Would an Ethos 21mm and x1.6 barlow replace a N31 & E13??

[bond19]

Arghh, sounds like a nightmare just from reading the question, so Let me try and better explain.

Lets suppose I own both an Nagler 31mm and an Ethos 13mm. Why not just swap both for a Ethos 21mm and pair this with a x1.6 barlow? Effectively replacing the two eyepieces with just the one eyepiece and barlow combination.

Here's my logic (of lack of maybe?)...

I believe the E21 essentially kills the N31 (well expect for a bit of FOV going to the Nagler).  So that's one EP down.

If I then x1.6 barlowed the E21 that takes me down to an effective E13, so that's another eyepiece down.

I guess I need to better understand how does the TFov get affected by all this, and am I missing something else here (or are my sums simply way off?). 

I read this somewhere which seems to explain that the TFov will not be relatively altered... (sorry for the highlights, I just copied n pasted)

For example, let's assume you have a 1000mm focal length telescope, if only for easy math...

A 20mm 52 degree AFOV plossl without a barlow results in 50x magnification, 52 degree apparent field of view, and 1.04 degree true field of view.
A 20mm 52 degree AFOV plossl paired with a 2x barlow results in 100x magnification, 52 degree apparent field of view, and 0.52 degree true field of view.

A 10mm 52 degree AFOV plossl without a barlow also results in 100x magnification, 52 degree apparent field of view, and 0.52 degree true field of view.       

 

I guess another thing to consider. Would the reliance on a barlow be detrimental to such high quality glass, and therefore would I be better of (IQ wise) using individual eyepieces instead? 

Does this make sense and what do you guys and girls think? Is this a logical plan or can you provide reasons this would not work? Thanks in advanced, and I realise this is a lot to get your head around. I'm just not qualified enough to make an informed decision.

[John]

Your maths seems OK to me.

The Antares 2" 1.6x barlow does indeed work very well with the Ethos eyepieces, or at least the older version does. It's smaller and lighter than the Powermate 2" 2x as well so the combinations are a little more focuser friendly. It does not seem to vignette the FoV of the 8mm or 13mm Ethos but I've not tried it with the 21mm, which has a larger field stop of course.

The slight snag with the Antares barlow is that the latest version requires masses of inward focuser movement. Having had some success with an early version I bought a new one a while back but could not get my Ethos eyepieces to focus in either my newtonian or my refractors. It seems that adding the compression ring eyepiece fitting and the 2" filter thread has resulted in the optical elements moving within the body of the barlow which has in turn had this impact on the focal position. Shame really.

If you could get one of the earlier versions of the Antares 1.6x 2" barlows (I've pictured them both below) then it would be worth a try.

[bond19]

Funnily enough,  I've just received in the post the Anatres x1.6 barlow with compression ring to lock in place. Thanks for the heads John. I'll have to return it if it doesn't come to focus.

[spaceboy]

I may be wrong but with it being a barlow you may find it pushes the eye relief back a bit to where your not going to appreciate to larger FOV ?? I know it increased the ER in my 20mm ES 100°. It is useable but not as comfortable as with out the x1.6 barlow.

[bond19]

Another thought?

How would a Ethos 21mm work in replacing a 41mm Panoptic?

Of course the e21 would have much better reach. But how would the reduced Tfov of the Ethos compare to the Tfov of the Panoptic? I calculate I'd be losing out on 0.85 degree Tfov between the two. What is this in real term views (discounting the moon as reference as I'm not good with this comparison due to 'moon illusion'). I'm more interested in relating this loss of 0.85 degree Tfov to starfields and objects like M45 etc.

Can anyone who understands this please advise?

Muchos gracias

[bond19]

Continued from above...

I guess what I'm asking is would you take a loss of .85 degree fov to get much more reach or would lower power and wider fog win out?

Thanks

[Stu]

Here you go, so you can see visually. I'm assuming in the TV102?

I think you would miss the fov of the Pan on M45 and M31 at a dark site, plus other stuff like the North America Nebula but not many other objects are that big.

I use to find the 41 quite washed out unless under dark skies so I stick to a 31 nag now (plus 21e of course :-) ), mind you I have some shorter focal length fracs so can get wider views still.

Stu

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[Stu]

Here's the NAN

It's a little tight but the darker sky in the 21 may be better...

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[bond19]

Have I mis-calculated? I was using the formula apparent FOV of view of eyepiece / ep magnification.

However if I follow the formula on Tele Vues site (eyepiece field stop / telescope focal length x 57.3) then the whole situation reverses?

With TV's formula I GAIN Tfov of 2.08 from changing to the Ethos 21mm verses the Panoptic 41mm Panoptic.

Both formulas cannot be correct and both show wildly different results? 

[John]

Stu and I think the same on this I see

I'd certainly miss having a "max Fov" eyepiece and mines a 31mm Nagler to compliment the Ethos 21. I thought the big Ethos would replace the big Nagler but there have been a number of occasions when the N31 has framed a large DSO like the Veil Nebula in a more appealing way than the E21 could.

I've not used the 41mm Panoptic but my slightly LP skies would lead to a washed out look I feel.

You may get a range of views on this issue though as there is no "right" or "wrong" way and there is quite some investment involved in holding on to another big eyepiece.

With the TV 102 I make the Pan 41 as delivering 3.1 degrees @ 21.4x and the Ethos 21 2.38 degrees @ 41.9x.

[bond19]

Thanks BigMakStutov,

That's a real big help.. Where can I find the app your referencing?

[bond19]

You've nailed it John. It all comes down to the heavy investment in these expensive glass, and trying to get strike the right balance.  I just want a good range but I'm limited to a finite budget. 

[Stu]

It's definitely a balance, and does depend on your skies.

If I lived somewhere really dark I would have a 41 pan back again I think, but where I normally go, the skies just don't merit it and they appear more washed out in the 41.

The app is excellent, SkySafari. Various versions available, don't buy the Pro unless it is on promotion down to £11.99 or similar which it does every now and then. Plus is pretty good anyway.

I'll check the field stops and see what numbers I get. This calculation is more accurate than the Afov/mag one, but I've used the latter for my image circles (which agree with John's numbers)

You usually find that this calc gets inaccurate for the longer focal lengths in each format eg 24mm pan in 1.25" and above 31mm nag territory in 2" because that's when the field stop starts to be limited by the dimensions of the barrel etc

Stu

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[Stu]

Here's a little table I've just thrown together comparing various long focal length TV eyepieces.

The two different calculations are fairly close, but show a more significant delta for the 31mm and longer, the 21 ethos is very similar both ways because it is not limited by field stop

I hope this a) displays correctly, is correct, and c) is of some use ;-)

Cheers,

Stu

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[Stu]

The sunglasses were supposed to b 'b)' :-)

The 21mm Ethos would actually be cracking I reckon. It gives an exit pupil of 2.4 which is excellent for galaxy and general DSO observing whilst still giving a decent fov.

I would still be tempted to keep a 41mm Panoptic though, to give you max for those large objects under dark skies.

This graphic shows you the difference on the Veil. They are all quite tight to be honest so perhaps not worth focusing on getting the whole lot in, you need around 3.6 degrees for that. In order smallest to largest these are 21e, 31nag and 41 pan in the TV102, then just to show off, the 31 nag in my 4" f5 Genesis ;-)

Note that this still uses the afov/mag calc so the reality will be slightly narrower fovs

Cheers,

Stu

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[bond19]

Wow, you really are swimming inside your FOV with your Genesis!!

Thanks for the table, that's a super nice gesture. One question, what is Delta and how do I learn to understand it?

The eastern veil comparison really helped visualise the various compromises each EP offer.

Huge thanks

[Stu]

Wow, you really are swimming inside your FOV with your Genesis!!

Thanks for the table, that's a super nice gesture. One question, what is Delta and how do I learn to understand it?

The eastern veil comparison really helped visualise the various compromises each EP offer.

Huge thanks

Sorry, all I meant by delta was the difference between the two ways of calculating the actual fov. The method using the girls stop is smaller than the afov/mag method by this amount. Delta just means difference really

Stu

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[bond19]

It's been quite a crash course recently and sometimes it hard to keep up. But I've leant so much since joining SGL and just wish I could return the kind gestures shown by all.

[John]

...... just wish I could return the kind gestures shown by all.

Just starting the thread helps others. It's an issue relevent to many I'm sure   

[Stu]

It's been quite a crash course recently and sometimes it hard to keep up. But I've leant so much since joining SGL and just wish I could return the kind gestures shown by all.

You will do in time. Everyone started where you are now, I've learnt (and am still learning) huge amounts from SGL and will always be grateful for that.

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[bond19]

I have a good grasp now of how to calculate FOV.. But one thing I'm unsure about. How does introducing a barlow affect this.  Am I correct with the following calculations >

68 degree Panoptic 41mm (in f8.6 scope / fl 880mm) = fov 3.17 degree

68 degree Panoptic 41mm (in f8.6 scope / fl 880mm) plus x2 barlow = fov 1.58 degree

Basically, will a x2 barlow effectively 1/2 the Tfov in the same way that it provides a x2 magnification gain using the same eyepiece?

Once I have the maths out of the way, I will be delighted to share my observations, now that I've had chance to use the scope quite a bit (teaser... WOW!!!)

[Stu]

Hi,

Yes, basically you do the same calculation but with the new effective focal length of the eyepiece eg

41mm Pan with x2 Barlow becomes 20.5mm

With x1.6 Barlow becomes 25.6mm

Does that help?

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[John]

.....Basically, will a x2 barlow effectively 1/2 the Tfov in the same way that it provides a x2 magnification gain using the same eyepiece?

Yep, thats it !

It's worth noting that a few barlow / eyepiece combinations result in the apparent field of view being reduced or vignetted a bit so the TFoV would be less as well. Powermate's don't do this.

[bond19]

All clear, thank you both