Field Stop Size and AFOV of 1.25" 32mm/40mm Eyepieces

[Rocket_the_Raccoon]

Hi! Everyone, I have a grab-and-go short refractor with a focal length of 400mm. My lowest power eyepiece right now is an OMCON Plossl 25mm (~Celestron silver-top Plossl) which gives ~3 degrees of TFOV. As I live in a heavily light polluted city and the OTA only has a red-dot finder, I need the widest possible field for finding objects. Star-hopping with a 3 degrees field can sometimes be very difficult.

So I am looking for an economical 32mm or 40mm, like the ones from Skywatcher, Orion, and GSO, which I believe they are all the same (just different brands). Then I came across some contradicting info. online. According to Orion:

http://www.telescope.com/mobileProduct/Accessories/Telescope-Eyepieces/32mm-Orion-Sirius-Plossl-Telescope-Eyepiece/pc/3/c/47/8728.uts

http://www.telescope.com/mobileProduct/Accessories/Telescope-Eyepieces/40mm-Orion-Sirius-Plossl-Telescope-Eyepiece/pc/3/c/47/8730.uts

The field stop diameters are both 28.5mm. This gives a TFOV of (28.5 / 400) x 57.3 = 4.1 degrees. However, another online source did a "lab test" on the Skywatcher versions:

http://astro-okulare.de/English/indexe.htm

and found out the AFOV of the two (32mm, 40mm) are actually 43 degrees and 36 degrees respectively. These give TFOV of 3.44 degrees and 3.6 degrees using the forumla, AFOV / magnification = TFOV.

I wonder which is true?

Thanks!

 

[Louis D]

First off, do you have 2" focuser?  If so, move up to 2" eyepieces and diagonal, and you can get to about a 6+ degree TFOV.  If not, you are stuck with a 27-28mm maximum field stop diameter in a 1.25" focuser.

The old 35mm Celestron Ultima Plossl went slightly bigger by putting the field stop above the eyepiece shoulder.  This required quite a bit more in-focus, however.

You could also try this 1.25" to 2" adapter if you have enough in focus and don't mind severe vignetting.

[michael.h.f.wilkinson]

A 28.5mm field stop is rather large, I feel. As fr as I know, the biggest field stop possible without vignetting in 1.25" format is 27mm or perhaps 27.5. The Panoptic 24mm sports a 27mm, I measured my Antares 25mm 70 deg EP at 27.3mm. That means a maximum TFOV of 3.87 deg or 3.91 deg respectively.

[Ricochet]

If you are observing from a heavily light polluted city then I think that you may be better off maxing out the 1.25" barrel with a 24mm 68° eyepiece rather than either of the Plossl options which will show quite a bright background sky. 

With regards to the TFoV calculations, the field stop based one is the accurate one assuming the field stop measurement is correct. AFoV/mag is just a approximation and eyepiece distortions will cause a difference, however half a degree sounds like an awful lot. Drift timing is probably required to find what the actual TFoV is. 

[Rocket_the_Raccoon]

Thank you everyone for your inputs!

22 hours ago, Ricochet said:

If you are observing from a heavily light polluted city then I think that you may be better off maxing out the 1.25" barrel with a 24mm 68° eyepiece rather than either of the Plossl options which will show quite a bright background sky. 

You are right, Ricochet. I forgot the fact that the background sky will become brighter as the magnification decreases. It seems that I will have to spend quite a bit more than originally planned to obtain a wider field as a wide field 25mm is more expensive than a basic 32mm/40mm Plossl. :-(

22 hours ago, michael.h.f.wilkinson said:

A 28.5mm field stop is rather large, I feel. As fr as I know, the biggest field stop possible without vignetting in 1.25" format is 27mm or perhaps 27.5. The Panoptic 24mm sports a 27mm, I measured my Antares 25mm 70 deg EP at 27.3mm. That means a maximum TFOV of 3.87 deg or 3.91 deg respectively.

Michael, may I ask how do you feel about your Antares W70 25mm? This is the only wide field 25mm that is within my budget. Thanks.

[michael.h.f.wilkinson]

16 minutes ago, Rocket_the_Raccoon said:

Thank you everyone for your inputs!

You are right, Ricochet. I forgot the fact that the background sky will become brighter as the magnification decreases. It seems that I will have to spend quite a bit more than originally planned to obtain a wider field as a wide field 25mm is more expensive than a basic 32mm/40mm Plossl. :-(

Michael, may I ask how do you feel about your Antares W70 25mm? This is the only wide field 25mm that is within my budget. Thanks.

I use it only in my 14x70mm finder, where the astigmatism (in this fast F/5 scope) is not a real problem. It should be OK in a slow scope, I guess, but the MaxVision 24mm 68° I have is much better.

[Rocket_the_Raccoon]

Thanks, Michael. I just read your "review" of the Antares from another threads. The OTA thar I am going to use the eyepiece on is in fact similar to yours, 70mm f/5.7 and my intended use of it is similar too. Worst case scenario, I can use it on a 50mm finder that I am building as you have mentioned the easy placement of cross-hair.