Eye piece advice

[jambouk]

Post on behalf of an astronomy friend who doesn't use SGL.

She has a 150mm Skywatcher Newtonian. She has the stock 10mm and 25mm EPs and a stock 2x Barlow.

She wants something lower magnification and wider FoV. She lives under a moderately light polluted sky, like me, on the edge of the city.

Someone has recommended a 40mm Teleview eye piece to her. She is mid-40s.

Any suggestions, up to and including the Teleview price range?

Thanks

James

[Charic]

The TV 40 will be a nice EP to own however at only 43° fov and for over £100 you could buy larger for more money, or my favourite, a panaview 32mm with 70° fov. Better still for under £100 buy two BST Starguiders, the 8mm and either 18 or 25mm . These will better the supplied EPs with their 60° fov.

[brantuk]

A 35mm Panoptic would work - not sure how well - might still be a little wide. But a 28mm - 32mm Swan/Uwan would deffo be good - that's what I used in my old 150P way back.

[Ruud]

Do you know the focal ratio of the telescope and whether or not it will take 2 inch eyepieces?

The exit pupil (EP) should be around 5mm, preferably not bigger (because of sky brightness), and certainly no bigger than her own pupil size. You calculate the exit pupil by dividing the focal length of the eyepiece by the focal ratio of the telescope.

A 40 mm eyepiece in an f/5 would give an exit pupil of 8 mm. That's too big.

A 40 mm eyepiece in an f/8 would give an exit pupil of 5 mm. That's good.

A 32 mm 50°,  a 24 mm 68° and a 40 mm 43° eyepiece will all show a view very close to the maximum true field that you can get from a 1.25" focuser.

In a 2" focuser you could fit a 40 mm 68° eyepiece for a much wider view. But you have to take the exit pupil into account.

[jetstream]

My 32mm TV plossl forces the eye away from the eyecup to get a proper view...the 40mm Plossl may be the same- and if it is I would look elsewhere. I'm just curious- what objects will she like to view with the new EP? Will it be used with an OIII?

[jambouk]

Thanks.

She is going to borrow my Baader Hyperions and see how she's gets on with them in the first instance, the lowest mag is a 36mm.

JD

[jambouk]

Assuming it is the 150P, she should get a pretty wide FoV with that combination, but just need to see how the eye relief works for her:

[staffop]

How do you do that Jambouk?

[jambouk]

A screen shot from this website:

http://www.12dstring.me.uk/fov.htm

JD
 

[staffop]

Thanks Jambouk