ED vs SD vs Achro

[bomberbaz]

Been doing a little reading, yes I know it's a dangerous hobby but I have found that many of the larger binocular that I have been looking at are achromat. A few notably Vixen and APM produce ED or SD using FPL 51 or 53 glass to help eliminate chromatic aberration.

I don't pretend to fully understand how this works and TBH, I am not unduly bothered about the mechanics, but what I am interested in is, is it really worth it as the apparent difference between apo and ED is anything up to £1400 depending on the aperture you choose. 

I am hoping to use some should I go ahead for wider field viewing, extended nebula, asterisms, some larger clusters, my idea was I would be viewing from around x20 to x50.

I have other scopes for higher powered work and planetary.

I am aware there will be some false colour on apo but on the listed target group, would it be really so bad as to push one towards ED.

I realise this is a choice I need to make for myself and that is why I am asking for other peoples thoughts as I am aware of the old buyers remorse and don't want to fall victim of it.

Can anyone tell me of there own experiences of results using ED and Achro bins against the type of objects I have listed above.

TIA all

Steve

Edited November 25, 2021 by bomberbaz

[vlaiv]

54 minutes ago, bomberbaz said:

I don't pretend to fully understand how this works and TBH, I am not unduly bothered about the mechanics, but what I am interested in is, is it really worth it as the apparent difference between apo and ED is anything up to £1400 depending on the aperture you choose. 

Depends on use case scenario

54 minutes ago, bomberbaz said:

I am aware there will be some false colour on apo but on the listed target group, would it be really so bad as to push one towards ED.

I think you have something mixed there.

Let's first get our term straight.

APO is currently used to usually denote triplet lens. It means apochromatic performance - or "color free" performance, and in reality - only good triplet lens provides true apochromatic operation in each use case scenario.

ED/SD is doublet scope - one that has good color correction due to exotic types of glass used.

ED/SD doublet is in essence achromatic scope - except for used exotic glass types that help eliminate most of chromatic aberration. Some of ED/SD doublets are apochromatic as far as visual use. Where they fall short is photographic use as cameras tend to be more sensitive to wavelengths that cause CA issues - deep red and violet (long and short wavelengths).

If you plan on using scope for visual only and for low power - then go with ED/SD doublet and faster one - like 100mm F/6 model, or 100mm F/7 model with FPL-51. These still have some chromatic aberration left - but you won't be able to see it unless you look at bright planets at high power (not something that you'll do if I understand correctly).

If you want good all round scope - for both DSO and planets - get FPL-53 doublet with a bit more focal length - like F/8-F/9. Like any doublet - slower the scope less chromatic aberration will be visible.

If you want all around scope for visual and photo, or you want color free faster scope that is also capable of planetary views - go with F/6-F/7 APO triplet.

Triplets cool slower than doublets and that is one of the reasons why people prefer doublets for visual.

[Peter Drew]

For relatively low power viewing of DSO's the chromatic aberration of achromats is hardly an issue.  If high powers are required then the advantages of ED and SD optics are worth paying for.  For large apertures, higher magnifications, weight and cost, reflecting binoscopes are hard to beat.     🙂

[vlaiv]

Oh, this is bino discussion

Disregard my post above - I thought we are talking about scopes.

Don't know much about binos.

[bomberbaz]

6 minutes ago, Peter Drew said:

For relatively low power viewing of DSO's the chromatic aberration of achromats is hardly an issue.  If high powers are required then the advantages of ED and SD optics are worth paying for.  For large apertures, higher magnifications, weight and cost, reflecting binoscopes are hard to beat.     🙂

Thanks Peter, coming from yourself that is as much as I need to know. 

4 minutes ago, vlaiv said:

Oh, this is bino discussion

Disregard my post above - I thought we are talking about scopes.

Don't know much about binos.

No worries vlaiv, I appreciate your input anyway.

[PeterW]

Think of the exit pupil, sometimes you want to maximise for a brighter view, not just add aperture which just changes the scale. What sized stuff are you after? Would you also be better suited with a smaller set for larger objects?

 

Peter

[bomberbaz]

11 hours ago, PeterW said:

Think of the exit pupil, sometimes you want to maximise for a brighter view, not just add aperture which just changes the scale. What sized stuff are you after? Would you also be better suited with a smaller set for larger objects?

 

Peter

Cheers Peter, I have given exactly that ^^ a lot of thought. I would be aiming towards asterisms like the coathanger, hercules keystone and similar. Double cluster, pleiades and things of a similar size.  M42, North American neb, the whole veil etc.  Basically bigger stuff of around 1 degree and more.  I would be looking forward to panning slowly across the river of the milky way, that would be amazing. 

Also done quite a bit of thinking on glass to go with it and two sets with exit pupils at 4mm exit pupil & mag x20 and 1.5mm EPupil & x 55 would cover the bases, option to add an inbetween set later for 2.5mm x 32. < (I already have one ep in this size). Also probably add a couple of astronomik UHC filters to the throw for viewing extended nebula.

The above would offer FOV ranging from 3.4 degrees down to 1.5. I have other bins for large fov objects.

Still at planning stage yet but things are coming together.

[PeterW]

You may want to add a larger exit  pupil and more aggressive filtration… https://www.cloudynights.com/topic/722322-pushing-the-filter-envelope-observing-galactic-nebulae-with-handheld-binoculars-under-suburban-skies/

Peter

[Robindonne]

Isnt the term ED also used in triplets scopes/binos?

[PeterW]

ED just refers to the use of low dispersion glass (fluorite or other) that help optical designers make systems that can deliver chromatic aberration free views… how it’s used, what materials etc vary widely and how “colour free” does too…. Best to see what people think of specific products as to how well they perform.

 

Peter

[bomberbaz]

3 hours ago, PeterW said:

You may want to add a larger exit  pupil and more aggressive filtration… https://www.cloudynights.com/topic/722322-pushing-the-filter-envelope-observing-galactic-nebulae-with-handheld-binoculars-under-suburban-skies/

Peter

Just had a good read of that Peter, thanks for the link.  Very interesting read but also some differentials in results depending on aperture, exit pupil, filter and skies used under.

J.Gardavsky seemed to get some striking and unexpected results using a 15x85 and HB filter, presumably with a near 6mm exit pupil.  I keep my exit pupils to below 6mm due to my age and gradually fading eyes.

From memory I remember reading an article which gave a rule of thumb for exit pupils when using filters, these being; HB filter 4-6 wzit pupil with 5 being optimum, (this is the exit pupil I used when recently seeing the horsehead), 4-2 for the OIII with 3 being preferred and finally UHC having the broadest spectrum as you would expect at 5-1 with 2 giving best results.

However as with all rule of thumb there are always caveats that break them, brightness of object, aperture used, darkness of sky etc.  An example of this is the heart or soul nebula, very dim and diffuse and would respond best to a large exit pupil (5 to 6) and a good UHC filter (OIII would also work here). Another good example being blue snowball planetary, filter not really needed because at high power the sky is so dark its inconsequential. However I am only thinking about low powered viewing up to as earlier mentioned x55, this latter DSO isn't being considered.

I did do a quick check on 30mm eyepieces at 1.25 after your post above, as you do 😅 and came up with the Vixen NPL 30mm which would offer 5.1 exit pupil, x16 but with no gain on fov over the smaller 24mm. However it is a lovely piece of glass that gives very contrasty views on the moon and I already have 2 of the 25mm for my binoviewers.

However I think if I go ahead I shall stick with my current train of thought and do some testing on some nebula and other DSO to gauge results before throwing lots of cash around.  Also I should mention I do not intend viewing from town skies, I have a reasonable decent dark site just a few miles away.

But that article is certainly very thought provoking and gives me a lot of food for thought. I have bookmarked that page for future reference, cheers.

Steve

[PeterW]

Good to hear it’s been of interest. I live under ugh skies, so plan to throw all the filtration I can….

Peter