Eyepieces for slow scopes

[Mark in Macc]

There seems to be a consensus that with slow scopes that just about any eyepiece will give decent views. However, surely scatter is scatter, irrespective of how fast or slow the scope is, or am I missing something? For planetary observing, isn't the best advice to always try to minimise scatter by using quality, simple design eyepieces such as orthos?

[russ.will]

Doesn't that make sense for observing everything? Less is always more, etc. 

I think the rule of thumb is that the narrower light cone of a long focal length scope, is less likely to exacerbate various aberrations in a given EP design. Thus the difference between designs, or the expense spared in the implementation of  the same design from different manufacturers is compressed, particularly with respect to sharpness across the field.

There seems to be lots of ways (and opinions) on how to skin various contrast/scatter cats and I'd suggest that's a different issue and one more related to finesse of construction. Some claim multi-layer coatings reduce scatter, whilst Brandon explicitly don't multi-layer their coating to reduce on-axis scatter. I saw somewhere that edge blackening after the field stop is irrelevant, whilst others claim is mandatory. Some are all over baffling, whilst some don't bother and you find fine EPs in all of those camps.

Russell

[badhex]

Hmm. I too am slightly confused by the ideal EPs to use in fast vs. Slow scopes. Many people seem to have a quotable consensus but I'm not sure what the actual theory is beyond the ever present 'less is more'. Your light cone theory does explain a few things though Russ.

Would this extend to the thought then that in general fast scopes might benefit more from EPs with fewer elements and simpler designs? Or am I extrapolating incorrectly?

[Dude_with_the_tube]

it is not only scatter in fast vs slow scope. it is cheaper to produce a slow F ratio scope then a fast one.Slow F ratio scope will be less affected by coma and other aberrations as a result,will require less perfectly corrected eye pieces,where fast scope will give you coma and other optical issues what NEED to be corrected somehow to give you that perfect image and as a result,on fast scope, you will use coma corrector and also optically corrected eye pieces to get rid of all impurities.Obviously to produce well corrected eye piece you will need multiple lenses,etc, what on another hand builds up the cost of the EP as you cant just stuck up a few simple lenses and get perfect image thats why companies who have spent lots of money,man power etc are eager to have them patented to avoid them being copied as to have a ready made example saves you all the hassle of development.(Televue for example) and as wider the field of view of the eye piece is,as more complex and perfect build will be required.Slow scopes dont need all this as in long focal length scopes,coma is not present ,optically image is good as it is from beginning and as a result you can walk away with cheaper alternatives and still have that excellent image.Also it is far more cheaper to produce an eye piece what will work for slow F ratio scopes as to build a complex and well corrected eye piece for fast scopes.

Ortho is an old design and very simple one.Orthos will work on all focal ratio scopes because of narrow FOV you do not require much of corrections.You can do the test yourselves.Take an ortho and put it in a fast scope.You will see  no coma,little to no scatter etc,where if you will use an eye piece with lets say 60 degree fov but made by ....  you will see what is happening there.Then use one of the good quality and known brand eye piece to see what optical corrections have been achieved in these.Well known companies will ALWAYS tell that eye piece will work in scopes up to  XX focal ratio,whereas eye pieces made in some shed in the land of raising sun will just say that eye piece is great and bla bla bla but will not mention any specific details.

As such, we are back to square one:Slow scopes you can use any type of EP be that an ortho or plossl or erfle or ultra modern eye piece,but in fast scopes to achieve perfect flat,coma and aberration free image you will have to use better quality eye pieces.

hope this sort of explains.

[Dude_with_the_tube]

as for the :Less is better for planetary.

I sort of agree on this as i am one of the believers of less glass= better. Thats why a lot of planetary observers use orthos or monocentrics to achieve they ultimate goal,however,with these days modern alternatives in likes of Pentax XW,Televues you can achieve the same result as you have with orthos,but without suffering the short eye relief if that is an issue and actually also having a larger FOV then orthos or monos,realistically we are talking of such a small increase or decrease of some ultra fine detail what in most days you will be hard pushed to justify.Obviously there are "maniacs" out there who will be more than happy to donate they kidney to obtain what they think is the best eye piece in the world just to get that tiny itsy bitsy increase in detail in one night out of 200 nights,but this is the interesting part of this hobby i think .You try one,then another and the train goes on.If this wasnt the case,then why we would need those 1/10 or higher accuracy mirrors for dobsonians,Zeiss or takahashi lenses or telescopes and all that massive assortment of available eye pieces starting from a few quid and ending in thousands