Eyepieces

[mikeyggg]

Hi,

I currently have a celestron 32mm nexstar, 25mm and 10mm elux, and a barlow.

I'm thinking of getting a 40mm omni, which i suppose would give me an equivalent of 20mm with barlow.

just wondering if it was worth it, ie, how much difference is there between 25mm and 20mm, or between 32mm an 40mm, in terms of actual view?

or would it be more effective me getting a 6mm for very high mags, (although when i put the barlow on the 10mm, its a pretty dim image).

any help appreciated.

eyepieces used with 6" dob by the way.

cheers,

mike..

[palefire]

I remember being advised that you generally get no wider field of view with a 40mm over a 32mm plossl (unless you're going to go for 2" wider angle eyepieces).

[MikeP]

Maximum practical magnification is usually estimated to be 40 to 50 times per inch of aperture, so in your case 240x to 300x. I believe your scope's focal length is 1219mm hence 4mm to 5mm will give magnification as high as you can go most of the time - as you have discovered by barlowing the 10mm.

Something around 6mm will give you 200x which is probably a more sensible maximum, although for something as bright as the moon, you probably could go higher.

Mike

[mikeyggg]

ok, that makes sense actually.

i looked at m13 with 10mm and barlow and image was almost to dark and grainy to be seen.

hmmm..

in that case, if its of no advantage to have a 40mm, i guess a 15mm with barlow would give me 7.5mm which might be just a bit better that the 10mm with barlow.

but then i suppose putting the barlow on the 32mm would give me almost the 15 equivalant!

its really quite difficult to decide on the best overall spread!!??

maybe i should sell my single eyepieces and buy a set. then i suppose all the decisions are taken care of!

[MikeP]

One useful rule of thumb that I came across recently regarding choice of focal lengths is "each eyepiece should be about 70% of the focal length of the next biggest". So, for example if you decided your longest eyepiece for wide low magnification views would be 30mm, then the rule would imply you'd need 21mm, 14.7mm, 10.3mm, 7.2mm and possibly 5mm (depends on the maximum magnification your scope is capable of).

Rounding them to 30, 20, 15, 10, 7.5 and 5 - you could buy 30, 20 and 15 and Barlow the 20 and 15 to get the 10 and 7.5. You may or may not decide to buy a 5 as well. I'm sure you see the principle.

I wrote the above in answer to a similar question. It may help you decide.

Mike

[ringz]

If you are using a 1.25" focuser then you have a hardware limit to the longest focal length you can use.

A 32mm Plossl with 50° apparent field, for example, is bordering on the longest focal length that is physically possible in the 1.25" tube.

To squeeze in 40mm they have to reduce the apparent field from 50° to 40° to accommodate the longer focal length.

This means that the actual field of view through the scope is the same, BUT the 32mm has more magnification and so the object you are looking is shown with a darker background.

You actually gain nothing by using a 40mm.

--

Martyn

[mikeyggg]

thanks for the advice folks.

ok...going to sit down with calculator and follow MikeP's system!

cheers..

(does anybody agree that there's a definate degree of retail therapy when purchasing eyepieces / filters etc......dangerous eh?)

[John]

(does anybody agree that there's a definate degree of retail therapy when purchasing eyepieces / filters etc......dangerous eh?)

Definately - it's very addictive !

I've lost count of how many different types of eyepiece that I have tried over the years I've been observing

At least your F8 scope is not too fussy about what eyepieces you use - shorter focal length scopes seem to need more expensive eyepieces to perform really well (at least thats my excuse ..... )

John