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Reged: 08/30/07

Loc: Fenton, MI
Re: ETX125 and 2" accessories new [Re: cavefrog]
      #5571926 - 12/15/12 01:42 AM

Just trying to honor his wishes - I dont know if he wants his address made public for the time being.

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Carpal Tunnel

Reged: 12/31/08

Loc: Western New York
Re: ETX125 and 2" accessories new [Re: Jaimo!]
      #5574020 - 12/16/12 11:40 AM Attachment (48 downloads)

I doubt you are actually getting 1.25° TFOV, while mathematically possible, in practice I would believe it is quite less. If you have the time, I'd be interested in your results if you made a few measurements using the "drift method" with couple of different eyepieces. An eyepiece with a max field for 1.25" barrel: 32mm Plössl, 40mm Plössl or 24mm SWA - all which yield ~0.9° TFOV; and also measurements with your 1.25° TFOV setup; also, the 26mm Meade 4000 Super Plössl that come with the scope as a standard. This could be very helpful for the entire ETX community as this subject as been brought up many times over the years, experimentally you could put this issue to rest in one evening. I would do it, but I got rid of my ETX about a year ago. Here is a good link for information on using the drift method. What do you think? Jaimo!


The chances of me getting out in the winter to do a FOV test using the drift method ranges from zero to none. I live in a snow belt area and lake effect clouds pretty much "closes out" any observing from mid-November through early March.

However, I did set up a FOV test inside my home. First, I determined mathematically the FOV of my 1.25" - 40mm. (29mm. field stop) and 2" - 32mm. (39mm. field stop) eyepieces when used on ETX-125.

Theoretical FOV results:

1.25" - 40mm .... 52.2 arc-minutes (0.87 degrees)
2.00" - 32mm .... 70.8 arc-minutes (1.18 degrees)

Next, I positioned a yard stick 42.6' from the ETX-125. Using the 1.25" - 40mm eyepiece, the reading was 8" ... and for the 2.00" - 32mm eyepiece, the reading was 10.1" ... with no discernible vignetting along the edge. Doing the math:

Actual FOV Results:

1.25" - 40mm .... 54 arc -minutes (.9 degrees)
2.00" - 32mm .... 68.4 arc-minutes (1.14 degrees)

Also, theoretical FOV from photo port (27mm opening) .... 48.6 arc-minutes (.81 degrees)

My conclusions ... The theoretical and actual FOVs in the 1.25" - 40mm are essentially the same. The actual FOV in the 2.00" - 32mm eyepiece falls slightly short of the theoretical FOV. However, the use of the 2" - 32mm eyepiece does represent an actual increase in the viewing area of 62% ... over the maximum realized with an 1.25" eyepiece. Vignetting is also an important issue and although none was noticed during testing, almost all optical systems suffer from this problem. Was testing during the daytime and not at infinity focus a factor? I don't know the answer. Also, why does a 2.00" eyepiece seem to work well in the ETX-125 when the optical train is restricted by the 27mm opening in the photo port? Any comments, thoughts or ideas on this test are certainly welcomed. BTW, here's a shot of my EXT-125 set up for use with 2.00" eyepieces:

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Pentax Syntax

Reged: 11/03/12

Loc: Maryland
Re: ETX125 and 2" accessories new [Re: *skyguy*]
      #5579742 - 12/19/12 10:10 PM

Interesting. I have observed that a 2" diagonal and eyepieces do increase the field but I see some vignetting with a 30 Pentax XW and Meade 30 QX Wide angle. The system is still useful though. Thanks for posting.

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Reged: 10/11/07

Loc: Exit 135 / 40° North
Re: ETX125 and 2" accessories new [Re: Pentax Syntax]
      #5611142 - 01/07/13 11:16 PM

Excellent work Skyguy! Sorry it has taken me so long to get back...


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