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Wider AFOV EP versus shorter tube.

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#1 Ziguy

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Posted 28 January 2020 - 08:55 PM

I'm looking for a telescope but I'm torn away between a long FL (for planet observation) and a short FL (for its wide field of view). I know such a telescope doesn't exist but I'm wondering if it would be possible to compensate a bit the small FOV of a long scope by using very wide EP.

 

Considering those two setups:

 

- 20 mm / 100º AFOV EP on a 1000 mm tube (50x, 2º FOV)

- 20 mm / 50º AFOV on a 500 mm tube (25x, 2º FOV)

 

How the view will differ?


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#2 areyoukiddingme

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Posted 28 January 2020 - 09:38 PM

Maybe you can say some more about the specific scope(s) you have in mind?

 

While it is true that a short focal length is necessary for the widest views, it is not also necessarily true that a short focal length scope can't do very well on planets.

 

Also, you seem to be focused on the lowest power part rather than planetary given your mention of the two eyepieces.

 

I suggest you simulate here: https://astronomy.to.../field_of_view/

 

This will give a picture that frames field of view under different scenarios.

 

In fact, larger aperture will be a much bigger effect than focal length on planetary performance, all else being more or less equal.


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#3 vtornado

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Posted 28 January 2020 - 09:48 PM

Although the true field of view is equivalent, we have the the following issues.

Fast telescopes have a sharply angled light cone which is hard on eyepieces,

I believe it produces astigmatism in the outer parts of the field.

 

In general a well corrected 50-60 degree eyepiece is less expensive than

a well corrected 100 degree eyepiece.  I can buy a top shelf 20mm plossl for $120.

A top shelf 20mm Ethos  will be $850

 

A 20 mm plossl weighs about 3 oz.

A 20mm Ethos weigns 2 lbs (can challenge a light duty mount).

 

In a reflector the faster the scope, the more coma will be introduced.

In a refractor the faster the scope the more field curvature their is.

 

Some folks don't like 100mm AFOV eyepieces the field is too wide.  (I don't have one too expensive)

 

Scopes with short tubes are easier to mount.


Edited by vtornado, 28 January 2020 - 10:08 PM.


#4 Eddgie

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Posted 28 January 2020 - 10:11 PM

I'm looking for a telescope but I'm torn away between a long FL (for planet observation) and a short FL (for its wide field of view). I know such a telescope doesn't exist but I'm wondering if it would be possible to compensate a bit the small FOV of a long scope by using very wide EP.

 

Considering those two setups:

 

- 20 mm / 100º AFOV EP on a 1000 mm tube (50x, 2º FOV)

- 20 mm / 50º AFOV on a 500 mm tube (25x, 2º FOV)

 

How the view will differ?

Yes. You already have answered your own question. If you were content with a field  that was no wider than what the 1000mm scope could offer, than that is the end of the discussion.

 

Now the question is whether you should or not.  

 

First, a 100 Degree field eyepiece will cost double the cost of an eyepiece that would give the same field in the shorter focal length scope.

 

If it were a refractor, the difference in tube lenght would mean less stability and very importantly for planetary observing, usually less comfortable observing position.  This does not sound like a big deal but if you raise the longer telescope up high enough to keep from having to sit on the ground when viewing near zenith, then the mount becomes even more unstable and focusing can get difficult.

 

Wind loading can also be a factor for longer telescopes.

 

Now all of this being said, if the only thing you are going to do with it is look at planets, then the flexibility of the shorter focal length is not important.

 

If the scope though is going to do other things, then a shorter telescope can be much better because you can get wider fields with cheaper eyepieces.

 

You've already answered your own question though.. Yes, both have the same field and if that field is all you need and the cost of such an eyepiece is within your budget range, then the 1000mm would work as well as the 500mm scope.


Edited by Eddgie, 28 January 2020 - 10:12 PM.


#5 Jon Isaacs

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Posted 28 January 2020 - 10:32 PM

I'm looking for a telescope but I'm torn away between a long FL (for planet observation) and a short FL (for its wide field of view). I know such a telescope doesn't exist but I'm wondering if it would be possible to compensate a bit the small FOV of a long scope by using very wide EP.

 

Considering those two setups:

 

- 20 mm / 100º AFOV EP on a 1000 mm tube (50x, 2º FOV)

- 20 mm / 50º AFOV on a 500 mm tube (25x, 2º FOV)

 

How the view will differ?

 

 

I assume these two telescopes are of the same aperture. In any event, the views will be very different.

 

The biggest difference is the factor of two in magnification. At 50x, everything will be twice the size but extended objects (nebulae, galaxies, planets) will be one quarter as bright. The much wider AFoV will also be apparent.

 

But, the 500 mm scope with a 10 mm, 100° eyepiece will provide essentially identical view to the 1000mm focal length scope with the 20 mm 100° eyepiece.

 

And the 500 mm focal length scope with the 20mm 100° eyepiece will provide a 4 degree TFoV.  To achieve this with the 1000 mm scope, it would require a 40 mm 100° eyepiece. Such eyepieces don't exist..

 

But specific scopes are needed for a meaningful discussion. 

 

Jon



#6 OneGear

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Posted 28 January 2020 - 10:54 PM

Although the true field of view is equivalent, we have the the following issues.

Fast telescopes have a sharply angled light cone which is hard on eyepieces,

I believe it produces astigmatism in the outer parts of the field.

 

In general a well corrected 50-60 degree eyepiece is less expensive than

a well corrected 100 degree eyepiece.  I can buy a top shelf 20mm plossl for $120.

A top shelf 20mm Ethos  will be $850

 

A 20 mm plossl weighs about 3 oz.

A 20mm Ethos weigns 2 lbs (can challenge a light duty mount).

 

In a reflector the faster the scope, the more coma will be introduced.

In a refractor the faster the scope the more field curvature their is.

 

Some folks don't like 100mm AFOV eyepieces the field is too wide.  (I don't have one too expensive)

 

Scopes with short tubes are easier to mount.

A short telescope with heavy eyepieces is much harder to balance, not easier.  Further, similar correction is cheaper in a higher f/ratio.  And as stated, one doesn't need the most expensive ep for a given afov to achieve the goal of wide, sharp views.

 

I think the OP's query is brilliant and deserving of discussion.  I've often wondered if my money might be better spent on wider eyepieces over shorter focal lengths, given the scopes and mounts I already own.



#7 SeattleScott

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Posted 28 January 2020 - 11:18 PM

Such a telescope does indeed exist. The TV NP-101 is a well-known example. See, this is what happens when you don’t set a budget!

The focal length is in the 500’s range, so it can go wiiiide. It is a four element Apo so it has the color correction to go high. With the new Vixen HR and Tak TOE series, you get top shelf planetary eyepieces down to 1.6mm in FL. That gives what, about 330x? So if you can be satisfied with a four degree FOV at the low power end, and 330x or so at the high power end, you can have your cake and eat it too. It will just run you $4000 for the scope, and maybe $800 for the 31 Nagler and 1.6 HR eyepieces. Or save a couple grand and buy used.

Of course there are less expensive variations. A $600 AT102 won’t go four degrees wide, but it will come close. Maybe an Explore Scientific 34mm 68 AFOV for low power, and barlow a 5mm Paradigm for high power. Not the ultimate setup I described first, but a lot cheaper, and still can go pretty wide, and pretty high on planets.

Ultimately the Apo refractor is intended to answer this specific dilemma-wanting wide field and good planetary performance. Unfortunately they aren’t cheap. But they are as cheap as they have ever been.

Scott
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#8 Jon Isaacs

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Posted 29 January 2020 - 02:28 AM

A short telescope with heavy eyepieces is much harder to balance, not easier.

 

 

The statement was short tubes are easier to mount and in general, I think this is true.  A tube that is twice as long requires a heavier duty mount.

 

As far as a shorter tube being easier or harder to balance, the equations are the same, it's a teeter-totter. People sometimes have trouble with shorter tubes because the focuser can limit where the scope is mounted...  

 

Such a telescope does indeed exist. The TV NP-101 is a well-known example. See, this is what happens when you don’t set a budget!

The focal length is in the 500’s range, so it can go wiiiide. It is a four element Apo so it has the color correction to go high. With the new Vixen HR and Tak TOE series, you get top shelf planetary eyepieces down to 1.6mm in FL. That gives what, about 330x? So if you can be satisfied with a four degree FOV at the low power end, and 330x or so at the high power end, you can have your cake and eat it too. It will just run you $4000 for the scope, and maybe $800 for the 31 Nagler and 1.6 HR eyepieces. Or save a couple grand and buy used.

 

waytogo.gif

 

I have owned an NP-101 for 10 years.  It's an amazing scope that is corrected for the field curvature that normally affects shorter focal length refractors, it provides fields as wide as 4.9 degrees that are essentially perfect. 

 

5976318-NP-101 Boulevard CN.jpg

 

 

One caveat: As mentioned, faster focal ratios with the steeper light cone angles are tougher on eyepieces and it's been my experience that the NP-101 is the most demanding scope of eyepieces I have ever used, more demanding than a fast Newtonian. I use a 3.5mm Nagler with a 2x Barlow for the highest magnifications, this gives about 310x.  Unless the Tak TOA and HR eyepieces have actually been proven in an NP-101, I think it's best to stick to TeleVue eyepieces which are proven and actually tested for the NP-101.    

 

NP-101 is an amazing telescope but it's an expensive proposition, not only for the scope itself, but for the eyepieces that allow it do what it can do. 

 

Jon



#9 luxo II

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Posted 29 January 2020 - 03:29 AM

I'm looking for a telescope but I'm torn away between a long FL (for planet observation) and a short FL (for its wide field of view). I know such a telescope doesn't exist 

Firstly this is a slightly different question to the example you propose, and Jon Isaacs responded to the example.

 

Short scopes pose one set of problems, long scopes solve some but create other problems, and vice versa. In short there is no telescope that "does it all" and solves all the issues - if there was we'd all be using it. However this is an old problem and there are some solutions using one telescope:

 

1. Dual Classical Cassegrain/Newtonian - take a fast parabolic mirror say f/3-f/5 and drill a hole in the centre - it can serve as a Newtonian, and put a convex secondary mirror in front of the diagonal and bingo, you can have an f/15 cassegrain. This is how big observatory telescopes solve the problem, but has never been popular with amateurs due to the mechanical precision required to make it work well.

 

2. The Barlow  lens - start with a medium to fast scope say f/5 and make that effectively f/10 (2X) or f/15 with a 3X Barlow lens; several recent variants on this exist (eg Televue Powermates etc). Easy, but never quite as good as a long mirror or objective lens in the first place.

 

3. There's another - a folded newtonian - take a standard fast newtonian, say f/4 and insert a convex parabolic mirror with a small hole in that; the light is reflected back to the primary for a second bounce, then back up through the hole to the newtonian focus. Only this time its f/15 or more.


Edited by luxo II, 29 January 2020 - 03:43 AM.

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#10 Ziguy

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Posted 29 January 2020 - 11:47 AM

I'm looking for a telescope but I'm torn away between a long FL (for planet observation) and a short FL (for its wide field of view). I know such a telescope doesn't exist but I'm wondering if it would be possible to compensate a bit the small FOV of a long scope by using very wide EP.

 

Considering those two setups:

 

- 20 mm / 100º AFOV EP on a 1000 mm tube (50x, 2º FOV)

- 20 mm / 50º AFOV on a 500 mm tube (25x, 2º FOV)

 

How the view will differ?

Thanks for all the comments but I just wanted to understands how the views will differ.

 

So if I understand correctly, I will see the exact same part of the sky (same FOV) but with the 100 AFOV EP, the view in the EP will be stretched by a factor of 2.

 

Assuming the same aperture, will the stretched view also be dimmed?

 

screen_shot_2020_01_29_at_11_36_31_df95d


Edited by Ziguy, 29 January 2020 - 11:50 AM.


#11 JMW

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Posted 29 January 2020 - 12:56 PM

If one has well corrected eyepieces with wide apparent field of view such as the Ethos and APO refractors what advantages are there for a longer focal ratio refractor?

 

I use my refractors for both visual and imaging so I find the shorter length for mounting and wider fields of view an advantage.

 

I would use my C11 EdgeHD if trying to image an object that has a very small scale.



#12 areyoukiddingme

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Posted 29 January 2020 - 12:58 PM

Brightness is a function of the exit pupil of the eyepiece in relation to the aperture of the scope. The scope's aperture is constant here, so it's just the focal length of the eyepiece that matters. The magnification should not matter.

 

 

With focal length differing, then I'd think it would have to mean that there's an equal amount of light in both eyepieces, but the light is spread out further in the 100 degree eyepiece. But it is a matter of contrast--the background sky will be darker too in the 100 degree because the magnification is higher.



#13 MitchAlsup

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Posted 29 January 2020 - 02:11 PM

Considering those two setups:

 

- 20 mm / 100º AFOV EP on a 1000 mm tube (50x, 2º FOV)

- 20 mm / 50º AFOV on a 500 mm tube (25x, 2º FOV)

 

How the view will differ?

While the true FoV will be the same, the 50º FoV will STILL look like a soda straw

while the 100º one will look like a port hole into space.

 

The thing you are not recognizing is that you are NOT limited to a single scope !!!

For the largest FoV you want a really fast scope (like F/3) and short lie 10"-13" (some might say 6"-8").

 

For planetary you want a bit more focal length but you can deal with this by making a 30" F/3 !



#14 Jon Isaacs

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Posted 29 January 2020 - 02:28 PM

If one has well corrected eyepieces with wide apparent field of view such as the Ethos and APO refractors what advantages are there for a longer focal ratio refractor?

 

I use my refractors for both visual and imaging so I find the shorter length for mounting and wider fields of view an advantage.

 

I would use my C11 EdgeHD if trying to image an object that has a very small scale.

Color correction depends on focal ratio and aperture so to achieve the same level of correction, the faster scope will need to use better glass, more lenses.. do something.  Other aberrations increase with shorter focal ratios.

 

The TV NP-101 has been mentioned as an example of a very fast refractor with very good correction.  The actual design is a dual-ED 4 element design that begins with a relatively slow doublet in front and a matched ED field flattener/reducer at the rear.  You get a 4 inch F/5.4 that has a very flat field and excellent color correction for a mere $4000.  

 

Just how it comes to an 100 mm F/9 FPL-53 doublet like the Skywatcher/Orion, I have never made that comparison but it's clear you pay a lot extra to get that fast objective.

 

Jon



#15 Eitau

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Posted 29 January 2020 - 02:46 PM

The statement was short tubes are easier to mount and in general, I think this is true.  A tube that is twice as long requires a heavier duty mount.

 

As far as a shorter tube being easier or harder to balance, the equations are the same, it's a teeter-totter. People sometimes have trouble with shorter tubes because the focuser can limit where the scope is mounted... 

Also, a heavy eyepiece will exert less torque in a short telescope than a long one, due to the shorter lever arm.

Which is odd, since in my experience eyepiece changes definitely go more smoothly in long telescopes (due to the heavier-duty mounts, perhaps?).



#16 SeattleScott

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Posted 29 January 2020 - 07:47 PM

A longer tube has better leverage so it is easier to make fine movements with it. Consequently you can apply more tension since it is easier to overcome the tension. The greater tension makes eyepiece swapping easier. My two cents anyway.

Scott

#17 Jon Isaacs

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Posted 30 January 2020 - 03:42 AM

A longer tube has better leverage so it is easier to make fine movements with it. Consequently you can apply more tension since it is easier to overcome the tension. The greater tension makes eyepiece swapping easier. My two cents anyway.

Scott

 

Scott:

 

The longer tube means the eyepiece has more leverage on the tube, , more friction is required to keep the tube from moving.  I think the reason people often have trouble balancing short tubes is the focuser gets in the way mounting it at the proper point.  With a long tube, the focuser is a smaller portion of the length so the rings can be placed where they need to go without much trouble. 

 

Longer tubes do provide finer motions, that's why Dob with a 10 foot focal length is easy to track at high magnifications.  But it is no easier to balance.

 

 I have no problem balancing my ST-80 with the 31mm Nagler.. it's just a question of getting the friction right and the balance point right. 

 

Celestron ST-80.jpg

  

 

Jon



#18 SeattleScott

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Posted 30 January 2020 - 06:07 AM

Yes, that’s all well and good until you swap eyepieces! Then you need to apply tension to avoid losing the target. But I get what you are saying. In your case you were probably using the ST-80 as a dedicated wide field scope and didn’t swap eyepieces.

Scott

#19 Jon Isaacs

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Posted 30 January 2020 - 07:17 AM

Yes, that’s all well and good until you swap eyepieces! Then you need to apply tension to avoid losing the target. But I get what you are saying. In your case you were probably using the ST-80 as a dedicated wide field scope and didn’t swap eyepieces.

Scott

 

Scott:

 

I swap eyepieces with it all the time, no problem.. 

 

People seem to get bored my analytical explanations and this is the eyepiece forum and not the mounts forum so I'll refrain.  However, it can be shown that if you setup a scope one pound nose heavy (no eyepiece) and add sufficient friction to so it is balanced, it will be balanced for any eyepiece up to 2 pounds and never require more than 1 pound to move it. This is true regardless of tube length. 

 

The challenge with shorter scopes is getting that nose heavy.. With 2 pounds of friction, its easier.

 

Jon


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#20 droid

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Posted 30 January 2020 - 11:45 AM

question ;  the op states he wants the longer ffocal length scope for planetary......so why a 100 degree fov? for planets, light transmission ,and clean field, would be better...for planets.

 

long f ratios are very forgiving of cheaper eyepieces.

 

Now short focal length telescopes, they require very good eyepieces to deliver good images.



#21 Starman1

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Posted 31 January 2020 - 02:43 PM

question ;  the op states he wants the longer ffocal length scope for planetary......so why a 100 degree fov? for planets, light transmission ,and clean field, would be better...for planets.

 

long f ratios are very forgiving of cheaper eyepieces.

 

Now short focal length telescopes, they require very good eyepieces to deliver good images.

If the scope is not tracking, though, the wider apparent field eyepieces yield longer times between nudges.

That could be a good reason to use wider field eyepieces.

To wit:

In my 12.5", I have a 110° 3.7mm eyepiece, and I also have a 3mm 62° eyepiece

The former yields a 13.3' true field, while the latter yields a 6' field.

The passage time of a planet through the eyepiece is 53.2 seconds for the former and 24 seconds for the latter, both from edge to edge.

Assuming I nudge the scope to follow the planet when the planet nears the edge but doesn't quite get there, I can go 45 seconds before nudging with the first eyepiece, and only 18-20 seconds on the latter

That makes a big difference in the field and explains why I never use the 3mm on that scope, but why I use the 3.7mm instead.

The point is that a wider field is more compatible with a non-driven longer focal length.

And lest you think that planetary images are poor in the 110°, I note that I have seen white markings on Uranus, and albedo markings on Ganymede with that eyepiece.


Edited by Starman1, 31 January 2020 - 02:44 PM.

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