help

[Dan73]

i just purchased a 2mm eyepiece for my telescope. i was wondering if this was a good buy or a useless one. my scope has a focal length of 1200mm and a 150mm mirror and i was wonderng if there was any sort of limit to the size of the eyepiece's i can use. any information will be extremely helpful.

thanks, dan:)

[MikeP]

There is a rule of thumb that suggests a maximum magnification of 2 per mm of aperture. For your scope that means a max of 300x.

With a focal length of 1200mm, a 2mm eyepiece will magnify 600x.

Even if you look at something bright like the moon, the movements in the atmosphere will spoil the view. Most everything else will be too dim and you'll have a devil of a job finding them anyway with such a small field of view.

Sorry to rain on your parade.

What are you interested in viewing with your scope?

Mike

PS I should add that nights are few and far between when you can use anything like the theoretical maximum magnification for any scope.

[Dan73]

thank you. im rather new to this:)

i want to see deep sky objects and occasionally planets. do you have any idea what the limit in terms of mm i can go up to?

dan

[SteveP]

Hi Dan

Have you used your eyepiece yet? You'll see instantly the point which Mike rightly makes. Unfortunately, it's just too short a focal length for your particular scope.

My recommendation with your scope is that you'll be unlikely to use an EP much shorter than 8mm on a regular basis. This will provide a mag of 150x. You might be lucky a few nights a year to be able to use a 6mm.

Have a go yourself. On a decent night, focus on an object with the lowest powered EP you have and progressively use the more powerful ones. You'll get to a point where the quality starts to suffer as the magnification increase

Steve

[Chris H]

For DSO viewing as a rule keep the magnifiaction low something between about x40 (25mm) and x100 (12mm) times magnification, maybe a little higher is seeing permits. For planets you want a higher mag but not too high I would aim for either x200 (6mm) - x250 (5mm) higher than that you are beginning to get beyond your scopes capabilities and that is without taking the atmosphere into consideration. What is your location? If it is the UK its not often you could push a 6 inch scope much beyond x200. Hope this helps.

[Dan73]

thanks for your advice:) i didnt really know how low i could go so you have been helpful:)

dan

[AndyH]

Hi.

On my Tal 2M 150mm F8 FL 1200 I use the following.

Divide the FL(focal length) by the FL(number on the eyepiece)of the eyepiece. this gives the magnification.

For planets and moon = Between 150x(8mm) to 200x(6mm)

For deep sky stuff, eg: galaxies, star clusters etc. = Between 50x (24mm)to 100x(12mm)

That's a general rule of thumb. If the air above is unsteady(stars twinkling like crazy), you might not be able to go as high. When the air is steady, high mags can be used possibly.

All the best,

Andy.

[AndyH]

These are the eyepieces I use on it.

25mm = 48x (wide field of view and centring of objects. Then stick a higher mag 'piece in)

12mm = 100x (some star clusters look good at this mag)

9mm = 133x (as below, when air is unsteady)

8mm = 150x (reason as above. For some reason, 8mm eyepieces are not as common as the 9 or 7mm)

7mm = 171x (for planets and globular clusters, when air is steady)

6mm = 200x (rarely used)

Cheers,

Andy.

[lunator]

Hi Dan

I agree with the views already stated.

I have found that you can use magnifications upto x200 on a reasonably frequent basis so you can get a 6mm and you will get some use out of it, but anything shorter will only be used rarely.

Cheers

Ian

[Capricorn]

Keep hearing of the 2x Dia in mm but also reading people that try for it they are all disappointed.

Before they said 2x Dia in mm they said 1.5x dia in mm.

Suggest that you count 2x as marketing and 1.5x more like the real maximum. I would say that you will only get this maximum rarely, so don't expect it to be a viable option.

I am guessing that when they made good ED APO refractors that these (some) were cabable of 2x Dia in mm so then everyone else had to say they were as good and so claimed the same.

Lets be honest if you saw 2 scopes side by side and one said Mag = 300x, the other said Mag = 225x which would you go for.

Optically the optimum is equal to the mirror/lens dia in mm, so not even 1.5x.

[John]

I think the big challenge is that people think of telescopes as a magnifiying instrument first and foremost wheras, for astronomy purposes at least, it's light gathering that is their primary function. So "x300" grabs the attention while "gathers 200x as much light as the human eye" is largely overlooked.

[Dan73]

thank you Andy and everyone else. you have been really helpful. i am now going to investin a 8mm instead as this seems like the best and see how i go with that and then maybe i will try the 6mm when the weather improves in a few months:)

dan

[Moonshane]

hi Dan

not sure if you have one but a decent 2x barlow lens will effectively double your options.

this would make a 16mm eyepiece also an 8mm and so on.

if you had eg a 25mm and a 10mm with the scope (common 'standard lenses' I think?) then you'd have a 12.5mm and a 5mm too. if you ever get a refractor with a star diagonal, you get the 2x with it between the diagonal and the eyepiece and 3x with it between the focusser and the diagonal.

cheers

Shane