a 1.25" (31.75mm) barrel and a 32 mm eyepiece then the biggest AFOV you could get would be a bit less than 31.75/32=0.99 radians = 56.8°....call it 50°.   That is why you wont see a super wide field 32mm 1.25" eyepiece. 

 

If you are asking what is the biggest true FOV you could potentially get (FOV in object space) then that is a bit less than the (barrel diameter)/scope EFL.   So with a 910mm EFL  your max true FOVs is 31.75/910 = 0.035 radians = 2° with a 1.25" eyepiece and 50.8/910=3.2° with 2" eyepieces.  You cant reach these maximums because the eyepiece field stop must be smaller than its barrel diameter; so take a little off these figures. 

 

Sorry I might be confused too, wasn't sure if you were asking about AFOV or TFOV.

This makes total sense!! I was dividing by inches like a dummy. Thank you for clarifying!!

[Redbetter]

The formula is simple:  

 

TFOV = 57.3 * Field stop diameter / telescope focal length.

 

As Dave said the widest field 2" eyepieces will be th 41 Pan or 55 Plossl (or rough equivalents.)  These have 46mm field stops.

 

TFOV = 57.3 * 46 / 910 = 2.90 degrees. 

[havasman]

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

[Jon Isaacs]

Sure.  if you have a 1.25" (31.75mm) barrel and a 32 mm eyepiece then the biggest AFOV you could get would be a bit less than 31.75/32=0.99 radians = 56.8°....call it 50°.   That is why you wont see a super wide field 32mm 1.25" eyepiece. 

 

If you are asking what is the biggest true FOV you could potentially get (FOV in object space) then that is a bit less than the (barrel diameter)/scope EFL.   So with a 910mm EFL  your max true FOVs is 31.75/910 = 0.035 radians = 2° with a 1.25" eyepiece and 50.8/910=3.2° with 2" eyepieces.  You cant reach these maximums because the eyepiece field stop must be smaller than its barrel diameter; so take a little off these figures. 

 

Sorry I might be confused too, wasn't sure if you were asking about AFOV or TFOV.

 

You need to use the real numbers.  That would be the inner diameter of the eyepiece barrel.  For 1.25 inch eyepieces, the barrels are normally threaded for filters, that's 28mm.  What actually determines the field of view, is the field stop.  For a 1.25 inch eyepiece, the largest normal field stop (the ring you see at the edge of the field) is about 27.5mm.,  

TFoV = 180 deg/pi x field stop/focal length scope.  

 

For a 1.25 inch eyepiece:

 

TFoV max = 57.3 deg/radian x 27.5mm / 910mm = 1.73 degrees.

 

For a 2 inch eyepiece, the maximum field stop possible is 46mm, in your scope this would provide a 2.90 degree field of view.  

 

Dave Mitsky wrote

 

The 42mm GSO has a 65-degree AFOV and comes close to producing a maximum TFOV.

 

The 38mm Agena SWA has a 70-degree AFOV and yields a bit smaller TFOV.

 

 The 42mm GSO Superview claims a 65 degree AFoV but that is overly optimistic.  The one I owned measured at about 59 degrees.

 

The 38mm Agena 70 degree has a 45.7mm field stop so for all practical purposes, it provides the maximum possible field of view in a 2 inch eyepiece.  The 38m Agena 70 degree seems like it was just made for a scope like this and the 2.9 degree field should be just awesome.

 

Jon

[sg6]

Your scope is f/10 so the aperture is 1/10 the focal length.

The angle from the focal plane center line to the edge of the front lens is therefore 5.7 degrees.

 

Tan^-1(0.2) = 5.7 degrees. So that defines the light that will get through the lens and down the tube.

 

What now occurs is that the eyepiece only pickes up a part of the final image. And it is really that bit that is relevant as far as you field of view is concerned.

 

The "simple" way to determine what you see is determine the magnification: (Scope focal length)/(eyepiece focal length) = Magnification.

 

Then divide the eyepiece fiels - the 50, 52, 62, 68, 82 etc bit - by this magnification.

That result is what you see/get.

 

So say you use a nice easy 12mm Paradigm that has a field of 60 degrees.

Magnification is 900/12 = 75x

Field of view you get is 60/75 = 0.8 degrees.

 

One problem I often suspect is that it is all fine asking here but outside at night in the dark with say a selection of 5 eyepieces you have to be able to make a determination then. So all these calculators etc are basically unavailable to you.

 

Often you will find you need to work backwards: M42 is say 1.25 degrees, scope is 900mm focal length which eyepiece do you drop in?

 

So you know the field you need but which eyepiece does that require?

 

As a quick example and lets change to Plossls of 50 degrees:

1.25 object needs say 2 degrees for comfort/ease.

2 degree field in a 50 degrees EP is 25x.

25x in your scope is 900/25 = 36mm eyepiece. Not available in the plossl design or range at 50 degrees.

So you will have slight difficulty is easily fitting all of M42 in a single view.

[Dave Mitsky]

The 42mm GSO Superview claims a 65 degree AFoV but that is overly optimistic.  The one I owned measured at about 59 degrees.
 

The 38mm Agena 70 degree has a 45.7mm field stop so for all practical purposes, it provides the maximum possible field of view in a 2 inch eyepiece.  The 38m Agena 70 degree seems like it was just made for a scope like this and the 2.9 degree field should be just awesome.

 

I now remember hearing that about the 42mm Superview.

 

I picked up a 38mm Agena SWA for ASH at the 2019 Stellafane swap meet.  It's quite a nice eyepiece for slow telescopes and is used primarily with the club's 17" f/15 classical Cassegrain, where it produces a slightly smaller true field of view of almost 25 arc minutes.

[Jon Isaacs]

I now remember hearing that about the 42mm Superview.

 

I picked up a 38mm Agena SWA for ASH at the 2019 Stellafane swap meet.  It's quite a nice eyepiece for slow telescopes and is used primarily with the club's 17" f/15 classical Cassegrain, where it produces a slightly smaller true field of view of almost 25 arc minutes.

 

Dave:

 

I have owned the 26 mm and 32 mm of the Agena/Orion Q70s.  I've never had a chance to try them in a slow scope but the seem like they'd be very good at F/15, no need for a Panoptic. 

 

At F/7, they're not sharp in most of the field but they are sharp in center and with the right cup half full attitude, they can be enjoyed despite their faults.

 

Jon