Binocular lenses

[GrahamJ]

I have seen comments on this site regarding binoculars where props say 80mm is probably nearer 70mm,could someone explain this, & if this is the case is 50mm not nearer 40mm. I would be grateful if someone could enlighten me. Also I am sure when I looked at the Orion nebula through my 4" refractor, I saw a greenish hue, a fellow astronomer assured me this is not possible who's correct?

[GrahamJ]

Obviously I mean people not props sorry.

[Stu]

It depends on the quality of the binoculars really. Many cheaper ones have the stated sized objectives but internally they are stopped down ie not all that light can pass through the entire light path and so they act like a reduced aperture. The limitations are often related to the prism size I think as these impact the cost more. Definitely worth researching more, check out www.binocularsky.com for lots of info.

Regarding the Orion Nebula, I think it is a very personal thing related to your eyes and brain. I don't see any colour in astro objects other than stars and occasionally on planets and a couple of small bright DSO's like the blinking planetary. Others I know see green and possibly other hues in the Orion Nebula and in sure they are correct, it's just I can't see it! I think aperture and a dark sky helps, so you are less likely to see in a four inch, but not impossible.

Stu

[Andy_E]

Certain DSOs can show (hints of) colour if your eyesight is pretty good. The ring nebula is a good example, faint pinky green hews can be seen on a good night. Curiously, the Orion nebula should be showing a pinkish colour (afraid it still looks grey to me!). Some multi-coatings can introduce a slight false colour cast to an image, but it's hard to say if this is the case here. I've heard of people picking out colour in a 14" dob. As for binoculars, afraid I use 42mm so can't be much help here I'm afraid!

[J_M_Franklin]

On a clear sky in very good conditions without light pollution the naked eye can see faint colour in the Orion Nebula if you have good eyes. In Binoculars this colour will be obvious.

As for the colours seen, considering that M42 is also an OII region then seeing green is highly possible, although i do not recall it ever being mentioned as a naked eye colour as most is washed out by the H ALPHA (Reds) that predominant the nebula to Human eyes. It is possible that in Bins with cheaper coatings some greens may get transmitted better than Reds but to be honest, i would put this more down to chromatic abberation caused by low quality lens either in the front of the Bins or in the eyepieces.

[Space Dragon]

As Bigmak says the cheaper lines of bins all seem to be internally stopped down probably to save costs re size of prisms and to hide abberations at the edge.

Even with some more expensive models this is evident, a 90 degree 100mm model was found to be operating at around 80mm and was then withdrawn by the importers.

Trades Desciption Act and all that....it's not really on.

I have seen hints of green in Orion in various instruments.

[GrahamJ]

Thanks for all your feedback. By the way the 4" I refered to was a Tal 100rs not binos, sorry for any confusion. But I have just got some 20x80 strathspey, they seem fine but cloud prevents a serious appraisal.