is this right?

[Megaman250]

Hi again,

Having trouble finding out what is the best size eyepiece to suit my telescope so here is my question if my scope is F900mm/A130mm that will be f6.92 so am I right to think the best EP for my scope will be an 7mm?

[baggywrinkle]

Use 2x130 (apeture) as the potential max magnification on a good night with excellent seeing etc etc.

So 900mm/7 would give you 128.5 times, so not too bad but if you Barlow it 2x then you would be on your theoretical max.

What do you intend to look at?

[Piero]

It all depends on what you want to look at. 

Here are two guides:

- http://stargazerslounge.com/topic/43171-eyepieces-the-very-least-you-need/

- http://stargazerslounge.com/topic/251506-a-guide-for-choosing-a-sensible-eyepiece-collection-using-the-exit-pupil/

Hope this helps

Best, Piero

[Charic]

Hi again,

Having trouble finding out what is the best size eyepiece to suit my telescope so here is my question if my scope is F900mm/A130mm that will be f6.92 so am I right to think the best EP for my scope will be an 7mm?

My scope is an f/5.9 so I chose 6mm as the eyepiece to match my focal ratio. (The scopes label says f6)

Your f6.92 would easily accept the 7mm as a guide to your highest magnification. You can go higher ( less than 7 ) but the gain will diminish rather than improve.

Also as a guide for your low power, multiply the focal ratio by the size of your eye pupil. The result in mm should give a rough guide. I chose a 32mm eyepiece.

[Mak the Night]

If I'm right, the Sky-Watcher Explorer with a 130mm aperture and a focal length of 900mm will have focal ratio of about f/6.9. 

If you divide the focal length of the telescope by the focal length of the eyepiece you want to use it should give you the magnification.

So a 10mm eyepiece will give you 90x, a 20mm 45x magnification etc. I used a 15mm a lot on my Explorer to give me 60x often tripled to 180x for lunar/planetary viewing with a 3x Barlow.  

With Plossls, I wouldn't go with a shorter f/l than 8mm with that scope. My 10mm Celestron Luminos would occasionally suffer from glare with the Explorer, but I think that's down to problems with the 10mm Luminos.

Sky-Watcher usually provide a 10mm and a 25mm with that scope, they are both useful focal lengths. 

[Megaman250]

Thanks everyone that has cleared a few things up as always a great help, So I think I maybe looking at the 8mm - BST Explorer ED Eyepiece for Jupiter and other planets and maybe a filter to bring out detail, will this combo be useful for DSOs too?

[John]

Thanks everyone that has cleared a few things up as always a great help, So I think I maybe looking at the 8mm - BST Explorer ED Eyepiece for Jupiter and other planets and maybe a filter to bring out detail, will this combo be useful for DSOs too?

The filters that some folks use to view the planets are different to the ones that are used to enhance the contrast of some DSO's, most notably nebulae.

Personally I don't use filters for the planets (or the moon) but I do find a UHC or O-III type filter useful on some nebulae. Quite dramatic on a few objects actually

The 8mm eyepiece will be good for viewing smaller DSO's such as planetary nebulae, globular clusters and small galaxies. For other DSO's a lower power, wider field of view is needed.

[Megaman250]

Ah I have just read that a low powered EP will be better for DSOs 

[John]

Ah I have just read that a low powered EP will be better for DSOs 

Eyepieces are like gears in a car - having 4 or 5 is somewhat more useful than just 1 or 2

(I think I nicked that from Sir Patrick Moore )

[Megaman250]

Yeah I have just been going through the thread's that Piero posted very useful info but as it is getting late my vision is starting to blur! Maybe a few hours kip before tomorrow's nights clear sky's I know it's a bit of a long shot but i'm going to try and go DSO hunting and then early morning Jupiter but I only have the standard 10mm and 20mm EPs with the skywatcher 130m