Televue Ethos 21mm, 13mm and 10mm

[Joves]

I just picked up a 90mm F5.5 achromatic doublet as a travel scope. can go on a camera tripod, takes 2" and has a dual speed focuser.  Should perform well with just the 3-6 zoom for travel where it'd show 165x - 82x.  Could throw in my Ethos too for 38x showing 2.6 degrees. 

The Ethos in this scope will also be my best chance of seeing the veil an NA nebs as it'd show a 2.3mm exit pupil.  I only found out recently that the UHC and O-III filters require a specific exit pupil to reveal these targets, although I dont know what size it is still..

Really, Graham? I was to aware of that. I've played around with both filters (and others) on different targets and in some cases they've helped, in other cases not. Had no idea (nor even really considered) that exit pupil would make a difference. Makes sense though, I guess...

[Joves]

Edit: I WASN'T aware of that

[Stargazer_00]

Yeah me either.  I never managed to see the Veil with either my 6" or 14" using my UHC filter and I was always using my widest, lowest possible power which has around a 5mm exit pupil in both scopes.

It didn't occur to me that there was a sweet spot on the filter itself where you needed a certain contrast level provided by a specific exit pupil size for it to actually work.

Really hoping someone in the know can chime in here and point to the relevenat maths which underpin this.

[Joves]

Very interesting. Would be a shame to own a heap of filters that are limited to not only sky conditions and targets, but also to EP exit pupil. Kinda makes using the right filter an even more painful sport!

[Stargazer_00]

I think it's more specifically the target that responds to the exit pupil rather than the filter.   I'm specifically interested in seeing the veil and the NA nebs.

The UHC itself absoutely works across all exit pupil ranges on every target, it just works best on specific targets at specific exit pupils.

for example, the horsehead nebula is only visible visually using a H-Beta filter AND a 5mm exit pupil.  Other exit pupils show nothing on this target to my knowledge.

[Stu]

There is some info on the Lumicon site here:

http://www.lumicon.com/store/pg/15-LUMICON-Nebula-Filters.aspx

Seems like there is a band of optimum sizes and it varies with sky conditions

Stu

[Joves]

Ah, that makes sense, Graham.

And Stu, that looks like an interesting read. I was playing around with different types of Astronomik filters last night on the Tarantula nebula and can't imagine seeing ANYTHING with the Hb filter, let alone something as faint as the Horsehead!

[Mark at Beaufort]

Only just picked up this thread. I have the 8mm, 13mm and 21mm Ethos which are excellent EPs across the whole range of my scopes. 4"  f6.95 Astro Tech APO Frac, 6" f5 Newt, 10" Dob f4.7 and finally the 180mm Mak/Cass f15.

The (worst) exit pupil that I have with these Ethos EPs is 4.4 in my 10" Dob so they can be switch between the scopes without any worry. The only recent decision I made regarding extra EPs was to buy the 10mm Delos which was decided on price against the 10mm Ethos. I have nearly a 3 degree FOV in my refractor which is great for clusters and can go to 337x with the 8mm Ethos in the Mak/Cass.

So yes Televue Ethos EPs are expensive but I feel whatever scope I might buy in the future I won't have a problem with the eyepieces.

[AlexB67]

Yeah me either.  I never managed to see the Veil with either my 6" or 14" using my UHC filter and I was always using my widest, lowest possible power which has around a 5mm exit pupil in both scopes.

It didn't occur to me that there was a sweet spot on the filter itself where you needed a certain contrast level provided by a specific exit pupil size for it to actually work.

Really hoping someone in the know can chime in here and point to the relevenat maths which underpin this.

Not that I know anything about filters in practice, but the fact you have an optimal exit pupil applies in any case filters or not, it can make the difference between detecting that faint fuzzy and not detecting it. In some cases it may well turn out to be sensitive to such a change ( M33 a good example without a filter where exit pupil plays a role to make the difference between seeing and not seeing it and it is sensitive to this)  To detect the object the trick is to maximise the contrast ratio between sky background brightness and that of the object features you are seeing. 

I'll have a stab at a bit of common sense here, but no doubt someone in the know will chime in and correct me if my reasoning has broken down entirely,   but I would think that filters it is still the same idea, now certain wavelengths get blocked and some transmitted, it is now all about optimising the contrast for what is left  that you see i.e. transmitted wavelengths. In a way a filter is a contrast enhancer I guess you could say, you get a form of improved contrast between what is left that you see ( transmitted light) versus a background.  In the end of the day, for the same aperture, a filter can never enhance light gathering and make you see extra things on that basis, but it can play with contrast, and contrast depends on exit pupil and in turn on magnification.

I'll be interested in hearing the expert opinions though and shoot down everything I've just said if need be, I may well be off target, mistaken, whatever the case.  No doubt there are some extras factors I've not considered either.

[Mark at Beaufort]

I have viewed the North American Nebula and both parts of the Veil Neb using a 66mm frac, 26mm Nagler and a Skywatcher UHC (exit pupil 4.4) from a dark site in North Wales. I am 65 so my exit pupil is not as good as a young person. In addition I have seen the Horsehead Neb using a 16" Dob, 19mm and 24mm Panoptic and an Astronomik H-Beta filter with exit pupils of 4.2 and 5.3