Eyepiece Out Put

[CanisMajorTom]

Why are some eyepieces better for certain objects than others?

And how can you work out what the maximum and lowest power eyepieces that will work for your scope are? :hmm:

[AstroTasmania]

Huge question there!

Quality of & type of optical glass, quality and type of anti-reflection coatings, number of lenses in an eyepiece, collimation etc all these factors influence the detail and sharpness. Also their focal ratio is important. To get super wide flat fields needs maybe 8 glass elements, and at every glass to glass and air to glass surface, light is lost and also refracted to the detriment of the image. To overcome these bad effects costs real money.

The more expensive the individual lenses and their coating are, generally the better they perform.

For planetary viewing one does not need an EP with so many lenses, often fewer is better, a good orthoscopic is an excellent planetary lens. A super wide lens for wide sky views would not be as suitable.

This is a huge topic but if you were to have two or three good quality plossl eyepieces of say:10mm, 16mm 25-30mm of a reputable make and a good apo Barlow lens, these would keep you going for a long time.

Hope this helps a bit!

Clear skies...

[beren]

Generally the max magnification used on any given night is 50x per inch of aperture depending on the stability of the atmosphere at your observing location.Formula for working out magnification.....

Magnification = telescope focal length ÷ eyepiece focal length

For reflector type scopes you have to be careful when selecting eyepieces that you don't choose a eyepiece that will give an exit pupil of 7mm or larger, you start to to see the secondary shadow otherwise.

Exit pupil = eyepiece focal length ÷ telescope f/L

When I brought my SCT I wanted to buy at least one good widefield eyepiece, one bit of advice i picked up was select an eyepiece that is double your focal ratio or produces a 2mm exit pupil .As it turned out its the most used eyepiece with my SCT . Also followed some other advice of selecting eyepieces with at least a 1.5x increment . So my set includes a 6mm, 9mm, 14mm, 22mm, 35mm with a 2x barlow. Pretty happy with the set covers low power views of 50-100x clusters etc, 100-150x DSOs, 150-200+ double stars/planets :thumbsupmate:

Deciding which brand or type of eyepiece to buy isn't easy, its very subjective for everyone so its best to try before you buy. Must admit when I brought a 22mm Televue nagler I was kicking myself for spending so much on one bit of kit but since using it i haven't regretted buying it .

[Radar]

Must admit when I brought a 22mm Televue nagler I was kicking myself for spending so much on one bit of kit but since using it i haven't regretted buying it .

One thing I have learnt with astronomy is 99% of the time you really do get what you pay for.

I have been down that road with all kinds of gadgets, and I found astronomy to be significantly smoother (and more enjoyable) when I spent my hard earned readies (though acquiring gear became significantly slower).

[Astrokid]

Why are eyepeices covered in coatings? What do the coatings do?

[AstroTasmania]

Hi Astrokid

All modern lenses have coatings, which serve various functions.
Here are a couple of reasons:

Hard coatings protect the relatively soft optical glass from being scratched, they also allow light to enter the lens with minimal light being reflected away.

Help prevent internal reflections, ghosting and various other artifacts.

Coatings are often in multiple layers and vary in wavelength thickness and material being vacuum deposited. They can filter out unwanted wavelengths.

I have left options open for others to comment.

Clear skies...

[Tenacious Del]

I always thought the coatings were simply to reduce the amount of light that gets reflected.

Is there any difference in the way someone should clean a lens that has a coating as opposed to one that doesn't have a coating?

[Mick]

I have to agree with beren, twice the focal length gives pleasing views. I use a 20mm SV most of the time in the SCT, it's a cheaper Ep ($40+) but it's crisp and clear in this f/10 scope. I have had a borrowed 13mm nagler VI ($400+) in my 300mm dob and that was awesome, it needs a good quality piece of glass at f/5. Be very gentle cleaning ep's coated or not, I find using a lens pen useful, I don't like using liquid solvents.

[Astrokid]

Thank you for those answers.