7 replies to this topic

[nblauss]

Hi, all.  I've seen a few posts on this, but - having had the question and now having answered it - I figured I'd put this up.

 

I've got an old Meade etx125.  It's been de-forked, and remounted on a GEM - which was a process.  But I've now replaced the plastic back with one of the Wegat ones, with 2" compatibility.  I wanted to know if a two inch diagonal was "worth it", given the pinch point coming out of the visual back.  So I ran some comparisons with several eyepieces, with both a 1.25" diagonal, and with a borrowed 2" - both from Meade.

 

At 12.7m of distance from the front of the scope to a meter stick, the diameter of the visual field was:

 

In the 1.25" category:

24mm 68 degree Explore Scientific: 19.9cm ~ 0.9 degrees TFOV

40mm Orion Plossl: 20cm ~ 0.9 degrees TFOV

 

In the 2" category:

30mm Skyline Erfle 68degrees: 23.7cm ~ 1.07 degrees TFOV

30mm Explore Scientific 82 degree: 28cm ~ 1.26 degrees TFOV

 

Please forgive me if I'm calculating something wrong.  I'm figuring these angles based on my eye being at the lens at the front of the telescope.

 

So you do get a significant increase in TFOV using the 2" back.  There was some slight chromatic aberration at the edges of the ES 30mm, and the field wasn't super flat all the way across, but I think I've decided that the 2" diagonal would be worth it.  While that 30mm ES is verging on being the same size as the OTA, for a scope with long focal length (my other scope is a 12" Dob), getting a wide angle view seems valuable.  Haven't had the chance to do a comparison at night, but I know its worth a try now.

 

 

 

 

[SeattleScott]

I think the real question will be vignetting. Probably get some sense of this during day but won’t really know until you get it under the stars.

Scott

[AhBok]

Good work on the test. I’ve done similar with my setup. I actually started with a 2” diagonal after performing daylight tests. I found the edge performance less appealing though, than using a 1.25” plossl. Perhaps using a higher quality 2” eyepiece than my SWAs would have improved that. I do find that I only really use the Mak for quick peeks at SS objects and some travel, reserving the larger DSOs for my Apo where 2” gear makes the most sense.

If a small bit additional vignetting, increased focal length and aperture reduction is acceptable to you, then enjoy!

[aeajr]

Hi, all.  I've seen a few posts on this, but - having had the question and now having answered it - I figured I'd put this up.

 

I've got an old Meade etx125.  It's been de-forked, and remounted on a GEM - which was a process.  But I've now replaced the plastic back with one of the Wegat ones, with 2" compatibility.  I wanted to know if a two inch diagonal was "worth it", given the pinch point coming out of the visual back.  So I ran some comparisons with several eyepieces, with both a 1.25" diagonal, and with a borrowed 2" - both from Meade.

 

At 12.7m of distance from the front of the scope to a meter stick, the diameter of the visual field was:

 

In the 1.25" category:

24mm 68 degree Explore Scientific: 19.9cm ~ 0.9 degrees TFOV

40mm Orion Plossl: 20cm ~ 0.9 degrees TFOV

 

In the 2" category:

30mm Skyline Erfle 68degrees: 23.7cm ~ 1.07 degrees TFOV

30mm Explore Scientific 82 degree: 28cm ~ 1.26 degrees TFOV

 

Please forgive me if I'm calculating something wrong.  I'm figuring these angles based on my eye being at the lens at the front of the telescope.

 

So you do get a significant increase in TFOV using the 2" back.  There was some slight chromatic aberration at the edges of the ES 30mm, and the field wasn't super flat all the way across, but I think I've decided that the 2" diagonal would be worth it.  While that 30mm ES is verging on being the same size as the OTA, for a scope with long focal length (my other scope is a 12" Dob), getting a wide angle view seems valuable.  Haven't had the chance to do a comparison at night, but I know its worth a try now.

I am not sure I understand.  Did you actually do this as a physical test or did you just calculate what you expect.

 

I also have an ETX 125 that I deforked and put on a Twilight 1 mount.  It was my understanding that the hole in the back was not large enough to justify 2" eyepieces so I have stayed with the stock back/diagonal.  If you actually tested this and measured the true FOV I would be interested in the Wegat you used.  A link and photo would be welcome. 

[nblauss]

I did this as a physical test.  I used this Wegat back (the first one shown).  I've yet to do the test at night (it's been crummy weather in Maine, and I have a big Dob that I've been using instead while I work out some balancing issues).  More to come if get the chance.

[aeajr]

Thanks.  I will be interested to read any updates you post.

 

Not worth the cost to me.  I have other options.

[AhBok]

The rear port on my ETX125 is 30mm. If you are as demanding as I am and have other options, I recommend something like a 32mm plossl with a decent 1.25” diagonal for cleanest non-vignetted views. You can get a bit wider views using the Wegat with a 2” diagonal, but vignetting, edge performance and aperture reduction effects are annoying for the more fastidious among us.

If none of that matters, you can get a bit wider field. This may be worthwhile for those with only this OTA.

[nblauss]

Well, I finally got the borrowed diagonal home on a clear night, and I didn't see much if any vignetting even in that 31mm ES eyepiece.  I tested it on the Orion Nebula, along with Rigel, and saw no noticeable dimming as they were moved to the edge of the field of view.  The field was not entirely flat, but for visual observing it seems to be worth a peak.

 

Anyway, I know a few folks were interested.

 

That's all I've got!