2 inch ep’s

[tripleped]

In general do 2” ep’s offer a big advantage over 1.25 for DSO  locating and viewing? My 10” skywatcher f 4.7 DOB accepts both sizes. I know that the 2 inchers can be pricey and quite heavy (might need an extra counterweight) but putting those factors aside are they worth it? Intuitively it would seem so but wondering what others have experienced .

[cloudsweeper]

2" EPs give more field of view, so make it easier to hop around the sky and see larger objects.  They are not necessarily very heavy.  Revelation Superviews and Baader Aspherics for example.  They might not be as good to the edge as heavier ones, but whether that matters is subjective!

Doug.

[John]

Doug has highlighted the main advantage with the 2 inch format - a wider field of view. In an F/4.7 scope though you also need reasonably well corrected eyepieces otherwise the distortion in the outer parts of the field of view, which can be from astigmatism from the eyepiece and / or coma from the newtonian optics, can make you wonder why you bothered investing in the larger field of view

For finding purposes "sharp across the whole field" might not be so important though.

 

 

 

 

 

 

Edited December 17, 2020 by John

[Louis D]

If you're hunting for planetary nebula with a low power, 2" eyepiece, having a well corrected field can help immensely in distinguishing tiny, fuzzy PNs at low power from merely bloated stars due to poor field correction.  Same goes for Uranus and Neptune.

[John]

Just to illustrate the wider field of view that the 2 inch format can deliver, below is the Orion's Sword area with the field diameters marked for the following eyepieces used with your Skywatcher 250 dob:

Green: The Vixen NPL 30mm plossl in the 1.25 inch format.

Yellow: The Explore Scientific 30mm ultra wide angle in the 2 inch format.

Edited December 17, 2020 by John

[tripleped]

Thanks to all of you for the insight. I always learn so much from posting on this forum. Having rekindled my love for astronomy after a 45 ish year hiatus I’ve got a lot of catching up to do so thanks again for sharing your know how and experience!

[pregulla]

Only above certain focal length 2" eyepieces offer larger field of view. For example if you want 82 degrees apparent field of view than for eyepieces with longer focal length than about 17mm you will need 2", if you want 70 degrees - than roughly above 24mm you need 2". At shorter focal lengths 2" or 1.25" by itself doesn't make a difference. 

[RT65CB-SWL]

39 minutes ago, pregulla said:

Only above certain focal length 2" eyepieces offer larger field of view. For example if you want 82 degrees apparent field of view than for eyepieces with longer focal length than about 17mm you will need 2", if you want 70 degrees - than roughly above 24mm you need 2". At shorter focal lengths 2" or 1.25" by itself doesn't make a difference. 

 

      

Here is what is I use for 82deg AFOV... TeleVue 13mm Nagler type 1 & 6. The type 1 is a heavy e/p
and the OTA does require re-balancing. It can be used with either a 1.25" & 2" nosepiece, whereas
the type 6 is 1.25".

 

For size comparison, my Nagler's alongside my TeleVue 13mm Plossl... (as shown on left).

Edited December 21, 2020 by Philip R

[John]

Yes, in some cases the 2 inch "skirt" is there as a convenience to enable a 2 inch scope connection to be used for a heavy eyepiece. The optics in these cases are still 1.25"

When the field stop of the eyepiece needs to be over 27mm, the 2 inch format barrel is required to accommodate that and the eyepiece is a "true" 2 inch one.

This graph sets out the limits for the 1.25 inch barrel :