Eyepieces choice for a telescope.

[Fightspit]

Hi,

I get recently a telescope Meade ETX105PE with a standard eyepice 26mm (super Ploessl serie 4000) and I want to have some advise for choosing eyepieces (range of zoom (nb of eyepiece), good zoom, do I take a barlow lense ?, serie 5000 eyepiece ?, FOV (super or ultra wide angle ?), etc ...)
About zoom, I know there is a limit but not his value, how to know the max.
I have not a particular limit of my budget but I want to get a good "quality/investissement".

That is all, what do you think.

[t00fri]

Hi,

I get recently a telescope Meade ETX105PE with a standard eyepice 26mm (super Ploessl serie 4000) and I want to have some advise for choosing eyepieces (range of zoom (nb of eyepiece), good zoom, do I take a barlow lense ?, serie 5000 eyepiece ?, FOV (super or ultra wide angle ?), etc ...)
About zoom, I know there is a limit but not his value, how to know the max.
I have not a particular limit of my budget but I want to get a good "quality/investissement".

That is all, what do you think.

The Primary Edtion (PE) ETX105PE is a Maksutov-Cassegrain folded design and reminds a lot of the famous Questar from a long time ago.

It has an D= 105 mm clear aperture. In general there is a golden rule for the maximum usable magnification M_max of any telscope.

It's

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M_max ~ 2 * D [mm]


Hence any magnification larger than ~200-250 is "empty". This maximum value can at best be used at VERY good seing conditions!

Since the focal length is f=1470 mm (f/14) the minimum eyepiece focal length one may use is

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f_min = f/M_max ~  6.5 mm


with your 26mm eyepiece, you correspondingly get about 57x magnification.

Moreover, the "apparent field" size d_app is calculated as

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d_app = d_eyepiece/ magnification


so with 225x magnification, and a ~ 6.4mm Ploessl typically having a 50 degree field, you get about 13 acrmins of visible field. Remember that the Moon has a diameter of ~30 arcmins.

So I would advice on VERY GOOD optics, i.e. e.g. another Super Ploessl of >= 6.4mm fl. There are also special wide angle eyepieces with fields as big as 82 degrees (e.g. NAGLER). But they tend to get VERY expensive. 65 degrees is a very convenient field (e.g. ERFLE type, 5-6 lenses) and is still not so expensive

I hope this helps a bit.

Bye Fridger

[Fightspit]

Thanks for the info

About barlow lenses, what do you recommend me ? A barlow with a "medium" eyepiece or a small eypiece (e.g. using barlow 2x and 18mm or using a 9mm)?

[t00fri]

Thanks for the info

About barlow lenses, what do you recommend me ? A barlow with a "medium" eyepiece or a small eypiece (e.g. using barlow 2x and 18mm or using a 9mm)?

Yes a Barlow lens is a possibility to have more "options" in magnification.
However, image quality is always lower than with a dedicated high-quality short focus eyepiece. So it's certainly a compromise. The Barlow MUST be achromatic (<=> 2 lenses) but the really good Barlows now have up to 4 lenses. Also it's sensible to choose the multiplication factor some odd number like 2.4 (not 2).

Another MOST crucial fact both with separate eyepieces and notably with Barlows is that ALL lenses have a high-quality COATING! Since eyepieces generally have 8-12 glass surfaces (!) it is paramount to minimize light reflection from each one!!!

Bye Fridger

[Fightspit]

Thanks for all explanations