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Wide angle eyepieces question


Goose0211
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I got my binoculars out last night for the first time in a while and it reminded me how great they are for general wide angled viewing.  If I look at eyepieces for my telescope to give a similar field of view is it best to look for a higher degree of view or can too high distort things too much?  So for example might an 82 degree 32mm be preferable over a 62 degree 40mm or not as they give a similar field of view?

I realise there are other factors but as a general rule?

Edited by Goose0211
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What scope do you have? Cheaper wise angle eyepieces can give distortions in the outer parts of the field (such as astigmatism), particularly when used with fast eg short focal ratio scopes.

Replicating a binocular field with a scope isn’t always cheap or easy; I use a Televue Genesis with a 31mm Nagler to give me a 5 degree field which is flat and fairly free from distortions.

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Since it has a 1.25" focuser, you're never going to be able to go wider than about 2.2 degrees when using a 32mm Plossl or similar.  To use either an 82 degree 32mm or a 62 degree 40mm would require a 2" focuser.

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6 hours ago, Goose0211 said:

So for example might an 82 degree 32mm be preferable over a 62 degree 40mm or not as they give a similar field of view?

As Louis says, you are limited to either 24mm 68 degree, 32mm 50 degree or 40mm 42 ish degrees. Depending on your skies, the 24 or 32mm are likely to give best views; the 24mm will give a smaller exit pupil and will darken the background a little if you have any light pollution.

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7 minutes ago, Goose0211 said:

ok thanks both - I hadnt realised the limitation with the 1.25. - will do some more rersearch.

It’s all to do with the maximum field stop diameter that you can fit inside a 1.25” barrel. You can’t have a 32mm 68 degree, 50 degrees afov is the max you can have at 32mm without going up to a 2”. In the large size you have eyepieces like the ES 30mm 82 degree which make use of the larger barrel to give a wider field.

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If you haven't already made the distinction, it's probably the true field of view (TFOV) that you are finding impressive with the binoculars - that is, how much of the actual sky they can show you at once. The apparent field of view (AFOV) instead describes how large that view appears to your eyes in angular terms. The AFOV is a property of the eyepiece alone and is quoted by the manufacturer (somewhere between ~40° and ~120°). The TFOV depends on both the eyepiece and the telescope it's being used in - specifically, on the focal length of the telescope and the field stop diameter of the eyepiece.

I also have the 150i and asked myself the same question about maximum TFOV. As Stu says, the maximum field stop diameter that will fit into the eyepieces we can use is (a little under) 1.25". And as Louis says, for our 750mm scopes that gives a maximum TFOV of around 2.2° of sky. Different eyepiece models will present that 2.2° to the eye at different apparent angular sizes, but it's only ever 2.2° max of actual sky. By comparison, standard binoculars might have TFOV of 4° or 5°, and widefield binos will be more. So yes, you can widen the view in a particular scope with a suitable eyepiece, and you can do more with a different scope, but you won't match the breadth of your binoculars - which is one reason why we use both.

1 hour ago, Goose0211 said:

will do some more rersearch

I'm guessing that if you have the 150i then, like me, you might not be wanting to spend hundreds to max out your TFOV. For example the ES 24mm/68° is very good, but it's into three figures, new.

As mentioned above, a 32mm plossl is a cost-effective way of achieving the maximum. This is a decent budget model.
Alternatives for a little more are the Vixen NPL 30mm (another plossl) and the BST Starguider 25mm.  They will both give you around 2° TFOV but, as Stu mentions, you may benefit in terms of contrast from the reduced exit pupil that comes with shorter focal length.

This might also be of use:

https://stargazerslounge.com/topic/377846-25-mm-bst-or-something-else/

 

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Maximum field of view with 1.25" eyepieces is 27/750 x 57.2958 = 2.06°, not the 2.2° people have mentioned.

That is still quite wide and good for the Beehive cluster, the Pleiades, etc.

Your binoculars will have a larger maximum field, but at a much lower power.  They won't have the resolution or light grasp of the telescope,

but they will have a wider true field.

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41 minutes ago, Don Pensack said:

Maximum field of view with 1.25" eyepieces is 27/750 x 57.2958 = 2.06°, not the 2.2° people have mentioned.

Sorry, I just did a quick calculation using the typical 52 degree AFOV of a 32mm Plossl assuming no distortion to get to 2.2°.  However, this is a minor quibble because neither value comes close to matching the typical 5° to 10° TFOV of handheld, 6x to 10x binoculars which the OP was trying to match, assuming TFOV and not AFOV was meant by the OP's phrase "they give a similar field of view".

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23 hours ago, Louis D said:

Sorry, I just did a quick calculation using the typical 52 degree AFOV of a 32mm Plossl assuming no distortion to get to 2.2°.  However, this is a minor quibble because neither value comes close to matching the typical 5° to 10° TFOV of handheld, 6x to 10x binoculars which the OP was trying to match, assuming TFOV and not AFOV was meant by the OP's phrase "they give a similar field of view".

Louis,

1) No 32mm Plössl has a 52° field--not one.  The average is somewhare in the 45-49.5° range, the latter with a bit more RD.  I've measured several different ones using the flashlight test.

I have seen 52° in shorter focal lengths, though, but a typical range of Plössls will vary from 47-52° across the series.

2) 27.0mm is the average field stop of a full-field 32mm Plössl, which, on a 750mm focal length, yields 2.06°.

 

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1 hour ago, Don Pensack said:

Louis,

1) No 32mm Plössl has a 52° field--not one.  The average is somewhare in the 45-49.5° range, the latter with a bit more RD.  I've measured several different ones using the flashlight test.

I have seen 52° in shorter focal lengths, though, but a typical range of Plössls will vary from 47-52° across the series.

2) 27.0mm is the average field stop of a full-field 32mm Plössl, which, on a 750mm focal length, yields 2.06°.

 

Great, but I think you're still quibbling over details not particularly relevant to the OP's original question.

I have measured my 32mm Orion Sirius and GSO Super Plossls to have 52° AFOVs each by the flashlight method and 51° and 50° AFOVs by the photographic method, and 49° eAFOV based on measured 27.1mm field stop.  Again, my measurements might be off by about 0.1mm.

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On 06/10/2021 at 13:34, Don Pensack said:

Hmm.  I measured those two eyepieces and got 49.8° on the Sirius and 49.5° on the GSO.

My GSO also had a field stop <27mm, so it's possible they're changed the eyepiece over the years.  That was ~2010.

I just measured the Sirius and GSO 32mm Plossl field stops directly using digital calipers after unscrewing each lower barrel to get clear access them.  I got 27.2mm for the former and 27.1mm for the latter.

I then did a comparative photographic analysis of each to the 27mm Panoptic accepting TV's 30.5mm FS diameter as gospel and arrived at 27.1mm for each.

At no point was there a doubt that they have at least a 27mm diameter field stop diameter, the only quibble would be about that last 0.1mm.

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9 hours ago, Louis D said:

I just measured the Sirius and GSO 32mm Plossl field stops directly using digital calipers after unscrewing each lower barrel to get clear access them.  I got 27.2mm for the former and 27.1mm for the latter.

I then did a comparative photographic analysis of each to the 27mm Panoptic accepting TV's 30.5mm FS diameter as gospel and arrived at 27.1mm for each.

At no point was there a doubt that they have at least a 27mm diameter field stop diameter, the only quibble would be about that last 0.1mm.

Good.  Then they have changed over the years.  That is compatible with a 50° field and edge distortion of 3%.

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