Minimum magnification

[OWK]

Dear members,

Ive had a telescope for 3months or so...c925 evo. It came with 40 and 13mm eps. After deliberation I added a baader Hyperion zoom mark 3. I love the convenience of it for a newbie until I settle on some focal lengths that I like to use.

i am researching my next EP. I want a minimum magnification, widest tfov, and larger afov I can afford both monetarily, and pupillary.

im 45 so figure my exit pupil will be 6mm. My scope is 2350 f10. So doing the math, it works out to be a 60mm EP for 40 magnification.

 

where do I get a 60mm EP? And it shouldn't theoretically fit in my scope with a 2inch diagonal. 

Can I buy a 30mm 2inch EP with a focal reducer? Will that work to give me max brightness and tfov in my set up?

Very confused!

regards

[John]

Apart from being very hard to find, a 60mm eyepiece will have some unwanted side effects such as showing the shadow of your secondary mirror when you observe.

What you need is an eyepiece that has the largest field stop diameter for the 2" fitting to get the widest field of view. Candidates would include the Tele Vue Panoptic 41mm, the Vixen NLVW 42m. Such eyepiieces will show you as much width of sky as your scope is capable of showing.

There are lower cost alternatives such as the Skywatcher Aero ED 40mm which show very nearly as wide a true field as you can get.

These are 2" eyepieces of course so you will need a 2" diagonal if you don't already use one.

 

[todd8137]

Not sure they even make a 60mm if it was wide field views you was after a sct would not be the way to go 

pat 

[Mak the Night]

TeleVue make a 55mm Plossl. At least you'd get a 5.5mm exit pupil for a 43x magnification. Although you'll also probably be able to see the corrector plate obstruction.

http://www.televue.com/engine/TV3b_page.asp?id=51&Tab=EP_EPL-55.0

[John]

Bresser do a 56mm plossl with a 52 degree apparent field for around £68.00:

http://www.telescopehouse.com/bresser-spl-56mm-52-50-8mm-2-00-inch.html

Again the secondary shadow issue might come into play though.

 

[YKSE]

60mm eyepiece or not, it depends very much if you're going to use the scope in dark site.

In most light-polluted backyards, eyepiece around 40mm should be about max exit pupil you use. As mentioned by John above, 42mm LVW, 41mm Panoptic and 40mm Aero have around 46mm field stop, that's the maximum FOV you can get out of 2" eyepiece.

Using f6.3 reducer do give you more options about exit pupil sizes, but wouldn't give you wider FOV, since the reducer has only 24mm clear aperture, using eyepieces with field stop larger than 30mm will most likely show noticeable vignetting.

As to longer 2" eyepiece, in addition to above mentioned 55/56mm plossl, there's revelation 50mm Superview which I have, and 50mm to 85mm(!) 2" in the States too, these long eyepieces do have very long eye relief, you'll need some time to find out right eye placement.

 

[baggywrinkle]

I tried a 40mm Celestron EP I have on my SCT 9.25 and I had the kidney bean effect if I did not line up my eye very carefully to the EP. So a 60mm would be much worse, if you could get one.

[YKSE]

For any eyepiece with long eye relief, moving the eye too close will result in darkening to blackout, the trick is to hold the eye at right distance or a little far away.

Kidney bean is the result of too much Spherical Aberration of the Exit Pupil(SAEP), which only some T1 Naglers are famous for. as far as I know.

[OWK]

Thanks everyone as always. So a 2inch 40mm EP will have a larger tfov than a 1.25 I presume?

 

if so I think I will replace the stock 40mm plossl with a 2inch  40mm and the largest tfov I can.

 

agree for a larger view I will need another scope...but that's for later!

 

regards

[YKSE]

Well, 2" eyepiece with 46mm field stop will have marginal larger TFOV than 32mm 1.25" with reducer, about 1.16° to 1.10°. The benefit with 2" 40mm, as least in my case, is that it's shows 4.5° TFOV and 5.3mm exit pupil in 80ED, which is very useful in dark sky for larger DSO like M31, Veil, North American, california nebula etc.

[OWK]

Thanks, and yes that's my scope. I'm about to buy the 2in diagonal.