10mm Ethos & coma.

[Daniel-K]

just a quick thought, me and mike are both pondering the same thing and it could lead to two 14mm delos  up coming for sale soon is coma more or less noticeable at higher mags in these 100deg EPs

[crashtestdummy]

from what I understand you the wider the fov degree wise the more coma you will get so a 10mm 70 degree will show less coma than the same 10mm 100 degree.all other things being equal ofcourse.obviously you are comparing two different eyepieces and coma will be less in a 10mm eyepiece than a 20mm eyepiece so I doubt it will be too bad.if it bothers you get a coma corrector or a narrower view

[John]

I can just see a touch of coma out by the field stop edge of my Ethos eyepieces with my F/5.3 12" dob. I don't use a coma corrector. Coma would be more extensive in a faster newtonian. The more off axis field you can see, the more coma you will see.

[Laurie61]

My understanding is mag does not affect coma as long as the eyepiece's are the same afov, so a 10 or 20 mm 70 degree EP will both show the same. Although the higher power eyepiece is looking at the field centre, where there's less coma, its higher mag shows the coma there more, so you have half the coma but twice the mag equals the same result.  

[Mike73]

Dan one thing I found about Ethos was that although I could fit more objects into that 100˚ it didnt and couldnt replace two EP's with different magnifications.

[John]

Dan one thing I found about Ethos was that although I could fit more objects into that 100˚ it didnt and couldnt replace two EP's with different magnifications.

Thats interesting.

I found my 6mm, 8mm and 13mm Ethos did replace 7mm, 9mm, 11mm, 13mm and 16mm Naglers - not quite a "1 for 2" deal but getting there !.

In shorter focal lengths I do like to have a tighter packed selection though.

Everybody will have slightly different preferences though, as we often say 

[estwing]

I would love to use a 8mm ethos in my dob john but the 31 nag wil have to come first and I couldn't live with all that view spoiled with coma

[John]

I would love to use a 8mm ethos in my dob john but the 31 nag wil have to come first and I couldn't live with all that view spoiled with coma

Do you use a coma corrector Calvin ?

[Mike73]

Thats interesting.

I found my 6mm, 8mm and 13mm Ethos did replace 7mm, 9mm, 11mm, 13mm and 16mm Naglers - not quite a "1 for 2" deal but getting there !.

In shorter focal lengths I do like to have a tighter packed selection though.

Everybody will have slightly different preferences though, as we often say 

I tried using a 13mm Ethos to cover everything upto my 20mm Nagler and I missed not having something in between, similar sort of jump for Dan going from a 10mm Ethos to 17.3mm Delos. Certainly not saying it cant/shouldnt be done just from my own experience it was too much of a stretch.

[jetstream]

To my eyes less.The 21mmE shows quite a bit of coma,the 17mmE much better and the 10mmE better again @ f4.8.For great galaxy contrast I reach for the 10mmE,and for some clusters....and for fine examination of the Veil neb and...I love my Delos 8mm & 6mm though too for clusters and interstingly enough galaxies lately(8mm).A story in itself.

[John]

I tried using a 13mm Ethos to cover everything upto my 20mm Nagler and I missed not having something in between, similar sort of jump for Dan going from a 10mm Ethos to 17.3mm Delos. Certainly not saying it cant/shouldnt be done just from my own experience it was too much of a stretch.

Fair enough Mike 

I'll have to get a 17mm Ethos and see how much use it gets between the 13mm and 21mm. 

[Stu]

To my eyes less.The 21mmE shows quite a bit of coma,the 17mmE much better and the 10mmE better again @ f4.8.For great galaxy contrast I reach for the 10mmE,and for some clusters....and for fine examination of the Veil neb and...I love my Delos 8mm & 6mm though too for clusters and interstingly enough galaxies lately(8mm).A story in itself.

I've little experience of this but my understanding of it was that higher mags show less coma I assume because they are not using the extremes of the mirror surface? I think was what was behind the original question so a 10mm ethos should potentially show similar coma to a 14mm Delos, higher mag but wider afov?

John, I have 13, 17 and 21 Ethoiseses and can't quite bring myself to get rid of any of them. Perhaps because of my different focal length scopes they each have their uses, but also the step from 13 to 21 just seems too big.

Stu

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[estwing]

Yes John once used I just can't view without it

[alan potts]

Estwing,

I imagine you are in the prime position to answer any question about coma as I have not noticed a faster scope on site. If it were of course slower like F5 you would either have to get out the block and tackle or be about 6 foot 9 inches. Super scope!

Alan

[estwing]

Alan I was so looking forward to finally taking it to fantastic dark sky last weekend at Dobfest but the weather didn't play ball, so many things to try,compair and report on... I will not be beaten cloud God!

[swamp thing]

I've little experience of this but my understanding of it was that higher mags show less coma I assume because they are not using the extremes of the mirror surface?

Have a read of this thread on CN pretty interesting stuff.

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5662033/page/0/view/collapsed/sb/5/o/all/fpart/all/vc/1

[John]

Have a read of this thread on CN pretty interesting stuff.

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5662033/page/0/view/collapsed/sb/5/o/all/fpart/all/vc/1

That post from Don Pensack is really interesting as is the thread as a whole. Don sets things out really clearly but also highlights the stuff that is not so readily explainable. 

[Daniel-K]

Alan I was so looking forward to finally taking it to fantastic dark sky last weekend at Dobfest but the weather didn't play ball, so many things to try,compair and report on... I will not be beaten cloud God!

it will soon come around again

[AlexB67]

John, I agree his posts are a wealth of info , but I do not entirely see it the same way that he says

Therein lies the mystery. Many, if not most, people report that the visibility of coma goes down as they raise the magnifications. That doesn't make sense from the standpoint of optics. So is there a reason or group of reasons why that might pertain?
 

The eye is also part of the optical train, when everything is added up it does make sense to me, there is a difference in coma as calculated, and that what is perceptible by the eye.  That is how I would put it as simply as possible,  but how could I argue with the great man   In a way it is not like he is denying it either, but seems to perhaps indicate it is a bit mysterious, when there is an actual basis behind it.

While the statement coma is independent of magnification is  true, mag cancelling out in one sense, and therefore  being independent. IMHO this is not quite true as far as the perception to the eye is concerned, not even bringing blurring due to  seeing conditions which will also be a factor, visual acuity needed to begin to detect  coma, exit puil brightness, mirror  etc, etc. it does drop out that perceptible coma is worse at lower mag than higher at the same FOV eyepiece. However if you have very good dark steady skies the difference will become less and less marked.  Many factors in it.

I suppose each to their own, but so far I feel happy with what my eyes tell me, albeit much less experienced than those of Don, but it tallies with what is discussed on telescope optics.net and there is a mathematical justification for it.

A small quote from there

On the other hand, at three times lower magnification (P=3), the 3-minute coma blur field radius also triples, to 1.8mm, with the coma this far off now somewhat worse than diffraction limited. At the field edge (assuming the same ~45° AFOV), it is as much as 1.9 wave P-V, or 10 arc minutes, same as at 1mm exit pupil. The difference is that both, comatic pattern deformation and energy lost to the rings are significantly greater at 1.9 than at 0.55 wave P-V. In other words, coma alone is more noticeable and detrimental at the lower magnification.

another handy rule, or little nugget if you like

Summing it up, stellar deformation due to coma is not apparent at any magnification within field radius less than three times the diffraction-limited coma field. The field radius at which coma becomes apparent, if sufficiently magnified, is given by F3/30 in mm, or tanα=F2/39 angularly, with the magnification needed to make it apparent to the eye (~5 arc minutes sagittal coma) being provided by an eyepiece of 1.4F mm focal length

The authors who wrote said pages  have a very deep understanding  and good explanation of many things all optical. If in doubt I've found it to often to clear up quite a few things many a time, it is a bit of a bible, even if a bit compact and hard going at times.

The full discussion can be found here

http://www.telescope-optics.net/newtonian_off_axis_aberrations.htm

As much as I find Don's posts a great wealth of information 99.99999% of the time. I do not see entirely  eye to eye on this very topic on this occasion, but what do I know, perhaps I should get my coat and hide now 

I suppose all good nice discussion anyway.

[Joves]

I've little experience of this but my understanding of it was that higher mags show less coma I assume because they are not using the extremes of the mirror surface? I think was what was behind the original question so a 10mm ethos should potentially show similar coma to a 14mm Delos, higher mag but wider afov?

John, I have 13, 17 and 21 Ethoiseses and can't quite bring myself to get rid of any of them. Perhaps because of my different focal length scopes they each have their uses, but also the step from 13 to 21 just seems too big.

Stu

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Ethoiseses

[Stu]

Ethoiseses

It seemed as good an option as any :-)

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[Joves]

It seemed as good an option as any :-)

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It has quite a ring to it!

[jetstream]

Eyepiece designers try to "trick" the eye all the time,trick it ito seeing as perfect as image as possible.This will be no different with the 100's and Televue has done a good job.Here is this link again,I wish someone else would take an interest in the info as it gives great insight into how our eyes work.http://www.holgermerlitz.de/globe/distortion.html,http://www.holgermerlitz.de/curv/pin_curvature.html.There are big decisions to be made when choosing which distortion to correct for and simultaneously balance them out (if possible) so that the majority of customers are satisfied.

[Dude_with_the_tube]

Get a paracorr Dan and keep your Delos,you will not get any better Ep`s as Delos and without coma corrector on 14" dob you will not gain any better or corrected views.unless you are desperate to get one or couple of Eps in 100 deg range then by all means do so,but this will only increase the need for coma corrector.been there,done that.To be honest,i am playing around with idea of getting rid of few 100deg Eps i have and replace them with narrower FOW like Delos and candidate for this will be my 9mm ES to be swapped out for 10mm Delos.but this is just an idea so far

[swamp thing]

Dan

If you're planning an increase in aperture. You will want Paracorr on a bigger faster scope. My 20" f/4 is messy without it.

I would want the Ethos over the Delos but I Barlow my eyepieces and a 10mm Delos isn't wide enough to do that with. At 460x the extra field of the ethos would be the deciding factor for me. My Nagler 12mm is cramped enough at 383x when Barlowed.